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Perovskite ferroelectric thin films for tunable microwave applications

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UNIVERSITY OF PUERTO RICO
PE R O V SK IT E FE R R O E L E C T R IC TH IN FILM S F O R TUNABLE
M IC RO W A V E A PPLIC A TIO N S
By
MENKA JAIN
A Dissertation In Partial Fulfillment O f The Requirements For The Degree O f
DOCTOR OF PHILOSOPHY
Program in Chemical Physics
Department o f Physics
Faculty o f Natural Sciences
Supervised hy
Prof. Ram S. Katiyar
San Juan, Puerto Rico
MARCH 2004
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UMI Number: 3133639
Copyright 2004 by
Jain, Menka
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P E R O V SK IT E F E R R O E L E C T R IC TH IN FIL M S F O R TUNABLE
M IC R O W A V E A PPLIC A TIO N S
ACCEPTED BY FACULTY OF THE CHEMICAL PHYSICS PROGRAM OF THE
UNIVERSITY OF PUERTO RICO
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE
OF
DOCTOR OF PHILOSOPHY
D r. R am S. K atiy ar
D r. Luis F. Fonseca
THESIS ADVISOR
THESIS COMMITTEE MEMBER
t.
D r. Fouad Aliev
D r. E dw in Q uinones
THESIS COMMITTEE M EMBER
THESIS COMMITTEE MEMBER
f
6
^
____
D r. M aria S. Baez
DEAN, FACULTY OF NATURAL SCIENCES
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A BSTRACT
Simple and cost effective sol-gel technique was utilized to deposit highly oriented
thin film perovskite ferroelectrics for their possible applications in timahle microwave
devices. We optimized the process methodology to synthesize highly (100) oriented
Bao.sSro.sTiOs (BST50) and Bao.eSro^TiOa (BST60) thin films on (100) strontium titanate
and ( 100 ) lanthanum aluminate substrates by sol-gel technique and to achieve best
possible dielectric properties for tunable microwave devices. The annealing temperature
and time were found to have effect on the epitaxial quality, phase transition behavior, and
dielectric properties o f the BST films. W ith increase in annealing temperature the grain
boundaries disappear and the grains coalescence occurs with increase in annealing time
and the epitaxy is improved. Eight element coupled microwave phase shifters were
designed using the standard lithographic method and phase shift & loss characteristics
were measured between 12-18 GHz range (Ku band). The degree o f phase shift was
increased from 221 to 308° (measured at 14.5 GHz in an electric field ranging from zero
to 30V/pm) with the improvement in the epitaxial quality o f the films. An insertion loss
o f 8.435 dB, phase shift in the order o f 320° (20-340V) and
k
value (phase shift per dB
loss) o f about 38 °/dB was achieved in Bao.6Sro.4Ti 03 films deposited on LAO (100)
substrate.
We have also studied the effect o f Mn doping on the degree o f texturing, surface
morphology, dielectric properties, and phase transition behavior o f BST50 thin films.
W ith 3 at % Mn doping the degree o f (100) texturing and grain size o f BST50 thin films
were markedly improved, which led to an increased tunability from 29% (undoped) to
39% (3 at % Mn doped); measured at 1 MHz and 2.34V/mm bias field. The phase shift
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measurements at 16 GHz showed that the degree of phase shift increases from 239° to
337° with 0 to 3 at% Mn doping. The insertion loss also increased from 5.4 dB (undoped)
to 9.9 dB (with 3 at % M n doped), so that there is no effective improvement in the k
factor, which remains in the range o f 33 - 44°/dB. The modification in the surface
morphology and film stoichiometry induced by M n doping is thought to play significant
role in the observed phase shifter characteristics.
It would be desirable for the ferroelectric thin films to have moderate dielectric
constant while having low losses in order to obtain high figure o f merit, hence hetero­
structured thin films o f BST with low loss materials like MgO and MgTiOs were
synthesized. We have therefore studied the effect o f thickness o f the BST50 and MgO
(and MgTiOs) layers on the structural, microstructural, & dielectric properties, and the
investigations on the leakage current characteristics o f these heterostructured thin films. It
was observed that the layer thicknesses play an important role in the dielectric, electrical,
and phase shifter properties. A significant improvement in the figure o f merit was
obtained for the BST:MT (-72 °/dB) and BSTiMgO (-8 7 °/dB) multilayer structures in
the Ku band region.
Finally, we studied another material, lead strontium titanate (PST), which is also a
potential candidate material for the timable microwave devices. PbojSro.TTiOs (PST30)
thin film films on LAO were highly (100) oriented and showed transition temperature as
in the case o f bulk. The preliminary results gave the phase shift o f 271° at 15.75 GHz and
the best corresponding value o f insertion loss o f 4.843 dB, which corresponds to figure o f
merit o f 56°/dB.
11
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The BST heterostmctured films have demonstrated a great promise for
applications in the tunable microwave devices and there is room for further enhancement
in the characteristic properties. Moreover, the work on the lead strontium titanate thin
films included in this thesis should provide a new insight in this important emerging area
o f microwave & related applications.
Ill
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ACKNOWLEDGEMENTS
First and foremost, my deepest and sincerest appreciation goes to my advisor
Prof. Ram S. Katiyar for his continuous guidance, support and enlightening discussions
during the years at the University o f Puerto Rico. I have learned a great deal from him,
which have promoted me to become a better researcher as well as a better person.
I would like to express my deepest gratitude to Dr. Amar Bhalla for his scientific
insight, continuous support, and guidance throughout my work. I benefited tremendously
from his depth o f knowledge in many diverse fields. His confidence in me has always
been a motivating factor to keep me striving to reach my goals. I am thankful to him for
supervising this thesis and constant discussions. I appreciate Prof. L. Fonseca, Prof. F.
Aliev, and Prof. E. Quinones for taking on the painful task o f reviewing my dissertation.
I gratefully acknowledge three-year financial support from the NSF fellowship.
I would like to extend my thanks to Prof. D.C. Agrawal, Prof. M. Tomar, Dr. S.B.
Desu, and Dr. S.B. Krupanidhi for collaboration and useful discussions. I would like to
thank my collaborators Dr. F.A. Miranda and Dr. F.W. Van Keuls, at NASA GRC, for
high frequency characterization, electrode deposition, providing answers to many
questions, and their constant support.
I am also thankfiil to Dr. S.B. Majumder, Dr. P. Bhattacharya, Dr. Yu.I. Yuzyuk,
Dr. S. Bhattacharyya, and Dr. B. Natesan for the discussions while working in various
projects. I would like to acknowledge the members o f Dr. Katiyar’s group, past and
present Rasmi Das, S. Bhaskar, A. Savvinov, William, Naha Karan, Sridevi,
Rajasekarakumar, S. Neito, A. Dixit, S. Das, N. Awasthi, Nora, Margarita, Itzier, Jose,
and many other group members that I caimot enumerate here. I would like to specially
IV
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thank my lab member and friend, William, for always taking the time to listen me
especially during those “coffee hours”, where everything starts with “you are wrong” and
ends with “no problem”. I would also like to thank Naba for his support and being a good
listener during my thesis writing.
I would like to give endless thanks to Mrs Katiyar (Auntiji) for her constant
support and love. She was always there whenever I needed help. Thanks for arranging
parties, by which I came to know a lot o f other families in PR.
I am especially grateful to my beloved family-Mummy (Ma), Papa, Payal di,
Puneet Jiju, Ruchi, Arpit, and Pallak-for always being there whenever I need. I owe all
the achievement to their extreme love, never failing prayers, unfaltering belief, and
support for me! Most importantly I thank my mother, who taught me to have confidence,
patience, belief in my dream, to whom I owe everything. I look up to my mother for her
never-ending support. I am thankful to my father to teach me great and useful skill o f
working with tools, which helped me a lot in fixing a lot o f things. I would like to thank
to my cousin brother Anuj for his support.
I would like to thank my friends in Puerto Rico, United States, and India for their
direct and indirect support. I am thankful to my loving fiiends-Anabelen, Mima, and
especially Jossie for providing me support whenever I needed.
Direct and indirect care and support from my beloved family and friends gave me
lot o f confidence, love, encouragement, and support, which means a lot to me throughout
those countless hard-working days and nights.
Above all, thanks to the GOD, who gave me the most support, without his
blessing everything was impossible.
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TABLE OF CONTENTS
A B S T R A C T ..........................
i
A C K N O W L E D G E M E N T S ...................................................................................................... iv
TABLE O F C O N T E N T S .......................................................................................................... v i
L IST OF T A B L E S
.............................................................................................................
L IST OF F IG U R E S .............................................................................................
x
x ii
LIST OF A B B R E V IA T IO N S .................................................................................................. x ix
D ISSER TA TIO N O R G A N IZ A T IO N ..................................................................................... xx
TH ESIS R E L A T E D PU B L IC A T IO N S A N D P R E S E N T A T IO N S ...............................xx i
Chapter 1: Introduction..................................................................................... 1
Ferroelectrics...................................................................................................................... 2
Phase Transitions In F erroelectrics................................................................................. 6
Ferroelectric D om ains...................................................................................................... 10
Modifications in B aT iO s.................................................................................................. 12
Applications o f Ferroelectrics..........................................
15
Microwave D ielectrics..................................................................................................... 16
1.6 .1 Linear-dielectrics for microwave d e v ic e s
........................................ 18
1.6.2 Tunable non-linear dielectrics for microwave d e v ic e s............................... 21
1.6.2.1 Devices based on tunable dielectrics: phase array
antenna and phase s h ifte rs ................................................................... 21
1.6.2.2 Properties o f dielectrics relevant to tunable microwave
device applications.............................................................................. 25
1.6.2.3 Suitable materials for tunable dielectric ap plications....................29
1.6.2.4 Properties o f barium strontium titanate (BST) relevant
to MW applications............................................................................. 32
1.6.2.5 Properties o f lead strontium titanate relevant to MW
applications.............................................................................................34
1.7 Literature re v ie w ............................................................................................................... 37
1.7.1 BST ceramics for tunable microwave device applications.......................... 37
1.7.2 BST thin films for tunable microwave device ap plications
.............42
1.7.3 Lead strontium titanate (PST) for tunable microwave d e v ic e s................... 54
1.8
Motivation for the present stu d y
........................................................................... 56
1.9 Statement o f the p ro b lem ................................................................................................ 58
1.10 R eferences.........................................................................................................................61
1.1
1.2
1.3
1.4
1.5
1.6
C h a p t e r 2 : E x p e r i m e n t a l T e c h n i q u e s .................................................................... 68
2.1 F abrication of th in films and p o w d e rs ..........................................................................68
VI
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2.1.1 Sol-gel technique...............................................................................................69
2.1.2 Sol-gel synthesis o f powders and thin films in this th e s is .......................... 76
2.1.2.1 Preparation o f Pure BST F ilm s........................................................79
2.1.2.2 Preparation o f manganese (Mn) doped BST film s ....................... 80
2.1.2.3 Preparation o f Heterostructured BST Thin F ilm s.........................81
2.1.2.4 Preparation o f Pure PST Thin F ilm s ............................................... 82
2.2 C haracterization te c h n iq u e s ......................................................................................... 85
2.2.1 Optical and vibrational stu d ies......................................................................... 85
2.2.1.1 Micro-Raman spectroscopy............................................................. 85
2.2.1.2 Film thickness measurements: spectroscopic reflectom etry
87
2.2.2 Structural and microstructural characterization............................................. 89
2.2.2.1 X-ray diffraction technique............................................................. 89
2.2.2.2 Rutherford backscattering spectroscopy (RBS) .......................... 91
2.2.2.3 Atomic force microscopy (A F M )................................................... 91
2.2.2.4 Scanning electron microscopy (S E M )............................................93
2.2.3 Thermal characterization o f m aterials............................................................93
2.2.4 Composition and depth profile analysis by X-ray photon
spectroscopy (XPS) ...........................................................................................95
2.2.5 Dielectric and electrical characterizations......................................................97
2.2.5.1 Interdigitated electrodes................................................................... 97
2.2.5.2 Dielectric and ferroelectric properties............................................97
2.2.5.3 Leakage characteristics.....................................................................99
2.2.6 High frequency phase-shifter m easurem ents................................................ 99
2.3 R eferences........................................................................................................................... 102
C h a p t e r 3 : S tu d ie s o f B a r i u m S t r o n t i u m T i t a n a t e T h i n film s f o r
M ic r o w a v e D e v ic e A p p l i c a t i o n s ............................................................................... 104
3.1 Sol-gel grow th o f grain oriented (B a,Sr)Ti 0 3 thin film s ......................................... 105
3.1.1 Introduction.......................................................................................................105
3.1.2 Experimental D e tails....................................................................................... 110
3.1.3 Results and D iscussion....................................................................................112
3.1.3.1 Thermal analysis o f sol-gel derived p o w d ers.............................. 112
3.1.3.2 Phase Evolution o f Gel Derived P o w d ers....................................112
3.1.3.3 Phase and Microstructural Evolution o f BST Thin F ilm s......... 113
3.1.3.4 Dielectric Properties.......................................................................125
3.1.3.5 High Frequency Phase Shifter S tu d ies..........................................125
3.1.4 C onclusions.......................................................................................................130
3.1.5 R eferences......................................................................................................... 131
3.2 Effect of A nnealing C onditions on b ariu m stro n tiu m titan ate T hin F ilm s
133
3.2.1 Introduction.......................................................................................................133
3.2.2 Experimental D e tails....................................................................................... 135
3.2.3 Results and D iscussions..................................................................................136
3.2.3.1 Structural and Microstructural Characterizations..................... 136
Vll
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3.2.3.2 Compositional analysis................................................................... 137
3.2.3.3 Dielectric and electrical properties................................................ 143
3.2.3.4 Phase shifter characteristics............................................................154
3.2.4 C onclusions....................................................................................................... 156
3.2.5 R eferences......................................................................................................... 157
C h a p t e r 4 : D o p in g E f f e c ts o n M ic r o w a v e P r o p e r t i e s o f B a r i u m
S t r o n t i u m T i t a n a t e T h i n F i l m s ................................................................................ 159
4.1 M anganese (M n) doping in thin b a riu m stro n tiu m titan ate thin film s
160
4.1.1 Introduction....................................................................................................... 160
4.1.2 Experimental D e tails........................................................................................162
4.1.3 Results and D iscussions.................................................................................. 165
4.1.3.1 Structural and microstructural characterization...........................165
4.1.3.2 Dielectric and electrical properties................................................ 171
4.1.3.3 Phase shifter eharacteristics............................................................173
4.1.4 C onclusions.......................................................................................................176
4.1.5 R eferences......................................................................................................... 177
4.2 G raded M anganese doped B arium S trontium T itanate T hin Film s ................... 179
4.2.1 Introduction....................................................................................................... 179
4.2.2 Experimental D e tails....................................................................................... 181
4.2.3 Results and D iscussion....................................................................................183
4.2.3.1 Structural and microstructural characterizations......................... 183
4.2.3.2 Dielectric and electrical pro p erties................................................185
4.2.4 C onclusions.......................................................................................................190
4.2.5 R eferences......................................................................................................... 190
C h a p t e r 5 : S tu d ie s o f M ic r o w a v e D ie le c tr ic P r o p e r t i e s o f B S T T h i n
F ilm s H e t e r o s t m c t u r e d w ith lo w lo s s d i e l e c t r i c s ............................................ 193
5.1 H etero-structured Bao.5Sro.5T i 0 3 :M gO T hin F ilm s .................................................194
5.1.1 Introduction.......................................................................................................194
5.1.2 Experimental D e tails....................................................................................... 197
5.1.3 Results and D iscussion....................................................................................199
5.1.3.1 Structural and microstructural characterization...........................199
5.1.3.2 Dielectric and electrical p ro p erties................................................204
5.1.3.3 High frequency Phase shifter stu d ies............................................ 209
5.1.3.4 Depth profile XPS stu d ies.............................................................. 212
5.1.4 C onclusions.......................................................................................................215
5.1.5 R eferences......................................................................................................... 215
5.2 H etero-strnctnred (Bao.5Sro.5)T i 0 3 :M gTi 0 3 T hin F ilm s ........................................218
5.2.1 Introduction.......................................................................................................218
5.2.2 Experimental D e tails....................................................................................... 220
5.2.3 Results and D iscussions.................................................................................. 222
V lll
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5.2.3.1 Structural and microstructural characterization............................222
5.2.3.2 Raman spectroscopic stu d ies.......................................................... 224
5.2.3.3 Depth profile XPS stu d ies...............................................................227
5.2.3.4 Dielectric and electrical properties................................................ 229
5.2.3.5 High frequency phase shifter characteristics................................ 232
5.2.4 C onclusions........................................................................................................ 236
5.2.5 R eferences.......................................................................................................... 237
Chapter 6: Lead Strontium Titanate thin Films for high frequency
phase shifter applications.............................................................................. 239
6.1 Introduction.........................................................................................................................239
6.2 Experimental D e ta ils......................................................................................................... 242
6.3 Results and D iscussions....................................................................................................243
6.3.1 Thermal analysis o f sol-gel derived p o w d e r................................................. 243
6.3.2 Structural and microstructural studies o f thin film s......................................245
6.3.3 Dielectric studies o f thin films..........................................................................248
6.3.4 High frequency phase shifter studies o f thin film s .......................................250
6.4 C onclusions.........................................................................................................................252
6.5 R eferences...........................................................................................................................253
Chapter 7: Conclusions and Fnture W o rk ................................................ 255
7.1 Conclusions..........................................................................................................................255
7.2 Future w o rk .........................................................................................................................259
IX
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LIST OF TABLES
1.1
Crystal classes.
3
1.n
Binary perovskite type oxides.
4
l.n i
Common aliovalent substituents.
14
1.IV
Frequency assignments to different bands.
17
l.V
Dielectric permittivity, figure o f merit {Qxf),
coefficients o f important linear dielectrics [9].
1.VI
Ferroelectric material vs current phase shifter technology [20].
23
I.Vn
Properties o f most commonly used
{Courtesy Dr. A.S. Bhalla].
30
1.Vni
STO thin film studies.
32
I.IX
Ceramic composite review.
39
1.X
Review o f pure, doped and composite BST thin films.
51
2.1
Various precursors used in the BST sol synthesis.
76
2.11
Additional precursor used for Mn doping.
80
2.ni
List o f precursor materials used in the PST sol synthesis.
83
2.rV
Details o f the PST thin films deposited in the present thesis.
84
3.1.1
Effect o f annealing temperature on (100) orientation o f BST60 film on
LAO (100) substrate.
109
3.1.11
Comparison o f the characteristics o f 8 element CMPS using BST60 thin
films deposited by sol-gel and other techniques.
130
3.2.1
Atomic concentration o f type A , B, and C BST(50/50) films,
determined from the slow scanned XPS spectra o f Ba 3 d 5/2, Sr 3d, Ti 2p
140
and
and potential
temperature
ferroelectrics
19
and O Is peaks.
3.2.n
Peakfit summary o f the transition behavior o f Type A, B, and C
BST(50/50) films determined after de-convolution o f capacitance vs
temperature plots using Gaussian peaks.
148
3.2.III
Phase shifter characteristics o f the Type A BST50 film annealed at 1050
°C/2hr.
155
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3.2.IV
Phase shifter characteristics o f Type C BST50 film aimealed at 1100
°C/6hr.
155
4.1.1
Characteristics o f the phase shifters using BST thin films.
176
5.1.1
Sample identification.
197
5.1.11
Room temperature dielectric properties o f the pure and heterostmctured
BST50 films measured at I MHz and applied electric field o f 25.3
kV/cm.
208
5.1.ni
Detailed phase shifter data o f the pure BST50 and heterostmctured
BST50:MgO films.
213
5.2.1
Sample identification.
221
5.2.11
High frequency phase-shifter characteristics o f the pure BST50 film.
235
5.2.n i
Phase shifter characteristics o f the Type A BST50:MT film.
236
5.2.1V
Phase shifter characteristics o f the Type B BST50:MT thin films.
236
6.1
High frequency characteristics o f the eight element coupled phase
shifter fabricated on PST30 film on LAO at 533kV/cm.
252
XI
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LIST OF FIGURES
1.1
(a) A cubic ABO 3 (BaTi 0 3 ) perovskite-type unit cell and (b) three
dimensional network o f com er sharing octahedra o f O^' ions.
3
1.2
Schematic Ps versus temperature curves for (a) second order phase
transition and (b) first order phase transition. There is a sudden jum p o f
Ps in the first order phase transition.
8
1.3
Perovskite BTO unit cell in centrosymmetric paraleectric phase (a), and
in the non-centrosymmetric ferroelectric phase (b) w ith spontaneous
polarization Ps.
8
1.4
Various phase transitions in barium titanate.
9
1.5
(a) The Cubic (high temperature and tetragonal (room temperature)
structures o f barium titanate, (b) Upon cubic to tetragonal phase
transition, the imit cell can take any o f the six equivalent combinations
o f strain and polarization. The arrow indicates the direction o f
polarization.
11
1 .6
(a) Schematic o f the subgranular stracture o f domains separated by 90 °
and 180 ° boundaries, (b) Ferroelectric domain pattem s in barium
titanate ceramic.
11
1.7
The effect o f isovalent substitutions on the transition temperatures o f
BTO ceramic [3].
14
1 .8
The microwave spectmm and its applications.
17
1.9
Frequency dependence o f K and Q values o f representative dielectric
materials.
20
1.10
Schematic representation o f various relaxation and resonance processes,
//H z.
^7
1.11
Phase diagram o f BaTi 0 3 -SrTi 03 system [34].
33
1.12
The temperature dependence o f % for several x values in Ba].xSrxTi03
(x=0-90) system [32].
33
1.13
Temperature dependent o f s / sq (or x) for a series o f applied electric
fields for BST5 ceramic at f=800 Hz [35].
34
Xll
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1.14
Variation o f Curie temperature o f PST solid solution [37].
36
1.15
Temperature dependent dielectric constant for ceramic and thin film
BST70 [52].
42
1.16
(a) Parallel plate and (b) interdigitated device structures.
46
1.17
Schematic o f stacked BST capacitor and relavent device integration
issues.
46
2.1
Schematic diagram o f hydrolysis and condensation reaction in alkoxide
73
M (0R )4.
2.2
Flow chart o f the sol-gel process o f BST sol and thin film preparation.
79
2.3
Experimental set up for Raman Spectroscopy.
86
2.4
Schematic o f the Filmetrics system.
88
2.5
Diffraction beam path in 0-26 mode.
88
2.6
Contact mode AFM operates by scanning a tip attached to the end o f a
cantilever across the sample surface, while monitoring the change in
cantilever deflection with a split photodiode detector.
92
2.7
Schematic representation o f an XPS spectrometer employing a
hemispherical electron energy analyzer and monochromatic X-ray
source.
96
2.8
IDT capacitor structure deposited on thin films.
98
2.9
Schematic set up o f the MMR cryostat with various accessories.
98
2.10
Schematic o f the eight element coupled phase shifter design deposited
on the BST films at NASA center.
101
3.1.1
Reflectance spectrum o f (100) BST60 film on LaAlOa (100) measured
by filmetrics unit. The film thickness was determined by simulating the
experimental spectrum.
111
3.1.2
(a) DTA and (b) TGA plot o f BTO dried gel powder. The heating rate
was 5°C/min and air ambient was used during heating.
114
xm
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3.1.3
X-ray diffractograms o f BTO dried gel heat treated at (a) 500*^C, (b)
600V , (c) 700°C, (d) 800V , (e) 90 0 V , and (f) lOOOV for 2 h in air.
115
3.1.4
X-ray diffractograms o f BST60 films annealed at (i) 600°C, (ii) 900°C,
(iii) 1000 *^0 , and (iv) 1100*^0 for 2 hours in air.
117
3.1.5
(a) The variation o f the percentage intensity o f the (llO)-peak; w.r.t.
(200) peak and (b) the variation o f intensity and FWHM o f the (200)
diffraction peak o f BST60 film with increasing annealing temperature.
118
3.1.6
AFM micrograph (1pm x 1pm) o f the single coated BST50 film on
LAO, (a) heated at 600 °C for 5min, & annealed at (b) 600 °C for 2h,
and (c) 1100 °C for 6 h.
120
3.1.7
AFM micrographs (2pm X 2pm) o f BST60 thin films annealed in air at
(a) 600 °C for 2 h, (h) 900 °C for 2 h, (c) 1100 °C for 2 h and (d) 1100
°C for 6 h.
121
3.1.8
Cross-sectional SEM o f the BST50 film deposited on STO substrate and
annealed at 1100 °C for 6 h.
123
3.1.9
Pole figure ofB S T 50 film on LAO.
123
3.1.10 Random and channeled speetra o f the BST 50 film on LAO substrate.
126
3.1.11 Variation o f capacitance and loss tangent o f BST60 thin film on STO
substrate as a function o f bias voltage. The measurement was performed
on 100 finger IDT electrode at 100 kHz.
126
3.1.12 Characteristics o f eight element CMPS using BST50 thin film on LAO
(254pm) substrate, variation o f (a) phase shift and (h) insertion loss as a
function o f bias voltage.
128
3.1.13 Characteristics o f eight element CMPS using BST60 thin film on LAO
(254pm) substrate, variation o f (a) phase shift and (b) insertion loss as a
function o f bias voltage.
129
3.2.1
AFM micrographs o f the BST films annealed at (a) 1050°C for 2h and
(b) 1100 °C for 2h and (c) 1100°C for 6 h respectively.
138
3.2.2
XPS scan o f BST50 film annealed at 1050 °C for 2h.
140
3.2.3
Slow scan XPS o f (a) Ba, (h) Sr, (c) Ti, and (d) O peaks.
141
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3.2.4
Experimental as well as simulated backscattering spectra from BST
(50/50) thin films on STO substrate aimealed at 1100 °C for 6 h.
142
3.2.5
Room temperature frequency dispersion o f capacitance and loss tangent
values o f type A BST50 thin film (annealed at 1050 °C/2h) measured at
different oscillation voltages.
145
3.2.6
Temperature dependent dielectric constant o f BST films annealed at (a)
1050 °C for 2h and (b) 1100 °C for 2 h and (c) 1100°C for 6 h.
146
3.2.7
Temperature dependent dielectric losses o f BST films annealed at (a)
1050 °C for 2h and (b) 1100 °C for 2 h and (c) 1100°C for 6 h.
146
3.2.8
Temperature-dependent capacitance for BST films annealed at 1050 C/2
h (a), 1100 C/2 h (b) and 1100 C for 6 h (c). The de-convoluted
Gaussian peaks are also shown in the figure (see text)
147
3.2.9
Temperature dependent hysteresis loops o f BST (50/50) films annealed
at different temperatures and time.
150
3.2.10
Hysteresis loops o f BST (50/50) films annealed at different
temperatures and time (measured at 100 K). The inset shows the
variation o f polarization with temperature.
151
3.2.11
Temperature dependent CV characteristics o f BST (50/50) films
annealed at different temperatures and time.
152
3.2.12
Temperature dependent dielectric properties o f (a) type A, (b) type B,
and (c) type C BST (50/50) films (measured at IMHz).
153
3.2.13
Degree o f phase shift and insertion characteristics o f BST (50/50) thin
films based CMPF phase shifter.
154
4.1.1
Flow chart o f the sol-gel process for M n doped sol and thin films.
164
4.1.2
X-ray diffraction pattems o f (a) undoped, (b) 1.0 at%, (c) 3.0 at%, and
(d) 5.0 at% M n doped BST thin films on (100) LAO substrates.
166
4.1.3
The X-ray pole figure distributions corresponding to the (220) reflection
(20 = 67°) o f undoped and 3 at% Mn doped BST films on (100) LAO
substrates.
168
4.1.4
AFM micrographs o f (a) undoped, (h) 1.0 at%, (c) 3.0 at%, and (d) 5.0
at% M n doped BST films.
170
4.1.5
The frequency dispersion o f the (a) dielectric constant and (b) the loss
172
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tangent o f undoped and M n doped BST films.
4.1.6
Time dependent dieleetrie breakdown data for Mn doped BST50 films.
174
4.1.7
(a) Increase in the leakage current characteristics o f the Mn doped BST
films with the increase o f M n content in films.
174
4.1.8
Characteristics o f an eight element CMPS using undoped and Mn doped
BST (50/50) thin films, variation o f (a) phase shift and (b) insertion loss
as a function o f bias voltage.
175
4.2.1
Schematic o f graded M n doped BST film structure.
181
4.2.2
X-ray diffractograms o f (a) pure and (b) graded Mn doped BST films.
183
4.2.3
AFM micrographs o f the (a) pure and (b) graded doped BST films.
184
4.2.4
Temperature dependent dieleetrie constant and loss tangent o f pure, 3
at% M n doped, and M n graded doped BST films.
187
4.2.5
Tunability and K factor o f pure and graded doped BST thin films as a
function o f temperature.
189
4.2.6
Room temperature leakage current characteristics o f pure, 3 at% Mn
doped, and Mn graded doped BST thin films.
189
5.1.1
X-ray diffractograms o f (a) pure BST50 film and (b) Heterostmctured
BST50:MgO films.
200
5.1.2
RBS channeling spectmm o f the Type III BST50:MgO film.
201
5.1.3
AFM micrographs o f the (a) pure BST50, (b) Type I, (c)
(d) Type H BST50;MgO films.
Type n, and
203
5.1.4
Cross sectional SEM o f (a) pure BST50 and (b) Type I
heterostmctured film.
BST50:MgO
205
Temperature dependent dielectric constant (a) and loss tangents (b) o f
the pure BST50 and BST50:MgO heterostmctured films.
206
5.1.5
5.1.6
Temperature dependent K factor o f the pure BST50 and
heterostmctured films.
BST50:MgO
208
5.1.7
Current-voltage characteristics o f the Type I, II, and HI
films at room temperature.
BST50:MgO
210
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5.1.8
Phase shift o f the pure BST and BST50:MgO films as a fimction o f bias
voltage.
211
5.1.9
Insertion loss values o f the pxu*e BST and BST50:MgO films as a
function o f bias voltage.
211
5.1.10
Depth profile XPS studies o f the Type n BST50:MgO film.
214
5.2.1
X-ray diffractograms o f the pure BST50 (a) and Type A & (b) Type B
(c) BST50:MT films.
223
5.2.2
AFM micrographs o f the BST50 (a), Type A (b), and Type B (c)
BST50:MT films.
225
5.2.3
Room temperature Raman spectra o f the pure BST50 (a), BST%):MT
(Type A) (b), and pure MT (c) thin films on LAO substrate. Peaks
corresponding to substrate are shown with the asterisk (*) mark and
those to MT are shown with circle (o).
226
5.2.4
XPS depth profile spectra o f the (a) Type A and (b) Type B BST50:MT
films.
228
5.2.5
Temperature dependent dielectric constants (a) and loss tangents (b) o f
the pure BST and Type A BST50:MT films.
230
5.2.6
Temperature dependent tunability and K factor o f the pure BST50 and
type A BST50:MT film.
231
5.2.7
Room temperature current-voltage characteristics o f BST:MT Type A
film compared with BST50:MgO (Type II and Type HI) film.
231
5.2.8
Bias voltage dependent phase shift (a) and insertion loss (b)
characteristics o f the phase shifter on Type A BST50:MT film at
different frequencies.
233
5.2.9
Bias voltage dependent phase shift (a) and insertion loss (b)
characteristics o f the phase shifter on Type B BST50;MT film at
different frequencies.
234
6.1
DTA and TGA plots o f the sol-gel derived PST30 powder.
244
6.2
X-ray diffraction pattems o f (a) PST30/Pt/Si and (b) PST30/LAO films.
246
6.3
AFM micrographs (1x1 pm) o f the spin coated PST30 films on the (a)
Pt/Si and (b) LAO substrates.
247
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6.4
Temperature dependent dielectric constant and loss tangents
PST30 films o n Pt/Si and LAO.
o f the
249
6.5
Temperature dependent dielectric constant and loss tangents
PST30 ceramic (prepared by solid state reaction method).
o f the
249
6.6
Phase shifter characteristics o f the PST30 film on LAO in terms o f (a)
Phase shift and (b) insertion losses at different frequencies.
251
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LIST OF ABBREVIATIONS USED IN THIS THESIS
AFM
BST
BSTx
BTO (orB T )
CMPS
CPW
DC
DRAM
DSC
DTA
FAME
FE
FOM
GEO
GHz
Hz
IDT
LAO
LEO
MEO
MHz
MIM
MT
Mw (or mw)
PB
PPG
PS
PST
PTO
RBS
RT
SEM
STO
TCC
TGA
VCO
XRD
XPS
Atomic force microscopy
Barium strontium titanate
BaxSri.xTiOa
Barium titanate
coupled microwave phase shifter
Coplanar waveguide
Direct current
Dynamic random access memory
Differential scanning calorimetric
Differential thermal analysis
Frequency agile materials for electronics
Ferroelectric
Figure o f merit
Geosynchronous earth orbit
Giga hertz
Hertz
Interdigitated
Lanthanum aluminate (LaAlOs)
Low earth orbit
Medium earth orbit
Mega hertz
Metal-insulator-metal
MgTiOs
Microwave
Paraelectrie
Parallel-plate capacitors
Phase shifter
Lead strontium titanate
Lead titanate
Rutherford backscattering
Room temperature
Scanning electron microscopy
Strontium titanate
Temperature coefficient o f capacitance
Thermal gravimetric analysis
Voltage controlled oscillators
X-ray diffraction
X-ray photon spectroscopy
XIX
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Dissertation organization
This thesis presents a comprehensive research effort for the development of
Barium Strontium Titanate (BST) and related thin films for tunable phase shifter
technology using low cost Sol-gel technique. The work focuses on the fabrication o f BST
thin films and optimization o f their dielectric properties for phase shifter applications.
These phase shifters are crucial components for the modem phased array systems. The
studies would therefore be extremely valuable in the development o f low cost BST thin
films with high performance.
A brief outline o f the contents o f each chapter is as follows;
A brief background o f the ferroelectric material and survey o f BaSrTiOs (BST)
thin film material properties for its potential application in tunable microwave devices are
presented in this Chapter 1. Material issues o f BST phase shifters are discussed in the
chapter. Motivation o f the thesis and he statement o f the problem are also presented in
Chapter 2. In the chapter 2, sol-gel technique and various characterization techniques
used in this study are discussed and summarized. B rief details o f the processing o f BST,
PST, and related thin films and powders are also provided in the chapter.
In Chapter 3, section 3.1, optimization o f parameters and approach to achieve
grain oriented BST thin films are presented. Effect o f annealing temperature on
stmctural, microstmctural, and dielectric properties are presented in section 3.2 o f the
chapter. Chapter 4 deals with the study o f uniform (Section 4.1) and graded (section 4.2)
Mn doping in BST thin films. Effects o f MgO and MgTiOs layers on the properties o f
BST thin films are presented in Chapter 5, in sections 5.1 and 5.2 respectively. Chapter 6
deals with the study o f the PbSrTiOs thin films as a potential candidate material for
XX
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tunable device. In the last Chapter 7, we provide a summary o f the present thesis and
discussions o f the future work to further improve the BST and related thin film
performance for high frequency devices.
Most o f the work presented in this thesis has formed part o f the following publications
and presentations:
List of Thesis Related Publications/Presentations;
Publications;
1.
Pbo.sSro.TTiOs Thin Film for High Frequency Phase Shifter Applications, M. Jain ,
R.S. Katiyar, and A.S. Bhalla, Applied Physics Letters, 2004. (Submitted)
2.
Study o f Local Disorder in Lead Strontium Titanate Ceramics and Composites by
Raman Spectroscopy, M. Jain , Yu. I. Yuzyuk, R.S. Katiyar, Y. Somiya and A.S.
Bhalla, Physical Review B, 2004. (Submitted)
3.
Sol-Gel Derived Textured Barium Strontium Titanate Thin Films For Microwave
Dielectric Applications, M. Jain , S.B. Majumder, R.S. Katiyar, A.S. Bhalla,
Proceedings o f the Electrochemical Society Meeting, 2004. (Submitted)
4.
Raman spectroscopy o f bulk and thin-layer (Ba,Sr)Ti03 ferroelectrics, R.S.
Katiyar, M. Jain and Yu. I. Yuzyuk, Ferroelectrics 2003. (Submitted)
5.
Investigations on sol-gel derived highly (100) oriented Bao.5Sro.5Ti 0 3 :MgO
composite thin films for phase shifter applications, M. Jain , S.B. Majumder, R.S.
Katiyar, A.S. Bhalla, F. A. Miranda, and F.W. Van Keuls, Applied Physics A,
(2003). (In Press)
6.
Tailoring o f BST and MgO layers for Phase Shifter Applications, M . Jain , S.B.
Majumder, R.S. Katiyar, A.S. Bhalla, F.A. Miranda, and F.W.Van Keuls,
Integrated Ferroelectrics. 2003. (In Press)
7.
Raman studies o f PbxSri.xTi03 ceramics and composites, M. Jain , Yu.I. Yuzyuk,
R.S. Katiyar, Y. Somiya, and A.S. Bhalla, Ferroelectrics 2003. (In press)
8.
Structural and Electrical Investigations o f Ferroelectric Lead Strontium Titanate
Thin Films and Ceramics, M. Jain , P. Bhattacharya, Yu.I. Yuzyuk, R.S. Katiyar,
and A.S. Bhalla, Proceedings o f Materials Research Society Fall meeting, 784
(2003) C l l . 15.1.
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9.
Structural and Dielectric Properties o f Heterostmctured BST Thin Films by SolGel Technique, M. Jain , S.B. Majumder, R.S. Katiyar, and A.S. Bhalla, Thin
Solid Films, 447-448 (2003) 537.
10.
Dielectric properties and leakage current characteristics o f sol-gel derived
(Bao.5Sro.5)Ti0 3 :MgTi03 Thin Film Composites, M. Jain , S.B. Majumder, R.S.
Katiyar, and A.S. Bhalla, Ferroelectrics Letters, 30 (2003) 99.
11.
Phase Transition Behavior o f Highly (100) textured Sol-Gel Derived
Bao,5Sro.5Ti 03 Thin Films, M. Jain , S.B. Majumder, R.S. Katiyar, and S.B. Desu,
Applied Physics A, 77 (2003) 789.
12.
Novel Barium Strontium Titanate (Bao.5Sro.5)Ti 0 3 :MgO Thin Film composites for
Tunable Microwave Devices, M. Jain , S.B. Majumder, R.S. Katiyar, and A.S.
Bhalla, Materials Letters, 57 (2003) 4232.
13.
Improvement in Electrical Characteristics o f Graded Manganese Doped Barium
Strontium Titanate Thin Films, M. Jain , S.B. Majumder, and R.S. Katiyar,
Applied Physics Letters, 82 (2003) 1911.
14.
Stmctural and Vibrational Properties o f Ferroelectric Phi.xSrxTi03 Thin Films and
Powders, M. Jain , A. Savvinov, P.S. Dobal, S.B. Majumder, R.S. Katiyar, and
A.S. Bhalla, Materials Research Society Symposium Proceedings, 748 (2003)
U3.17.1.
15.
Improved Dielectric Properties o f Hetero-stmctured Bao.5Sro.5Ti 03 Thin Films for
High Frequency Applications, M. Jain , S.B. Majumder, R.S. Katiyar, A.S.
Bhalla, D.C. Agrawal, F.W Van Keuls, F.A. Miranda, R.R. Romanofsky, and
C.H. Mueller, Materials Research Society Symposium Proceedings, 748 (2003)
U17.4.1.
16.
Raman Spectroscopy o f Ferroelectric Thin Films, R.S. Katiyar, A. Dixit, M. Jain ,
A. Savvinov, and P.S. Dobal, Materials Research Society Symposium
Proceedings, 748 (2003) US.10.1.
17.
Dielectric Properties o f sol-gel derived MgO:Bao.5Sro.5Ti 03 Thin Film
Composites, M. Jain , S.B. Majumder, R.S. Katiyar, D.C. Agrawal, and A.S.
Bhalla, Applied Physics Letters, 81 (2002) 3212.
18.
Synthesis and Characterization o f Lead Strontium Titanate Thin Films by Sol-Gel
Technique, S.B. Majumder, M. Jain , S.B. Majumder, R. Guo, A.S. Bhalla, and
R.S. Katiyar, Materials Letters, 56, 692, (2002).
19.
Electrical Characteristics o f Sol-Gel Derived (100) Oriented Bao,5Sro,5Ti 03 Thin
Films on LaA 103 (100) Substrates, S.B. Majumder, M. Jain , A. Martinez, R.S.
Katiyar, F.A. Miranda, F.W. Van Keuls, P.K. Sahoo, and V.N. Kulkami,
Ferroelectrics, 267 (2002) 409.
x x ii
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20.
Investigations on Sol-Gel Derived Bao.sSro.sTii.xMnOs (x~ 0 to 5 at %) Thin
Films for Phase Shifter Applications, R.S. Katiyar, M. Jain , S.B. Majumder, R.R.
Romanofsky, F.W. Keuls, and F.A. Miranda, Proceedings o f Materials Research
Society, Edited by S.C. Tidrow, J.S. Horwitz, X.X. Xi, and J. Levi, 720, H2.1.1
(2002).
21.
Evaluation o f chemical solution deposited BaxSri-xTiOa thin films on LaAlOs in
tunable microwave devices, F.W. Van Keuls, C.H. Mueller, R.R. Romanofsky,
J.D. Warner, F.A. Miranda, S.B. Majumder, M. Jain , A. Martinez, R.S. Katiyar,
and H. Jiang, Integrated Ferroelectrics, 42, 207 (2002).
22.
Highly textured chemical solution deposited Bao.sSro.sTii-xMnxOs (x~0 to 5 at %)
thin films for microwave dielectric applications, M. Jain , S.B. Majumder, A.
Martinez, R.S. Katiyar, F.W. van Keuls, and F.A. Miranda Integrated
Ferroelectrics, 42, 343 (2002).
23.
Process induced modification o f the high frequency dielectric behavior o f (100)
Textured BaxSri-xTiOa (x=0.5 and 0.6) Thin Films, S.B. Majumder, M. Jain , A.
Martinez, R.S. Katiyar, E.R. Fachini, F.W. Van Keuls, F.A. Miranda, P.K. Sahoo,
and V.N. Kulkami, Proceedings o f Materials Research Society, 688 , C7.8.1
(2002).
24.
Sol-Gel derived grain oriented BST thin films for phase shifter applications, S.B.
Majumder, M . Jain , A. Martinez, F. W. Van Keuls, F.A. Miranda, and R.S.
Katiyar, Journal o f Applied Physics, 90, 896 (2001).
C onference Presentations;
1.
Tailoring o f Perovskite Ferroelectric Thin Films for Tunable Microwave Devices,
R.S. Katiyar, M, Jain , S.B. Majumder, and A.S. Bhalla, 106‘^ Annual Meeting &
Exposition o f The American Ceramic Society, Indianapolis, Indiana, April 18-2H‘,
2004. (Invited talk)
2.
Temperature Induced Self-Assembled Nanostructured Microwave Dielectrics, M.
Jain , S. Bhattacharyya, and R.S. Katiyar, 106‘^ Annual Meeting & Exposition o f
The American Ceramic Society, Indianapolis, Indiana, April 18-21®*, 2004.
3.
Investigations o f PbxSri.xTiOs Thin Films and Ceramics for Microelectronic
Applications, M . Jain , Yu.I. Yuzyuk, R.S. Katiyar, Y. Som iya, A .S. Bhalla, F.A.
Miranda, F.W. Van Keuls, Materials Research Society Spring Meeting, Stui
Francisco, April 16-21®* 2004.
4.
BST/MgO Heterostrutured Thin Films with High Figure o f Merit for Tunable
Microwave Devices, M . Jain , S.B. Majumder, R.S. Katiyar, A.S. Bhalla, Annual
AP S March Meeting, Montreal, Canada, March 22-26**’, 2004.
X X lll
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5.
Enhanced Dielectric and Electrical properties o f BST Heterostructured Thin Films
by Sol-Gel Technique for Phase Shifter Applications, M. Jain , S.B. Majumder,
R.S. Katiyar, A.S. Bhalla, F.A. Miranda, and F.W. Van Keuls, Materials
Research Society Spring Meeting, San Francisco, April 16-21®‘ 2004.
6.
Tailoring o f Dielectric Properties o f Ferroelectric Films with Low Loss Dielectric
Material hy Sol-Gel Technique M . Jain , S.B. Majumder, R.S. Katiyar, A.S.
Bhalla, F.A. Miranda, and F.W. Van Keuls, Materials Research Society Fall
Meeting, Boston, December 1-5*’’, 2003.
7.
Structural And Electrical Investigations o f Ferroelectric Lead Strontium Titanate
Thin Films And Ceramics, M. Jain , P. Bhattacharya, Yu.I. Yuzyuk, R.S. Katiyar,
and A.S. Bhalla, Materials Research Society Fall Meeting, Boston, Dec 1-5*’’,
2003.
8.
Investigations o f PbxSri.xTiOa Thin Films and Ceramics for Microelectronic
Applications, M. Jain , Yu.I. Yuzyuk, R.S. Katiyar, A.S. Bhalla, 75'* AAAS and
Puerto Rico EPSCoR Annual Meeting, Condado, November 19-23'*’, 2003.
9.
Heterostructured BST;MgO Thin Films for Phase Shifter Applications, M . Jain ,
S.B. Majumder, and R.S. Katiyar A.S. Bhalla F.A. Miranda and F.W. VanKeuls,
75'* AAAS and Puerto Rico EPSCoR Annual Meeting, Condado, November 1923'*’, 2003.
10.
Sol-gel Derived Textured Barium Strontium Titanate Thin Films for Microwave
Dielectric Applications, R.S. Katiyar, M. Jain , S.B. Majumder, and A.S. Bhalla,
204^^ Electrochemical Society Meeting. Orlando, Florida, October 12-16*’’, 2003.
11.
Raman Studies o f Pbi-xSrxTiOa Ceramics and Composites, M. Jain , Yu. I.
Yuzyuk, R.S. Katiyar, Y. Somiya, A.S. Bhalla, 7(/* European Meeting on
Ferroelectricity. 3-8*’’ August (2003) Cambridge UK.
12.
Structural and Dielectric Properties o f Heterostructured BST Thin Films by SolGel Technique, M. Jain , S. B. Majumder, and R.S. Katiyar, International
Conference on Metallurgical Coatings and Thin Films, San Diego, California,
April 28*’’-M ay2"‘’, (2003).
13.
Dielectric and Electrical Properties o f (Bao.5Sro.5)Ti 0 3 :MgTi03 Thin Film
composites for Tunable Microwave Devices, M. Jain , S. B. Majumder, R.S.
Katiyar, and A.S. Bhalla, 705'* Annual Meeting & Exposition o f The American
Ceramic Society, Nashville, Tennese, April 27*’’ —30*’’, 2003.
14.
Structural And Dielectric Properties O f Heterostructured Bao.5Sro.5Ti 0 3 :MgO
Thin Films By Sol-Gel Technique, M . Jain , S. B. Majumder, R.S. Katiyar, T ^d
A.S. Bhalla, The 15^^ International Symposium on Integrated Ferroelectrics,
Colorado Springs, Colorado, March 9-12*’’, (2003).
XXIV
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15.
Microstructural and Vibrational Properties o f Ferroelectric Pbi.xSrxTiOa Thin
Films and Powders, M, Jain , A. Saw inov, P.S. Dobal, S.B. Majumder, R.S.
Katiyar, and A.S. Bhalla, Materials Research Society Fall Meeting, Boston, MA,
December 2-6*, 2002.
16.
Improved Dielectric Properties o f Heterostmctured Bao.sSro.sTiOs Thin Film
Composites for Microwave Dielectric Devices, M. Jain , S.B. Majumder, R.S.
Katiyar, A.S. Bhalla, D.C. Agrawal, V.N. Kulkami, F.W Van Keuls, F.A.
Miranda, R.R. Romanofsky, C.H. Mueller, and F. Femandez, Materials Research
Society Fall Meeting, Boston, MA, December 2-6*, 2002.
17.
Sol-gel derived modified Bao.sSro.sTiOs thin films for high frequency applications,
M. Jain , S.B. Majumder, R.S. Katiyar, R.R. Romanofsky, F.W. van Keuls, F. A.
Miranda, P A SI Science and Technology o f Ferroelectric Materials, Rosario,
Argentina, September 23’^‘^-October 2"“^, 2002.
18.
Sol-gel derived BaxSri-xTiOs thin films for microwave dielectric applications, M.
Jain , S.B. Majumder, R.S. Katiyar, R.R. Romanofsky, F.W. van Keuls, F. A.
Miranda, 14* AAAS & Puerto Rico EPSCoR Annual Meeting, Dorado, May 1719*, 2002.
19.
Phase transition behaviour o f highly (100) textured Sol-gel Derived Barium
Strontium Titanate Thin Films for Phase Shifter Applications, M. Jain , S.B.
Majumder, and R.S. Katiyar, 104‘^ Annual Meeting o f the American Ceramic
Society, St. Louis, April 28*-May 1®', 2002.
20.
Investigations on sol-gel derived transition metal doped (100) oriented BST
(50/50) thin film phase shifter applications, R.S. Katiyar, M. Jain , S.B.
Majumder, F.W. Van Keuls, and F.A. Miranda, Materials Research Society
Spring Meeting, San Francisco, April 1-5*, 2002.
21.
Highly Oriented Sol-gel Derived MgO/Bao.sSro.sTiOs Mutlilayer Thin Films for
Tunable Microwave Devices, M . Jain , S.B. Majumder, R.S. Katiyar, P.K. Sahoo,
and V.N. Kulkami, American Physical Society Meeting, Indianapolis, March 1822"^ 2002.
22.
Process induced modification o f the high frequency dielectric behavior o f (100)
Textured BaxSri-xTiOs (x = 0.5 and 0.6) Thin Films, S.B. Majumder, M. Jain , A.
Martinez, R.S. Katiyar, E.R. Fachini, F.W. Van Keuls, F.A. Miranda, P.K. Sahoo,
and V.N. Kulkami, Materials Research Society Fall Meeting, Boston, November
26-30* (2001).
23.
Barium Strontium Titanate Thin Films for Microwave Dielectric Applications.
M . Jain , S.B. Majumder, R.S. Katiyar, F.W. Van Keuls, and F.A. Miranda, P ‘
Congress on Integrating NASA Research and Education Projects in Puerto Rico,
Dorado, November, 14-17*, 2001.
XXV
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24.
Electrical Characteristics o f Sol-Gel Derived (100) Oriented Bao.sSro.sTiOs Thin
Films on LaAlOa (100) Substrates, S.B. Majumder, M. Jain , A. Martinez, R.S.
Katiyar, F.A. Miranda, F.W. Van Keuls, P.K. Sahoo, and V.N. Kulkami, l(f^
International Meeting on Ferroelectricity, Madrid, September 3-7*, 2001.
25.
Highly textured chemical solution deposited Bao.sSro.sTii.xM^Oa (x~ 0 to 5 at %)
thin films for microwave dielectric applications. M. Jain , S.B. Majumder, A.
Martinez, R.S. Katiyar, F.W. Van Keuls, F.A. Miranda, 2"^ Ferroelectric
Workshop Puerto Rico 2001, San Juan, June l-2"‘*, 2001.
26.
Sol-Gel Derived Highly Textured Bao.sSro.sTii.xMnxGs (x~0 to 5 at %) Thin films
for Microwave Dielectric Applications, S.B. Majumder, M. Jain , A. Martinez,
F.W. Van Keuls, F.A. Miranda, and R.S. Katiyar, 13* AAAS & Puerto Rico
EPSCoR Annual Meeting, Dorado, April 20 - 22" , 2001.
XXVI
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PE R O V SK ITE F E R R O E L E C T R IC TH IN FILM S F O R TUNABLE
M IC RO W A V E A PPLIC A TIO N S
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Dedicated to my Papa and Ma
fo r their extreme love, support, and patience
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CHAPTER 1
Introduction
Recent development in the thin film technology and device miniaturizations has
promoted strong renewed interest in tunable microwave (MW) dielectrics for microwave
device applications by DARPA (Defense Advanced Research Projects Agency) and
NASA (National Aeronautics and Space Administration). The focus o f the research work
is the development o f tunable dielectric materials for frequency agile radio frequency
(RF) and MW devices, such as tunable filters, voltage controlled oscillators, varactors,
delay lines, and phase shifters. This thesis deals with the investigations on the microwave
dielectric materials, mainly barium strontium titanate (BST) and other related materials,
for such high frequency applications.
In this chapter the basic background o f materials and phenomenology leading to
the understanding and development o f such materials & their characteristics are
described.
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1.1 Ferroelectrics
A dielectric material contains charge carriers that can be displaced, and charge
displacements within the dielectric can neutralize a part o f the applied electric field.
Some charges can drift through the material to be discharged at the electrodes, while
other charges are bound and can only oscillate to produce a polarization. Polarization is
expressed quantitatively as the sum o f the electric dipoles per unit volume [C/cm^].
Depending upon the crystal structure, the centers o f the positive and negative charges
may not coincide even without the application o f an external electric field. Such crystals
are said to possess a. spontaneous polarization (Ps). When the spontaneous polarization o f
the dielectric can be reversed by an electric field, it is called ferroelectric [1,2]. Not every
dielectric is a ferroelectric.
The term ferroelectric (FE) is derived from the analogy with ferromagnetic
materials. Since the application o f magnetic field to a ferromagnetic materials shows net
spontaneous magnetization, which can also be reversed. In ferromagnets, it is the electron
spin, which plays a major role [3]; whereas in ferroelectrics, it is the dipoles, which
contributes to net spontaneous polarization.
In general, crystals can be classified into 32-point groups according to their
crystallographic symmetry. The 32 point groups can be divided into two classes, one
centrosymmetric (with a center o f symmetry), which includes eleven point groups, and
the other noncentrosymmetric (without center o f symmetry), which includes twenty one
point groups as indicated in Table l.I. In the noncentrosymmetric point groups (except
the 432-point group), positive and negative charges are generated on the crystal surfaces
when appropriate stresses are applied. These materials are known as Tpiezoelectrics.
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Table 1.1 Crystal classes.
C ry sta l system
Symmetry
Polarity
Cubic
11
Centro­
symmetric
Non polar
Hexagonal
m3
rn .lrn
Not piezoelectric 432
non-
ceiitrnsymntftric/
£
4/mmm
4/m
3m
3
Ortho.
Mono.
mmm
2/m
FE
6mm
:
•
#
4
32
4
3m
222
6
4mm
3
2mm
2
m
.
T1 (B>
tM
1
42m
If polar vector can be switchable with field
o *>
Tri.
422
6
6m2
10-polar
(Pyro)
T 7 JT
(»
(i22
23
43m
6/m
Rhomborhedrai
1
n
“
10
non­
polar
6/mmm
Tetragonal
<b>
Figure 1.1 (a) A cubic ABO 3 (BaTi0 3 ) perovskite-type unit cell and (b) three
dimensional network o f comer sharing octahedra o f O^' ions.
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1
Among those piezoelectrics, some crystals have unique direction/axis and have dipole in
their unit cell. Such polar crystals having spontaneous polarization (defined by the value
o f the dipole moment per unit volume or by the value o f the charge per unit area on the
surface perpendicular to the axis o f spontaneous polarization) are called pyroelectric. All
pyroelectric crystals are piezoelectric, but not all piezoelectric crystals are pyroelectric.
Among the pyroelectric crystals, those whose spontaneous polarization can be reversed
by an application o f external electric field (not exceeding the breakdown limit o f the
crystal) are called ferroelectrics.
The most common crystal structure o f ferroelectrics is the perovskite structure
(ABO3) as shown in Figure 1.1. Perovskite structure provides an essential family o f
materials for a wide variety o f present day technologies and is likely to be an integral part
o f the future developments needed in high frequency communication and several other
integrated technologies.
Perovskite-type oxides can be divided into A^'^B^’^Os,
A^^B^^Oa types
and oxygen- and cation- deficient phases [4]. Some examples o f compounds having these
structures are presented in Table l.II. In the family o f perovskites not only the
compounds having the ideal cubic perovskite structure is included but also all the
compounds with structures which can be derived from the ideal one by small lattice
distortion or omission o f some atoms.
Table l.II : Binary perovskite type oxides [4].
Group
Type of oxide
1
NaNbOa, KTaOa, NaTaOa
2
3
Examples
a 3+b ^^03
BaTiOa, PbTiOa, SrTiOa
YCrOa, LaAlOa, YAIO 3
(no FE has been discovered in this group)
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Some o f the technologically important perovskite ceramics include barium titanate
(BaTiOa), lead titanate (PbTiOa), lead zirconate titanate (PZT), lead lanthanum zirconate
titanate (PLZT), lead magnesium niobate (PMN), potassium niobate (KNbOa), potassium
sodium niobate (KxNai.xNbOa), and potassium tantalate niobate (K(TaxNbi.x)Oa) etc.
The discovery o f ferroelectricity in barium titanate (BTO) ceramics was made in
the 1940s and was the first perovskite-type compound (ABO 3) shown to be ferroelectric.
The structure o f BTO is simple cubic with Ba^^ cations and O^' anions in a close packed
structure with a Tf*^ cation in the center (Figure 1.1(a)). This structure can also be
visualized from a slightly different point o f view, namely by taking the origin at the Ti
atom (Figure 1.1(b)). Each Ti atom is at the center o f the six O atoms, arranged at the
comer o f a regular octahedron. The octahedral are linked by their comers into a three
dimensional framework enclosing large holes, which are occupied by the Ba atom.
In order to have contact between the A, B, and O ions in a perovskite type
stmcture (ABO 3), R a + Ro should equal '^2(Rb + Ro), where, R a, R b, and R q are the ionic
radii o f the A, B, and O ions, respectively. When t is exactly equal to unity, the packing is
“ideal”. Goldscmidt showed that the cubic perovskite stmcture is stable only if a
tolerance factor, t defined by [ 1]
R ,+ R ^ = t-4 i{ R s + R o )
( 1 . 1)
has an approximate range o f 0.8< t <0.9. For t<\, the size o f the unit cell is govemed by
the B-site ion and as a result the A-site ions have more room for vibration. For t> \, just
the opposite situation occurs in the unit cell, i.e. in this case, B-site ions have more room
to vibrate and this ion can therefore “rattle” inside the octahedron.
In the BaTi03 perovskite stmcture, all TiOe octahedral are placed in identical
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orientations, joined only at their comers and fastened in position by barium ions. The
variation o f this comer linked octahedral such as tilt (1,2,3 axis) rotations give the
possibility o f several new families o f ferroelectrics. Tungsten bronze stmcture (tilting o f
the octahedral in the a-b plane) and Bi-layered stmctures (where comer linked oxygen
octahedral layers are separated by 81262 ^^ layers in the stmcture) are two o f such
families. The origin o f ferroelectricity is linked with the characteristics o f the oxygen
octahedron unit not only in the ideal perovskite stmcture but also in the derived stmctures
(tungsten bronzes and the bismuth titanate stmcture).
1.2 Phase Transitions In Ferroelectrics
•
Normal Ferroelectrics
Analogous to magnetic materials, in FE materials also, the ferroelectricity
disappears beyond a certain temperature. This temperature called the Curie-point (Tc).
Near the Curie-point or transition temperature, thermodynamic properties including
dielectric, elastic, optical, and thermal constants show an anomalous behavior and the
stmcture o f the crystal changes. For example, the dielectric constant in most FE crystals
has a very large value near their Curie points. This is usually called the dielectric
anomaly. Above the Curie point, the dielectric constant falls off with the temperature
according to the Currie-Weiss law [1] :
K =— =
^0
^ ;
T -0
(1.2)
where C is a constant, T is temperature, 0 is the Curie-Weiss temperature, and s reaches
its maximum at Tc, the transition temperature. For 2"*^ order transitions, 9^Tc and e
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declines steeply for T<Tc. For 1^* order transitions 9<Tc and £ discontinuously decreases
at this temperature.
Figure 1.2 shows the characteristics variation o f
versus temperature for the first
and second order phase transitions. In both the cases, Ps decreases with increasing
temperature and vanishes at the transition temperature, thus, above the Curie point, the
ferroelectric behavior disappears and the crystal becomes paraelectric (the permanent
dipoles inside the domains no longer exist). In the paraelectric regime, the spontaneous
polarization is zero but the permittivity remains high. Therefore, materials in the
ferroelectric regime exhibit memory effect via the hysteresis behavior, which is absent in
the paraelectric phase. Figure 1.3 shows the perovskite BaTiOa structure that is tetragonal
at T<Tc and cubic above Tc. W ith decreasing temperature from room temperature,
however, BTO undergoes a series o f complicated phase transitions. Figure 1.4 illustrates
these successive phase transitions.
Other kinds o f ferroelectric transitions are given below, where the dielectric
anomaly is not that sharp as in normal ferroelectrics (described above) and the CurrieWiess behavior is not applicable near the phase transition.
•
Diffuse Phase Transition Ferroelectrics:
Diffuse phase transitions often
occur in simple solid solutions o f oxide ferroelectrics and may be traced to local
fluctuations in cation site occupancy. A typical example is BaxSri-xTiOs solid solution
where diffuse behavior occurs owing to local regions with enhanced barium (higher
or strontium concentration (lower T^. Diffuseness depends critically upon preparation
conditions and careful attention to processing can often homogenize composition, and
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p
T' C
b
Figure 1.2 Schematic Ps versus temperature curves for (a) second order phase
transition and (b) first order phase transition. There is a sudden jump o f Ps in the
first order phase transition [5].
(a)
Ba
o °
o
Ti
Figure 1.3. Perovskite BTO unit cell in centrosymmetric paraleectric phase (a), and in the
non-centrosymmetric ferroelectric phase (b) with spontaneous polarization Ps.
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largely restoring the C u rie-W eiss b eh avior h y sharpening the transition behavior. In
m an y practical capacitor d ielectrics h ow ever, h eterogen eity is d eliberately introduced to
broaden th e C urie range and m aintain higher p erm ittivity over a w id er tem perature range.
•
Relaxor Ferroelectrics:
qualitatively
different.
T he
R elaxor b eh avior
broad
d ielectric
as
m axim u m
in
P b (M g i/3N b i/3) 0 3 , is
clearly
d o es
n ot mark
a
co n ven tion al p h ase transition in this system b eca u se it m o v e s to high er tem peratures as
th e m easuring freq uency increases. D eta ils o f relaxor ferroelectrics are n ot d iscu ssed here
as it is b eyon d the sco p e o f th is th esis.
Rhomboliedral
T etn ison al
Oitboifiombic
Cubic
10,000
M
5.000
Q
■—C l
a.
-150
-100
-50
0
50
100
Tanpemture (& )
F ig u r e 1.4. V ariou s p hase transitions in barium titanate.
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150
1.3 Ferroelectric Domains
Above the Curie temperature {T^, the unit cell in barium titanate (and other
similar materials) is cubic and has no net dipole moment at zero field. During cooling
below Tc, the strain is induced by the lattice distortion and spontaneous polarization is
produced along the c-axis o f the unit cell. Thus, at the phase transition the unit cell can
take any o f six crystallographically equivalent combinations o f strain and polarization
(Figure 1.5) [6 ]. The direction o f spontaneous polarization changes in the different
regions o f the ferroelectric. These local regions within which there is complete alignment
o f the electric dipoles are called ferroelectric domains. An internal structure o f
spontaneously electrically polarized domains is a characteristic feature o f the FE phase.
The planes along which individual domains conjoin are termed domain walls, and the
process o f polarization reversal or reorientation under high fields is accomplished by the
motion existing walls, or by the creation and motion o f new domain walls. There are 90
degree and 180 degree domains {Figure 1.6(a)}. FE domain patterns in barium titanate
ceramic are shown in Figure 1.6(b). In the FE phase o f a material, domain boundary
motion contributes to hysteresis and losses.
10
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' if^-,
_ o <">^'
t!ubic
(a)
Figure 1.5. (a) The Cubic (high temperature and tetragonal (room temperature)
structures o f barium titanate, (b) Upon cubic to tetragonal phase transition, the unit
cell can take any o f the six equivalent combinations o f strain and polarization. The
arrow indicates the direction o f polarization. [6 ]
(b)
90® boundarv
\
X
1S0“ boLoidaiy
(a)
I
Figure 1.6. (a) Schematic o f the subgranular structure o f domains separated by 90 °
and 180 ° boundaries, (b) Ferroelectric domain patterns in barium titanate ceramic [7].
11
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1.4 Modifications in BaTiOa
By suitable ion(s) substitution it is possible to synthesize other perovskite
compounds derived from the prototype BaTiOs perovskite. The choice o f the substituent
ion(s) is decided based on their valence state, ionic radii, range o f solid solubility, and
their chemical affinity to the host ion(s).
•
Isovalent substitutions.'
Isovalent additives such as Sr^^, is o f the same
valency as the replaced ion. Solid solubility o f these additives is usually high. This
isovalent doping in BTO, shifts the Curie temperature o f the (Figure 1.7).
a)
A-position substitutions
i.
Sr"^^: The Curie point o f BTO is decreased with solid solution o f Strontium in
place o f barium ion. It is inetersting to note that (Ba-Sr)Ti03 solid solutions have
higher peak values o f the dielectric constant than pure BTO.
ii.
Ca"^^: Ca^^ replaces Ba^^ in BTO to a varying extent depending on temperature.
Solid solution with upto 21 at% Ca are stable under normal firing conditions. The
admixture does not materially affect the Curie point, but strongly lowers the
tetragonal-orthorhombic transition temperatures. This is o f great practical value in
improving the temperature stability o f the piezoelectric, elastic and dielectric
properties o f BaTiOs for many engineering applications.
ill.
Pb^^: The substitution o f
for
in BTO raises the Curie point
monotonically toward that o f PbTiOs (490 °C). It also lowers the orthorhombictetragonal and rhombohedral-orthorhombie transition temperatures [ 8 ]. Complete
solid solution occurs between the two end member compounds.
12
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b)
B-position substitutions
Z / \ Sn+^ and H f ^
The three perovskites BTO, BaSnOs, and BaZrOs show complete mutual
solubility. In the metatitanates, substantial replacement o f Ti^'^ by any o f the three
ions Zr^"^, Sn^"^, or Hf^"* causes depression o f the Curie point below room
temperature. However, slight replacement causes a rise in the orthorhombictetragonal transition temperature, so that orthorhombic phase becomes stable at
room temperature.
c)
Simultaneous Isovalent substitution in both A and B positions:
Properties o f such ceramics can generally be anticipated from the effect o f each
additive
ion
individually
[9].
One
example
of
such
compound
is
Bai-xSrxTii.yZyOa.
•
Aliovalent substitutions:
Ions o f suitable size but unmatched
valency
(although limited in solubility) can cause significant changes in the nature o f the
dielectric and piezoelectric properties. Thus properties can be modified by addition of
foreign ion(s) substituting part o f host atom(s). This is known as doping. Donor dopants,
i.e. those o f higher charge than that o f the ions they replace, are compensated by cation
vacancies; acceptor, i.e. dopants o f lower charge than that o f the replaced ions are
compensated by oxygen vacancies. The common dopants in perovskite type ceramics are
presented in Table l.lll:
13
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Table l.III: Common aliovalent substituents.
A-site donors
La^^ Bi^", Nd^'
B-site donors
N b ^ \ T a " \ Sb'""
A-site acceptors
K^, Rb""
B-site acceptors
Fe^^ Mn^^ Mn^", Cr^^ La^^ Sc^'
100
10
19
20
25
50
35
AtOm %
Figure 1.7. The effect o f isovalent substitutions on the transition temperatures of
BTO ceramic [10].
14
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1.5 Applications of Ferroelectrics
The integration o f ferroelectric thin films with semiconductor has initiated the
development o f a variety o f integrated devices for a wide range o f microelectronic
applications. Ferroelectric materials apart from having high dielectric constant exhibit a
wide range o f phenomena e.g. ferroelectricity (electric field dependent polarization),
piezoelectricity (stress/strain dependent polarization), electro-optic effects (electric field
dependent optical birefringence) etc. Research efforts are directed to the development o f
thin ferroelectric films for applications in computer memories (non-volatile and dynamic
random access memories, NVRAM and DRAMs), tunable capacitor for high frequency
microwave applications, un-cooled infra-red detectors, micro-electromechanical systems
(MEMs) and electro-optic modulators [11-14]. The solid solution o f lead zirconium
titanate (PZT), strontium bismuth tantalite (SET) are attractive materials for memories,
strontium titanate (STO) and solid solution o f barium strontium titanate (BST) are
important candidates for microwave dielectric applications. BST solid solutions are also
considered to be attractive for DRAM and IR detectors along with lanthanum (La) and
calcium (Ca) doped lead titanate (PT), strontium barium niobate (SEN) and potassium
tantalate niobate (KTN) thin films are the most sought out materials for a variety o f
pyroelectric and electro-optic devices.
This thesis is devoted for the development o f FE thin films for tunable microwave
devices. This section o f the chapter overviews the background o f the subject in relation to
the structure-property relation o f various aspects o f MW dielectric materials, synthesis
and engineering o f the special composition samples and the current status o f this field.
15
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Microwave dielectrics are key in realization o f low-loss temperature-stable
resonators and filter for satellite and broadcasting equipment, and in many other tunable
microwave devices like phase shifters, filters etc.
The term microwave is used to define (loosely) a band o f frequencies in the EM
spectrum. Most microwave systems are located in the 300 MHz to 30 GHz range. The
microwave spectrum is broken down into various bands. Figure 1.8 shows the microwave
spectrum and its applications. Various bands and their frequency range are presented in
Table 1.IV.
1.6 Microwave Dielectrics
Microwave (MW) dielectrics have found a major role in the communication
systems. Conventionally for such applications, ultra low loss and low permittivity
dielectrics (linear dielectrics) are desirable (in polycryatllaine form). Such materials have
also found their use in single crystal form in the integrated superconducting MW devices.
In the recent trends in MW applications o f materials, the goals o f the new programs are to
develop and improve components and subsystems for frequency agile communications,
remote sensing and several defense related applications. Hence, there are increasing
needs for the frequency agile materials for electronics (FAME). Most low K solids are
highly linear dielectrics and therefore untunable with electric field and frequency. On the
other hand, the nonlinear dielectrics (with high K and high losses) can be tuned with the
application o f electric field and hence can be used in the tunable MW devices such as
phase shifters, filters etc.
16
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Table 1.1V: Frequency assignments to different bands.
Frequency
(GHz)
1 to 2
2 to 4
4 to 8
8 to 12
12 to 18
18 to 27
27 to 40
40 to 75
75 to 110
Band Designator
LB and
S band
C band
X band
Ku band
K band
Ka band
V band
W band
Wavelength in free space
(centimeters)
30.0 to 15.0
1 5 tp 7 .5
7.5 to 3.8
3.8 to 2.5
2.5 to 1.7
1.7 to 1.1
1.1 to 0.75
0.75 to 0.40
0.40 to 0.27
0,1
■Long-rai^ ituli(8iE>' ssaich ratbr
IfHF W (I4«83)
■1
L-latwl
J
■ C
c l l « E » r p h 3 «
■A ir tfaftic wwstoI i i a i ^ i K f e r
' TmpaMilEr
S-Baiid
' Spacis 1clcKic|i>-
MitTOwav’c 0vxii5
a
C"B0iid
x^Bwd
Kw-Baftd
©
;SsiteffiteHpftnk
- 5 10
Steellite cfoft'jilsnk
Airboime fitcooittml tadar
Pialicc tadar
Tckpwiw rclagf
■Futtms satellite dowttliric
■Peitfe .radar
■F trtw satellite aj^ink
K-B#nd_
K8‘B«n£l__
loot
■Missile-scekimg radar
Figure 1.8 The microwave spectrum and its applications [15].
17
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1.6.1 Linear-dielectrics for microwave devices
One o f the most important applications o f linear dielectric materials is in
dielectric resonators. Dielectric resonators are the prime components in various
communication and microwave systems as those provide for very effective size reduction
for microwave components compared to that for air filled cavities. In such applications,
the material should fulfill the following requirements: 1) high linear dielectric constant
(Sr), 2) minimum possible dielectric loss (tan 6 ), (3) low temperature coefficient
(temperature stability) o f dielectric properties, and 4) very low thermal expansion
coefficients. To have such an ideal material available is highly improbable, but suitable
materials can be designed by using simple guidelines o f crystal chemistry.
The wavelength o f the electromagnetic wave in a dielectric material is reduced by a
factor o f
as shown in the equation (1). For the size reduction o f device, the
dielectric constant must be high
............................................ (1.3)
where Ag is the wavelength in the dielectric material, Ao is the free space wavelength and
£r is the relative permittivity.
At MW frequencies, according to the classical dispersion theory o f dielectric [16],
the real part o f the dielectric constant (AT) is unchanged, and the dielectric loss increases
with frequency (/). Therefore, the product (Q.J) describes the basic properties o f each
dielectric material and is also regarded as the F.O.M o f an MW material (Figure 1.9).
Another important property is the temperature coefficient o f resonant frequency, Zf or
TCF, is the parameter which indicates the thermal stability o f the resonator; that is, how
much the resonant frequency drifts with changing temperature. The origin o f ^ is related
18
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to the linear expansion coefficient a, which affects the resonator’s dimensions, and its
dielectric constant and is related through the equation,
Tf =
Where
.(1.4)
a + -
is the temperature coefficient o f permittivity.
Several family o f compounds such as Ti 0 2 , spinels, and perovskites etc. have
been examined for MW dielectric resonators, but the perovskites provided the wide range
o f suitable materials, possibility o f tailoring the parameters to meet the device
requirements. They are aligned and/or coupled to create devices for signal propogation
and processing. Examples o f such passive components used in wireless communications
are dielectric resonators, phase shifters, phased array antennas, transmission lines,
bandpass filters and coplanar waveguides. The properties in case o f some candidate
materials are listed in Table l.V. All these devices require high K materials for
miniaturization and fabrication in the thin film form. In addition, these materials must
have very low losses, tanS- 10'^-10'"*, and a very low temperature coefficient o f
resonance, on the order o f a few ppm/°C.
Table l.V Dielectric permittivity, figure o f merit (Qxf), and temperature coefficients o f
important linear dielectrics [17].
Materials
&
MgTiOs-CaTiOs
Ba(Sn,Mg,Ta )03
Ba(Zn,Ta )03
Ba(Zr,Zn,Ta )03
(Zr,Sn)Ti04
Ba2Ti0902o
Ba0-Pb0-N d203-Ti02
21
25
30
30
38
40
90
Q x/
(GHz)
55,000
200,000
168,000
100,000
50,000
32,000
5,000
ppm/"C
+/-10
+/-5
+/-5
+/-5
+/-5
+ 10/+2
+/-10
19
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
References
[18]
[19]
[20 ]
[21 ]
[22 ]
[23]
[24]
10Q •
ii<3HPbO—Wttt Qj ^ TIOi
M|TEO,-CiTiO,
g
*■
K
a
m m h
fOCUKIII
1
B i r s n ,M ^ T ^ „
Q x f* m o a a
S
8^Zr,Z«,Ta|0,,
QXf-100,000
iftjOiO
<a^.3«STI0,,
O X f-5 0 ^
p ilgTlO^-CftTIO),
aojOM
K liO - P to - « d tO ,- - T iO g ,
i
S
IQ
t3
14
T«
1t
Figure 1.9 Frequency dependence o f K and Q values o f representative
dielectric materials. [17]
20
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1.6.2 Tunable non-linear dielectrics for microwave devices
The strong renewed interest in tunable dielectrics for microwave applications has
been promoted by the recent expansion in microwave communications. Ferroelectric
materials, most often titanate based, demonstrate a spontaneous electric polarization at
temperatures below a critical value known as the Curie temperature. It is desirable to
operate devices slightly above this temperature, where the dielectric constant is high,
dielectric loss are comparatively lower, and hysteresis effects are less pronounced. In the
sub-Curie temperature region o f operation, these ferroelectrics exhibit a dielectric nonlinearity with the application o f a dc electric field (ie. dielectric permittivity can be fast
controlled by electric bias field) [24,25]. This feature makes them particularly attractive
for use as frequency agile tunable microwave electronic components, including phase
shifters, varactors, tunable filters, and antennas.
1.6.2.1 Devices based on tunable dielectrics: phase array antenna and phase shifters
M ost o f the beams scanning antennas in commercial use today are mechanically
controlled. This has a number o f disadvantages including: limited beam scanning speed,
limited lifetime, reliability and maintainability o f the mechanical components such as
motors and gears. Thus, electronically controlled antermas are becoming more important
with the need for higher speed data, voice and video communications through
geosynchronous earth orbit (GEO), medium earth orbit (MEG) and low earth orbit (LEO)
satellite communication systems and point-to-point and point-to-multipoint microwave
terrestrial communication systems. Additionally, new applications such as automobile
21
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radar for collision avoidance can make use o f antennas with electronically controlled
beam directions.
Phased array antennas are well known to provide such electronically scanned
beams. Antenna arrays typically comprise several radiating elements, each having a
dedicated phase shifter, which is the key component o f phased antenna arrays. By
adjusting the phase shifts for each o f the individual radiators, it is possible to control the
direction o f the composite main beam. There are several benefits to phase anterma arrays.
For example, the direction o f the antenna main beam can be scanned very rapidly in
comparison with a mechanically rotated antenna. In addition, it does not require moving
parts and can be constructed as a planar or conformal structure. A disadvantage o f using a
phased array, however, is that each radiating element requires its own phase shifter. High
performance phase shifters that are currently available are too expensive for most
communication applications. Thus, at present, phased array antennas are primarily used
for military and satellite operations where the high cost o f the antennas can be justified. A
typical array may have several thousand elements and that many phase shifters and
drivers. The cost o f the phase shifters can be as high as 40% o f the total cost o f the
phased array and hence there is a need for a small, low cost microwave phase shifter for
electronic scanning applications. Such a device would significantly reduce the cost and
increase the availability o f electronic scanning antennas, both for military and civilian
applications. Therefore, reducing the cost and complexity o f the phase shifters, drivers,
and controls is an important consideration in the design o f phased arrays.
The most commonly used phase shifters (PS) are ferrite and semiconductor based
phase shifters. Ferrite PSs are very slow to respond to control voltages. Semiconductor
22
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based phase shifters are much faster, but they suffer from high losses at MW frequencies
and have limited power-handling capabilities. At this point, phase shifters using
ferroelectric materials have been proposed [26-28], which offer a variety o f benefits to
overcome these difficulties. Some advantages o f FE based PS include high speed,
increased microwave power, moderate microwave losses, lower cost and size, as well as a
possibility to operate at much higher frequencies. Comparison o f various issues in FE
phase shifters vs present PS technology are presented in Table 1.VI.
Table l.V I: Ferroelectric material vs current phase shifter technology [28].
Issues
FE
Cost
Low
Good after >10^, 040
V/pm
bias
cycles
Good, >1W
Reliability
Power
handling
Switehing
speed
Radiation
tolerance
DC power
consnmption
RF loss
Switching
energy
Linearity
Semiconductor
Expensive
Very
good
properly packed)
(if
Low power, few tens
of watts
Very fast (<10'"’ Fast at low power
sec)
(<10'® s)
Excellent
Poor
(good
if
radiation hardened)
Low
(hundred Low (few MW)
volts, no current)
~5dB/360“ @
band
Unknown
Unknown
K
~1.5dB/bit @ K band
Few joules
IMD intersect +35 to
+40 dBm
Ferrite
MEMS
Very expensive
Excellent
Low
Questionable
Very high (kW)
Low power <1W
Slow <l ps
Slow ~5ps
Excellent
Excellent
Eligh
(large
current)
few
hundred MW)
<ldB/360” @ X
band
Tens of joules
Negligible
Unknown
IMD intersect +80
dBm
~2.3dB/337.5° @
Ka band
~10 nano joules
A ferroelectric phase shifter (FE-PS) is an electric delay line, in which the amount
o f electric delay, i.e. phase shift, is controlled by the dielectric constant, and hence the
scan angle o f a radar beam may be controlled by changing the dielectric constant o f the
material [29]. Bulk ferroelectric materials have been used for many years for phase
shifter. The differential phase shift (or phase shift) o f a simple microstrip FE phase shifter
23
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(using bulk FE material), A 0 (in degree), where the wave is traveling through FE only,
is related to change in dielectric constant by [29]:
=(^ ~ 7 ^ )x ^ ^ ^
......................................... (1.5)
7VC
Where S2 is the dielectric constant without bias and Si is the dielectric constant after some
fixed bias voltage. L is the length o f the F E ,/is the frequency o f operation, and c is the
velocity o f light in free space.
Excluding the return loss, the intrinsic loss o f a phase shifter is given by [29]:
a = - 101o g jA ,if+ |5 ,,|^ )-(J5 ),
( 1.6 )
where S u and S 21 are the measured reflection and transmission coefficients, respectively.
The figure o f merit {k, or FoM) o f a phase shifter can be defined as the quotient o f the
differential phase shift, A 0, and the insertion loss,
K = ^ ( d e g / d B ) .............. ................................(1.7)
a
Hence,high phase shift and low losses are aimed
for the high performance (high
k)
of a
phase shifter. To design an effective microstrip FE-PS, one needs a good impedance
match for the microwave signal to travel from a 50 Q circuit through a low impedance FE
material, a DC block to prevent loss o f microwave energy through the DC biasing circuit.
Thin film FE material has very different properties from bulk or thick film
material, including flat temperature response profile, giving FE thin films, a good
controllability over wide temperature range. The difficult challenge o f thin film
ferroelectrics comes in controlling the growth or deposition o f the material during
manufacture. It is necessary to increase the performance o f thin film FE-PS through
improving ferroelectric material and/or device design. Several successful demonstrations
24
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o f coplanar waveguide (CPW) HTS/FE phase shifters have been reported. However, for
some applications, (such as reflectarray antennas) the large ground planes are unsuitable
because o f specular reflections and other effect. Thin film FE-PS based on microstrip and
coupled microstrip configurations have also been tried by Keuls et.al. [28]. Coupled
microstrip design was found to provide more phase shift per unit length at a given voltage
than the simple microstrip. However, coupled phase shifters (CMPS) require careful
design because they are inherently filters with a limited bandwidth. The coupled
microstrip geometry can be excited in two modes: even and odd. The propogation
constant is given by,
(1-8)
where Zois the free space wavelength, Vp is the phase velocity, Seven -CJCe-mr and
Saddleo/Co-airl Ce-air and Cg-air arc obtained by replacing all dielectrics with air. In odd
mode, the electric field lines are concentrated in the FE film. By applying a dc voltage
between the two mictrostrips, the Sr o f the ferroelectric between the lines are tuned with
lower voltages than those required in the single microstrip line.
1.6.2.2 Properties of dielectrics relevant to tunable microwave device applications:
Ceramic dielectrics consist o f atoms and ions, where the latter contribute largely
to the dielectric losses. The frequency at which a dielectric is used has an important effect
on the polarization mechanisms, notably the “relaxation “ process or time lag displayed
by the material in following field reversals in an alternating circuit. Figure 1.10 shows
schematically the electric polarization effects on dielectric properties at different
25
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
frequencies. The variation o f dielectric loss with frequency coincides with the change in
dielectric constant, since the two are related by the polarization mechanisms. There are
four primary contributions: electronic, ionic, dipole or orientation, and space chargerelated. The total polarizability o f the dielectric can be represented as the sum o f these:
a =ae+ai+ad+as
(1.9)
It is the response o f these polarization mechanisms to periodic fields that determine the
frequency dependence o f the dielectric properties o f solid. For each polarization
mechanism, there exists a sufficiently high frequency above which
the particular
mechanism will not be able to follow the alternating field and thus will cease the
contribution to the polarization and dielectric properties.
The loss contribution is maximized at a frequency where the applied field has the
same period as the relaxation process. To state the matter simply, losses are small when
the relaxation time and the period o f the applied field differ greatly,
a) relaxation t i m e » field frequency, loss is small
b) relaxation t i m e « field frequency, loss is small
c) relaxation time = field frequency, loss is maximized
The situation (a) generates little loss, as the polarization mechanism is much slower than
the field reversals, and the ions cannot follow the field at all, hence creating no heat loss.
The inverse occurs in the situation (b), where the polarization processes can easily follow
the field frequency, with no lag. In the case (c), however, ions can follow the field, but
limited by their relaxation time, and thus generate the highest loss with frequency.
26
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Vibrations
space
________ _________
charges
j dip<>ite ] ( atoms
Z'
0
4
S
4
S
36
20
hgf
g'
0
16
12
v
•V"
Relaxations
Resonances
Figure 1.10. Schematic representation o f various relaxation and resonance processes,
/7Hz.
27
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Dielectric loss (loss tangent, tan Si:
Dielectric loss o f a material is the most important factor for the realization o f
deviee. Microwave loss has been attributed to the damping o f the polarization mode by
impurities and the anharmonicity o f lattice vibrations, i.e., phonons excited at the MW
frequency can be scattered by impurities into an acoustic mode. The loss tangents
(intrinsic to the material) serves to dissipate or absorb the incident microwave energy and
therefore, is desired to be in the range o f 0.01 or less for the specific design o f the
antennas. The low loss tangents serves to decrease the insertion loss and hence increase
the phase shifting per decibel o f loss and the operating frequency o f the device can be
extended by reducing the loss tangent [30].
Tunability: Tunahility is defined by the equation:
r(% ) = 100 X
(1.10)
Q
where Co and Cy are the capacitance values at zero and maximum dc bias voltages.
Figure o f m erit (FOM): K fa c to r
For tuning applieations, both tunability and tan5 must be considered when
comparing the relative merits o f different film compositions and varactor configurations.
A figure o f merit, K, is given by:
C -C
K = - ^ ----- ^
C q. tan S
Where tanS is measured under zero dc bias.
28
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(1.11)
For high frequency tunable device such as phase shifter, high permittivity is not
desired due to the device impedance matching purposes, which lead to less efficient
power transfer in the device and thereby degrading the device performance. Moreover,
high dielectric loss tangent also results in inferior device performance. Thus, it is
important to select materials with low microwave loss and high dielectric tunability, so
that the figure o f merit is high. Also, other desirable material’s characteristics are high
breakdown voltage and low water absorption, which are a function o f density.
1.6.2.3 Suitable materials for tunable dielectric applications
The perovskite family provides the suitable materials for tunable MW dielectrics
e.g. SrTiOs, KTaOa, CaTiOs, and several solid solutions e.g. (Ba,Sr)Ti 0 3 , (Pb,Sr)Ti03
etc. and their doped and modified compositions (for the frequency and field agile MW
electronics). Table l.V II lists some o f the commonly used and potential ferroelectrie
materials for such applications and their characteristics. Electrically tunable MW
components based on the thin films o f STO and BST have been demonstrated in past few
years [31,32]. Much work on tunable MW devices has been done using the STO thin
film, which requires cryogenic operation [33,34]. The discovery o f high temperature
superconductivity (HTSC) in complex metal oxides stimulated a special interest in STO
thin film ferroelectrics. Table l.V m below summarizes the results o f few studies.
The STO films show timable characteristics at large field at room temperature
[35]. Thus for room temperature applications, barium strontium titanate and related thin
films are preferred. Properties and literature survey o f commonly studied BST, and a
possible candidate material, lead strontium titanate, are presented in the next section.
29
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CD
■D
O
Q.
C
oCD
Q.
■CDD
C/)
(/)
3
O
5
O
o
■D
cq
'
Table l.V II: Properties o f most commonly used and potential ferroelectrics [Courtesy Dr. A.S. Bhalla].
Structure
Working
temp
Dielectric
Permittivity
Dielectric
loss
Tunability
Remarks
Perovskite
4-lOOK
-20,000
(-2,000 in film)
-10-^
(10'^-10'^ in
film)
95%
(79% in film)
FOM -2 0 0 (-2 0 in film);
incipient ferroelectric; 110
K phase transition.
Perovskite (solid
solution)
RT
-2,000
(-2 0 0 in film)
-10'"
(10'^-10'^ in
film)
87%
(50% in film)
Property varies by
composition; Oxygen
deficiency; large temp
dependency.
Perovskite
4-lOOK
-4,500
(4.2K, lOkHz)
10'"-10-^
-63%
K(Tai.^Nbx)0 3 , e.g..
x-0.35
Perovskite (solid
solution)
RT
-2 4 0 at RT
(-2 0 0 in film)
lO'^-lO'^
-
(Sr3ai-xNb2)06, e.g.,
x-0.5
Tungsten Bronze
solid solution
<400K
lO'^-lO'^
-
(Ba2-xSrxKi.yNayNb5)Oi5,
Tungsten Bronze
solid solution
10‘^
-
Material
Materials Currently used
SrTiOs
i
3
CD
"n
c
Sri.^axTiOa, e.g. (x=0.5)
CD
■D
O
Q.
C
a
Candidate FE materials
KTaOa
§
"D
O
Highly suitable for liquid
phase epitaxy; incipient
ferroelectric; no twinning
or ferroie transitions; better
control o f stoichiometry
Q.
§
O
■D
CD
3
C/)
C/)
o'
3
(BSKNN)
300-500
(at 100 MHz)
<470K
100-600
(at 100 kHz)
30
Stoichiometric film can be
made; LPE and lift-off tech
useful.
Film can be made; LPE
suitable; high quality
material synthesis is
possible
Interesting and high
performance compositions
CD
■D
O
Q.
C
oCD
Q.
■CDD
for various frequency
range are possible
MPB compositions in
Tungsten Bronzes
Various systems with
various interesting features
are possible; MW data is
scarce
Tungsten Bronze
solid solution
Pyro niobates and tantalates
Ca, Sr, La, Nd,..
Pyroclore
4K to
above RT
50-
10'^-10-^
Possible/data
not available
A2Nb207
Pyroclore
250 K and
above
>500 (365 Hz)
10'^
Possible/data
not available
>170 K
>300 K
Bi2(Mg2/3Nb4/3)07
Pyroclore
>190 K
Jncommensurate systems,
e.g., Rb2 ZnCl4
Special temperature regions
in TB systems
Tungsten Bronze
Below and
~RT
<10'^
Very high Efield agility
20-150 K
300-1000
Special Sn doped
antiferroelectric PZT
composition
31
Possible/data
not available
-10'^
No
quantitative
data
k=50-100
and Q=5,000 at
10 GHz has been
measured, single crystal
films/crystals have been
grown
Pb compositions decrease
the variable operating temp
range down to 30 K
Good epitaxial films are
possible to produce
Very high field agile k are
possible; change o f k by a
factor o f 4-5
High agility possible, films
by LPE, large crystals can
be grown
Field (and frequency)
agility upto 50 GHz,
k=800-600 for E=0-7
V/pm
Table l.VIII: STO thin film studies.
Author [Ref|
Tunability
tanS at 77 K
Raymond [36]
Kozyrez [37,38]
Galt [39]
2.5:1
1.6:1
2:1
0.01 at 1-2 Ghz
0.03-0.05 at 3Ghz
0.01 at 6-10 Ghz
1.6.2.4 Properties o f barium strontium titanate (BST) relevant to MW applications
The solid solution o f barium strontium titanate has been studied intensively in
past years for its potential application in novel electronic devices. BaxSri.xTiOs (BST)
undergoes a ferroelectric phase transition at Curie temperature {T^ that depends on the
Ba:Sr ratio, [40]. The phase diagram o f BST solid solution is shown in Figure 1.11.
Below Tc, BST is true FE with re-orientable, spontaneous polarization [41]. Figure 1.12
shows temperature dependent dielectric constant o f BST ceramics with various Ba/Sr
ratio. BST exhibits a large electric field dependent dielectric constant (Figure 1.13)
(tunability) [41,42]. It is well known that the tunability is enhanced for BST in the FE
phase (below and near T ^, however, the dielectric losses are usually higher below the
Curie temperature, due to domain and wall motion in FE phase. For the tunable
microwave device applications paraelectric region, just above the Curie point is
preferable, where the dielectric constant is high, dielectric loss are comparatively lower,
and hysteresis effects are less pronoimced. Barium strontium titanate with Ba content
-0.5 is used in the development o f microstrip phase shifters, which has a Curie point just
below 0°C, which allows normal operation above the Curie point in the paraelectric
region.
32
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400
300
y4
X
200
'^'^2
X l T - '* - ''* '
100
0/
0.0
4 m rn ^ '
3m
,1
0.2
0.4
0.6
0.8
X (BaTiOj concentration)
1,
1.0
Figure 1.11 Phase diagram o f BaTiOs-SrTiOs system [43].
m
MS
m
-m
~m
T-
Figure 1.12 The temperature dependence o f x for several x values in Bai.xSrxTiOs
(x=0-90) system [41].
33
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r,
Figure 1.13 Temperature dependent o f s / sq (or x) for a series o f applied electric
fields for BST5 ceramic at f=800 Hz [44].
1.6.2.5 Properties of lead strontium titanate relevant to MW applications:
There have been numerous efforts to integrate thin films o f high dielectric
constant materials into electronic devices, by academic groups that focus on the
exploration o f new material systems and industrial groups that highlight the integration
and reliability issues. In the previous sections, the properties necessary for the selection
o f the materials for high frequency tunable devices has been mentioned. The perovskite
materials such as SrTiOs and BaxSri-xTiOs (BST) have been studied extensively for such
applications. SrTiOs is a good material to work at cryogenic temperatures, while BST is
an attractive material for room temperature MW applications. Recently, FbxSri.xTiOs
(PST) has also been considered as one o f the potential candidate material for the future
34
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tunable microwave device components such as resonators, filters etc. when used in
paraelectric region. In this section, properties o f PST, relevant for microwave device
applications and few relevant literature review are presented.
Smolenskii was the first to study PST system in 1950 [45]. Later Nomura and
Sawada carried out more detailed investigations o f these solid solution and established a
complete series o f solid solution from PbTiOa to SrTiOa [46]. Lead titanate is
ferroelectric below 490 oC and it was reported that the Curie temperature decreases as the
Sr content in PTO increases (Figure 1.14). SrTiOs (STO) is one o f the few titanates that is
cubic at room temperature, but the dielectric constant is lower and is not tunable. The
addition o f lead (Pb) into STO makes its dielectric constant higher and temperature o f
crystallization lower than barium strontium titanate (BST). Thus it is important to study
this potential candidate material for tunable microwave devices, which can be formed at
comparatively lower temperatures.
There is another consideration in the selection and study o f PST over BST in the
future interests point of views. In the case o f BST, the high Tc end member BT (rc~120
°C, and one o f the transitions orthorhombic to tetragonal is right at room temperature) has
three phase transitions and the most common room temperature (RT) application
composition fluctuations o f BST (due to solid solution nature). This results in large
dielectric losses, which are not easy to control unless highly reproducible chemical films
are prepared. A small inclusion o f BT or BT rich compositions will increase the losses o f
BST substantially due to the room temperature ferroelectric nature o f BT. On the other
hand in PST, the high Tc end member PT {Tc ~ 490 °C) has only one transition and for the
RT application the material is more forgivable to small composition fluctuations. Even
35
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
small nanoscale inclusions o f ST or PT may not influence the dieleetrie properties much
at RT as both are stable and low loss compounds at room temperature. The most common
Pb vacancy problem in Pb-compounds is well studied and can be easily controlled. Also,
the materials could be comparatively easier to obtain in good stoichiometry composition
thin films desirable for the tunable capacitors.
For room temperature tunable device applications, compositions o f PbxSri.xTiOs
(PST or PSTx) with x=0.3-0.5 are attractive as their Curie temperature is around room
temperature, hence it is expected that it will show properties desirable for room
temperature microwave devices.
SOO
m
P(Cubic)
300
ZOO
100
F(Tetragonal)
-WO
-ZOO
Figure 1.14 Variation o f Curie temperature o f PST solid solution [46].
36
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.7 Literature review
1.7.1 BST ceramics for tunable microwave device applications
Towards the goal o f affordable high performance electronic beam steering o f
antennas at mw frequencies, FE materials such as barium strontium tutanate (BST) have
been o f recent interest. In particular, the composition-dependent Curie temperature (Tc)
and the non-linear field dependent dielectric permittivity o f BST make it attractive for
tunable devices, such as voltage-controlled oscillators, tunable filters, and phase shifters
[47-50]. Phase shifters using BST, which has high tunability, low loss tangent, and high
power handling capability, are promising as a replacement for traditional ferrite and
semiconductor device phase shifters.
Conventionally, BST is used as a bulk material to form the entire substrate that
results in high control voltages [51,52]. Labeyrie et.al. measured dielectric constant and
losses o f BST ceramics with different Sr content and showed that ferroelectric
compositions o f BST had high dielectric constant (~6000) and high losses (10’') in 2-18
GHz range and thus cannot be used unless material properties are improved [53]. Peng
et.al. [54] utilized BST ceramics in the phase shifters and demonstrated the phase shift o f
-149° at 1.58 GHz. Flaviis et.al. used thinner ceramics o f BST in ferroelectric microstrip
based phase shifter and demonstrated a phase shift o f 165° with the reduced required
voltage to change dielectric constant and losses below 3dB at 2.4 GHz [52]. Geyer et.al.
showed the decrease in dielectric constant and loss tangent by making BST ceramic
composites with non-ferroelectric low loss materials [55]. In the effort o f improving
properties o f BST Sengupta et.al. prepared BST60 composite ceramics with Zr 0 2 , AI2O 3,
and MgO [29]. Four element coupled phase shifter using BST-MgO composites showed
37
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
phase shift o f 360 ° with 3 dB losses at 5 GHz [29]. Some o f the research efforts on the
study o f BST ceramic composites are summarized in Tables 1.IX.
There are several disadvantages o f using bulk ceramics in high frequency
applications. In general, very high voltages are required to apply large electric fields in
the material and additionally bulk based devices are heavy. From the point o f view o f
device integration, as well as the ability to achieve substantial fields with reasonable
applied voltages, incorporation o f BST in thin film form to the microwave components is
desirable.
38
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CD
■D
O
Q.
C
oCD
Q.
■CDD
C/)
(/)
O
O
■D
cq
'
O’
Q
CD
■D
O
Q.
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a
o
■o
o
CD
Q.
■CDD
(/)
(/)
Table l .I X : Ceramic composite review
BST composition/substrate
K
Frequency
Tunability (Electric
field, kV/cm))
Loss
BST60:MgO bulk
0.25 wt%
0.5 wt%
1 wt%
5 wt%
20 wt%
30 wt%
60 wt%
2654
2405
2275
2006
966
519
102
250 kHz
-do-do-do-do-do-do-
24 (20 kV/cm)
11.5 (-do-)
17 (-do-)
16 (-do-)
16 (-do-)
14 (-do-)
8 (-do-)
0.000592
0.000513
0.000535
0.000764
0.000859
0.000592
0.000434
BST45;MgO
0 wt%
1 wt%
5 wt%
10 wt%
20 wt%
30 wt%
60 wt%
1205
943
817
656
343
363
80
10 GHz
15.2
3.71
4.10
3.90
4.00
4.78
3.7
BST50:MgO
0 wt%
1 wt%
5 wt%
10 wt%
20 wt%
30 wt%
60 wt%
1099
1004
851
616
463
84
1526
25
5.1
5.5
6.4
5.8
3.7
20.8
.138
.0079
.0071
.0066
.0064
.0063
.0041
0.018
0.0105
.0103
.0087
.0085
.0066
.0181
0.02
.0155
BST55:MgO
0 wt%
1519
11.6
.015
10 GHz
39
Ref.
[56]
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1 wt%
5 wt%
10 wt%
20 wt%
30 wt%
60 wt%
1464
1290
1079
704
527
100
10 GHz
6.52
8.63
9
8.8
9.5
6.5
.0167
.0135
.0121
.008
.091
.039
56.3
16
12
15.4
15
16.6
15.8
15
10
.035
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0.5 wt%
1 wt%
5 wt%
10 wt%
20 wt%
30 wf/o
60 wt%
Bao.6-xSro.4Cax)Ti03
(x = 0.10, 0.15, 0.20)
BSCT+Zr02
1 wt%
2 wt%
3 wt%
BST60
BST60:Al2O3
0.4 wt%
0.8 w f^
1.2 wt%
1.6 wt%
BST60 (bulk)
BST60+20 wt% MgO (bulk)
1002
2152
2005
1949
1669
1431
871
636
118
10 GHz
[57]
-2600
22 (lOkV/cm)
0.009 (FOM 20)
-2250
-2000
1900
1750
30
28
27
26
0.003 (FOM 60)
0.0015 (FOM 130)
0.008
0.010
56.7
0.009
15.8
0.002
5160
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1.7.2 BST thin films for tunable microwave device applications
In the last few years, thin films o f ferroelectric materials have attracted attention
because o f their potential applications in microwave components. The integration o f thin
films are necessary not only to reduce size and weight o f the devices, but also to have
configurations that are compatible with existing planar microwave circuits. Thin films
offer additional advantage over bulk materials for the high frequency applications, since
large electric fields (0-200 kV/cm) can be applied in thin films (~0.5 pm) using low bias
voltages (0-10 V). Thin films have much broader temperature coefficient o f capacitance
(TCC) as compared to their bulk counterpart, i.e. the dielectric anomaly is not as sharp as
that in bulk (Figure 1.15). Although much research work has been done to develop BST
thin films for tunable microwave dielectric devices, but high dielectric losses and low
tunability o f these films at high frequencies have restricted its practical application. It
100000
10000Ceram ic
1000:
100
''" ^ T h l n Film
t=100nm
T em perature (K)
Figure 1.15 Temperature dependent dielectric constant for ceramic and thin film
BST70 [61].
42
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
remains a challenge to synthesize the thin film with higher dielectric tunability, moderate
dielectric constant and low loss tangent. Development o f BST thin film o f suitable
compositions having the above stringent properties will permit the development o f lowcost phase shifters, tunable oscillators, and tunable filters etc.
Film composition pronouncedly affects the dielectric properties. For MW
applications, large dielectric tunability and low dielectric loss tangent are the two critical
parameters needed for optimal device performance. The choice o f Ba/Sr ratio in the BST
films for MW applications depends on the intended temperature o f operation o f the
device. As mentioned before the Curie temperature o f the BST system changes linearly
with Sr contents on BTO. Among the BaxSri.xTiOa films, compositions with 0.4<x<0.6
are the compositions studied my most o f the researchers [62,63] for room temperature
tunable dielectric applications as their Tc lies around room temperature.
In determining the performance o f a ferroelectric thin film for tunable microwave
devices, both, the quality o f thin film material as well as design o f the integrated device
play an important role. Conventionally, BST thin film capacitors are integrated to
microwave circuitry either in parallel-plate or planar electrode configuration (Figure
1.16). BST parallel-plate capacitors (PPC) Figure 1.16 (a) are usually formed by two
parallel plate capacitors in series connection for process concern. BST films are deposited
directly on a bottom electrode on substrate, followed by the top electrodes, creating
metal-insulator-metal (MIM) structures. The distance between the electrodes is the
thickness o f the film. In this configuration, the control voltages typically scales with the
film thickness and thus, devices can effectively be tuned according to the tuning
capability o f BST thin films with relatively low DC control, which makes them attractive
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
for most microwave and millimeter-wave applications. The major issues in the vertical
structure are the choice o f the bottom electrode, thin film deposition process and
associate parameters, choice o f top electrode, electrode patterning etc. The relevant
device integration issues o f parallel plate capaeitor design have been summarized in
Figure 1.17. The main limitation for BST in PPC configuration is that the total device
loss at microwave frequency is quite high which is probably due to the BST material
degradation during the process flow. Moreover, such BST PPC requires six-layer mask
in fabrication, which is not desirable for low cost applications.
Most tunable microwave resonators and phase shifters that use ferroelectric films
use coplanar electrode configurations to integrate the ferroelectrie films into the devices.
Examples o f planar electrode configuration include gap eapaeitors, interdigitated
capacitors (Fig 1.16(b)), coplanar waveguide and eoupled microstrips. For the interdigital
capacitors, BST films are directly deposited on the low loss single erystalline appropriate
followed by top interdigital electrode metallization (Figure 1.16(b)). In general,
interdigital devices are simpler to fabricate and integrate into circuits, but suffer from
reduced tunability (due to large fringing electric field in the air) and relatively higher
tunning voltages. Smaller spacing between the fingers are desired to increase the
tunability at relatively lower voltages. Since the proeess flow o f BST interdigitated
devices is fairly easy, we exclude the possibility o f process induced damage to BST thin
film, which is an important factor needed to be concerned in parallel plate BST structure.
In addition, easier fabrication o f DDT electrodes drastically reduces the total cost o f
phased array systems.
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
While the concept o f using ferroelectric films for tunable microwave devices is
also not new, the practical implementation requires the ability to fabricate high-quality
thin films and the demonstration that the microwave dielectric properties o f thin films are
attractive.
Most electrically tunable MW devices that use FE thin films as an active
dielectric layer are based on planar configurations. To achieve high tenability and low
loss tangent, the FE thin film are usually grown on epitaxially on single crystalline
substrates with low dielectric constant and dielectric loss, such as LAO (Sr =25,
tan6=6xl0'^) and MgO (Sr=9.5, tan5=3.3xlO'^). By using planar configurations, high
tunabilities are obtained on ferroelectric films that maintain in-plane epitaxy with the
substrate and are only minimally strained [64,65]. However, it is not easy to grow
epitaxial BST films onto substrates with large lattice mismatch, such as MgO (6.4%
between MgO and BST60). Chen et.al. [66,67] and Gao et.al. [68 ] have examined the
interface between epiatxial BST60 films on MgO and LAO substrates. As grown film on
MgO had high degree o f orientation and crystallinity with flat sharp interface. Many
period edge dislocations were observed [69], which indicated that the lattice mismatch
strains were completely released at the interface with such dislocations. Twins exist in
LAO substrate surface, which were found beneficial to release the strain energy between
the BST film and the LAO substrate [70], so that the total energy in this system can be
reduced during film growth. The tunability o f BST films strongly depends on the state o f
the strain that arise tfom the lattice parameter mismatch between film and the substrate.
45
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
BST
Electrodes
Electrodes <
Substrate
Substrate
> Vertical polarization
> Horizontal polarization
> High tunability
> Low tunability
> Complex fabrication
> Easier fabrication, grown
directly on substrate
> Low breakdown voltage
> High breakdown voltage
Figure 1.16 (a) Parallel plate and (b) interdigitated device structures.
Bottom Electrode issues
High growth tomporaturos
BST varactor on Si
BST Aim issues
Oxidation resistance
Sufficient conductivity
Top Electrode
Stoichiometry
Surface roughness
Ba/Sr ratio
Adhesion
Orientation
Thermal stresses
Stress
Patterning
Structure
Adhesion layer
Diffusion Barrier
Enhance electrode adhesion
Electrode-substrate reaction
Withstand BST deposition
conditions
Adhesion, stress
Si Substrate
Ease of processing
E ase of processing
Figure 1.17 Schematic o f stacked BST capacitor and relavent device integration
issues.
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Epitaxial films are typically comprised o f 50-70-nm grains that arc epitaxially oriented
relative to the substrate and have very smooth surfaces, with rms roughnesses 0.3-0.5 nm
[71,72]. C.L. Chen et.al. observed high dielectric constant, high tunability and low losses
in PLD grown epiatxial thin films, which suggested that epitaxial films can improve the
dielectric properties o f the films for room temperature tunable MW elements [63]. To
improve the epitaxial quality o f the BST thin films, post-annealing process is usually
required. Deposition o f ferroelectric films directly on dielectric substrates (such as LAO)
also enables the additional option o f post-annealing the films at higher temperatures.
Knauss et al [73] demonstrated that post-annealing BST thin films on LAO substrates at
900°C substantially decreased strain in the films, which led to higher tunabilities, and Sr
versus temperature profiles which more closely resembled bulk behavior. The tunability
in that case increased from 20% to 32% for BST35 films on LAO substrates after
annealing treatment [73]. Carlson et.al. [62] reported that annealing the film at higher
temperatures relaxes the residual compressive stresses and thereby improve tunability
from 36% to 52% o f BST60 film deposited on LaAlOs substrate. Al-Shareef et al and
Raymond et al [74,75] demonstrated that increasing the post-annealing temperatures o f
BST and STO films to 1100°C caused substantial increases in grain size, Cr, and
tunability. Kim et.al. measured the microwave properties o f BST40 films on MgO
substrate at 10 GHz and showed that the tunability & figure o f merit increased to 50 %
and 7 respectively, when stress in the film was minimum [76]. Chang et.al. observed
reduction in the dielectric constant and losses by using an optimized buffer layer between
the film and the substrate to reduce (or relieve) strain in the BST film due to lattice
mismatch [77].
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Degree o f orientation o f the films was also found to affect the dielectric properties
o f BST films. For example, Carter et al showed that parallel-plate varactors o f highly
(100) oriented BST (80/20) layers (0.5-pm thick) fabricated by pulsed laser deposition
have a 40% tunability with five volts dc bias at 2 GHz, and tan5 values from 0.067 to
0.10 [78].
Doping has been found to have a significant effect on the microwave dielectric
properties o f the BST thin films. Acceptor ions (such as Fe^^, Mn^^, Mn^^) provide
electron traps, but they may cause oxygen vacancies and holes to compensate their
effective negative charge, whereas donor ion (such as W^^) reduce the oxygen vacancies,
but they may generate (Ba,Sr) vacancies and free electrons to compensate their effective
positive charge [79]. Several researchers have shown that the addition o f some dopants
(with small concentrations) such as Fe^"^, Mn^"^, Mg^^, Ni^"^, and La^"^ could dramatically
reduce the dielectric loss tangents o f the BST thin film [80-83]. Usually it has been
observed that with the reduction in the losses o f the BST thin films is accompanied by the
reduction in tunability. Kim et.al. observed that with 1 mol% Ni-doped BST thin films
tunability and losses were reduced to 54.2%, and 0.0183 respectively [84] as compared to
63 % and 0.0275 that in pure BST film, so that FOM o f lm ol% Ni doped film was 30 as
compared 23 for pure BST film measured at 100 kHz. Upto 5 mol% Mg doping in
BST60 films Joshi et.al. observed the reduction in tunability to 17.2 % and losses to
0.007 as compared to 28% and 0.013 for pure BST60 on Pt substrates [80]. W ith further
Mg doping the losses increased as observed in case o f La doping. Cole et.al. prepared 1,
5, and 10 mol% La doped BST60 films [85]. Among those films, 1 mol% La doped film
showed lowest losses o f 0.019 with 19% tuning, and with further increase in dopant
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
concentration tunability decreased drastically and losses were increased [85]. It was
usually observed that the dielectric tuning decreases with the addition o f acceptor dopants
in BST film. Considering the trade off between dielectric tunability and the values o f
dielectric losses, dielectric constant, and film resistivity {tan6 = [pcoSoSr]where p i s the
resistivity, and Sr is the relative permittivity, [ 86 ]}, the concentrations o f the aceeptor
dopants must be optimized to obtain the best overall properties for use in the tunable
device applications.
In the case o f BST bulk, addition o f low loss materials like MgO, AI2O 3 etc. was
found to significantly reduced the loss o f pure BST ceramics. Recently, there have been
few efforts o f making such composites in thin film form. For example, Chong et.al.
prepared doped BST50 targets with 10-40 % AI2O 3 and deposited the thin film by PLD
technique [87]. W ith the increase in the AI2O 3 contents in the target, dielectric constant,
dielectric loss, and tunability was found to decrease and reaehed to 870, 0.11, and 15.9
respectively for 40 % doped BST measured at 7.7 GHz. The FOM increased from 7.33
for pure BST50 to 14.15 for 40% AI2O 3 doped BST50 film. Lee et.al. observed dielectric
constant and loss tangent o f 5mol% MgO doped BST70 films deposited by RF sputtering
were 372 and 0.0037 as compared to 329 and 0.011 (at 100 kHz) for the pure BST films
grown on Pt/TiN/Si 02 coated on AI2O 3 substrates [88 ]. Chang et.al. deposited BST60
thin film by PLD technique [81] using MgO doped BST60 targets with 1-60 % MgO. The
dielectric losses o f the composite thin films were best for the 20 % MgO doped BST film
(Q~75) than the pure BST film (Q~ 20) however, the tuning o f the films were only 10%
measured at 8 GHz. Composite thin films o f BST60 with 40 % MgO was prepared by
Rivkin et.al. [89]. Lowest loss value o f 0.006 was obtained in the film aimealed at 950 °C
49
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
at 8 GHz, however the tenability redueed to 2% for the film, whereas the highest
tenability o f 17.7% was obtained for film annealed at 1200 °C but the losses also
increased to 0.027.
These results show that the exact quantity o f acceptors needed to compensate a
film depends on the details o f growth technique. In addition, good dielectric and
insulating properties are not standalone requirements, other material properties, such as
film structure, microstructure, surface morphology, also affect device performance and
long-term reliability. Although studied in a scattered way, other parameters, such as film
structure, microstructure, nature o f the film-substrate interface etc. are reported to
influence device performance and long term reliability. To fully evaluate and understand
the properties, the influence o f acceptor concentration on the microstructural, surface
morphological, properties must be assessed and correlated with the film’s dielectric and
insulating properties. Moreover, most o f the measurements are only done at low
frequency regime. The dielectric properties o f these films at microwave frequencies have
rarely been reported, however in most o f these studies, it was tacitly been assumed that
the high frequency behavior would resemble the trend o f low frequency behavior o f these
films.
Few results are summarized in the Table l.X .
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CD
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Substrate
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Tech.
PLD
Loss
Kfacto
r
Freq.
E
Tunability (%)
(E. Field)
IMHz
2290
65 (57kV/cm)
-
-
30GHz
-
-
-
-
IMHz
4660
52 (57 kV/cm)
-
-
Phase
shift
(field in
kV/cm)
FOM
(PS*)
Ref.
-
437dB
[62]
-
BST 50 on LAO
PLD
IMHz
1430
33 (23.3 kV/cm)
0.007
47
-
-
[63]
BSTSOonLAO
PLD
100 MHz
-
42 (80 kV/cm)
0.008
52.5
-
-
[90]
IMHz
2000
47 (30 kV/cm)
0.008
58.7
B
S
c
BST60 on MgO
PLD
250” (400)
537dB
BST70
MOCVD
H
1/3
OQ
u
BST40onPl/Si
Polymeric
BST80onPt/Si
precursor
C/3
o
[67]
50 MHz
BST70onLAO
Q.
Sol-gel
680
(/)
(/)
H
00
[93]
-
-
4000
-
0.018
-
-
IMHz
3500
Sol-gel
[92]
0.01
0.04
BSTSOonLAO
BST with BT-BST70/Pt/Si
-
-
-
3800
Compositionally graded
-0.004
749
BST60onLAO
■CDD
60 (0.175)
-
100 kHz
BSTSOonLAO
CD
<5dB
23.67GHz
46.9 (80 kV/cm)
-
[94]
-
-
[96]
-
-
[80]
0.008
55
0.009
-
-
-
-
100 kHz
520
40 (230 kV/cm)
0.03
100 kHz
450
28.1 (200 kV/cm)
0.013
-
Mg doped BST60 on Pt/Si
0 mol%
MOSD
'a<o
a
1 mol%
100 kHz
386
17.2 (200 kV/cm)
0.007
a
20 mol %
100 kHz
205
7.9 (200 kV/cm)
0.009
51
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100 kHz
283
12.1 (-do-)
0.019
5 mol%
100 kHz
204
3.49 (-do-)
0.019
10 mol%
100 kHz
200
1.2 (-do-)
0.030
BST60 on LAO
100 kHz
1736
66 (lOOV)
0.01S3
10 GHz
730
S8 (-do-)
0.040
100 kHz
2093
63 (-do-)
0.0030
10 GHz
1820
S6 (-do-)
0.006
0 mol%
o
MOSD
-
-
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[85]
-
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[99]
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274“
39.1 °/dB
121°
27.S°/dB
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1% Mn-BST50
19.6 GHz
-1.4 dB
S7°
40.7 °/dB
1% Mn-BST60
IS GHz
-2.1 dB
114°
S4.3°/dB
-
-
168° (400)
37.3 °/dB
274° (400)
39.1 °/dB
7S° (S33)
38.S °/dB
-
-
-
[95]
-
-
-
[29]
in
B
BSieOonLAO
1-2 % Mn-BST50
PLD
PLD
BSTSOonLAO
BST40onLAO
■CDD
BST: 1%Magnesia on MgO
(/)
(/)
BST60/ MgO
1 mol% Mg-BST60/MgO
15 GHz
1-10 GHz
-
-
-
30 (67 kV/cm)
14 GHz
-
-
10 GHz
-7.01 dB
406
0.02S
348
0.022
BST60/Pt/Si
4S0
SO.l (300 kV/cm)
0.013
1 mol% Mg-BST60/ Pt/Si
423
43.0 (-do-)
0.009
O.S MHz
1200
23 (30 kV/cm)
-do-
926
24 (30 kV/cm)
BST60
1 wt % Magnesia -BST60
-
-
-1.9S dB
16 GHz
MOSD
O.OOS
-
-4.SdB
14.3 GHz
PLD
-4.4 dB
PLD
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1.7.3 Lead strontium titanate (PST) for tunable microwave devices:
Somiya et.al. investigated PST ceramics with x=0.2, 0.25, and 0.3 with transition
temperatures o f -59, -24, and 10 °C respectively [101]. Dielectric tunabilities o f 3, 15,
and 70 % were measured for PST20, 25 and 30 respectively under 20 kV/cm at 10 kHz.
Dielectric losses were below 0.001 for all these ceramics [101].
Thin film studies are important for the integration in the devices. Kang et.al.
found that at room temperature, dielectric constant maximizes at 50 mol% Sr contents
(sr=1350, tan6=0.037) in PST thin films grown on Pt/Si substrates [102]. Loss values for
all the studied compositions (l<x<0.7) were found to be below 0.05 at IkHz and leakage
current density was minimum for PST30 film. In another study, Naik et.al. investigated
PST40 film deposited on Pt/Si substrate and found broad dielectric anomaly centered
around room temperature [103]. Spontaneous polarization and Raman modes studies
indicated the presence o f tetragonal phase up to -1 4 0 °C, which was believed to be due to
the distribution o f various phases in the film. Chung et.al. reported PST thin films as a
promising candidate material for ULSI DRAM capacitor and other microelectronic
device [104]. Dielectric constant and tan5 were found to be 330 and 0.04 respectively.
Kim et.al. studied polycrystalline PST (x=0.2-0.8) thin films deposited on Pt/Si substrates
and found that the dielectric constant continuously increases with increasing Pb contents
in the film [105]. For PST40 film, Sr, tan 8 , tunability, and FOM were 335, 0.0174,
47.89% (at 250 kV/cm), and 27.52 respectively measured at 100 kHz. Pontes et.al.
studied polycrystalline PST films with x=0.4-1.0 deposited on Pt/Si substrates and found
that dielectric constant was maximum for PST40 film (-750) and the loss was 0.06
54
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measured at 1 MHz [106]. With increasing Sr content, tenability was found to reduce and
maximum FOM ~6 was observed for PST40 films measured at 100 kHz.
All the films in various studies as mentioned above were polycrystalline in nature.
However, epitaxial tbin films o f this material may improve its dielectric properties as
observed in case o f BST tbin films. Karaki et.al deposited polycrystalline PST50 film on
Ir/Si02/Si and epitaxial PST films on Ir/SrTiOs and NbiSrTiOs substrates [107]. The
epitaxial film was found to have larger Sr (2000), higher tunability (-7 0 % at 200 kV/cm)
as compared to polycrystalline film. Kim et.al deposited PST50 films on LaNiOs (LNO)
and Pt/Si substrates and found that PST film on LNO was highly (100) oriented with Sr,
tunability, and FOM o f 483, 60% and 29.5 respectively as compared to 368, 52%, and
25.8 respectively for polycrystalline film on Pt/Si substrate [108]. This change in the
dielectric properties o f PST films grown on various substrates was attributed to the
change in the film stress. Recently, Lin et.al observed the effect o f strain on the
dielectric properties o f PST tbin films by inducing anisotropic in-plane strain in the film
using (110) NdGaOs substrate [109]. Tunability was 33 % along PST [100], where the
strain was larger as compared to 48 % along PST [010], where the strain was smaller.
These studies show that PST is a promising candidate material for high frequency
tunable devices. However, high quality epitaxial films are needed to grow to improve
dielectric properties. High frequency studies o f these films are completely absent in the
open literature.
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1.8 Motivation for the present study
The presented literature review in earlier sections elucidated the current status o f
barium strontium titanate (BST) ferroelectric thin films (prepared by various deposition
techniques) for tunable devices operated at microwave frequencies.
From the point o f view o f device integration, as well as the ability to miniaturize
the device, incorporation o f BST in thin film form with properties similar to bulk (high
tunability and low loss) is desirable. To attain this, BST thin films should be epitaxially
grown on lattice matched substrates having sharp film-substrate interface with minimal
interdiffusion. Reduction/absence o f grain boundaries in textured/epitaxial films
respectively would reduce the losses, especially at microwave frequencies and also it is
expected that these films would have higher tunability as compared to their
polycrystalline counterpart. The growth o f epitaxial quality thin films by physical vapor
deposition (PVD) techniques or by metallorganic chemical vapor deposition (MOCVD)
technique has been demonstrated. However, it remains an open challenge to grow
stoichiometric, epitaxial BST thin films over large (viz. 2" wafer). Using chemical
solution deposition technique, synthesis o f highly stoichiometric BST films with uniform
film thickness deposited over large substrate area is possible, however, it is challenging
to grow epitaxial film by any solution deposition route.
In the last decade, BST thin films have been studied extensively for their use in
tunable microwave devices. Most o f these studies were limited to evaluate material
parameters at lower (<1 MHz) frequencies. To test these thin films for practical purpose,
in fact, the material should be evaluated at microwave frequencies. Moreover, as
mentioned earlier, design o f device architecture is also equally important to evaluate the
56
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actual thin film quality. In that respect, in fact, a device prototype is required to he made
to evaluate the film quality. This approaeh is attraetive as it gives the right feedback to
evaluate the true structure property relationship. There are very few attempts in the
literature.
Since tunability is a funetion o f applied voltage, the film should withstand large
bias voltage, and henee leakage eurrent should be minimized. Aliovalent doping is an
attractive method to reduee the leakage eurrent density. However, to maintain the eharge
neutrality with the addition o f aliovalent dopant, eationic/anionic vaeaneies are
introdueed in the lattice. Systematie studies are required to investigate the effeet o f such
dopants on the dielectric constant, tunability, and loss tangents. It is also required to
investigate how the epitaxial growth is related wit the dopant indueed lattiee distortion
due to the ereation o f point defeets. Hardly there is any attempt made so far to correlate
the dielectric properties, leakage and growth characteristics o f doped BST thin films.
For many praetical applieations, the temperature coeffieient o f material
parameters should be minimum. For example, the tunability or loss tangent should not be
a strong function o f temperature. Most o f the bulk materials have very sharp transition
behavior, therefore they have large temperature eoeffieient o f eapacitance, loss, and
tunability. If the grown thin film imitates the bulk behavior, it would also have similar
problems. Very limited experimental attempts have been identified in the literature that
addresses these issues. Design o f suitable experiment strategies are elearly required to
address these issue.
There are several studies on the effeet on dieleetrie properties, especially losses
by making eomposites o f BST ceramies with other low loss material. However such
57
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studies are completely absent in the thin Film form. In the ceramic composites, by
tailoring the low loss dielectric volume contents, the effective dielectric constant can be
timed. Viewing this light, synthesis o f such composites in thin film form seems
promising. However, it may be challenging to retain textured growth as well as control th
tenability of such composite structures. Therefore, it is required to fabricate composites in
thin film form to tailor the losses o f BST films without compromising the tunability much
to achieve high figure o f merit o f the tunable devices at high firequencies. More research
is clearly required to explore these altematives.
Understanding o f the structure-property relation is important to design novel
materials with better material characteristics as well as device performance. As for
example, although BST thin films are extensively investigated for tunable microwave
device applications, limited attempts have been made so far to identify any alternative
ferroelectric material for high frequency applications. More research is required to
explore other ferroelectric materials in thin films form, which could be an alternative or
better candidate than BST.
1.9 Statement of the problem
Motivated by the above stated facts, following areas have been identified that
need further research to synthesize barium strontium titanate and related thin films for
their possible applications in microwave applications. The aims o f the studies are stated
as follows:
1.
To synthesize high quality textured barium strontium titanate (BST) thin films
by sol-gel technique.
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Sol-gel technique is a cost effective technique is used to deposit homogeneous and
highly stoichiometric thin films. The preparation o f the films are very sensitive to
process parameters. A systematic study o f the effect o f process parameters on phase
formation behavior aimed towards synthesizing highly oriented and textured films
needs to be carried out. Hence, the first part o f the thesis was devoted to identify
and optimize the process parameters to obtain grain oriented highly textured, device
quality BST thin films.
2.
Evaluation of the BST thin film properties at high frequency by fabricating
prototype phase shifter
Most o f the earlier studies on the electrical properties o f BST thin films were
carried out only at low frequencies. However, high frequency properties o f the films
may differ from that measured at low frequencies. Hence, in order to choose the
best material for room temperature microwave tunable devices, it is needed to test
these thin films at high frequencies.
3.
Study the effect of aliovalent dopant, manganese (Mn), on the epitaxial growth,
electrical, and dielectric properties of the BST thin films.
Electrical and dielectric properties o f a ferroelectric capacitor depend on factors
such as, the crystalline quality, microstructure o f the film and its distribution,
concentration and type o f the point defects in the thin film etc. Studies on the effect
o f aliovalent dopant on structure, microstructure, and dielectric properties o f BST
thin films are needed to be studied for better insight o f the structure-property
59
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relation. These studies will help to understand the structure-property relationship o f
the material and to develop high performance material for tunable device
applications.
4.
Heterostructures of BST thin films with MgO and MgTiOs
Very low loss non-ferroelectric materials, such as MgO and MgTiOa have been
found to have considerable effect o f the electrical and dielectric properties o f the
BST ceramics. However, such studies are scarce in the thin film form. Hence the
effect o f making composites in the BST thin films with various volume contents o f
the BST and low loss materials are needed to be is studied in details at low and high
frequencies.
5.
Thin Films of lead strontium titanate (PST) as a potential candidate material
for application in tunable microwave devices.
PbxSri.xTiOs (PST) with x<0.3 has Curie temperature below room temperature.
Therefore it is possible to expect that PST films will show the properties similar to
paraelectric materials (like BST) with a high dielectric constant. There are limited
reports where PST has been proposed as a perspective material for tunable devices
applications. Thin films o f PST for tunable microwave devices has not been tested
at high frequencies so far, it is therefore interesting to grow PST thin films using
cost effective sol-gel technique and characterize these films at low and high
frequency to actually test the potentiality o f this material for tunable microwave
device applications.
60
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[102] D.H. Kang, J.H. Kim, J.H. Park, K.H. Yoon, Mat. Res. Bull., 36 (2001) 265.
[103] V.M. Naik, D. Haddad, R. Naik, J. Mantese, N.W. Schubring, A.L. Micheli, and
G.W. Auner, J. Appl. Phys., 93 (2003) 1731.
[104] H.J. Chung, J.H. Kim, and S.l. Woo, Chem. Mater., 13 (2001) 1441.
[105] K.T. Kim and C.H. Kim, Thin Solid Films, 420 (2002) 544.
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[106] F.M. Pontes, S.H. Leal, M.R.M.O. Santos, E.R. Leite, E. Longo, L.F.B. Soledade,
A.J. Chiquito, M.A.C. Machado, and J.A. Varela, Appl. Phys. A, (2003) (in Press).
[107] T. Karaki, J. Du, T. Fujii, and M. Adachi, Jpn. J. Appl. Phys., 41 (2002) 6761.
[ 108] K.T. Kim and Chang II Kim, Thin Solid Films, 447-448 (2003) 651.
[109] Y. Lin, X. Chen, S.W. Liu, C.L. Chen, Jang-Sik Lee, Y. Li, Q. X. Jia, and A.S.
Bhalla, Appl. Phys. Lett., 84 (2004) 577.
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CHAPTER 2
Experimental Techniques
In this chapter, thin film deposition technique and their characterization techniques used
in the present study are described.
2.1 Fabrication of thin films and powders
BST thin films have been fabricated using various techniques, including metalorganic chemical vapor deposition (MOCVD) [1-3], R f sputtering [4,5], ion-beam
sputtering [6 ], pulsed laser deposition (PLD) [7-10], and sol-gel technique [11-17] etc..
W et chemical methods such as sol-gel [11-17] and metal organic decomposition (MOD)
[18,19] have also been successfully used in the preparation o f BST thin films. Two
differences between dry and wet deposition methods are cost o f apparatus and
decomposition rate. The dry methods require a high vacuum system, which makes them
more expensive than the wet methods that do not require a vacuum. Deposition rates are
typically faster in the wet methods than the dry methods.
For many capacitor applications, cost is the driving factor because thin-film
devices often are competing against low-cost discrete devices. In this case, chemical
solution deposition (CSD) techniques are preferred because they offer a relatively simple,
low-cost approach to fabricating thin films that is compatible with commercial processing
equipment and techniques such as spin coating and photolithography. In the present
study, sol-gel technique was used to prepare thin film and powders.
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Sol gel offer significant general advantages in fabrication o f electronic materials
given below [20 ,21 ]:
1.
High purity is attainable via purified precursors.
2.
Uniform materials o f controlled complex compositions can be obtained by the use
of suitable homogeneous precursor mixtures.
3.
Relative low processing temperatures may be used, minimizing the reaction with
substrates.
4.
Films may be prepared by simple spin coating technique without the need to
involve vacuiun chambers or high-energy cost o f vacuum evaporation.
5.
Large area substrates (eg. Semiconductor wafers) can be rapidly coated.
2.1.1 Sol-gel technique
Sol-gel technique is one o f the most widely used and very attractive process for
producing high purity ferroelectric, non-ferroelectric oxides, and ferromagnetic materials.
The technique dates as far back as the middle 1800 s with studies on silica gels, and
ferroelectric thin film deposition has been developed in the year since 1985. Today solgel methods are reaching their full potential, enabling the preparation o f new generations
o f advanced materials not easily accessible by other methods yet using mild, low-energy
conditions.
Sol-gel processing is a wet chemical route for the synthesis o f a colloidal
suspension o f solid particles or clusters in a liquid (sol) and subsequently for the
formation o f a dual phase material o f a solid skeleton filled with a solvent (wet gel)
though sol-gel transition (gelation). Thus, a sol can be described as either a stable
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suspension o f submicron particles dispersed in a liquid or a solution o f metallorganic
precursors dissolved in water. The first step o f the sol-gel process includes the selection
o f the precursors o f the desired materials. The most important reagent (precursor) in the
sol-gel process is a hydrolysable organometallic, called metal-alkoxide. The general
formula for a metal-alkoxide is M(OR)n, where the metal atom (M) with valence (n) is
bonded to alkoxy group (OR). An alkoxy group contains an alkyl group (R) such as
methyl (CH 3), ethyl (C 2H 5) etc. The precursor, by its chemistry, lead to the reaction
towards the formation o f either colloidal particles, or polymeric gels. The colloidal
particles obtained can be precipitated and treating accordingly (cold/hot pressing and
sintering) to produce desired ceramics, while the polymeric sol-gel process is used to
synthesize high purity nano-crystalline powders, thin films, and fibers.
Sol-gel processing is very useful and important when the production o f
reproducible homogenous complex ceramics is necessary. As a matter o f fact, this
technique is often considered as the route to “better ceramics through chemistry”.
Sol-gel chemistry
Sol-gel process utilizes organic chemistry to produce inorganic materials. Figure
2.1 shows schematically the chemical reactions that occur in a metallic alkoxide during
the sol-gel process. In this example, the metal (M) has a coordination number o f four and
is surrounded by four alkoxy (OR) ligands, where R represents the alkyl group. (For
simplicity, a single compound is used in this example).
The first step in the polymerization process is the hydrolysis o f the metal
alkoxides. This reaction occurs when metal alkoxides reacts with water to form
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hydrolyzed molecule. Water is usually diluted in solvent before adding it to the sol to
prevent very rapid and inhomogeneous reactions.
M(OR)n + H 2O
M(OR)n-i(OH) + ROH
......................(2.1)
The final reaction can be represented by the equation:
M(OR)n + nHzO ^ M(OH)n + nROH
During hydrolysis,
(hydrolysis)
......... (2.2)
ions replace the alkyl groups and alcohol is given off as a by­
product. Complete hydrolysis usually results in the formation o f precipitates, which are
used in the preparation o f powders. For thin film preparation, sols are usually only
partially hydrolyzed in order to maintain a spinnable sol. The hydrolyzed molecules can
then link together in the condensation reaction. This can be achieved in one o f the two
ways depending on the degree o f hydrolysis (as shown in figure, condensation occurs via
the reaction between an alkoxy and a hydroxy group or between two hydroxy groups). In
the condensation reactions, either water or alcohol is given off as a byproduct, depending
on the reaction path. The first is a dehydration reaction, which can be written as follows:
2M(OR)n-iOH ^ M20(0R)2n-2 + H 2O
(condensation)
......... (2.3)
The second is a dealcoholation reaction represented by the equation:
M(OR)n + M(OR)n-iOH ^ M 2 0 (0 R) 2n-2 + ROH (condensation)
......... (2.4)
Both, continuous hydrolysis and condensation reactions in the sol eventually lead to the
to polymerization o f the sol with M-O-M bonds (the backbone o f the oxide) joining the
metals in an inorganic cross-linked structure or a gel. The advantage o f application o f
metal alkoxides in comparison with other starting materials lies in formation o f alcohol as
the only byproduct o f the reaction.
M(OR)n +nHX -> MXn + nROH
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(2.5)
Rapid hydrolysis o f alkoxide can also be slowed down by reacting them with various
cbemical modifiers popularly known as chelating agents, Acetic acid, acetyl acetone etc
are the widely used cbemical modifiers in sol-gel system. However, the characteristics
and properties o f a particular sol-gel inorganic network are related to a number o f factors
that affect the rate o f hydrolysis and condensation reactions, such as pH, temperature and
time o f reaction, reagent concentration, catalyst nature and concentration, and drying.
When the sol particles aggregate, or interknit into a network, a gel is formed. Gel is
amorphous and contains trapped volatiles (water, alcohol etc.). Upon drying (at around
100 °C), these trapped volatiles are driven off and the network shrink as further
condensation can occur. After drying, unhydrolyzed alkoxy groups may still remain in
the gel, which are removed using a pyrolysis heat treatment at temperatures between 300
to 400 °C. The pyrolized gel is still amorphous and must be given a crystallization heat
treatment to yield the desired oxide.
In multicomponent systems, a nonrandom distribution o f elements can occur. To
achieve maximum homogeneity, control o f the networking building rate o f the different
components is necessary. This can be achieved by several techniques such as the complex
formation o f precursors, controlled addition o f water, choice o f proper solvent etc.
Thin film deposition
For the thin film deposition, a parent sol is prepared by co-mixing the individual
solutions in required stoichimetric proportion. Prior to deposition, the parent sol is further
diluted to make coating sol. Several different techniques, such as dip coating or spin
coating can be used for depositing the thin films on the cleaned substrates/
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%
OR
Sol
=
R O _ iv i— OR
OR
OR
I
I
Hydrolysis
HO— M— OH
RO— M— OR
OH /
H+
J
Jr
r
OR
Condensation
+ROH
OR
HO— M— OH
+
HO— M— OH
OR
1.
OR
OR
I
I
OR
I
OH
OR
OH
I
I
ot
0*H
HO— M— O— M— OH
+H p
L
or
2.
HO— M— O— M— OR
+ ROH
Gel
Rigid network forming backbone of oxide
F ig u r e 2.1
Schematic diagram o f hydrolysis and condensation reaction in alkoxide
M(0R)4.
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In dip coating, sol viscosity and withdrawal speed o f the substrate determine the film
thickness, whereas in spin coating technique, the molar concentration o f the sol, spin
speed and duration o f spirming determines the film thickness. Due to its process
simplicity, films are deposited mostly by spin coating technique. In the present work also,
spin coating technique was used to deposit thin films o f ferroelectric materials.
Heat treatments of films
Drying and final annealing are the two heat treatment steps are followed for the
final processing o f the films by the sol-gel technique. Various thermal processes occur in
stages upon heat treatment o f the deposited films. These stages involve many complex
reactions between the substrate and film, the film and the atmosphere, and within the film
itself. Up to approximately 150 °C, loss o f solvent, physically absorbed water, weakly
bound ligand molecules occur with shrinkage o f the film resulting from capillary
contraction. From 150-400 °C pyrolysis o f the organic residues in the film begins, and
tiny pores are left in the film. Densification o f the film occurs generally between 400-600
°C, by collapse o f the small pores. At higher temperatures, complete consolidation o f the
film is achieved by a viscous flow sintering mechanism [22]. All the sol-gel deposited
films are amorphous after deposition and needed to he post annealed at relatively higher
temperature for crystallization.
Film quality
In addition to the fundamental requirement o f stoichiometry and crystal structure,
the physical quality o f the ferroelectric film also has strong influence on its electrical
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properties [23]. There are several factors, which determine the quality o f the film, which
includes thickness, cracks, surface roughness, inhomogeneous nucleation, surface energy,
etc of the films. However to obtain a good quality film the precursor chemistry should be
properly adjusted. During sol-gel processing, adhesion o f the film to the substrate should
be ensured throughout all the processing steps. Proper cleaning o f the substrate surface,
and the introduction o f the OH groups onto the substrate surface can enhance film
adhesion. The selection o f precursor material, solvent, and chemical additives has
immense influence on the electrical characteristics o f sol-gel films. Diffusion between
substrate and film and stresses resulting from film-substrate interaction also affect the
film quality.
Films become stressed due to the removal o f organics. The other possible
contributions o f stress are due to the lattice parameter mismatch between film and
substrate and also due to change from amorphous to crystalline state. This intrinsic and
extrinsic stress often destroys the ordered growth influenced by the single crystalline
substrate. Therefore most o f the sol-gel films are reported to be polycrystalline in nature.
Moreover it has been experimentally verified that micro-in homogeneity in the deposited
films results homogeneous nucleation within the film resulting equiaxed grains [24].
Also, under certain condition crystallization starts from the surface o f the films resulting
polycrystalline growth [25].
When lattice-matched substrates having identical crystal structure and similar
thermal expansion coefficient are used the substrates promotes epitaxial growth. The hot
substrates impart the atom mobility o f the deposited species and also act as an effective
heterogeneous nucleation site to control the growth orientations o f the deposited films
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[26]. For practical application point o f view, ferroelectric films are needed to be
deposited on a variety o f substrates, which influence the electrical characteristics.
Important issue addressed in the present study is the synthesis o f epitaxial/highly
textured perovskite thin films by sol-gel technique. In this thesis a systematic approach
has been made to understand the effect o f several process variables on the growth
characteristics o f sol-gel films.
2.1.2 Sol-gel synthesis of powders and thin films in this thesis:
Precursor used in sol-gel synthesis of powders and thin films
To prepare the sols from precursors, barium and strontium acetates were co­
dissolved in heated acetic acid. Ethylene glycol was added for their complete dissolution.
Ti-IV-iso-propoxide was dissolved in acetic acid through continuous stirring and added
to Ba-Sr complex solution at 50 °C. List o f precursor materials used are listed in Table
2 . 1.
Table 2.1: Various precursors used in the BST sol synthesis.
Chemical
Company
Formula weight
Yield (%)
Barium acetate
Alfa Aesar
255.43
99
Strontium acetate
Alfa Aesar
214.72
99
Ti-IV-isopropoxide
Alfa Aesar
284.26
97
Acetic acid
Alfa Aesar
60.05
99
Ethylene glycol
Alfa Aesar
62.07
99
Typical preparation for the 0.01 M Bao.sSro.sTiOs sol is given below.
Chemical formula: Bao.sSro.sTiOs (BST50),
1.
Ba acetate;
= 1.290 gm
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2.
Acetic acid:
= 2.7 gm
Ethylene glycol:
= 1 .1 4 gm
Sr acetate:
3.
= 1.084 gm
Acetic acid:
= 2 .7 gm
Ethylene glycol:
= 1.14 gm
Ti-IV-isopropoxide:
= 2 .9 3 g m
Acetic acid:
= 1.2 gm
(Stirred well inside the glove box)
The three solutions were mixed together in a flask and heated upto 110 °C. The solution
was stirred for 20 minutes and then cooled to room temperature to get the final sol named
as Parent sol.
Powder preparation
To derive gel powders the parent sol was kept overnight in an oven maintained at
80°C.
The dried gel was cmshed in a mortar pestle to get fine amorphous powders,
which are subsequently annealed at higher temperatures to form nano-crystalline
ceramics.
Preparation of Thin Films
In the present work, films have been synthesized by spin deposition technique.
Prior to film deposition, the parent sol was diluted to required strength (in the present
case 0.35 M) with suitable solvent to get the coating sol for thin film deposition.
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Calculations for obtaining the coating sol of 0.035 M from the parent sol:
Volume o f the parent sol = 9.5 ml (suppose)
Strength o f the parent sol = Si = 0.01/9.5x 10'^ = 1.0526 M
Vi = known volume o f the parent sol (usually 2 ml)
Ni = Known strength o f the parent sol = Si = 1.0526 M
N 2 = Required strength o f the coating sol (0.035 M, in case o f BST sols)
Since Vi x Ni = V 2 x N 2
(2.6)
Therefore, Required volume, ¥2 = 6 .015 ml
Therefore, volume o f the solvent (acetic acid) to be added in the 2 ml parent sol
= 4.015 ml
Sol thus obtained is the coating sol o f 0.35 M. It is then filtered using 0.2 pm filter, to be
used for film coating.
The substrates used for the epitaxial growth o f the BST thin films were (100)
oriented single crystal LaAlOa (LAO) supplied by MTI Corporation (for low frequency
measurements) and from NASA (for phase shifter m easurem ents). This substrate is used
because o f the close lattice match with BST (aBsi= 3.947A and aLAo= 3.79A) and since it
is common substrates for microwave devices. The as-received substrates are cleaned with
soap solution followed by ultrasonification and degreasing with alcohol and ethyl
alcohol. Coating sol was spun coated by depositing single drop o f the solution onto a
cleaned substrate spinning around 2500 rpm for 8-10 seconds. The as-deposited film spin
coated on the substrate is amorphous and can retain a significant organic fraction, which
is highly dependent on the size and reactivity o f the initial precursor species. Typically,
deposition o f each coating is immediately followed by pyrolysis. In this way, each
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coating layer is crystallized before the deposition o f next coating. After the coating o f the
final layer, the film is finally annealed at the desired temperature in air. General flow
chart o f the sol-gel process is given in Figure 2.2
Barium acetate dissolved
in acetic acid and mixed
with ethylene glycol
Stronitum acetate dissolved
in acetic acid and
mixed with ethylene glycol
C o n tin u o u sly s tir r e d
Ba-sol
C o n tin u o u sly stir r e d
Sr-sol
Continuously stirred
fo r 30 minutes
Ti-IV isopropoxide
dissolved
in acetic acid
C o n tin u o u sly s tir r e d
Ti-sol
BST-sol
Diluted to 0.35 M for coating
Spin coated on substrates
R e q u ir e d no. o f tim es
Fired at 600 ®C for 5 minutes
Annealed at proper temperature
Figure 2.2 Flow chart o f the sol-gel process o f BST sol and thin film preparation.
2.1.2.1 Preparation o f Pure B ST Films
Parent sol o f BST was prepared as described above. The coating sol o f 0.35 M
was spin coated on the cleaned LAO substrates using multilayer approach. Film was
coated required number o f times. After the deposition o f each coating, the films are kept
in a preheated furnace at 600 °C for 5 minutes, for the removal o f organics and
crystallization. In this way, each coating layer is crystallized before the deposition o f next
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coating. After the final coating BST films were annealed at different temperatures. The
annealing conditions used in the different studies will be described in the respective
chapters.
2.1.2.2 Preparation o f manganese (Mn) doped B ST film s
2.1.2.2.1 Uniformly Mn Doped
Parent sols were prepared with Mn doping o f 1, 3, and 5 at % doping in BST50.
To prepare M n doped BST sol, stoichiometric amount o f manganese acetate, dissolved in
acetic acid was added to BST50 sol through continuous stirring. For thin film deposition,
the as-prepared sol was diluted to a concentration o f 0.35 M by adding acetic acid. The
precursor films were coated on (100) oriented single crystalline LAO substrates by spin
coating technique. Films were coated at 2500 rpm for 10 seconds for the deposition o f
each layer. After the deposition o f each coating, the films were immediately thermally
treated in a preheated furnace at 600 °C for five minutes. On finishing the final coating
procedure, all the films were annealed at 1050 °C for 2h.
Table 2.II: Additional precursor used for Mu doping.
Chemical
Company
Formula weight
Yield (%)
M n (Il)-acetate
Alfa Aesar
245.08
Mn 22%
Acetic acid
Alfa Aesar
60.05
99
Ethylene glycol
Alfa Aesar
62.07
99
2.1.2.2.2 Graded Mu Doped
To prepare Mn doped BST50 sol, stoichiometrie amoimt o f manganese acetate,
dissolved in acetic acid was added to BST50 sol through continuous stirring. For thin
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film deposition, all o f the pure BST50 and M n doped BST50 as-prepared sols were
diluted to a concentration o f 0.35 M by adding acetic. A multi-layer graded structure was
made starting from 1.0 at%, 3.0 at%, and 5.0 at% M n doped layers on cleaned LAO
substrate followed by the deposition o f 3.0 at% and 1 at% Mn doped BST50 coatings.
Each individual layer is about 100 run thick yielding a total film thickness o f about 500
nm. Finally, the graded as well as uniform BST50 films were annealed at 1050 °C for 2h
in air followed by the normal furnace cooling down to room temperature. Such a graded
structure is expected to have improved overall electrical characteristics compared to the
films prepared by introducing uniform doping.
2.1.2.3 Preparation o f Heterostructured B ST Thin Films
BST50 sol was prepared in the similar way as described previously, using barium
acetate, strontium acetate, and Ti-IV-isopropoxide as precursor materials. MgO sol o f 0.2
M was prepared by mixing magnesium ethoxide and methoxy ethanol.
Deposition of heterostructured thin films and annealing condition
BST thin film heterostruetures were prepared by the different sequential
deposition o f MgO and BST layers (and hence the different BST/MgO volume ratio),
with the aim to achieve composite BST films with the desired dielectric properties. MgO
was chosen primarily because o f its low dielectric constant and low loss tangent and it
also influences the crystallinity o f the grains and microstrueture o f the composite. As a
result the dielectric properties and tunability o f the composites can be tailored.
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BST:MgO hetero-structured thin films were prepared by depositing MgO and
BST layers. The precursor films were coated on (100) oriented single crystalline
lanthanum aluminate (LAO) substrates by multiplayer spin coating approach. Films were
coated at 2500 rpm for 10 seconds for the deposition o f each layer.
Different types o f BSTSO/MgO layer sequences were followed for the preparation
o f heterostructured BST film. In Type I film, thick coating o f each BST and MgO were
deposited alternately on the LAO substrate, with BST50 as the first and the terminating
layers to get the BST:MgO volume ratio o f ~ 80:20. For Type II film, coatings o f both
BST50 and MgO o f different thicknesses were deposited altematively with MgO as the
first and BST as the terminating layers to get the BST:MgO volume ratio o f ~ 68:32. For
Type n i film, thin coatings o f both BST50 and MgO were deposited altematively with
MgO as the first and BST as the terminating layers to get the BST:MgO volume ratio o f
~ 62:38 (more MgO in the film as compared to Type I and n films).
After the deposition o f each coating, the films were immediately thermally treated
in a preheated fiimace at 600 °C for five minutes. On finishing the final coating
procedure, all the films were annealed at 1100 °C for 6 h.
2.1.2.4 Preparation o f Pure PST Thin Films
Precursors and Preparation of PST sol
Precursor solution o f PST was synthesized from lead acetate tri-hydrate,
strontium
acetate,
and
titanium
isopropoxide
(list
presented
in
Table
2.III).
Stoichiometric amount o f lead acetate was dissolved in acetic acid. The solution was
refluxed at 110 °C for 1 h to remove the crystallization water. Separately strontium
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acetate was dissolved in warm acetic acid and addition o f ethylene glycol was found
effective for complete dissolution o f strontium acetate in acetic acid. Inside moisture
controlled glove box; stoichiometric amount o f titanium isopropoxide was dissolved and
chelated with acetic acid.
The strontium acetate and titanium alkoxide solution were
mixed with lead acetate solution through continuous stirring at a temperature o f about 90
°C. The precursor sol thus prepared is termed as parent sol.
Table 2.III: List o f precursor materials used in the PST sol synthesis.
Chemical
Company
Molecular weight
Yield (%)
Lead acetate
Alfa Aesar
379.33
99
Strontium acetate
Alfa Aesar
214.72
99
T i-lV-isopropoxide
Alfa Aesar
284.26
97
Acetic acid
Alfa Aesar
60.05
99
Ethylene glycol
Alfa Aesar
62.07
99
Typical preparation for the 0.01 moles Pbo jSro TTiOs sol is given below:
Chemical formula: Pbo.sSrojTiOa (PST30),
4.
5.
6.
Ph acetate:
= 1.149 gm
Acetic acid:
= 2.7 gm
Ethylene glycol:
= 1 .1 4 gm
Sr acetate:
= 1.518 gm
Acetic acid:
= 2.7 gm
Ethylene glycol:
= 1.14 gm
Ti-IV-isopropoxide:
= 2.93 gm
Acetic acid:
= 2 .7 gm
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After mixing them in proper conditions, parent sol is obtained. It is then diluted to
required strength (0.035M) to get the solution for spin coating.
Preparation of PST thin films
The coating sol was spun coated on cleaned platinized silicon (Pt/Si) and LAO
substrates at an rpm o f 3000 for 5s. Just after deposition the films were fired at 400°C for
5 min. for organic removal and crystallization into perovskite phase. The coating and
firing sequence was repeated for 10 times to attain a film thickness o f about 400 nm.
After the final coating, the films were annealed for complete perovskite phase formation
and better crystallinity. Depositing and armealing conditions o f the PST films are
tabulated in Table 2.IV below.
Table 2.IV: Details o f the PST thin films deposited in the present thesis.
PST composition
Substrate used
Auuealiug temperature
PT to ST
Platinized Si
750 °C/2hr
PST30
LAO
900 °C/3hr
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2.2 Characterization techniques
2.2.1 Optical and vibrational studies
2.2.1.1 Micro-Raman spectroscopy
When an intense beam o f monochromatic light from laser impinges upon a
material, scattering occurs in all directions. If the frequency o f the scattered light being
the same as that o f the original light (vo), the effect is known as Rayleigh scattering.
Another type o f scattering that occurs simultaneously at frequencies both higher and
lower than vq and with considerably diminished intensities, is known as Raman effect.
The difference, Av, between incident and scattering frequencies are equal to the actual
vibrational frequencies o f the material [27].
The experimental setup for Raman spectroscopy, used in our studies is shown in
Figure 2.3. The samples were excited with argon Innova 90 Plus laser from Coherent
Radiation Inc. with 514.5 nm radiation. The laser beam traveled through a set o f filters,
polarization rotator, an iris and then it was brought to sample through the Olympis BH2UMA microscope with an 80x objective equipped with a NEC NC-15 camera. The laser
(~ 3 pm spot size) was focused on the sample. For temperature dependent measurements,
sample is focused before each scan. Scattered light from the sample was passed through a
set o f mirrors and brought to the triple grating spectrophotometer ISA Model T64000
equipped with a liquid nitrogen cooled CCD detector. The backscattering spectra were
recorded using a Raman microprobe system interfaced to a computer. Data analyses were
carried out using the software packages, namely Peakfit v4.02 (Jandel Scientific) and
Origin 7.0 (Origin Lab Corporation).
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Riunnn s|iectroiiielerISAT64000 with triple grating
PMT
Photomultiplier tube
CCD
NECNC15 CCD
Color Camera
Sample (micro
R ara^ setup)
Cold cathode detector
|_
ISA Microscope
L..i
Filter
Low temperature cell:
MMR Tech ciyostat or
Litikam Inc. ciyostat <
Polarization
rotator
F
Backscattering geometry
' Sample (macro Raman setup)
90-degree geometry
High temperature fumace
Figure 2.3. Experimental set up for Raman Spectroscopy.
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2.2.1.2 Film thickness measurements: spectroscopic reflectometry
Filmetrics F20 system was used for the film thickness measurement. Optical
technique like spectroscopic reflectometry (SR) is typically the preferred method for
measuring thin film thickness because it is accurate, non destructive, and requires little or
no sample preparation.
Schematic o f the Filmetrics system is shown in Figure 2.4. The Filmetrics system
illuminated the sample with a tungsten-halogen white light source with an effective
measurement range from 400nm to 850nm. When light is incident on the film (on a
substrate), both the top and bottom o f the film reflects, as the light crosses the filmsubstrate interface and fraction o f the light is reflected by the interface. Light reflected off
o f a material in air is then given by
[{n + \ f + e ]
(2.7)
where n and k are the film’s refractive index and extinction coefficient, respectively.
To fit the experimental reflection curve, initially, the refractive index values and
approximate thickness values o f the substrates and films are entered in the software. The
reflectivity spectrum o f the sample was compared to an internal mathematical reflectivity
spectrum for the given sample. A calculated curve was then fitted to the measured curve,
by changing the fed parameters to achieve the best fitting (or minimum error) to get the
corresponding thickness o f the calculated model.
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FJbar O ptic C a b le
LH)Nt SsurcD
SpsciromBler
ReflwrUKl
Aasembty
.fi ,V |.
I I '
I ,1 '
I II *
Light In
T '■ T
' :
!
i
. ,
i )
TrananitnBd and
di
» _ _ i 4 r — kaA
r6riiSwic^3 n^giifc
Figure 2.4. Schematic o f the Filmetrics system.
Scattwed-radis'M ofl.
diaphragm
\
\
I
'J
26
8 Glancing angle
26 Diffraction angle
a Aperture angle
Diffractomet&r bsftm p th in 0/28 mode
Figure 2.5. Diffraction beam path in 0-20 mode.
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2.2.2 Structural and microstructural characterization
2.2.2.1 X-ray diffraction technique
X-ray diffraction is a versatile, non-destructive analytical technique for
identification and quantitative determination o f the various crystalline compounds,
known as 'phases', present in solid materials, powders and thin films. The physical
properties o f thin films must relate to their structure and it is the X-ray diffraction
technique is used to analyze the structure, crystallinity, orientation o f the crystallites,
stress and strain etc o f thin films. This technique can be used to study the changes in the
above properties as a function o f temperature, composition, doping etc in the material.
When the beam o f CuK<x radiation is incident on the sample, it is diffracted off by
the planes o f atoms, which satisfy Bragg’s equation,
nX = 2 d s in 9
(2.8)
Where n is the order o f diffraction, X is the wavelength o f radiation, d is the spacing
between planes o f atoms and 9 is the angle o f radiation with the diffraction plane (Figure
2.5), The diffracting X-rays interact with the variation o f electron density inside the
sample. For the crystalline material, the periodic repeating electron density will give rise
to well defined diffraction peaks whose widths are determined by the crystalline
"quality". "High quality" crystalline material will give rise to sharp peaks (high
frequency) whose widths are limited by the instrumental resolution. While "poor quality"
crystalline material will give rise to broader more diffuse diffraction peaks (low
frequency). By "quality" is meant the length scale over which the crystal order exists and
also the reproducibility o f crystalline spacing. The widths o f the peaks in a particular
pattern provide an indication o f the average crystallite size. Large crystallites give rise to
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sharp peaks, while the peak width increases as crystallite size reduces. Peak broadening
also occurs as a result o f variations in J-spacing caused by micro-strain.
The phase formation behavior and degree o f orientation o f the annealed films in
the present study were investigated using a Bruker GADDS X-ray diffractometer. X-ray
difffactograms were recorded using CuKa radiation; with 20 typically in the range 20 to
80
scan speed o f 3 7m in and slits o f lm m -2m m -2m m -lm m widths. The various peaks
corresponding to hkl planes o f the films (/powders/ceramics) and the substrates were
identified using the JCPDS files.
Lattice parameters for the tetragonal (a=b^c) and cubic (a=b=c) systems were
calculated from the X-ray difffactograms using the relation:
^hki
a
b
c
(2-9)
Where dhki is the distance between adjacent planes in the set {hkt) {called as interplanar
distance} and 'a \ 'b \ and ‘c ’ are the lattice parameters inx, y, andz-axis respectively.
To determine the preferred orientation o f the crystallites in polycrystalline
aggregates is referred to as texture analysis, and the term texture is used as a broad
synonym for preferred crystallographic orientation in the polycrystalline material,
normally a single phase. The preferred orientation is usually described in terms o f pole
figures. The most common representation o f the pole-figures is sterographic or equal area
projections. Measurement o f texture (the non-random or preferred orientation o f
crystallites) involves measurement o f the variations in intensity o f a single Bragg
reflection as the sample is both tilted (psi) and rotated (phi). The result is plotted as a
'pole figure', in which the contours indicate intensity levels as a function o f sample
orientation.
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2.2.2.2 Rutherford backscattering spectroscopy (RBS)
RBS technique can be used to study the film stoichiometry and interdiffusion in
thin films. In this technique high energy ions, typically low MeV 4He, are bombarded
onto the sample surface at a known angle. The backscattered ions are measured for their
energies and intensities to determine the qualitative and quantitative compositional
analysis, respectively. Also, concentrations can be measured as a function o f depth into
the sample upto several hundred nm.
The BST thin films prepared in this research were analyzed by RBS at nuclear
laboratory at ITT Kanpur, India, by Dr. V.N. Kulkami. The experimental spectra were
simulated by the RUMP program to analyze composition and thickness.
2.2.2.3 Atomic force microscopy (AFM)
AFM is a microscopic technique that measure the morphology and properties o f
surfaces on the atomic scale. It can be used to measure surface topography, surface
hardness, and elastic modulus. The interaction that is monitored in AFM is the van der
Waals force between the tip and the surface; this may be either the short-range repulsive
force (in contact-mode) or the longer-range attractive force (in non-contact mode). For
AFM, a sharp tip, typically made from Si3N 4 or Si is placed at the end o f a cantilever with
a very low spring constant. The nanoscope AFM head employs an optical detection
system in which the tip is attached to the underside o f a reflective cantilever. A diode
laser is focused onto the back o f a reflective cantilever. As the tip scans the surface o f the
sample, moving up and down with the contour o f the surface, the laser beam is deflected
off the attached cantilever into a dual element photodiode.
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Feedbutcik Loop Malnlains
Ccmttant C anulm ir DofhtcElon
Controilm
il»eironic«
Ui»«r
Scanner
Pctector
ileefiontea
Split
Ph.ototltorfe
Msctor
CanMlevar &ftp
Sample
Figure 2.6. Contact mode AFM operates by scanning a tip attached to the end o f a
cantilever across the sample surface, while monitoring the change in cantilever
deflection with a split photodiode detector.
The photodetector measures the difference in light intensities between the upper
and lower photodetectors, and then converts to voltage. The tip is scanned over a surface.
Feedback, from the photodiode difference signal through software control from the
computer, enables the tip to maintain either a constant force or constant height above the
sample.
In the present work, the surface morphologies o f the thin films were imaged using
contact mode atomic force microscopy (Nanoscope UIE, Digital Instruments). Schematic
o f an Atomic Force Microscope (AFM) set up is shown in Figure 2.6. When tip is in
contact with the sample, the interaction is dominated by relatively short-range inter­
atomic forces. The force on the tip is repulsive with a mean value o f 10
N. This force is
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set by pushing the cantilever against the sample surface with a piezoelectric positioning
element. The deflection o f the cantilever is sensed and compared in a DC feedback
amplifier to some desired value o f deflection. The voltage that the feedback amplifier
applies to the piezo is a measure o f the height o f features on the sample surface. It is
displayed as a function o f the lateral position o f the sample.
Effects on the surface morphologies o f the films were analyzed with various
parameters like: different growth conditions, composition and doping level. This
technique was also used to get the surface roughness and the grain size o f the films.
2.2.2.4 Scanning electron microscopy (SEM)
The cross sectional analysis o f few films was carried out in two places; at the
University o f Puerto Rico (By Mr. Oscar Resto) and Arrizona State University. A
question that was raised by AFM images o f the heterostructured films was whether or not
we can identify and observe the different component layers in the cross section o f the
films.
2.2.3 Thermal characterization of materials
TGA, DSC-50 and DTA-50 from Schimadzu Corporation were used for the
measurements. Built-in software TA-50 from Shimadzu Corporation was used for data
analysis and control o f the equipments.
Thermogravimetric analysis (TGA): TGA is a technique that permits the continuous
weighing a sample as a function o f temperature and/or as a function o f time at a desired
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temperature.
It
is
used
to
characterize
materials
through
their
characteristic
decomposition patterns (loss o f organics etc) and its kinetics. In this study dried gels
(powders) were used for TG analysis. Samples with weights o f 5-15 mg were placed in a
Pt pan. Weight decrease over the temperature range was recorded.
Differential thermal analyzer (DTA) and differential scanning calorimetry (DSC):
Differential thermal analysis is the measurement o f the difference in temperature
between a sample and a reference as heat is applied to the system. DTA involves heating
the sample and comparing its temperature to that o f an inert standard. If the sample
experiences an exothermic or an endothermic reaction, it will be at higher or lower
temperature than the standard respectively. In these way reactions such as dehydration,
reduction, oxidation, phase transition, decomposition, and crystallization can be
correlated to a temperature range. DSC is useful to make the same measurements as DTA
and has the added capability to measure heat capacities.
DTA/DSC in conjunction with TGA is used to identify the various thermal events
associated with the transformation o f amorphous gel powder to crystalline powder during
heat treatment. These methods are used to decide the intermediate thermal treatment
temperature during thin film deposition by sol-gel technique. Experiments were done
typically in the temperature range o f 20-900 °C at a constant heating rate ranging from 510 deg/min.
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2.2.4 Composition and depth profile analysis by X-ray photon
spectroscopy (XPS)
XPS is a surface sensitive technique that provides information about the chemical
states and the concentration o f the elements comprising the surface layers o f a solid.
Scematic o f an XPS spectrometer is shown in Figure 2.7. The sampling depth o f this
technique is usually 30-50 nm. This technique has been used to determine the distribution
o f elements and average surface compositions o f elements in the surface layer o f the film
within the analysis area (typically o f several mm^).
The Einstein relation gives the energy o f a photon;
E = hv
(2.10)
Where, h is the Plank's constant (6.62x10’^"^ J s), a n d /is the frequency (Hz) o f the
radiation.
In XPS the photon is absorbed by an atom in a molecule or solid, leading to
ionization and the emission o f a core (inner shell) electron. The kinetic energy
distribution o f the emitted photoelectrons can be measured using any appropriate electron
energy analyzer and a photoelectron spectrum can thus be recorded. For each element,
there will be a characteristic binding energy associated with each core atomic orbital i.e.
each element will give rise to a characteristic set o f peaks in the photoelectron spectrum
at kinetic energies determined by the photon energy and the respective binding energies.
The presence o f peaks at particular energies therefore indicates the presence o f a specific
element in the sample under study.
The most commonly employed x-ray sources are those giving rise to: Mg Ka
radiation: hv = 1253.6 eV, A1 K« radiation: h v = 1486.6 eV. The emitted photoelectrons
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will therefore have kinetic energies in the range o f ca. 0 - 1250 eV or 0 - 1480 eV. Since
such electrons have very short Inelastic Mean Free Path (IMFP, it is a measure o f the
average distance travelled hy an electron through a solid before it is inelastically
scattered) in solids, the technique is necessarily surface sensitive.
Energy Analyzer
Electron
Detector
j Lens System
Computer
Amplifier and
Ratemeter
Source Slit
Crystal
-4—
X-Ray
Monochromator
Sample
Printer
X-Ray
Anode
Figure 2.7. Schematic representation o f an XPS spectrometer employing a
hemispherical electron energy analyzer and monochromatic X-ray source.
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2.2.5 Dielectric and electrical characterizations
2.2.5.1 Interdigitated electrodes
For low frequency measurements (IkH z-lM H z) interdigitated capacitors consists
o f 50 and 100 fingers respectively were deposited on the films (at NASA facilities) that
were 7 mm long, 20 pm wide, and spaced 15pm apart (Figure 2.8).
2.2.5.2 Dielectric and ferroelectric properties
Dielectric properties o f the films (1 kHz to 1 MHz) were measured using the HP
4294 A impedance analyzer and interdigitated design (Figure 2.8). Frequencies were
varied from 1 kHz to 1 MHz and the measurement oscillating voltage was 500 mV. The
dielectric constant (Cr) o f the films was extracted from the capacitance using conformal
mapping technique originally developed by Gevorgian et. al [28].
For the temperature dependent dielectric measurements, samples were placed in a
MMR cryostat. The low temperature MMR refrigerators operate using the JouleThompson effect. When a gas, such as nitrogen, is allowed to expand through a porous
plug or fine capillary tube at high pressure (-1800 psi), the gas cools on expansion. The
MMR K-20 programmable controller provided a controlled cycling and temperature over
a temperature range 70-700 K. Schematic set-up o f the MMR cryostat with various
accessories are shown in Figure 2.9.
The ferroelectric hysteresis measurements were carried out using a virtual ground
mode. The Radiant HVS6000 ferroelectric tester was used for such measurements [29].
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IDT electrodes --------- ►
Dielectric film
-►
Substrate
IDT electrodes —
VV d t 1. a - 1 5 tim
(»ap. b “ 20 nm I R
D ielectric film
Figure 2.8. IDT capacitor structure deposited on thin films.
To V olt K e te r o r I n s tn « » e n ta tio n
R e g u la to r
E l e c t r ic a l
C oaxial
Copper Tubing
d / 2 " m ln .)
-Vacuum
Rubber Tubing
F ilte r
E le c t r ic a l
H arness — <
Gas Line
Flow Meter
»1R Progranw able
Tem perature C o n tr o lle r
Figure 2.9. Schematic set up o f the MMR cryostat with various accessories.
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The total charge flows as a result o f the switching o f the spontaneous polarization, the
linear capacitive component and the resistance leakage o f the film is collected by an
integrator to ascertain the area under the current versus voltage curve. This total response
is then renormalized per unit area and then graphically displayed for the user
interpretation. The collection o f all charge flow by the test system allows the instrument
to separate out each o f these effects and thus obtain a true picture o f the film’s remanent
polarization response.
2.2.S.3 Leakage characteristics
The leakage current is an important characteristic o f thin film ferroelectric
capacitors. The leakage current measurements were performed as a function o f applied dc
voltage and time held under a has field using Keithley electrometer. Current
measurements versus applied voltage typically involved increasing the voltage to +150 V
with a 1.0 V step and a 60 sec hold at each voltage. For the current density measurements
with respect to time, a selected bias voltage was held for a period o f time ranging from
1000 to 10,000 seconds.
2.2.6 High frequency phase-shifter measurements
To fabricate phase shifters, the BST films were metallized using electron beam
evaporation at NASA Glenn Research Center with a 15 nm chromium (Cr) or titanium
(Ti) adhesion layer followed by a 2 - 2.5 pm thick gold (Au) film. Standard lift-off
chemical etching technique was used to fabricate Au/BST/LAO phase shifters. Finally, a
Cr/Au ground plate with the same thickness was e-beam evaporated on the back o f the
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substrate. The phase shifter design consists o f n coupled microstrip sections in series.
Each section functions as a single pole broadband filter whose passhand shifts with dc
bias applied to the ferroelectric. The phase shift is proportional to n. The circuits used in
the present study were eight element coupled section phase shifters (CMPS) [28]. Figure
2.10 shows the schematic diagram o f the eight element coupled sections designed on 254
p,m thick LAO substrate. A schematic o f a single coupled microstrip section is shown
enlarged in the Figure 2.10. These phase shifters are fairly narrowband (about 12 %
bandwidth) and the optimal frequency fopt, depends upon the dielectric permittivity (€r)
and thickness o f the ferroelectric film. These phase shifters were modeled primarily using
Sonnet’s em® and, to a lesser extent, EEssof Touchstone® and Zeland’s IE3D®
electromagnetic simulators. An analytical model o f a CMPS based on the quasi-TEM
variational expressions o f Koul and Bhat [30] in combination with the transmission line
method o f Crampagne, et al. [31] has been developed by R. Romanofsky et al. [32] to
facilitate the design. The phase shifters were optimized to minimize microwave losses
and maximize relative insertion phase shift. The phase shift is a function o f the dielectric
constant o f the BST in the gap {SrBsi) between microstrips. However, the relation between
phase shift and SrssT is non-linear, with phase shift per SrBST increasing near the lower
frequency edge o f the passhand.
The measurements were taken in vacuum to avoid the dielectric breakdown in air
under high dc fields between coupled micro-strip sections. The performance o f these
CMPS at microwave frequencies was evaluated by measuring the transmission (S 21) and
reflection (S n ) scattering parameters in Ku band using an HP 8 5 IOC network analyzer.
All loss measurements include the losses due to the SMA launchers ( -0 .2 5 dB).
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2
□
F ig u r e 2 .1 0 . Schematic o f the eight element coupled phase shifter design deposited on
the BST films at NASA center.
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2.3 References:
[1]
Y.A. Jeon, E.S. Choi, T.S. Seo, and S.G. Yoon, Appl. Phys. Lett., 19 (2001) 1012.
[2]
Y. Gao, S. He, P. Alluri, M. Engelhard, A.S. Lea, J. Finder, B. Melnick, and R.L.
Hance, J. Appl. Phys., 87 (2000) 124.
[3]
S. Saha, D.Y. Kaufinan, S.K. Streiffer, and O. Auciello, Appl. Phys. Lett., 83
(2003) 1414.
[4]
T.G. Kim, J. Oh, Y. Kim, T. Moon, K.S. Hong, B. Park, Jpn. J. Appl. Phys., 42
(2003)1315.
[5]
H.T. Lue, T.Y. Tseng, and G.W. Huang, J. Appl. Phys., 91 (2002) 5275.
[6 ]
Y. Gao, A.H. Mueller, E.A. Irene, O. Auciello, A. Krauss, and J.A. Schultz, J.
Vacuum Sci. & Tech. A: Vacuum, Surfaces, and Films, 17 (1999) 1880.
[7]
C.M. Carlson, T.V. Rivkin, P.A. Parilla, J.D. Perkins, D.S. Ginley, A.B. Kozyrev,
V.N. Oshadchy, and A.S. Pavlov, Appl. Phys. Lett., 76 (2000) 1920.
[8 ]
B.H. Park and Q. Jia, Jpn. J. Appl. Phys., 41 (2002) 7222.
[9]
W. Chang, J.S. Horwitz, A.C. carter, J.M. Pond, S.W. Kirchoefer, C.M. Gilmore,
and D.B. Chrisey, Appl. Phys. Lett., 74 (1999) 1033.
[10]
J. Sok, S.J. Park, E.H. Lee, J.P. Hong, J.S. Kwak, and C.O. Kim, Jpn. J. Appl.
Phys., 39 (2000) 2752.
[11]
J.G. Cheng, J. Tang, J.H. Chu, and A.J. Zhang, Appl. Phys. Lett., 77 (2000) 1035.
[12]
S.I. Jang, B.C. Choi, and H.M. Jang, J. Mater. Res., 12 (1997) 1327.
[13]
F.De Flaviis, D. Chang, N.G. Alexopoulos, and O.M. Stafsudd, IEEE MTT-S
Digest, (1996) 99.
[14]
S.U. Adhikary and H.L.W. Chan, Materials Chemistry and Physics, 79 (2003)
157.
[15]
D.M. Tahan, A. Safari, and L.C. Klein, J. Am. Ceram. Soc., 19 (1996) 1593.
[16]
J. Kim, S.I. Kwun, and J.G. Yoon, IEEE Proceedings, (1995) 423.
[17]
A.S. Sigov, K.A. Vorotilov, A.S. Valeev, and M.I. Yanovskaya, J. Sol-Gel
Science and Technology, 2 (1994) 563.
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[18
D.S. Kil, J.B. park, D.S. Yoon, C.R. Song, H.J. Cho, Y. Kim, Y.S. Yu, J.S. Roh,
and H.K. Yoon, Jpn. J. Appl. Phys., Part 1, 40 (2001) 3260.
[19
K. Arita, E. Fujii, Y. Shimada, Y. Uemoto, T. Nasu, A. Inoue, A. Matsuda, T.
Otsuki, and N. Suzuoka, Jpn. J. Appl. Phys., 33 (1994) 5397.
[20
G.H. Haertling, Ferroelectrics, 119 (1991) 51.
[21
R.W. Schwartz, B.C. Brinker, D.B. Dimos, R.A. Assink, B.A. Tuttle, et.al.
Integrated Ferroelectrics, 2 (1992) 243.
[22
G. Yi and M. Sayer, Ceramics Bulletin, 70 (1991) 1173.
[23
E. Dien, J.B. Briot, M. Lejeune, and A. Smith, J. European Ceram. Soc., 19
(1999) 1349.
[24
M.C. Gust, N.D. Evans, L.A. Momoda, M.L. Mecartney, J. Am. Ceram. Soc. 80,
2828 (1997).
[25
M.J. Lefevre, J.S. Speck, R.W. Schwartz, D. Dimos, and S.J. Lockwood, J.
Mater. Sci., 11 (1996) 2076.
[26
D.S. Yoon, C.J. Kim, J.S. lee, W.J. Lee, K. No, J. Mater. Res., 9 (1994) 420.
[27
K. Nakamoto, Infrared and Raman Spectra o f Inorganic and Coordination
Compounds, (John W iley and Sons, New York 1978).
[28
S. Gevorgian, E. Carlsson, S. Rudner, L.D. Wemlund, X. Wang, and U.
Helwersson, IEEE Proc. Microm. Antennas Propag., 143 (1996) 397.
[29
RT6000 HVS Radiant Manual, Radiant Technologies Inc., New Mexico (1994).
[30
S. K. Koul, and B. Bhat, IEEE M TT-S Digest, 489 (1981).
[31
R. Crampagne, M. Ahmadpanah, and J.L. Guiraud, IEEE Trans. Microwave
Theory Tech., 26 (1978) 82.
[32
R. Romanofsky and A.H. Quereshi, IEEE Trans. Mag., 36 (2000) 3491.
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CHAPTER 3
Studies Of Barium Strontium Titanate Thin Films For
Microwave Device Applications
This chapter is divided into two sections. In the first section (3.1) o f this chapter, growth
o f oriented barium strontium titanate (BST) thin films is described. The second part
(section 3.2) deals with the study o f annealing effect on the dielectric properties o f the
BST thin films for tunable microwave device applications.
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3.1 Sol-gel growth of grain oriented (Ba,Sr)Ti03 thin films
3.1.1 Introduction
For a wide range o f optical and electronic applications highly oriented/single
crystalline inorganic thin films are preferred as compared to their polycrystalline
counterparts. As for example in case o f lead titanate (PbTiOs) thin films the pyroelectric
coefficients are optimal along their polar (001) direction [1]. For optical wave-guide
devices where high optical transmission is desirable, elimination o f high as well as low
angle grain boundaries in the films are o f utmost importance to minimize the scattering
loss. Oriented growth o f LnAlOs (Ln = Nd^^, Gd^'*') was claimed necessary to serve the
purpose o f a chemical barrier layer sandwiched between cuprate superconductors and
sapphire substrates [2 ].
Broadly the epitaxial thin films can be classified according to three categories [3]
namely highly oriented polycrystalline films with complete ‘c’ axis orientation and
random ‘a ’ as well as ‘b ’ axis orientation (type-I), complete ‘c ’ axis orientation with low
angle grain boundaries in ‘a ’ as well as ‘b ’ axes (type-II) and single crystalline thin films
(type-in). Among these three categories the type-I and type-II types o f films may have
both high as well as low angle grain boundaries, whereas, type III films should not have
any grain boundaries except the presence o f defects such as twins, stacking faults,
dislocations etc which may arise to relax stress between film and substrate during the
growth o f such films. The oriented growth mechanisms o f physical vapor deposited films
are distinctly different from that o f solution derived films [4] as the former case the
nucleation initiates on the surface o f the heated substrates insitu, followed by their
growth. If the substrate used in this case is latticed matched with the film then the
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oriented growth is more probable due to the high adatom mobility o f the condensate, bi
contrast, in case o f solution-derived films, the nucleation is initiated in a solid amorphous
film prepared by multiple coating and firing cycle. The nucleation in the later case is
therefore not limited to the film-substrate interface as it is equally probable to be initiated
from the surface as well as on impurity particles in the amorphous film. The random
orientation o f the crystalline films are obvious if the nucleation event initiates at the film
surface or on the impurity particles and therefore in order to obtain an oriented film by
any solution growth technique it is o f utmost importance to restrict the nucleation event at
the interface o f the amorphous film and its lattice matched substrate.
Limited reports are available in the literature that describes the growth o f highly
oriented/epitaxial inorganic thin films by solution growth technique. The usual
preparation recipe o f solution grown polycrystalline thin films usually involves the
following steps: The first step is to form an amorphous gel coating on selected substrate
from a multi-component precursor solution containing metallo-organic precursor and/or
acetates/nitrate salts dissolved in organic/aqueous solvents. An intermediate firing step is
involved to bum out the residual organics followed by a final beat treatment at relatively
higher temperature (as compared to the organics bumout treatment) to synthesize
polycrystalline films o f desired composition. As mentioned earlier that the mecbanism/(s)
leads to highly oriented/epitaxial quality inorganic thin film in solution grown films are
distinctly different from that o f vapor phase epitaxial growth. In the first instance it has
been reported that even if the film and substrate has identical crystal stmcture with
smaller lattice mismatch (eg. tetragonal ( 001 ) Zr(Y )02 film on ( 001 ) zirconia substrate),
the amorphous film is first transformed into a randomly oriented polycrystalline film.
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However, with further heat treatment at relatively higher temperature, oriented nano­
crystallites form at the film substrate interface, which eventually grows along the
interface followed hy through the film to consume randomly oriented grains [5,6]. In the
second case, even if there is no obvious match either in crystal structure or lattice
parameters between the film and substrate, oriented growth is still possible and in such
case the amorphous film is first converted into polycrystalline film which eventually
break into isolated epitaxial grains upon further heat treatment. These isolated epitaxial
grains act as seed layer, which induces epitaxy to the subsequently coated thin film layers
during the repeated coating and firing cycles [7,8,2]. Finally, the third mechanism
involves a liquid phase processing, where upon a high temperature heat treatment
procedure epitaxial films form via a liquid phase [9].
Many efforts have so far been made to grow epitaxial/highly textured ferroelectric
thin films by solution growth technique. However, the mechanism leading to such
textured growth o f these multi-component ferroelectric films remains poorly understood.
Some o f the significant findings have been tabulated in Table 3.1.1. Several factors seem
to he responsible to yield oriented growth in case o f solution prepared films. Schwartz et.
al. [10] deposited BTO thin films on structure matched lanthanum aluminate (LAO)
substrate where the lattice mismatch was around 5.38%. After heat treatment the film had
equiaxed grains and was polycrystalline in nature, leading to the conclusion that mere
structure match between the substrate and the film is not sufficient to obtain epitaxial
growth. Gust et. al. [11] deposited BTO film on MBE grown BTO thin layer (which
exhibited both good crystal chemistry and excellent lattice matching). However, in this
case also polycrystalline BTO thin film with equiaxed grains were formed. Compared to
107
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BST, it is relatively easier to obtain epitaxially grown lead based perovskite thin films
such as lead titanate (PT), lead zirconate titanate (PZT) etc on lattice-matched substrates
[12,13]. The as deposited CSD films are amorphous and upon heat treatment at higher
temperature it transforms into the perovskite phase. The phase formation sequence from
amorphous phase to perovskite phase is distinctly different in the case o f PT, PZT when
compared to BST systems. Thus, in the case o f the lead based system an intermediate
oxygen deficient pjrochlore phase forms which subsequently transforms into the
perovskite phase upon heating at higher temperature; in contrast, in the case o f BTO and
STO, several intermediate compounds form at lower temperatures and react at higher
temperature to form the perovskite phase. Seifert et. al. [14] showed that the initial
crystallization o f the fluorite may be advantageous to obtain highly oriented PT thin films
at low temperature. The films, however, had entrapped porosity and thereby needed
higher annealing temperature to form smooth, epitaxial films. In contrast, several
intermediate metastable phases form prior to the crystallization o f perovskite BTO and
BST ceramics and the formation o f these intermediate phases depend upon the precursor
used. For the acetate-based system, we have observed (as described later) and also others
have reported [10] that BTO crystallizes from the interaction between the intermediate
BaCOa and TiOa phases. If the underlying substrate is STO or LAO, then considering
only the lattice match, the nucleation o f perovskite BTO phase is equally probable at the
TiOa/BaCOa interface as well. In other words, the nucleation events are equally probable
throughout the bulk o f the film as in the film -substrate interface. Nucleation throughout
the bulk o f the film yields a polycrystalline film and therefore it is important to time the
process variable/(s) to limit the nucleation events at the film-substrate interface.
108
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Table 3.1.1. Effect o f annealing temperature on (100) orientation o f BST60 film on LAO
( 100 ) substrate.
Seed layer
heated at
Film heated at
Film annealed at
(100) film
orientation^*
Reference
-
600 °C/5min
700 °C/2hours
42
Present study
-
- do -
900 °C/2hours
92
- do -
-
- do -
1000 °C/2hours
407
- do -
-
-do-
1100 °C/2hours
516
-do-
-
- do -
1100 °C/6 hours
4783
- do -
800 °C
800 °C/5min
-
10-34
Ref. [10]
800 °C
- do -
700 °C/2hours
32-37
- do -
1000°C
- do -
- do -
50
- do -
a. (100) orientation was calculated from the formula, page 2363 in ref. [10].
(1 0 0 )
orientation = {I(ioo)/I(uo)fiim}/{I(ioo)/I(iio)powder}
fri the case o f CSD films, nucleation o f perovskite phase is reported to take place
at the film surface as well as on the intermediate phase in addition to the substrate surface
and results in the formation o f randomly oriented equiaxed grains [13]. According to
these published results it appears that the nature o f the precursor, coating layer thickness,
heat treatment temperature after each deposition and also the final annealing temperature
are important parameters to obtaining oriented/epitaxial growth o f BST films using the
CSD technique.
The focus o f this section o f chapter one is to grow highly textured BaxSri.xTiOa
(BST) thin films and understand the factor/(s) control the growth behavior. The structural
109
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qualities o f these films were characterized by XRD analysis in conjunction with RBS
channeling measurements.
We have also characterized these films in terms o f their
dielectric properties at moderate frequency range (1 kHz to 1 MHz). The quality o f these
films at microwave frequency ( > 1 0 GHz) has been evaluated by fabricating proto type
phase shifters to be used in steerable antennas. To the best o f our knowledge, this was the
first attempt to use sol-gel derived grain oriented BST thin films to fabricate phase shifter
devices.
3.1.2 Experimental Details
Barium acetate, strontium acetate, and titanium iso-propoxide were used as
precursors to synthesize BuxSri.xTiOa (with x = 0.5 and 0.6, i.e. BST50 and BST60) sols
were prepared as mentioned in the section 2.1.2.1. For thin film deposition the diluted sol
(0.35 M) o f the desired composition was used. The heating schedule o f the film was
decided on the basis o f the thermal analysis o f BaTiOs gel. Thin films were deposited on
(100) (LAO) substrates using spin coating technique. We have adopted a two-step heat
treatment temperature for the BST thin films. Just after each deposition, the films were
inserted into a preheated furnace kept at 600°C and fired for 5 min. for the removal o f
organics. The coating and firing cycles were repeated required number o f times.
An optical thin film thickness measurement unit (F-20, Filmetrics Inc.) was used
to evaluate the thickness from the measured reflection spectrum o f the annealed films.
The unit simulates the experimental spectra by iterating the user provided refractive
indices o f the film material and the substrate and also the thickness and extinction
coefficient o f the film [15]. As shown in Figure 3.1.1, reasonably good match is obtained
110
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between the experimental and simulated spectra. Thicknesses for BST50 and BST60
films annealed at 1000-1050 °C for 2 hours were 348 nm and 350 nm respectively (fitting
error less than 0.05). The phase formation behavior, the degree o f orientation, and the
quality o f in-plane epitaxy o f the deposited films were studied using X-ray diffraction
technique. The surface morphology o f the films was studied using an atomic force
microscope (AFM). The low frequency (1 kHz-1 MHz) dielectric properties o f BST thin
films were measured using an HP 4294A impedance analyzer and inter-digital (DDT)
capacitor designs. The capacitors consist o f 50 and 100 fingers respectively that are 7 mm
long, 20 pm wide, and spaced 15pm apart (as mentioned in Chapter 2, section 2.2.5.1).
The dielectric constant (Sr) was extracted from the capacitance using conformal mapping
0.32
(100) BST (60/40) film on LAO (100)
0.28
O 0.24
Measured spectrum
- Calculated spectrum
O
C
(0
0.20
0.16
0.12
0.08
500 550 600 650 700 750 800 850 900 950
Wavelength (nm)
Figure 3.1.1. Reflectance spectrum o f (100) BST60 film on LaAlOa (100)
measured by filmetrics unit. The film thickness was determined by simulating the
experimental spectrum.
Ill
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technique originally developed by Gevorgian et. al [16]. Phase shifters on these films
were fabricated at NASA Glenn Research Center using standard lift-off chemical etching
technique as described in Chapter 2, section 2.2.6. The performance o f these was
evaluated by measuring the transmission (S 21) scattering parameters between 15 to 17
GHz using an HP 85IOC network analyzer.
3.1.3 Results and Discussions
3.1.3.1 Thermal analysis of sol-gel derived powders
Figure 3.1.2 shows (a) the differential thermal analysis (DTA) and (b) thermo gravimetric
analysis (TGA) plots o f the BaTiOs (BTO) gel powder. The endothermic peak at around
50°C and corresponding weight loss on the TGA curve are due to the evaporation o f
solvents. The exothermic peak and corresponding weight loss around 300 °C are due to
the oxidation o f residual organics. The weight loss is constant beyond 350 °C, indicating
that the organics are completely removed around this temperature. The exothermic peak
around 400 °C and relatively broad exothermic features at 600 °C, 880°C, and at 1070 °C
could be due to the crystallization o f various intermediate phases.
3.1.3.2 Phase Evolutioii of Gel Derived Powders
It is known that for sol-gel derived BTO several intermediate phases form prior to
the transformation o f the amorphous phase into the perovskite phase. The nature o f these
intermediate phases and the crystallization sequence depends upon the precursors used
[17]. Therefore, to identify the phase formation sequence we heat-treated the BTO gel
powders at temperatures ranging from 400 °C to 1000 °C for 2 hours and the phases o f
112
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the powders were identified by X-ray diffraction. Figure 3.1.3 shows the X-ray
difffactograms o f the heat-treated powder in air at different temperatures for 2 hours
each. It was found that at 400°C the powder begins to crystallize into BaCOs (pattern 5378 JCPDS file) and the crystallinity improves when the temperature is increased to 500
°C (Fig. 3.3a). When the temperature is increased to 700 °C, the anatase Ti 02 (pattern 211272 JCPDS file) begins to crystallize. The anatase Xi02 phase has been marked in the
diffractogram along with the BaCOs phase in Fig. 3.1.3(c). Ti 02 starts reacting with
BaCOs to form perovskite BaTiOa (pattern 5-626 JCPDS file) at a temperature around
700 “C (marked by asterisk). W ith further increase in annealing temperature, the
perovskite phase fraction increases, and finally at 1000 °C, the powder completely
transforms into the perovskite phase. Annealing o f the films was routinely accomplished
by introducing amorphous films into a furnace pre-heated to the desired temperature.
3.1.3.3 Phase and Microstructural Eyolution of BST Thin Films
XRD measurements
Since the crystallization pathway influences the nucleation, it is import£int to
know whether the crystallization sequence is similar or different in case o f thin film and
gel derived powder, both prepared from the same sol. In the case o f twice - coated BST60
film deposited on LAO (100) substrate and heat treated at 600°C for 5 min between
coatings, only the peaks from the substrate were apparent in XRD, indicating the film is
either amorphous or nano-crystalline in nature. The coating and firing cycle was repeated
ten times and the film remained amorphous or nano-crystalline in nature.
113
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35
- (a) BaTiOj gel
30
25
20
> 15
10
5
0
-5
400
200
0
600
800
1000
1200
Temperature (°C)
100
wm
90
V)
(0
80
jC
70
o
O)
60
50
0
100
200
300
400
500
600
700
800
Temperature (°C)
Figure 3.1.2. (a) DTA and (b) TGA plot o f BTO dried gel powder. The heating rate
was 5®C/min and air ambient was used during heating.
114
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o I * Perovskite BTO
’"I oAnataseliO,
' BaCO,
3
(6
800X
■
tfl
c
■
o
o
*
1
*
oCM
T"
J lyl | o
1
700X
(b)
*4iwW
IO go ^
500X
20
25
30
35
40
45
50
■r^
T
-■
»- C
CO
W
0O
^
55
60
2© (degree)
Figure 3.1.3. X-ray diffractograms o f BTO dried gel heat treated at (a) 500*^0, (b)
600^C, (c) 700°C, (d) 800V , (e) 900°C, and (f) lOOOV for 2 hours in air.
115
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Fig. 3.1.4 (a) shows the X-ray diffractograms o f the BST60 films annealed at (i) 600°C,
(ii) 900°C, (iii) 1000°C, and (iv) 1100°C for 2 hours. In comparison to the powder derived
from the same sol, we have found that thin film crystallizes directly into the perovskite
phase without the formation o f any intermediate compounds. It is also worth pointing out
that the present films are annealed by introducing films into a pre-heated furnace to the
desired temperature. This offers a near rapid thermal annealing process, maintaining a
relatively higher rate o f temperature rise. Alternatively, in the case o f thin films it may
also he possible that the intermediate products form but quickly react to form perovskite
BST at relatively lower temperatures. The film was found to be oriented along (100)
direction. Misaligned planes with weaker intensities are detected in films with lower
annealing temperature; however, with an increase in the annealing temperature the film
becomes aligned predominantly into (100) direction. In the film annealed at 600 °C for
2h, the (110) peak had -1 4 % the intensity o f the (200) peak.
The variation o f percentage intensity o f the (110) peak with increasing annealing
temperature is shown in Figure 3.1.5(a). The integrated intensity and FWHM o f (200)
diffraction peak o f BST60 is plotted separately in Fig. 3.1.5(b). As shown in the figure,
the intensity increases and the FWHM values decrease with an increase in annealing
temperatures indicating that the epitaxial quality and the crystallinity o f films improve
with the increase in annealing temperature.
116
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f
(a )
§
£1,
^
BST (60/40)
on LAO (100)
* substrate
O
o
CO
3
re
o'
o
T“
(0
c
o
o
o'
o
CO
.
1 L.............................. 1
I
1
20
1
30
1
1
40
1
1
50
1
1
60
(iv) T
(iii)
(ii)
(i)
1 1
70
J
1
80
2 0 (degree)
Figure 3.1.4. X-ray diffractograms o f BST60 films annealed at (i) 600'’C, (ii)
900°C, (iii) 1000°C, and (iv) 1100°C for 2 hours in air.
117
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15
12
9
a.
E
<
6
a
E
<
3
0
600
700
800
900
1000
1100
Annealing Temperature (°C)
0.7
2.0x10
Intensity of (200) peak
a — FWHM of (200) peak
0.6
1.6x10
(A
0.4
d>
0L_)
O)
o
■O
0.3
X
0.5
a
O
1.2x10
>*
w
c 8.0x10
o
C
0.2
4.0x10
0.0
550
0.1
650
750
850
950
1050
1150
Temperature ( C)
Figure 3.1.5. (a) The variation o f the percentage intensity o f the (1 10)-peak w.r.t.
(200) peak and (b) the variation o f intensity and FWHM o f the (200) diffraction
peak o f BST60 film with increasing annealing temperature.
118
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AFM Studies
Figure 3.1.6(a) shows that surface morphology o f the single time coated BST50
film heat- treated at 600 °C for 5 minutes, where the film appears to be nanocrystalline in
nature. After annealing the film at 600 °C for 2 hours (Fig 3.1.6(b)) appreciable grain
growth occurs and the growth is expedited with further increment o f aimealing
temperature and time (Fig 3.1.6(c)). The above-mentioned sequence o f microstructure
evolution remains unaltered in case o f thicker films. Thus as shown in Fig. 3.1.7(a), a
film after multiple deposition and annealed at 600 °C for 2 h has fine-grained structure at
the surface. The grain size increases with the increase in armealing temperature and
columnar grains are observed in the films annealed at 900 “C for 2 hours (Fig. 3.1.7 (b)).
These grains are highly oriented along (100) direction as envisaged from XRD analysis.
With additional increase in the aimealing temperature (1100 °C), the grain boundary
concentration is reduced and the (100) orientation is improved (Fig. 3.1.7(c)). Also it can
be noted that the film still possesses some fraction o f porosity left at the surface, and
longer annealing duration is required to completely remove the porosity from the film.
Fig. 3.1.7(d) shows the AFM micrograph o f BST60 film aimealed at 1100 °C for 6 hours.
As expected, it was found that the grains coalescence occurs with an increase in
aimealing time and the epitaxial quality is improved as the elimination o f grain
boundaries takes place. This observation supports the X-ray diffraction data presented
above. Based on the above experimental observation, the following mechanism is
proposed that yield highly (100) textured BST thin films on LAO substrate. At lower
temperatures the BST film crystallizes into perovskite phase and the perovskite grains are
textured predominantly along the ( 100 ) direction.
119
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BST50, single coating
on LAO, 600 "C/5min
BST50, single coating
on LAO, 600 “C/2h
BST50, single coating
on LAO, 1100 »C/6 h
Figure 3.1.6. AFM micrograph (1pm x 1pm) o f the single coated BST50 film on LAO,
(a) heated at 600 °C for 5min, & aimealed at (h) 600 °C for 2h, and (c) 1100 °C for 6 h.
120
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X = 0.5 jim/div
z = 135 nm /div
Figure 3.1.7. AFM micrographs (2pm x 2pm) o f BST60 thin films annealed in air at
(a) 600 °C for 2 h, (b) 900 °C for 2 h, (c) 1100 °C for 2 h and (d) 1100 °C for 6 h.
121
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W ith the increase in annealing temperature, the oriented grains grow along the interface
or within the bulk o f the film to consume the misaligned grains. Higher temperatures and
longer annealing times are required to improve the textured quality o f the films.
Table 3.1.1 compares the degree o f epitaxy obtained in the present work with that
reported in the literature using a similar kind o f precursor materials [10]. It is to be noted
that in our case the degree o f texturing is particularly improved at lower annealing
temperature. The postulated growth mechanism was tested by examining the crosssectional scanning electron microscopy.
Cross-sectional SEM analysis
Fig. 3.1.8 shows the cross-sectional SEM micrograph o f the BST film deposited
on STO substrate with intermediate heating at 600 °C for 5 minutes and annealing at
1100 °C for 6h. As can be seen (Fig. 3.1.8), the film has only one or two grains along its
thickness direction. Probably, all these grains maintain epitaxial relation with the
substrate, however to ascertain the epitaxial relation further, cross-sectional HRTEM
analysis are needed to be performed. However, Ifom this microstructure, one can see that
each o f these grains are fractured along their respective cleavage planes, which indicates
that each o f these grains are single crystalline nature. Now, apparently each individual
grain maintains nearly identical growth direction amongst themselves and thereby
globally (as indicated by XRD) the film exhibits (100) orientation. This fact could be
ascertained again by doing SAED on individual grains through cross-sectional high
resolution turmeling electron microscopy (HRTEM) measurement.
122
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15.0kV 10.6mm x40.0k SE(U) 9/ 13/02 17:26
I I I I I I
1.OOum
F ig u r e 3 .1 .8 . Cross-sectional SEM o f the BST50 film deposited on STO substrate and
annealed at 1100 °C for 6h.
r p=azimuthal angle
X=Tilt angle
F ig u r e 3 .1 .9 . Pole figure o f BST 50 film on LAO.
123
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Apparently it is indicative that each o f these oriented grains have low angle grain
boundary, which motivated us to call this kind o f growth mechanism as grain oriented
textured growth.
Pole-figure analysis
In order to identify the in-plane epitaxial relation o f these grain-oriented BST
films we have performed the X-ray pole figure and RBS o f the BST50 films. Figure 3.1.9
shows the X-ray pole figure distributions corresponding to the (220) reflection (20 = 67°)
o f a BST 50/50 film on LAO (100) substrate. As shown in the figure the film has four
strong (220 ) reflections coupled with their narrow intensity distributions which indicate
that the normal vectors o f the (220 ) plane are aligned in the x-y plane o f the substrates,
with the polar angle o f about 44.77° from the surface normal o f the LAO (100) substrate
as expected for a cubic crystal structure. The measured angle between two BST (220)
poles and the surface normal matches well with the calculated value (45°). This result
confirms that the films developed in ( 100 ) direction and have excellent in plane epitaxy
with the substrate as well.
RBS m easurem ents
Figure 3.1.10 shows the random and channeled spectra for the BST50
film. Theoretical simulation o f the random spectrum gives the film thickness about 330
nm having a composition Bao^sSro.syTiOs. The channeled spectrum shows a large
reduction in the backscattering yield indicating an excellent lattice matching o f the
substrate and the deposited film. The minimum yield which is measured as a ratio o f the
124
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channeled to random )delds at the surface is 8 %, a value which is slightly larger than the
Xmin value for the substrate.
3.1.3.4 Dielectric Properties
For a grain oriented grown BST50 film on a LAO substrate annealed at 1050 °C
for 2h, Fig. 3.1.11 shows the variation o f the capacitance and tandW iih the bias voltage.
At zero bias field the estimated Sr (at 1 MHz) o f the BST50 and BST60 films was found
to be about 1000, while maintaining a dissipation factor (tanS) as low as 0.014. From the
reported values in literature, the dieleetric constant o f polycrystalline BST60 film rarely
exceeded 800 [18,19].
The higher dielectric constant in the present work is attributed to the highly
oriented nature o f the film. In general for a tunable structure, a figure o f merit “A" faetor”,
is defined by equation 1.7 (Chapter 1). The tunability o f the BST50 film at room
temperature was ~29 % (at applied voltage o f 35V) with ta n S o - 0.014 at zero applied
voltage. The K factor o f the deposited film is 20 (at room temperature).
3.1.3.5 High Frequency Phase Shifter Studies
Encouraged by the fact that these films exhibit attractive tenability and low loss at
moderate frequency, we were interested to evaluate the performance at microwave
frequencies. For this purpose, the oriented grain BST 50 and BST60 films have been used
to fabricate coupled micro-strip phase shifter to be used in low cost steerable antennas for
communication applications.
125
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Energy (MeV)
0.6
0.8
1.0
g
o Random LAO/BST (50/50)
+ Channeled LAO/BST (50/50)
300
2
1.4
1.2
T
350
250
■a 200
0)
.H
2 150
o
Z
100
50
0
100
1----300
200
400
500
Channel
Figure 3.1.10. Random and channeled spectra o f the BST 50 film on LAO substrate.
0.06
BST50 film on LAO
i.exio"""
0.05
1.5x1O'"
0.04
0.03
CO
c
n
0.02
0.01
•40
-30
-20
-10
0.00
40
Bias voiatge (V)
Figure 3.1.11. Variation o f capacitance and loss tangent o f BST60 thin film on
STO substrate as a fimction o f bias voltage. The measurement was performed on
100 finger IDT electrode at 100 kHz.
126
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Fig. 3.1.12(a) shows the phase shift and Fig. 3.1.12 (b) shows the insertion loss as a
function o f bias voltage at three different frequencies for BST50 thin films on 254 pm
LAO substrate. The maximum phase shift obtained at 14 GHz was 150° and the
corresponding insertion loss was 5.75 dB. The maximum /rvalue o f BST50 film is 26.08
°/dB (13.3V/pm). However, we could not increase the bias voltage beyond 100 V
because o f the unexpected breakdown o f the sample. The film was annealed at 1000 °C
for 2 hours; and an optical micrograph o f this sample shows porosity on the surface o f the
film. Surface porosity is known to have a deleterious effect on the phase shift
characteristics o f BST50 thin films. Better phase shift has been obtained for BST60 thin
film annealed at 1000 °C for 2 hours, when the bias voltage was increased up to 340 V
(Fig. 3.1.13(a)). The maximum phase shift obtained at 15.5 GHz was 320.4°, and the
insertion loss was 8.435 dB at this frequency (Fig. 3.1.13(b)). The calculated figure o f
merit,
k,
is 37.98 °/dB. Table 3.1.11 compares the characteristics o f 8 element CMPS
(254pm thick LAO design) using sol-gel derived BST 60/40 films deposited by us and
the similar BST60 films deposited by others using pulsed laser deposition and rf
sputtering techniques.
127
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160
d)
2
O)
140
■o
100
(100) BST (50/50)/LAO (100)
120
CM
(0
o
15 GHz
^ 1 4 . 5 GHz
^ 1 4 GHz
o
(0
nj
JC
Q.
0
20
40
60
80
100
120
Bias voltage (V)
-4
(b) (100) BST (50/50)/ LAO (100)
QQ
i- A —
-5 -
X
U)
<4-
o
0)
T3
O)
re
-6
^
V -
-
- * - 1 5 GHz
14.5 GHz
14 GHz
-7 -
-8
20
40
±
60
80
100
120
Bias voltage (V)
Figure 3.1.12. Characteristics o f eight element CMPS using BST50 thin film on
LAO (254|im) substrate, variation o f (a) phase shift and (b) insertion loss as a
function o f bias voltage.
128
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350
(a) (100)BST(60/4G)/LAO(100)
300
O)
0)
250
X
200
T3
150
100
17 GHz
16 GHz
15.5 GHz
50
0
50
0
100
150
200
250
300
350
Bias Voltage (V)
-6
. (b) (100)BST(60/40)/LAO(100)
CQ
2 ,
s
CO
4-
-7
o
■o
3
- ■■■
------- -
-8
If
i y
-9
V ’
17 GHz
16 GHz
15.5 GHz
C
a -10
O)
(0
S
-11
0
_L
_L
JL
50
100
150
I
200
250
I
I
300
350
Bias voltage (V)
Figure 3.1.13. Characteristics o f eight element CMPS using BST60 thin film on
LAO (254iJ,m) substrate, variation o f (a) phase shift and (b) insertion loss as a
ftinetion o f bias voltage.
129
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Table 3.1.II: Comparison o f the characteristics o f 8 clement CMPS using BST60 thin
films deposited by sol-gel and other techniques.
Deposition
Substrate
Film
Thickness
fopt
technique
(254jim)
composition
(nm)
(GHz)
PLD
LaAlOs
BST50
350
14.3
-do-
-do-
-do-
700
15
Tuning
Loss
K
(dB)
(7dB)
4.6
43.7
6.43
34.7
7.01
42.7
8.4
38
9.3
36
201 °
(400V)
223°
(360V)
BST
RF
271°
750
-dosputtering
14
(40/60)
(400V)
320°
Sol-gel
-do-
BST60
350
15.5
(340V)
342°
-do-
-do-
BST60
350
15.5
(340V)
As shown in Table 3.1.II, it is indeed encouraging that the sol-gel derived films
exhibit better tuning characteristics as compared to the films deposited using other
techniques. However, the losses are still higher than optimum, result in moderate
k
value.
In the following chapters, effect o f annealing temperature, doping and other approaches
will be presented in an effort to further improve the microwave dielectric characteristics
o f BST films deposited by sol-gel technique.
3.1.4 Conclusions
In the present work, oriented grain BST50 and BST60 thin films were deposited
on SrTiOa (100) and LaAlOs (100) substrates sol-gel technique. It was demonstrated that
sol-gel is an attractive economic technique to obtain highly textured films. These films
130
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were characterized in terms o f their phase formation behavior and structural growth
characteristics using X-ray diffraction. Texture quality o f the film was analyzed using Xray pole figure and RBS channeling measurements. Microstructural evolution o f the films
was studied using AFM and SEM (cross-sectional) studies. Based on XRD and AFM
analysis, a tentative growth mechanism o f the film was proposed. Dielectric properties o f
these films were evaluated in terms o f tunability and loss tangent in the frequency range
1kH z-1MHz. From the dielectric measurements at 1 MHz we have obtained a
1000
with no bias, and -29% tunability and low dielectric loss (tan6~0.014 or less) resulting in
K factor o f -2 0 at room temperature. Eight element coupled micro-strip phase shifter
(CMPS) was fabricated on these films in NASA facilities. The phase shifters were tested
in terms o f their degree o f phase shift and insertion loss characteristics. A reasonably
good phase shift in the order o f 320° (20-340V) and acceptable /rvalue, (/r - 38.0 °/dB)
achieved on BST60 films at room temperature and 15.5 GHz, compares well with the
BST films grown by other film deposition techniques.
3.1.5 References
[1]
K. lijima, R. Takayama, Y. Tomita, and I. Ueda, J. Appl. Phys., 60 (1986) 2914.
[2]
K.J. Vaidya, C.Y. Yang, M. DeGraef, and F.F. Lange, J. Mater. Res., 9 (1994)
410.
[3]
C.K. Barlingay and S.K. Dey, Appl. Phys. Lett., 61 (1992) 1278.
[4]
E.G. Bauer, B.W. Dodson, D.J. Ehrlich, L.C. Feldman, C.P. Flynn, M.W. Geis,
J.P. Herbison, R.J. Matyi, P.S. Peercy, P.M. Petroff, J.M. Phillips, G.B.
Stringfellow, and A. Zangwill, J. Mater, Res., 5 (1990) 852.
[5]
K.T. Miller, C.J. Chan, M.G. Cain, and F.F. Lange, J. Mater. Res., 8 (1993) 169.
[6 ]
A. Seifert, F.F. Lange, and J.S. Speck, J. Am. Ceram. Soc., 76 (1993) 443.
131
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[7]
K.T. Miller, and F.F. Lange, J. Mater. Res., 6 (1991) 2387.
[8]
F.F. Lange, Science, 273 (1996) 903.
[9]
S.G. Golden, F.F. Lange, D.R. Clarke, L.D. Chang, and C.T. Necker,
“Metalorganic depostion o f high critical current thin films in the Bi-Sr-Ca-Cu-O
system on {100} LaAlOa substrates”
Phys. Lett., 61 (1992) 351.
[10]
R.W. Schwartz, P.G. Clem, J.A. Voigt, E.R. Byhoff, M.V. Stry, T.J. Headley, and
N.A. Missert, J. Am. Ceram. Soc.., 82 (1999) 2359.
[11]
M.C. Gust, N.D. Evans, L.A. Momoda, and M.L. Mecartney, J. Am. Ceram. Sac.,
80 (1997) 2828.
[12]
J.H. Kim, A.T. Chien, F. F. Lange, and L. Wills, J. Mater. Res., 14 (1999) 1190.
[13]
J.H. Kim and F.F. Lange, J. Mater. Res., 14 (1999) 1626.
[14]
A. Siefert, F.F. Lange, and J. S. Speck, /. Mater. Res., 10 (1995) 680.
[15]
Operations Manual for the Filmetrics F20 (version 2.2.5,1999).
[16]
S. Gevorgian, E. Carlsson, S. Rudner, L.D. Wemlund, X. Wang, and U.
Helwersson, IEEE Proc. Microm. Antennas Propag., 143 (1996) 397.
[17]
A.C.
Pierre,
“Introduction
Boston/Dordrecht/London, 1998).
[18]
J.G. Cheng, X.J. Meng, B. Li, J. Tang, S.L. Guo, J.H. Chu, M. Wang, H. Wang,
and Z. Wang, Appl. Phys. Lett., 75 (1999) 2132.
[19]
S.I. Jang, B.C. Choi, and H.M. Jang, J. Mater. Res., 12 (1997) 1327.
to
Sol-gel
Processing”,
132
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(KAP,
3.2 Effect of annealing conditions on barium strontium titanate
thin films
In this section a possible relationship between the microstructure and the
dielectric properties o f sol-gel derived highly (100) oriented Bao.sSro.jTiOs (BST50) thin
films have been investigated in order to prepare them with properties comparable to those
o f the bulk material. BST thin films exhibited orthorhombic to tetragonal transition in
addition to the commonly observed tetragonal to cubic transition., which explained the
commonly observed degradation o f the dielectric behavior, when compared to those o f
the bulk material in terms o f grain size, compositional in-homogeneity (measured in
terms o f Sr/Ba ratio) between the grain bulk and grain boundary, and mechanical stresses.
3.2.1 Introduction
Epitaxial/highly textured Bai-xSrxTiOa (x = 0.4-1.0) (BST) thin films with large
dielectric tunability and low dielectric losses are attractive candidates for various
frequency agile microwave electronic devices [1-2]. For these tunable microwave
devices, coplanar tuning configuration on highly oriented BST thin films have been
found more compatible with planar microwave circuitry. Near Curie temperature, BST
has larger tunability and both dielectric losses and the dielectric constants are known to
be strongly temperature dependent. The temperature dependence o f dielectric constant
could pose a serious concern for practical applications viz. carrier signal drifting in and
out o f resonance in hot and cold days [3]. As compared to ceramics/single crystals, the
temperature dependence o f the capacitance o f BST thin films were reported to be quite
different and the observed behavior may be grouped into three broad classifications: (i)
133
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unlike the bulk ceramics, the capacitance vs temperature plot does not exhibit any
anomaly indicating the absence o f any ferroelectric to paraelectric phase transition (the
cubic phase is retained down to room temperature) [4], (ii) a broad dielectric anomaly
indicating a diffuse phase transition is observed [5] and due to the broad nature o f C vs. T
plot, the other two low temperature ferroelectric (tetragonal to orthorhombic and
orthorhombic to rhombohedral) transitions have rarely been identified [4, 6,7]. (iii) The
permittivity is lower in thin film and the Curie temperature is often reported to be either
higher or lower than the corresponding bulk composition [8,9]. A variety o f possible
factors including the film microstructure (in terms o f grain size, interface layer etc),
compositional heterogeneity, strain state (compressive or tensile) may be responsible for
the observed discrepancy between the bulk and thin film dielectric transition behavior,
however, no systematic study has so far been attempted to correlate the transition
behavior with the structural and micro-structural features o f BST thin films [8-11].
In the present section o f this chapter, the synthesis and characterization o f highly
(100) oriented Bao.sSro.sTiOa thin films on (100) LaAlOa substrates by sol-gel technique
is discussed. The X-ray photoelectron spectroscopy (XPS) in conjunction with the
Rutherford backscattering spectrometry (RBS) depth profile analysis indicated that the
grain boundary regions were Ba deficient whereas the bulk o f the grains maintain
nominal Sr:Ba (1:1) ratio. A diffuse nature o f the phase transition was identified in the
films as apparent from the temperature dependent capacitance data measured at 1 MHz in
a temperature range o f 80 K to 425 K. The paraelectric (cubic) to ferroelectric
(tetragonal) phase transitions were readily identified in all these films, however, the
134
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ferroelectric (tetragonal) to ferroelectric (orthorhombic) phase transition was observed
only in BST films annealed at higher temperatures for a longer duration.
3.2.2 Experimental Details
Barium acetate, strontium acetate, and titanium-IV isopropoxide were used to
prepare BST50 precursor sol as described in section 2.1.2.1. BST50 film was deposited
onto cleaned lanthanum aluminate (LAO) substrate by spin coating technique. Multilayer
coating approach was used to achieve thickness o f ~500 nm. The films were finally
annealed in the temperature range o f 1050 to 1100 °C for 2-6 h for improved
crystallization and textured growth into (100) direction. The phase formation behavior,
degree o f orientation and quality o f in-plane epitaxy o f the deposited films were studied
using the X-ray diffraction technique. RBS measurements were performed using a 2 MeV
Van de G raff accelerator. The spectra have been simulated using RUMP software [12] to
obtain the thickness and the depth profile o f the deposited layers. Further studies were
carried out utilizing the XPS technique (Physical Electronics PHI5600 ESCA system)
with M gK a (1253.6 eV) radiation and 45° measuring angle to ascertain the nature and
chemical composition o f the films. The slow scan XPS was performed on barium (Ba
3d5/2), strontium (3d), titanium (Ti 2p), and oxygen (O Is) peaks in the binding energy
ranges 775-785 eV, 130-140 eV, 453-469 eV, and 526-536 eV respectively. All peaks in
the spectra were calibrated with respect to carbon C ls (285.5 eV) peak. The curve fitting
o f the slow scan XPS spectra was carried out using a nonlinear least squares fitting
program with a Gaussian/Lorenzian function. The surface morphology o f the films was
studied using an atomic force microscope (AFM) (Nanoscope Ula multimode AFM
135
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Digital Instruments). The dielectric properties (at 1 MHz) o f BST thin films were
measured by an impedance analyzer (HP 4294A) using an inter-digital capacitor (IDC)
design. The capacitor consisted o f 50 fingers that were 7 mm long, 20 pm wide, and
spaced 15 pm apart (as mentioned in Chapter 2, section 2.2.5.1). The hysteresis
measurements were performed using a hysteresis loop tester (Radiant Technologies). A
computer controlled thermal stage (MMR Technologies) was used to measure the
temperature dependent dielectric and ferroelectric properties. Phase shifters shifters as
described in Chapter 2, section 2.2.6 were fabricated on these films at NASA Glenn
Research Center using standard lift-off chemical etching. The performance o f these was
evaluated by measuring the transmission (S 21) scattering parameters between ~14 to 15.5
GHz using an HP 85IOC network analyzer.
3.2.3 Results and Discussions
3.2.3.1 Structural and microstructural characterizations
X-Ray Diffraction studies
It is reported in literature that in the case o f BST films deposited by sol-gel
technique, minimum temperature o f crystallization is approximately 600 °C [13]. It is
also documented that the deposition o f BST thin films with good quality required high
processing temperature (600 °C or above), because o f the difficulty in the formation o f
pure perovskite phase [14,15]. However, in section 3.1, in the case o f BST thin films
annealed at low temperatures (~600 °C), misaligned planes with weaker intensities o f
BST peaks were detected; and with an increase in the annealing temperature the films
became aligned predominantly into (lOO)-direction. Thus, in this section, BST50 thin
136
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films deposited on LAO (100) substrates were annealed at 1050 °C and 1100 °C for 2h
(type A and B respectively), & 1100 °C for 6 h (type C) in air. The X-ray diffractograms
o f these BST 50 thin films showed (as presented in chapter 3, section 3.1) that all the
films were highly ( 100 ) textured.
AFM studies
Figure 3.2.1 shows the AFM micrographs o f (a) type A, (h) type B and (c) type C
BST50 films. It may he noticed that the type A film has finer grain size (~ 150 nm) and
the grain size increases with the increase in annealing temperature (Fig. 3.2.1h) as well as
annealing time (Fig. 3.2.1c). The increase o f annealing temperature and the annealing
time was found to increase the grain size o f BST thin films appreciably and thereby
reducing the volume ratio o f grain boundary region:bulk grain region.
3.2.3.2 Compositional analysis
XPS analysis
Planar IDT electrodes are deposited on the surface o f the film, therefore it is
important to characterize the surface o f the film in terms o f its compositional uniformity.
In case o f both for PZT and BST type perovskite films it is reported that the surface o f
the films are degraded either during the film annealing or reaction with atmospheric
moisture [16,17]. In these cases the films were deposited by r-f sputtering and pulsed
laser deposition techniques and it has been reported that both Ba 3 d 3/2 and Ba 3 d 5/2 peaks
can be deconvoluted as two peaks and Ba3d peaks from the surface decomposed phase on
BST have higher binding energy than that from BST in the perovskite phase.
137
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1 .00
2.00
2.00
1.00
1.00
2 .0 0
2 .0 0
1.00
2 .0 0
Figure 3.2.1 AFM micrographs o f the BST films annealed at (a) 1050°C for 2h and
(b) 1100 °C for 2h and (c) 1100°C for 6h respectively.
138
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To understand the physical origin and the nature o f the decomposed phases a detailed
analysis was undertaken and it was concluded that the surface decomposed phase could
be the various possible amorphous oxides o f barium.
Figure 3.2.2 shows the XPS spectra o f the Type A BST50 film in the binding
energy range o f 0-1100 eV. All peaks in the spectra were calibrated with respect to
carbon C ls (285 eV) peak. Quantitative atomic composition analysis for all the BST50
films under study was performed. The slow scan XPS was recorded on barium (Ba Bds/i),
strontium (Sr 3d), titanium (Ti 2p), and oxygen (O Is) peaks in the binding energy ranges
775-785 eV, 130-140 eV, 453-469 eV, and 526-536 eV respectively. Figure 3.2.3(a)
shows the slow scanned XPS spectrum o f Ba atom at the surface o f BST (50/50) film.
The singular Ba 3 ds/2 peak at the binding energy -7 8 0 eV confirms the presence o f Ba in
perovskite phase. Fig. 3.2.3(b), (c) and (d) shows the slow scanned XPS spectra o f Sr, Ti
and O peaks respectively. The intense peak at lower binding energy (0 2 ) (529.8 eV) in
the deconvoluted oxygen spectrum (Fig. 3.2.3(d)) is identified from 0 1 s electron in the
perovskite BST film. The other peak at relatively higher binding energy (0 1 ) (531.4 eV)
could be from absorbed oxygen in the form o f -C O or -C O 2 molecule [16-17]. From the
slow scanned Ba 3 ds/2, Sr3d, Ti2p, and 0 1 s XPS spectra, Ba, Sr, Ti, and O contents were
measured and as shown in the Table 3.2.1. From the XPS chemical analysis, it was
observed that all the films were Sr affluent and Ba deficient at the film surface. The Ba
deficiency is relatively more in the film annealed at higher temperature. Since the film
surface was found to be Sr affluent, it was therefore assumed that in the grain boundary
region Sr:Ba ratio was more than in the grain interior [18]. XPS, in the present
configuration (with measuring angle 45°, exposed to a circular area o f diameter 3mm)
139
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probes around 20-30 A region in depth and therefore it is more likely that the atomic
concentration at the grain boundary is reflected in the XPS analysis.
T able 3.2.1: Atomic concentration o f type A , B, and C BST(50/50) films, determined
from the slow scanned XPS spectra o f Ba 3 d 5/2, Sr 3d, Ti 2p and 0 1 s peaks.
Film
Ba
Sr
Ti
O
Sr/Ba
Type A
8.98
11.74
17.94
51.37
1.31
Type B
5.61
8.54
14.68
48.43
1.52
TypeC
8.73
12.69
18.37
53.55
1.45
Film
(Ba+Sr)/Ti
(B a+ Sr+ T i)/0
Type A
1.16
0.75
Type B
0.96
0.60
Type C
1.17
0.74
BST (50/50)
8
6
^
a.
a
4 ..
1000
800
600
400
200
Binding energy (eV)
Figure 3.2.2. XPS scan o f BST50 film annealed at 1050 °C for 2h.
140
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0
»:(a)
Ba
:(b)
W
Z
Binding energy (eV)
Binding energy (eV)
02
SM
S33
522
531
S3S
Sa
SX
Binding energy (eV)
Binding energy (eV)
Figure 3.2.3 Slow scan XPS o f (a) Ba, (b) Sr, (c) Ti, and (d) O peaks.
141
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S»
RBS Analysis
The Rutherford backscattering spectrometry was utilized to carry out depth profile
analysis o f the constituent elements o f BST thin films (Fig. 3.2.4). The simulated layer
structure (shown in the inset o f Figure 3.2.4) was as follows: the first layer (400 A thick)
was with Ba:Sr ratio o f 0.3:0.7, the second layer (2900 A thick) had Ba:Sr ratio o f 0.5/0.5,
and finally the third layer corresponded to the substrate. It was observed that the 40 nm
surface layer was indeed Ba deficient and Sr affluent in composition. The bulk o f the film
was stoichiometric and identical to the nominal composition in terms o f atomic
constituents and cation to anion ratio.
E n e i g y (M e V )
0.6
0.8
1.0
1.2
1.4
100
(400 «A) BaaaSr.,Tt03
80
(2000 «A) Ba,jSit,T10,
SrTiOj
^
—
E x p e rim en ta l
S im u lated
20
100
200
300
400
500
Channel
Figure 3.2.4 Experimental as well as simulated backscattering spectra from BST
(50/50) thin films on strontium titanate substrate annealed at 1100 °C for 6 h.
142
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The non-flat tail o f the RBS spectra could be due to various possible reasons including
the interdiffusion o f the constituent elements into the substrate or vice versa, asymmetric
scattering from the rougher surface o f the film or channeling formation from highly
crystalline film. The cross sectional SEM analysis o f these films exhibited a sharp film
substrate interface indicating minimal interdiffusion. However, since the measured
surface roughness is in the range o f 15-60 nm and these films are highly (100) oriented,
the asymmetric scattering from the rougher surface and/or the channeling effect could he
responsible for the observed non-flat tail o f the RBS spectra.
3.2.S.3 Dielectric and electrical properties
The dielectric characteristics o f BST films were measured in terms o f their
frequency dispersion o f capacitance and loss tangent in a frequency range o f 1 kHz to 1
MHz.
Bias field effect on dielectric properties
Figure 3.2.5 shows the room temperature frequency dispersion o f capacitance and
loss tangent o f type A-BST50 film measured at various suh-switching oscillation voltage
levels. The increase in dielectric constant (capacitance) with the increase in oscillation
voltage could he due to de-pinning o f ferroelectric domain walls from the defects [19].
The increase in loss tangent with oscillation voltage also indicates that the film is not
fully paraelectric at room temperature.
143
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Phase transition and loss measurements
Temperature dependent dielectric constants and dielectric losses for the type A,B,
and C films are plotted in Figures 3.2.6 and 3.2.7 respectively. A broad dielectric
anomaly o f the permittivity vs. temperature indicated more than one phase transition in
all the films (Fig. 3.2.6). Among the possible factors for the diffused nature o f the phase
transition, compositional heterogeneity, grain size, and unrelieved strains could play
significant role [9, 20].
To obtain a better insight o f the problem it was felt necessary to deconvolute the
broad dielectric peak that is believed to be composed o f several peaks indicating multiple
phase transitions in the films. There is, however, no definite line shape known for the
dielectric behavior as a function o f temperature in the ferroelectric phase. The observed
dielectric data was fitted utilizing Gaussian function in the Type A, B and C, BST films
(figure 3.2.8) allowing the fact that the surface composition is different compared to the
bulk. The results are shown in Table 3.2.II. Peaks 1 and 3 were believed to be due to the
cubic to tetragonal {T^ and tetragonal to orthorhombic {Tc") transitions respectively from
the stoichiometric bulk region o f the grain whereas peaks II and IV were the same
transitions from the Sr richer grain boundary regions.
144
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0.10
3.13x10*
1V
0.08
900 irV
3.03x10™
TDOmV*
U.
2.93x10®
0.06
500tnV
to
c
re
0.04
2.73x10®
0.02
2.63x10®
0.00
1(f
10®
Frequency (Hz)
Figure 3.2.5 Room temperature frequency dispersion o f capacitance and loss tangent
values o f type A BST50 thin film (annealed at 1050 °C/2h) measured at different
oscillation voltages.
145
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4400
,TiO, film s o n LAO
1 0 5 0 ' C/2hr
3900
1100' C/2hr
1100' C/6hr
c 3400
iS
(0 2900
c
o
u 2400
'C
u 1900
0>
«
Q 1400
900
400
75
125
175
225
275
325
375
425
Temperature (K)
Figure 3.2.6 Temperature dependent dielectric constant o f BST films annealed at
(a) 1050 °C for 2h and (b) 1100 °C for 2 h and (c) 1100°C for 6 h.
0.16
film s 0 1 LAO
0.14
0.12
eo
C
B
0.10
0.08
0.06
0.04
0.02
0.00
75
125
175 225 275 325
Temperature (K)
375
425
Figure 3.2.7. Temperature dependent dielectric losses o f BST films aimealed at (a)
1050 °C for 2h and (b) 1100 °C for 2 h and (c) 1100°C for 6h.
146
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■7B-11
7B-11
6 b- I t
■6B-11
® ^ T S p e :s ;
5e-11
•5e-11
3e-11
■3e-11
2e-11
•2e-11
ie -1 1
• 6 e - ll
6e-11
5e-11
■ 5 e -ll
4e-11
•4 e -li
38-11
•36-11
2e-11
•2e-11
150
60
250
350
2e-10
.7 5 6 -1 0
1 .5 6 -1 0
•1 .7 5 6 -1 0
(b) Type B
1 .5 6 -1 0
.2 5 6 -1 0
16 -1 0
1 6 -1 0
7 .5 6 -1 1
7 .5 6 -1 1
5 e -1 l
•2.56-11
2 .5 e-1 1
u
a
■aM
1 .7 5 6 -1 0
1 .5 6 -1 0
1 .5 6 -1 0
'3
a
es
U
- • 1 6 -1 0
7 .5 6 -1 1
7 .5 6 -1 1
•2.56-11
2 .5 6 -1 1
200
4e-10
3.5e-10
3e-10
2.5e-10
2e-10
1.5b-10
le-10
5b-11
300
4e-lG
3.5e-lG
3e-10
2.5e-lG
2e-lD
1.5 e-10
le-1 0
5 e -ll
(c )T y p e C
0
0
3.58-10
3e-10
2.5e-10
2e-lG
l.Se-lG
le-lG .........137.15/
5 e -ll
G
-S e ll
IGG
262.39
176.66
........... ^
^ ......
•3.58-10
■3e-10
2.5e-10
2e-10
l.Se-10
le-10
58-11
0
5 e -ll
T e m p e ra tu re (K )
Figure 3.2.8 Temperature-dependent capacitance for BST films aimealed at
1050 °C/2 h (a), 1100 °C/2 h (b) and 1100 °C/6 h (c). The de-convoluted
Gaussian peaks are also shown in the figure (see text)
147
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Table 3.2.II: Peakfit summary o f the transition behavior o f type A, B, and C BST(50/50)
films determined after de-convolution o f capacitance vs temperature plots using Gaussian
peaks.
Film
Peak 1
Peak 2
Peak 3
Peak 4
A
289.6
232.2
180.0
131.5
B
309.2
236.4
184.9
134.9
C
262.3
207.5
177.6
137.1
As can be seen from the Table 3.2.11, the Curie temperature {T^, for type- A, B, and C
thin films were larger than the Tc o f BST (50/50) bulk ceramics (-230 K) [7]. Since the
lattice parameter and thermal expansion coefficient o f BST50 (3.947 A and 10.5 X 10’
^/°C respectively) is larger than the corresponding lattice parameter and expansion
coefficient o f LAO substrate (3.787 A and 10.0 X lO 'V c respectively), it was expected
that the BST50 thin films will be under tension upon cooling the film after annealing. The
shift o f Tc to higher temperature as compared to the bulk ceramics was believed to be due
to the retained tensile strain in the film [8 ]. The fluctuation o f transition temperature in
the range o f 262 K to 309 K for these films were thought to be due to the relative
magnitude o f the retained tensile strain in these films. The transition temperature from the
portion o f the film material in the grain boundary region was less as compared to the bulk
grain region in case o f type A, B and C films. This was in fact expected as the RBS
analysis in conjimction with the XPS analysis indicated that the grain boundary region
had Sr affluent composition. W ith the increase in annealing temperature as indicated by
XPS analysis (Table-3.2.1) the Sr segregation at the grain boundary region was further
increased and therefore, as expected the transition temperature from these regions were
148
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further lowered as compared to bulk grain region (for example 237 K from 309 K for
type-B film as compared to 232 K from 290 K for Type-A film).
Hysteresis measurements
The temperature variation o f the hysteresis loop for the types A, B, and C-BST50
films is shown in Figure 3.2.8. At room temperature, all the films exhibited slim
hysteresis loop characteristics o f nearly paraeleetric material. However at lower
temperatures (up to 100 K) they exhibited polarization hysteresis characteristics to
ferroelectric materials (Figure 3.2.10). Under the identical measurement conditions it was
observed that the polarization was systematically increased and the coercive voltage was
reduced with the increase in annealing temperature and time. This improvement could be
correlated with the systematic increase o f grain size and thereby ease o f domain wall
movement resulting larger polarization and reduced coercive voltage. The temperature
variation o f remnant polarization for type B and C films is plotted in the inset o f Figure
3.2.10. The steep change corresponding to the temperature -185 K and -3 0 0 K for type B
and 270 K and 175 K for type C films were indicative o f orthorhombic to tetragonal and
tetragonal to cubic transitions in agreement with the information extracted from the
temperature dependent capacitance measurements.
149
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1050 «C/2hr
U
zl
a
•pNal
u
iS
"o
Ph
0.012
100 K
0.006
0.000
.y '
-0.006
-0.012
0.012
150 K
0.006
y
0.000
A
-0.006
-0,012
0.012
200 K
0.006
>
0.000
-0.006
-0.012
0.012
250 K
0.006
J
0.000
-0.006
-0.012
0.012
300 K
0.006
0.000
-0.006
-0.012
0.012
325 K
0.006
0.000
-0.006
-0.012
•40-30-20-10 0 10 20 30 40
1100 “C/6hr
1100«C/2hr
0 .0 2
0.04
100 K
0.01
0 .0 2
0.00
0 .0 0
-0.02
-0.04
0.04
- 0 .0 1
■8:85
150 K
0.00
-0.02
0 .0 0
0.01
■8:85
200 K
200 K
0.02
0.00
0.01
/
0.00
/
- 0.01
0 .0 2
-
■8:85 2 5 0 K
250 K
0 .0 2
0.01
0 .0 0
y
0 .0 0
-
/
-
0.01
■8:85 3 0 0
0 .0 2
-0.04
0.04
K
0 .0 0
0 .0 0
-
0.01
0 .0 2
-0.04
0.04
■8:85
32SK
0.01
0.02
0.00
0.00
- 0 .0 1
-
300 K
0 .0 2
0.01
-
ISO K
0.02
0.01
-
100 K
-
0 .0 2
-40-30-20-10 0 10 20 30 40
325 K
0.02
0.04
-40-30-20-10 0 10 20 30 40
Bias voltage (V)
F ig u r e 3.2.9 Temperature dependent hysteresis loops o f BST (50/50) films annealed at
different temperatures and time.
150
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
^ 1 0 5 0 C/2hr(typeA)
1100 C/2hr (type B)
CM
• —1100 C/6hr (type C)
E
0
1c
o
1E3
ni
_N
T ypeC
w
o
Q 0.006
Q.
100
200
300
Temperature (K)
-40
-30
-20
-10
10
20
30
40
Bias voltage (V)
Figure 3.2.10
Hysteresis loops o f BST (50/50) films annealed at different
temperatures and time (measured at 100 K). The inset shows the variation of
polarization with temperature (see text)
Capacitor-voltage characteristics: Tunability and K factor
The temperature variation o f the capacitance-voltage data for types A, B, and CBST50 films is shown in Figure 3.2.11. Dielectric tunahility, loss tangents (at zero hias
voltage) and K factor as a function o f temperature for these films is shown in Fig. 3.2.12.
It was observed that the tunability was maximum around the transition temperature, while
losses were lower in the paraeleetric region o f the films, making the K factor maximum
just above room temperature. These observations made these films suitable for high
frequency tunable devices, which are operated at room temperature.
151
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1050“C /2hr
1100 '>C72hr
1 .2 x 1 0 ’
1100“C76hr
100 K
1 .1 x1 0
1 . 0 x 1 0 ’“
9 .0 x 1 0
10
1 0
150 K
’
3 .5 x 1 o ’ "
300 K
3 .0 x 1 o ’
325 K
1 .3 x 1 0 ’
Bias voltage (V)
Figure 3.2.11 Temperature dependent CV characteristics o f BST (50/50) films
annealed at different temperatures and time.
152
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33
0.07
28
0.06
0.05
g23
100 150 200 250 300 350
Temperature (1^
= 18
.n
ra
0.04 eo
. ■
(a) Type
/
i 13
0.03 -2
X '" n
0.02
I-
8
0.01
3
100
150
200
250
300
3
Temperature (K)
70
0.28
50
40
60
0.24
‘Q 30
^20
50
0.20
10
0 100
JQ
40
"
R>
c 30
3
h-
0.16 eo
150 200 2ffl 300 350
Tempeiatiro (K)
/
^
(b) Type
-------------7
0.12
S
0.08
/
20
100
0.04
150
200
250
300
0.00
350
Tem perature (1^
0.25
70
40r
O30
60 20
10
50 0
100 150 200 250 300 350
>* 40 -
0.20
^
Tenveiatiiie(K)
(c) Type
!5 30
re
c
3
\
■ ■*
20 - ^
0.05
10 0
0.15 eo
C
re
0.10
1
100
150
200
250
300
0.00
350
Temperature (1^
Figure 3.2.12 Temperature dependent dielectric properties o f (a) type A, (b) type B,
and (c) type C BST (50/50) films (measured at IMHz).
153
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3.2.3.4 Phase shifter characteristics
Figure 3.2.13 shows the phase shifter characteristics o f BST50 films annealed at
1050 °C/2h (type A) and 1100 °C/6 h (type C) respectively in air. As shown in the figure,
the degree o f phase shift (S 21) was significantly higher for the type C film at all voltage
level. However, the insertion loss (magnitude o f S21) was also higher for the film and
therefore the figure o f merit,
k
value, (defined by the ratio o f phase shift and insertion
loss) for Type C film remained lower as compared to the Type A film annealed at 1050
°C/2h at all voltages. The phase shifter characteristics o f the Type A and Type C films at
various frequencies are presented in Table 3.2.Ill and Table 3.2.1V respectively.
350
- ^ 1 0 5 0 "C/2hr
- • — 1100 ®C/6hr
300
14.5 GHz
250
s
O) 200
0)
■O
150
(0
>4o 100
0)
(A
n
50
O)
0
0
100
200
300
Bias Voltage (V)
-12
-15
400
Figure 3.2.13 Degree o f phase shift and insertion characteristics o f BST (50/50) thin
films based CMPF phase shifter.
154
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Table 3.2.III: Phase shifter characteristics o f the Type A BST50 film annealed at 1050
°C/2hr.
Freq
(GHz)
Electric
field
(kV/cm)
Phase shift
0
400
225.149
14.1
Average
Insertion
loss (dB)
(7dB )
43.868
Insertion
loss at E
field
(dB)
-5.1922
52.3267
-6.9119
38.85
43.8099
-5.0505
43.84
52.2156
-6.3134
41.798
43.0408
-4.9887
43.395
50.7717
-5.7632
44.31
42.062
-5.0642
42.22
46.1226
-5.5679
42.11
41.27
-5.0072
41.94
41.48
-5.3354
39.56
(”/dB)
K
43.36
-5.132
533
268.56
400
221.409
14.2
-5.054
533
263.89
400
216.487
14.3
-5.029
533
255.372
400
213.825
14.4
-5.084
533
234.467
400
209.9985
14.5
-5.088
533
211.067
Table 3.2.IV: Phase shifter characteristics o f Type C BST50 film annealed at 1100
°C/6hr.
(7dB)
29.6486
Insertion
loss at E
field
(dB)
-16.842
-11.227
28.9356
-11.085
29.31
316.59
-10.7088
29.5635
-10.004
31.65
400
308.29
-10.498
29.366
-9.4406
32.66
400
303.04
-10.695
28.3347
-9.5154
31.85
Freq
(GHz)
Electric
field
(kV/cm)
Phase
shift
0
Average
Insertion
loss (dB)
t^av
(7dB)
13.9
400
385.74
-13.0104
14.8
400
324.86
15.0
400
15.2
15.4
155
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K
22.90
3.2.4 Conclusions
In the present chapter, it was demonstrated that the microstructure and the
composition heterogeneity between the grain boundary and bulk o f the grain o f solution
derived textured BST thin films play a major role in determining their phase transition
behavior. The increase o f annealing temperature and the annealing time was found to
increase the grain size o f BST thin films appreciably and thereby reducing the volume
ratio o f grain boundary region:bulk grain region. A diffuse nature o f the phase transition
was identified in the films as apparent from the temperature dependent dielectric constant
measured at 1 MHz in a temperature range o f 80 K to 425 K. The paraeleetric (cubic) to
ferroelectric (tetragonal) phase transitions were readily identified in all these films,
however, the ferroelectric (tetragonal) to ferroelectric (orthorhombic) phase transition
was observed only in BST films annealed at higher temperatures for a longer duration.
The observed dielectric anomaly was correlated with the microstructural features and the
compositional heterogeneity. Temperature dependent dielectric properties o f the films
measured at IM Hz showed the potentiality o f the films for high frequency tunable
devices. Phase shifters were fabricated on the films and were characterized in the
frequency range o f 13.9-15.4 GHz. It was observed that the degree o f phase shift
increased with increasing annealing temperature and time, however, insertion losses also
increased in the film. To increase the figure o f merit o f high frequency tunable devices,
two approaches are discussed in the following chapters, a) improve the degree o f phase
shift for the film annealed at lower temperature and b) to decrease the insertion losses for
the films annealed at higher temperatures and time.
156
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3.2.5 References
[1]
D.S. Kom and H.D. Wu, Integrated Ferroelectrics, 24, 215 (1999); R. Babbitt, T.
Koscica, W. Drach, and L. Didomenico, Integrated Ferroelectrics 8 , 65 (1995).
[2]
F.W. Van Keuls, C.H. Muller, R.R. Romanofsky, J.D. Warner, F.A. Miranda,
S.B. Majumder, M. Jain, A. Martinez, R.S. Katiyar, and H. Jiang, Integrated
Ferroelectrics, 42, (2002).
[3]
R.J. Cava, J. Mater. Chem. 11, 54, (2001).
[4]
M. H. Frey, and D.A. Payne, Appl. Fhys. Lett. 63, 2753, (1993). J. G. Cheng, X.J.
Meng, B. Li, J. Tang, S.L. Guo, J. H. Chu, M. Wang, H. Wang, Z. Wang, Appl.
Fhys. Lett. 75, 2132 (1999).
[5]
H.D. Wu, F.S. Bames, Integrated Ferroelectrics, 22, 291, (1998).
[6 ]
L.A. Knauss, J.M. Pond, J.S. Horwitz, D.B. Chrisey, C.H. Mueller, and R. Treece,
Appl. Fhys. Lett. 69, 25 (1996).
[7]
V.V. Lemanov, E.P. Smirnova, P.P. Symikov, and E.A. Tarakanov, Fhys. Rev. B.
54,3151 (1996).
[8 ]
C.M. Carlson, T.V. Rivkin, P.A. Parilla, J.D. Perkins, D.S. Ginley, A.B. Kozyrev,
V.N. Oshadchy, and A.S. Pavlov, Appl. Fhys. Lett. 7 6 ,1920, (2000).
[9]
W. Chang, J.S. Horwitz, A.C. Carter, J.M. Pond, S.W. Kirchoefer, C.M. Gilmore,
and D.B. Chrisey, Appl. Fhys. Lett. 74, 1033 (1999).
[10]
T.M. Shaw, Z. Suo, M. Huang, E. Liniger, R.B. Laibowitz, and J.D. Baniecki,
Appl. Fhys. Lett. 75, 2129, (1999).
[11]
D.M. Tahan, A. Safari, and L. C. Klein, J. Am. Ceram. Soc. 19, 1593 (1996).
[12]
L.R. Doolittle: Nucl. Instr. Meth. B 9, 3334 (1985).
[13]
D. Bao, Z. Wang, W. Ren, L. Zhang, and Xi Yao, Ceram. Inter., 25, 261 (1999).
[14]
C.S. Hwang, S.O. Park, H.J. Cho, C.S. Kang, S.I. Lee, and M.Y. Lee, Appl. Fhys.
Lett., 67, 2819(1995).
[15]
X. Chen, W. Lu, W. Zhu, S.Y. Lim, and S.A. Akbar, Surface and Coatings
Technology, 167, 203 (2003).
[16]
Y. Fujisaki, K. Tori, M. Hiratani, and K. K. Abdelghafar, Appl. Surf. Sci. 108,
365, (1997).
[17]
Y. Fujisaki, Y. Shimamoto, and Y. Matsui, Jpn. J. Appl. Fhys. 38, L 52, (1999).
157
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[18]
S. B. Desu and D. A. Payne: J. Am. Ceram. Soc., 73, 3391 (1990).
[19]
Dragan Damjanovi, Rep. Prog. Fhys. 61 (9), 1267 (1998).
[20]
W.J. Kim, H.D. Wu, W. Chang, S.B. Qadri, J.M. Pond, S.W. Kirchoefer, D.B.
Chrisey, and J.S. Horwitz: J. Appl. Fhys. 88 , 5448 (2000).
158
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CHAPTER 4
Doping Effects on Microwave Properties of Barium
Strontium Titanate Thin Films
This chapter is divided into two sections. First section, section 4.1 describes the effect o f
uniform manganese doping on the dielectric properties o f the barium strontium titanate
thin films for tunable microwave devices. In section 4.2, effect o f graded manganese
doping on electrical and dielectric properties o f BST thin films is described.
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4.1 Manganese (Mn) doping in barium strontium titanate thin
films
In the present section we have studied the effect o f manganese (Mn) doping on
the nature o f epitaxial growth and electrical behavior o f ( 100 ) oriented Bao.sSro.sTiOs
(BST) thin films. The degree o f texturing and quality o f the in-plane epitaxy o f BST thin
films on lanthanum aluminate (LAO) was found to be improved for upto 3 at% Mn
doping. These films were characterized in terms o f their electrical properties and
dielectric behavior at low (IkH z-lM H z) and microwave frequencies. We have fabricated
eight-element coupled microstrip phase shifters and tested them in terms o f their degree
o f phase shift and insertion loss characteristics. The phase shift increases from 239°
(undoped) to 337° with 3 at% M n doping. However, the insertion loss also increases (5.4
to 9.9 dB respectively) with the increase in dopant concentration so that effective
k
factor
(defined as phase shift/insertion loss) does not improve significantly and remains in the
range o f 33-44 °/dB. The observed electrical properties are correlated with the structure
and microstructure o f the M n doped BST films.
4.1.1 Introduction
Barium strontium titanate (BaxSri-xTiOa) thin films in paraeleetric state are
attractive candidates for tunable dielectric devices such as voltage controlled oscillators
(VCO ), tunable
filters, phase-shifters, tunable matching networks and frequency
multipliers. In general high dielectric tunability and low dielectric loss at microwave
frequencies are required for microwave applications. Moreover, for impedance matching
purposes the dielectric constants o f the films should be kept low (-500) and the films
160
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should have low leakage current densities (-100 nA/cm^). Fabrication o f phase shifter
utilizing bulk BuxSri.xTiOs remains unsuccessful due to high voltage requirement as well
as higher dielectric constant o f the capacitor. The dielectric constant o f BST thin films
are low as compared to their bulk counterpart due to finer grain size, presence o f
interfacial layers and residual stress [1]. In addition, its dielectric properties can be tuned
at moderate voltage compatible with modem electronic circuitry. To achieve low
dielectric loss, BST thin films should he grown with low defect densities and it has been
reported that epitaxially grown film on lattice-matched substrates has better tunability as
compared to polycrystalline thin films [2]. However, epitaxially or textured thin films
known to have higher leakage current densities as compared to polycrystalline thin films
[3]. Attempts have been made to improve the electrical characteristics o f BST thin films
by adding impurity elements. Dopants such as manganese (Mn^"^) replaces titanium
(Ti"*^), which acts as an acceptor, have been was found to prevent the reduction o f Ti^^ to
Ti^^ by neutralizing the donor action o f oxygen vacancies. Up to 1 at % M n doping was
found to increase the dielectric constant (measured at 100 kHz) o f imdoped BST from
1736 to 2093, the loss tangent was reduced from 0.0153 to 0.0033 whereas the tunability
was reduced from 66 to 63 % [4]. Magnesium doping (Mg^"^) up to 5 at% in
Bao.6Sro.4Ti 03 thin films was found to yield single-phase perovskite thin films, however,
excess Mg^^ addition up to 20 at% was foimd to yield multi-phase material. The
reduction o f dielectric constant, loss tangent, and leakage current densities with Mg
doping were attractive for phase shifter application; however, the tunability o f undoped
BST was also found to be decreased from 28 % to 7.9 % with 20 at % Mg doping [5].
Al^^ doping was found to modify the microstructure o f Bao^Sro.eTiOs ceramics with an
161
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increase in grain size for up to 1 at % aluminum (Al) doping, and decrease in grain size
with further Al addition [6 ]. The dielectric constant was found to decrease with Al
addition. Boron doping (B^^) was foimd useful to initiate liquid phase sintering and
impart better sinterability and lower surface roughness o f BaxSri-xTiOs thin films. Boron
was found to form a highly insulating B 2O 3 layer at the grain boundary, which was
claimed to be responsible for the lower leakage current densities o f Boron-doped BaxSri.
xTiOa thin films [7].
From the literature review, it appeared that the addition o f impurity element had
significant impact on the phase formation behavior o f BaxSri.xTiOs, microstructure and
electrical characteristics o f BST thin films. In this section o f the chapter, highly (100)
oriented undoped and Mn doped BaxSri-xTiOa, i.e. Bao.sSro.sTii-xMnxOB (x = 0.0 to 5.0 at
%) thin films were synthesized by an economic sol-gel process on ( 100) lanthanum
aluminate (LAO) substrates. The effect o f manganese doping on the phase transition
behavior, electrical, and dielectric properties o f the sol-gel derived BST (50/50) films
were examined.
4.1.2 Experimental Details
Pure BST50 sol was prepared as described in chapter 2, section 2.1.2.1. To
prepare Mn doped BST sol, stoichiometric amount o f manganese acetate, dissolved in
acetic acid was added to BST50 sol through continuous stirring. Three Mn doped BST50
sols (1, 3, and 5 atomic %) were prepared as described in section 2.1.2.2.1. For thin film
deposition, the as-prepared sol was diluted to a concentration o f 0.35 M by adding acetic
acid. The precursor films were coated on (100) oriented single crystalline lanthanum
162
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aluminate (LAO) substrates by spin coating technique. Films were spin coated at 2500
rpm for 10 seconds for the deposition o f each layer. After the deposition o f each coating,
the films were immediately thermally treated in a preheated furnace at 600 °C for five
minutes. On finishing the final coating procedure, all the films were annealed at 1050 “C
for 2h. General flowchart o f the process is shown in Figure 4.1.1.
An optical thickness measurement unit (F-20, Filmetrics Inc.) was used to
evaluate the thickness o f the aimealed films from the measured reflection spectrum. The
phase formation behavior and degree o f orientation o f the annealed films were
investigated using a Bruker GADDS X-ray diffi'actometer. The quality o f in-plane
epitaxy o f the films was evaluated by pole figure analysis. The surface morphology o f the
films was studied using an atomic force microscope (AFM) (Nanoscope Ula, Digital
Instrument). The low frequency (IkH z - IM Hz) dielectric properties o f BST thin films
were measured at room temperature by an impedance analyzer (HP 4294A, Agilent Tech.
Inc.) using an inter-digital transducer (IDT) design. The IDT consists o f 50 fingers that
are 7 mm long, 20 pm wide, and spaced 15 pm apart. The leakage current characteristics
o f the films were measured using a computer interfaced source measure unit (6517A,
Keithley Instrument) utilizing Labview 5.1 software. An eight element coupled
microstrip phase shifter (CMPS) was deposited on the films and the performance o f these
was evaluated by measuring the transmission (S 21) scattering parameters between 15 to
17 GHz using an HP 85IOC network analyzer.
163
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Strontium a ceta te
[ Barium a ceta te
1d is s o lv e d in a cetic
1 acid and eth y len e
g ly co l
^................. i ..................
1d is s o lv e d In a cetic
1 acid and eth y len e
1
g ly co l
i------------- ^------ ----
Titanium
Isop rop oxid e
d is s o lv e d in acetic
acid
Sr- so l
Ba - so l
|Vln a ceta te d issolve< i
I
in a c e tic acid
I
BST so l
Ti- s o
Continuously stirred
.....
Mn d o p ed BST parent s o l
^ ......
Diluted to 0.35M/L
with acetyl a c e to n e
S p h cdM ed on LAO
su b str a te s
I
Fired at tem perature
600°C/ Sm ins
i
=
A n n ealed at 105(FC for tw o h ours
Figure 4.1.1 Flow chart o f the sol-gel process for Mn doped sol and thin films.
164
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.1.3 Results and Discussions
4.1.3.1 Structural and microstructural characterizations
X-ray Diffraction Studies
Figure 4.1.2 shows the x-ray diffractograms o f undoped, 1.0, 3.0, and 5.0 at% Mn
doped BST thin films on LAO (100) substrates. The undoped and Mn doped
Bao.sSro.sTiOs films were found to be highly oriented along the (100) direction. The Au
lines from the electrodes and diffraction lines from the substrate are also marked in the
figure. The BST diffraction lines were indexed according to the JCPDS File No. 5-378.
The sharp points on the (200) diffraction ring (marked by small arrows in the figure)
indicate the textured nature o f the undoped film (Fig. 4.1.2(a)). The appearance o f other
misaligned planes (eg. 110, 220 etc) in Fig. 4.1.2(a) indicates that the undoped film is not
fully (100) oriented. The (100) texture does not improve much up to 1 at% M n doping.
(Fig. 4.1.2(b)). However, strong (100) texture is clearly evidenced in 3 at% Mn doped
BST film (Fig. 4.1.2(c)) and it deteriorates with further Mn addition (Fig. (4.1.2(d)).
Figure 4.1.3 shows the X-ray pole figure distributions corresponding to the (220)
reflection (26 = 67°) o f undoped and 3 at% M n doped BST films on (100) LAO
substrates. Undoped film displayed (Fig. 4.1.3(a)) considerable diffraction intensity in a
ring containing four spots and in the center region near x =
corresponding to a degree
o f in- plane disorder. On the other hand 3 at % M n doped film (Fig. 4.1.3(b)) exhibited
improved crystalline quality with four spots at the expected %= 45° in a low intensity
background. These results indicated that all the films were textured in (100) orientation
and texturing & in-plane epitaxial quality o f the BST films improved up to 3 at% Mn
doping and deteriorated with further Mn addition.
165
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lST(i
(b)
--' ■'.''i-'.
lisT-.a
;Au(20p>
■:V
Figure 4.1.2. X-ray diffraction patterns o f (a) undoped, (b) 1.0 at%, Mn doped BST thin
films on (100) LAO substrates.
166
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m ^ m
W W
AU(2W)
: ■
m:xi
•t^s'vv
"¥*
;\-v-.-.'; -^v■■.■
~ Ail(200) ^
'its^-i
Figure 4.1.2. X-ray diffraction patterns o f (c) 3.0 at%, and (d) 5.0 at% Mn doped BST
thin films on (100) LAO substrates.
167
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(a)
Figure 4.1.3. The X-ray pole figure distributions eorresponding to the (220) reflection
(20 = 67°) o f undoped and 3 at% M n doped BST films on (100) LAO substrates.
168
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A F M Analysis
In chapter 4, the study showed the influence o f grain size on dieleetric losses.
Thus, the objective o f the microstructural studies in this chapter o f doped BST films was
to determine if dopant influenced the grain size and loss value in the films. Figure 4.1.4
shows the AFM micrographs o f the undoped and Mn doped BST thin films. As shown in
AFM micrographs, undoped film has fine-grained structure at the surface o f the film.
Grain size was found to be increased in 1 at% M n doped film (Fig. 4.1.4(b)). The
concentration o f high angle grain boundaries was found to he markedly reduced in 3 at %
Mn doped film (Fig. 4.1.4(c)). The elimination o f high angle grain boundaries indicated
the highly textured nature with in plane epitaxy o f 3 at % M n doped BST film as
envisaged also by X-ray diffractograms and pole figure analysis. W ith further Mn doping
the film exhibited granular microstrueture with high angle grain boundaries at the surface
o f the film (Fig. 4.1.4(d)). The surface roughness o f the film increased from 14.3 to 27.5
run with the improvement o f the (100) texturing with M n doping up to 3 at %. The exact
nature o f the improvement o f (100) texturing with M n doping content up to 3 at% was
not clearly understood. The solubility o f Mn was limited in BST lattice, hence, when the
doping content was more than 3 at%, it was precipitated to grain boundaries thereby
reducing the grain boundary mobility. This explained the grain size reduction in 5 at %
Mn doped BST film. Up to 3 at%, Mn^"^ acted as an acceptor replacing Ti"*^ at B site and
to maintain the charge neutrality oxygen vacancies were created in the lattice. Probably,
due to the presence o f vacant lattice sites, the grain boundary mobility was increased
resulting in the observed improvement o f the (100) texture quality o f the BST films.
169
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t.m
tm
I.Gtft
Figure 4.1.4. AFM micrographs o f (a) imdoped, (b) 1.0 at%, (c) 3.0 at%, and (d) 5.0
at% M n doped BST films.
170
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4.1.3.2 Dielectric and electrical properties
Dielectric Constant and Dielectric Loss Measurements
Frequency dispersion o f capacitance and loss tangent o f Mn doped BST films
were measured in a frequency range o f 1 kHz-1 MHz. The dielectric constants o f the
films were calculated from the measured capacitance using the conformal mapping
results o f Gevorgian et al [9]. Fig. 4.1.5 shows (a) the frequency dispersion o f the
dielectric constants and (b) the tan 6 o f undoped and M n doped BST films. The dielectric
constant increased up to 3 at% Mn doping and decreased with further Mn addition. The
corresponding tan5 were in the range o f 0 .001 - 0.020 and there was no systematic
variation o f the tan5 with Mn doping. The increase in dielectric constant could be due to
the improved (100) texture o f the BST films up to 3 at% M n doping. Tsu et. al. [10]
reported that since the volume o f dielectric polarization is proportional to the size o f the
grain, the dielectric constant is increased with the increase in grain size. As mentioned
earlier, up to 3 at % Mn doping, the grain size was increased in BST thin films, which
may result in the observed increase in dielectric constant. For the undoped and Mn doped
BST thin films, the relatively high dielectric constants and its low frequency dispersion
indicated the absence o f any low dielectric constant interfacial layers.
Dielectric breakdown/leakage characteristics
The leakage current behavior o f M n doped BST thin films were measured by
measuring the current at a constant dc bias o f 7 V as a function o f time. Figure 4.1.6
shows that the leakage current systematically increased with the increase o f Mn content
in BST thin films.
171
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16501600c
m
in
c
o
u
u
■c
o
0)
a>
Q
(a )
—
0 at% Mn
- A - 3 at% Mn
5 at% Mn
155015001450-
,
•
*
14001350-
\
13000
2x10®
4x10®
6x10®
8x10®
1x10®
Frequency (Hz)
0.022
0.018
0.014
lO
c
n 0.010
—
0 at% Mn
-A-3at %Mn
—
5 at% Mn
0.006
0.002
-
0.002
10 ®
Frequency (Hz)
Figure 4.1.5. The frequency dispersion o f the (a) dielectric constant and (b) the loss
tangent o f undoped and Mn doped BST films.
172
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The current (I)-voltage (V) characteristics also showed similar trends (Fig. 4.1.7). The
increase o f leakage current with Mn doping could he due to the fact that Mn^^ acted as an
acceptor in BST lattice replacing Ti"*^ and thereby created oxygen vacancies to maintain
the charge neutrality. The creation o f charged vacancies in turn introduced mobile charge
carriers (in this case electrons), which increased the leakage current densities.
4.1.3.3 Phase shifter Characteristics
Highly textured undoped and M n doped BST films were used to fabricate eightelement coupled micro-strip phase shifters for low cost steerahle anteimas in
communication applications. The degree o f phase shift and insertion loss (in the
frequency range o f 13.5 to 15 GHz) o f imdoped and Mn doped BST thin films as a
function o f bias voltage are shown in Figure 4.1.8. The phase shift increased
systematically from 239° for undoped to 337° for 3 at% Mn doped BST50 film and
decreased with further Mn addition The degree o f phase shift, insertion loss, and the
figure o f merit
(k ),
for undoped and M n doped BST films are tabulated in Table 4.1.1. As
shown in the table, although there was a significant improvement o f the degree o f phase
shift up to 3 at % M n doping, the insertion loss also increased (5.4 to 9.9 dB respectively)
with the increase in dopant concentration and as a result the effective
k
factor did not
improve significantly and remained in the range o f 33 - 44 °/dB. The improvement o f the
degree o f phase shift could he due to the improvement o f (100) texturing up to 3 at % Mn
doping.
173
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5% Mn
Mn doped BST (50/50) on LAO
TV
5 10
'
3% Mn
10
1% Mn
,-11
10“
Time (s)
Figure 4.1.6
films.
10®
Time dependent dielectric breakdown data for Mn doped BST50
Mn doped BST (50/50)
5% M n
a
E
<
3% M n
c
sw>
3
o
,-11
_/■
1% M n
,-1 3
10
1
100
Voltage (V)
Figure 4.1.7 (a) Increase in the leakage current characteristics o f the M n doped
BST films with the increase o f M n content in films.
174
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Mn doped BST/LaAlO, (NASA)
350300250-
100
—
-
0%,
1%,
3%,
500
50
100
150 200 250 300
Bias Voltage (V)
350
400
Mn doped BST/LaA10,(NASA)
0%,
3%,
0
50
100
150 200 250 300
Bias Voltage (V)
350
400
Figure 4.1.8 Characteristics o f an eight element CMPS using undoped and Mn doped
BST (50/50) thin films, variation o f (a) phase shift and (b) insertion loss as a function
o f bias voltage.
175
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As mentioned earlier the surface roughness also increased with the increase in Mn
content up to 3 at %, therefore, the higher insertion loss with the increase in Mn content
could be either due to higher surface roughness or creation o f oxygen vacancies due to
the acceptor nature o f Mn doping in the BST lattice.
Table 4.1.1: Characteristics o f the phase shifters using BST thin films.
BST (50/50)
Frequency
(GHz)
Phase shift
Max. loss
(dB)
Max. K factor
(°/dB)
5.4
44
undoped
14.2
(°)
(at 339 V)
239.2
1 at% Mn
14.3
242.3
6.5
37
3 at% Mn
14.5
336.8
9.9
34
5 at% Mn
14.5
290.9
6.9
42
4.1.4 Conclusions
Highly (100) oriented undoped and M n doped BST thin films were successfully
grown on LAO (100) substrates by sol-gel technique. The results demonstrated that Mn
doping had a strong influence on the growth characteristics, microstructure, and electrical
properties o f BST thin films. Up to 5 at % M n doping BST thin films crystallized into
single-phase perovskite structure. The degree o f (100) texturing improved up to 3 at %
M n doping. Pole-figure analysis showed the improvement o f in-plane epitaxy upto 3 at %
M n doping. The grain size increased up to 3 at % M n doping. However the surface
roughness also increased with the grain growth. From the dielectric measurements it was
found that the dielectric constant o f the BST film increased up to 3 at % M n doping,
maintaining low dielectric loss values (tan5 ~ 10'^). Eight element coupled micro-strip
176
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phase-shifters were fabricated and tested in terms o f their degree o f phase shift and
insertion loss characteristics. Phase shift measurements showed that the degree o f phase
shift increased from 239° to 337° with 0 to 3 at% M n doping. The insertion loss also
increased from 5.4 dB (undoped) to 9.9 dB (3 at % M n doped) with doping content so
that there was no effective improvement in the factor, which remained in the range o f 33447dB. The relatively higher insertion loss in 3 at% M n doped films could he due to its
rougher surface morphology. The improvement in degree o f phase shift for 3 at % Mn
doped BST film was encouraging, however, the insertion loss needs to he minimized for
practical phase shifter applications.
4.1.5 References
[1] T.M. Shaw, Z. Suo, M. Huang, E. Liniger, R.B. Laihowitz and J.D. Baniecki, Appl.
P hys.L ett.,15 (1999)2129.
[2] F.A. Miranda, F.W. Van Keuls, R.R. Romanofsky, C.L. Mueller, S. Alteroritz and
G. Sxxhrammyam, Integrated Ferroelectrics, 42 (2002) 131.
[3] K. Ahe and S. Komatsu, Jpn. J. Appl. Phys., 32 (1993) 4186.
[4] H.D. Wu and F.S. Barnes, Integrated Ferroelectrics, 22 (1998) 291.
[5] M.G. Cole, P.C. Joshi, M.H. Frvin, M.C. Wood, and R.L. Pfeffer, Thin Solid Films,
374 (2000) 34.
[6] L. Wu, Y.C. Chen, Y.P. Chou, Y.T. Tsai, and S.Y. Chu, Jpn J. Appl. Phys., 38
(1999)5154.
[7] S.I. Jang, H.M. Jang, Thin Solid Films, 330 (1998) 89.
[8] S.B. Majumder, M. Jain, A. Martinez, R.S. Katiyar, F. W. Van Keuls and F.A.
Miranda, J. Appl. Phys., 90 (2001) 896.
[9] S. Gevorgian, F. Carlsson, S. Rudner, L. D. Wemlund, X. Wang, and U.
Helwersson, IEEE Proc. Microm. Antennas Propag., 143 (1996) 397.
177
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[10] R. Tsu, H.Y. Liu, W.Y. Hsu, S. Summerfelt, K. Aoki and B. Gnade, Mat. Res. Soc.
Symp. Proc., 361 (1995) 275.
[11] S.K. Koul, and B. Bhat, IEEE M TT-S Digest, 489 (1981).
[12] R. Crampagne, M. Ahmadpanah, and J.L. Guiraud, IEEE Trans. Microwave Theory
Tech., 26 (1978) 82.
[13] R. Romanofsky and A.H. Quereshi, IEEE Trans. Mag., 36 (2000) 6 .
[14] H.J. Gao, C.L. Chen, B. Rafferty, S.J. Pennycook, G.P. Luo and C.W. Chu, Appl.
Phys. Lett., 75 (1999) 2542.
178
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4.2 Graded Manganese Doped Barium Strontium Titanate
Thin Films
By depositing highly (100) textured graded manganese (Mn) doped Bao.sSro sTiOa
{BST (50/50)} thin films on lanthanum aluminate substrates using sol-gel teehnique, it
has been demonstrated that the graded acceptor doping is a promising technique to reduce
the temperature coefficient o f eapaeitanee (TCC), loss tangent, and leakage current o f
BST thin films. In the temperature range between 175 to 260 K the reported TCC o f
graded M n doped BST (50/50) films was less than 5.55 x 10'V k , which is comparable to
the best capacitors known so far. The lower temperature coefficient o f the capacitance o f
the Mn graded films was believed to be due to the induced compositional heterogeneity
resulting into a distribution o f the Curie temperature.
4.2.1 Introduction
Barium strontium titanate (BST) thin films have been studied for dynamic random
access memories (DRAMs) and frequency agile microwave electronics (FAME).
Extensive interest in the material properties viz. dielectric eharacteristics at moderate (<
IMHz) and high frequencies (> 2 GHz), tunability, leakage current densities are apparent
from the recent reports [1-3]. For single crystalline and polyerystalline bulk BST
ceramics, the dielectric permittivities are reported to be strongly temperature dependent,
with a sharp dielectric anomaly at the ferroeleetrie to paraeleetrie phase transition [4].
Moreover, due to the functional dependence o f the dielectric permittivity with
temperature, the resonant frequency o f a fabricated microwave device also becomes
strongly temperature dependent resulting into the carrier signal drift in an ambient
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surrounding [5]. The temperature dependent drift o f resonant frequency poses serious
problems in using bulk BST for the practical device applications. A broad dielectric
anomaly, indicative o f a diffused phase transition is often reported in case o f BST thin
films mainly due to the finer grain sizes, retained strain, composition heterogeneity etc
[6-8]. The smaller temperature coefficient o f capacitance (TCC) o f thin films as
compared to their single/polycrystalline counterpart could make thin films attractive for
several FAME devices. Limited attempts have so far been made to tailor the TCC o f the
BST thin films. Attempts have been made to lower TCC by fabricating paraeleetrie
Bao.isSro.TsTiOs and ferroelectric Bao.vsSroisTiOs thin film heterostructure separated by
MgO barrier layer [9]. Another approach utilizes compositionally graded BaTiOs,
Bao.gSro.iTiOa, Bao,8Sro.2Ti 03 and BaojsSro.asTiOs layers without using the diffusion
barrier MgO in te r-la y e r^ to broaden the dielectric anomaly. None o f these reports have
compared the dielectric anomalies o f the graded films with undoped BST films prepared
using the same experimental parameters. Hence, it was not possible to appreciate the
effect o f composition gradation on the electrical characteristics, as compared to those for
the uniform BST film. Recently, it has been demonstrated that the leakage current
densities are largely suppressed by graded La (donor) doped Bao.sSro.sTiOs (BST 50/50)
thin films [11], Significant reduction o f leakage current is also reported in Ni, Mn
(acceptors) doped BST thin films [12]. Moreover, it was also reported that the graded Fe
(acceptor)
dopings
improve
the
ferroelectric
and
leakage current densities
of
PbZro.53Tio.47O 3 thin films [13]. Another aspect o f composition grading is large
polarization dc offset o f ferroelectric films driven by an ac field. Direction o f the offset is
related to the direction o f composition gradient with respect to the substrate. These
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offsets have been reported to have a strong temperature dependence-giving rise to
possible pyroelectric applications in addition to other potential sensors, actuator and
energy converter applications [14].
Encouraged by these results, highly textured M n graded BST (50/50) thin films
were prepared on (100) LAO substrates to study their temperature dependent dielectric
properties. The primary focus o f this work has been to study the effect o f graded doping
on the TCC o f BST (50/50) thin films. Such a graded structure is expected to have
improved overall electrical characteristics compared to the films prepared by introducing
uniform doping. The effect o f graded doping on the dielectric and leakage current
characteristics o f BST thin films have also been highlighted.
4.2.2 Experimental Details
Graded M n doped BST50 thin films were prepared by sol-gel technique. The
details o f the preparation o f pure BST50 and Mn doped BST50 precursor sols have been
described in chapter 2 (Sections 2.1.2.1 and 2.1.2.2.1, respectively) [15-16]. The
Figure 4.2.1 Schematic o f graded M n doped BST film structure.
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precursor sol was diluted to 0.35M and spun coated on cleaned (100) LAO substrate at
2500 rpm for 5 seconds. A multi-layer graded structure was made starting from 1.0 at%,
3.0 at%, and 5.0 at% Mn doped layers followed by the deposition o f 3.0 at% and 1 at%
M n doped BST coatings. After each coating, the films were directly inserted into a
preheated furnace and fired at 600 °C for 5 minutes in air for organic removal and
crystallization. Each individual layer (each doping) was about 100 nm thick yielding a
total film thickness o f about 500 nm. Finally, the graded as well as uniform BST films
were annealed at 1050°C for 2h in air. The schematic o f the film structure is shown in
Figure 4.2.1.
The structure and orientation o f the annealed films were studied by X-ray
diffractometer (Siemens, D5000) using CuK<x radiation. The surface morphology was
characterized using an atomic force microscope (AFM) (Digital Instruments, Nanoscope
Ilia). For electrical characterization, an inter-digital capacitor (IDC) structure (as
described in chapter 2 section 2.2.5.1) was fabricated on the annealed films using
photolithography at the NASA Glenn Research Center facilities. The temperature
dependent dielectric properties and capacitance-voltage characteristics were measured by
an impedance analyzer (Agilent Tech. Inc., HP4294A) interfaced with a computer
controlled thermal stage (MMR Inc.). The leakage current characteristics o f the films
were measured using a computer interfaced source measuring unit (Keithley Inst.,
6517A), utilizing Labview 5.1 software.
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4.2.3 Results and Discussions
4.2.3.1 Structural and Microstructural Characterizations
Figure 4.2.2 shows the X-ray diffraction (XRD) patterns o f (a) undoped and (b)
Mn graded BST (50/50) thin films. Both these films are highly (100) oriented and
crystallized into the desired cubic perovskite structure.
Figure 5.2.3 shows the AFM images o f the pure and M n graded doped BST films.
The films had smooth surface (rms surface roughness in the range o f 6-14 nm) and a
typical bimodal grain size distribution with the average grain size (~ 90 nm) slightly
increased in case o f the M n graded BST films.
o
o
Substratepeak
6
(0
w
c
o
c
20
30
40
50
60
70
80
20 (degree)
Figure 4.2.2 X-ray difffactograms o f (a) pure and (b) graded M n doped BST
films.
183
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a.fid
1 .0 0
2 .0 0
2.00
1.00
Q
Z .W m .
Figure 4.2.3 AFM micrographs o f the (a) pure and (b) graded doped BST films.
184
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4.2.3.2 Dielectric and Electrical properties
Phase transition Studies
Figure 4,2.4 shows the temperature dependence o f the dielectric constant (K) and
loss tangent (tan5) o f the imdoped and M n graded BST (50/50) thin films, measured at 1
MHz. As shown in the figure the graded film undergoes a more diffused phase transition
as compared to the undoped as well as 3 at % M n doped BST thin films (which showed
the best characteristics among the Mn doped films). In the Chapter 4, section 4.1, it was
observed that the degree o f phase shift (measured at 339 V, 14.5 GHz) significantly
increased in uniformly doped 3 at% Mn doped BST thin films. Therefore, for effective
comparison
the
temperature
dependent
dielectric
constant
and
tan6
of
Bao.5Sro.5Tio.97Mno,o303 -x (uniformly doped) thin film is also included in Fig 4.2.4. The
dielectric constant is intermediate between the 3 at% Mn doped and undoped BST films.
However, the loss tangent o f the graded film is significantly lower than both o f these
films in the temperature range o f 80 to 250 K. Up to 325 K both graded and 3 at% Mn
doped BST thin films had lower loss tangents as compared to that o f the undoped BST
film. In the paraeleetrie state both these films obeyed the Curie-Weiss law [K= CcunJ (T0), where K is the dielectric constant, Ccurie is the Curie constant, T is the temperature,
and 0 is the Curie temperature]. The Curie constant and the Curie temperatures for both
these films were estimated from the slope and intercept o f linear fitted 1/K vs T plot.
These values along with the estimated transition temperature are tabulated in the inset o f
Fig. 4.2.4. It may be noted that for both the cases 0 < Tc indicates first order phase
transition. The TCC (= AC/CAT, where C is the measured capacitance) in the temperature
range o f 175 to 260 K is 5.5 x 10''*/°K for graded film, which is significantly lower than
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the corresponding TCC o f the undoped BST film (1.7 x 10'^ /°K)) and 3 at% M n doped
film (7.9 X 10'^^ /°K). Above the transition temperature the sign o f the TCC is changed for
all these films, however, its magnitude remains lower in graded film (1 x 10'^ /°K) as
compared to the undoped (1.7 x 10'V°K) and Mn doped BST films (1.713 x 10'V°K).
In the temperature range between 175 to 260 K the temperature coefficient o f
capacitance (TCC) o f the M n graded BST (50/50) film is less than 5.55 x 10'V k , which
is comparable to the compositionally graded value (2 x 10'"*) [9] or the best commercial
capacitors [17]. The lower TCC in graded doped films is due to the distribution o f Curie
temperature, which arises from the gradient doping. The nature o f the dielectric transition
in BST thin films is broader due to various reasons including finer grain size, retained
strain etc. The cubic to tetragonal ferroelectric transition is further smeared due to the
distribution o f the Curie temperature in graded-doped films, resulting in a relatively
lower TCC as compared to the undoped film value. It should be mentioned at this point
that the TCC values o f uniformly doped (1.0 and 5.0 at%) BST (50/50) films were larger
than the graded BST films.
Capacitor-voltage characteristics: Tunability and K fa cto r
The tunability and K factor (in a temperature range o f 100 to 375 K) o f undoped
and graded BST thin films were estimated from the C-V loop and measured tan6 (at 1
MHz frequency, 500 mV oscillation voltage and applying a dc bias o f (23 kV/em), and
the results are shown in Fig. 4.2.5. In the temperature range 175 to 300 K the values o f K
factor for the graded Mn doped BST film remains almost comparable to the undoped
BST film. Although the loss tangent is considerably less (see Fig. 4.2.4) for the graded
186
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K (G raded doped)
K (Pure BST)
K (3at% Mn doped)
tans (G raded doped)
tans (Pure BST)
tans (3 at% Mn doped)
eo
C
n
o 1200
S 1000
Temperature (K)
Figure 4.2.4 Temperature dependent dielectric constant and loss tangent o f
pure, 3 at% M n doped, and M n graded doped BST films.
187
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Mn doped BST film but the tunability is also lowered and as a result the K factor has not
improved. However, the reported room temperature tunability both for undoped and Mn
graded BST thin films in the present case is better or comparable to the existing literature
reports considering the fact that the applied surface field (23 kV/cm) is much less (due to
the limitation o f the instrument) than applied by the other workers [9,18,19]. Looking at
the XRD pattern o f these two films one can see that the undoped film is highly oriented
along (100) direction whereas the graded film has comparatively lower intensity for both
(100) and (200) diffraction peaks. Similar observation has also been reported in case o f
graded La doped BST (50/50) films [11]. The perceptible reduction in the peak intensity
o f the graded film could be due to its lower crystallinity. The better crystallinity is known
to yield better tunability [10] and since the graded doping reduces the loss tangent
appreciably, it is expected that it would in turn improve the K factor.
Dielectric breakdownAeakage characteristics
The tunability is known to be a function o f applied dc electric field. To withstand
large electric field, the leakage current o f BST films should be low. Figure 4.2.6 shows
that the leakage current is appreciably reduced in M n graded BST film and up to 150V
and varies linearly with the applied voltage. The undoped and 3at% Mn doped films on
the other hand exhibited dielectric breakdown (indicated by the sudden increase o f
leakage current) about 70 V and 50 V, respectively. Generally, when Mn^^, Ni^"^, Ni^"^,
Mg^^ ions (act as ‘B ’ site acceptor dopants in ABO3 perovskite BST lattice) substitute
Ti"^^, extrinsic oxygen vacancies are created in order to maintain the charge balance. The
acceptor dopants prevent reduction o f Ti"*^ to Ti^^ by neutralizing the donor action o f the
188
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% T (Graded doped)
A % T (Pure BST)
-n— K factor (Graded doped)
A K factor (Pure BST)
A
50 t
Si
n
c
3
I-
'A
.A
-10
100
150
A
A
200
250
300
400
350
Temperature (K)
Figure 4.2.5 Tunability and K factor o f pure and graded doped BST thin films
as a function o f temperature.
Mn doped BST (50/50) on LAO
0% Mn
Q.
3% Mn
10 "l
10’ 1
Graded Mn
10
.-12
1
10
100
V oltage (V)
Figure 4.2.6 Room temperature leakage current characteristics o f pure, 3 at% Mn
doped, and Mn graded doped BST thin films.
189
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oxygen vacancies. High temperature annealing in air (slightly reducing ambient) creates
intrinsic oxygen vacancies in BST films. The dopant ion carries extra negative charge and
compensates the positive charge o f the oxygen vacancies, as a result the concentration o f
free carrier (electrons) is reduced [20]. The decrease in electron concentration leads to
lower leakage current in graded doped BST films as compared to the undoped ones.
Moreover, the graded Mn doping is assumed to form intermittent dopant layers, which
act as limiters o f charge transport. Such localized impediments due to improved dopant
distribution may be responsible for reducing leakage current further in case o f graded Mn
doped BST films as compared to uniformly doped Bao.sSro.sTio.gyMno oaOa.x film.
4.2.4 Conclusions
In the present section, a graded doping approach has been followed to reduce the
temperature coefficient o f capacitance o f barium strontium titanate thin films. The graded
doping significantly reduced the loss tangent (80 to 325 K) and leakage current in BST
thin films. In the temperature range between 175 to 260 K the reported TCC o f Mn
graded BST (50/50) films is less than 5.55 x 10'V k which is comparable to
compositionally graded or the best capacitors.
4.2.5 References
[1]
T.M. Shaw, Z. Suo, M. Huang, E. Liniger, R.B. Laihowitz, and J.D. Baniecki,
Appl. Phys. Lett., 75 (1999) 2129.
[2]
R. Thomas, V.K. Varadan, S. Komareneni, D.C. Dube, J. Appl. Phys., 90 (2001)
1480.
190
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[3]
W J. Kim, W. Chang, S.B. Qadri, J.M. Pond, S.W. Kirchoefer, D.B. Chrisey,
and J.S.
Appl. Phys. Lett., 76 (2000) 1185.
[4]
V.V. Lemanov, E.P. Smirnova, P.P. Symikov, and E.A. Tarakanov, Phys. Rev.
B, 54(1996)3151.
[5]
R.J. Cava, J. Mater. Chem., 11 (2001) 54.
[6]
J.G. Cheng, X.J. Meng, B. Li, J. Tang, S.L. Guo, J.H. Chu, M. Wang, H. Wang,
and Z. Wang, Appl. Phys. Lett. 75 (1999) 2132.
[7]
W.J. Kim, H.D. Wu, W. Chang, S.B. Qadri, J.M. Pond, S.W. Kirchoefer, D.B.
Chrisey, and J.S. Horwitz, J. Appl. Phys., 88 (2000) 5448.
[8]
N.W. Schubring, J.V. Mantese, A.L. Micheli, A.B. Catalan, and R.J. Lopez,
Phys. Rev. Lett., 68 (1992) 1778; J.V. Mantese, N.W. Schubring, A.L. Micheli,
and A.B. Catalan,
Phys. Lett., 67 (1995) 721.
[9]
S. Gevorgian, P.K. Petrov, Z. Ivanov, and E. Wikborg, Appl. Phys. Lett., 79
(2001) 1861.
[10]
X. Zhu, N. Chong, H.L. W. Chan, C.L. Choyng, K.H. Wong, Z. Liu, and N.
M m g Appl. Phys. Lett., 80 (2002) 3376.
[11]
S. Saha, and S.B. Krupanidhi, Appl. Phys. Lett., 79 (2001) 111.
[12]
K. H. Ahn, S. Baik, and S.S. Kim, J. Appl. Phys., 92 (2002) 2651.
[13]
S.B. Majumder, B. Roy, S.B. Krupanidhi, and R.S. Katiyar, Integrated
Ferroelectrics, 39 (2001) 127.
[14]
I. Boerasu, L. Pintilie, M. Kosec, Appl. Phys. Lett., 77 (2000) 2231; D. Bao, X.
Yao, and L. Zhang, Appl. Phys. Lett., 76 (2000) 2779; F. Lin, G.W. Auner, R.
Naik, N.W. Schubring, J.V. Mantese, A.B. Catalan, and A.L. Micheli, Appl.
Phys. L ett, 73 (1998) 2838.
[15]
S.B. Majumder, M. Jain, A. Martinez, F.W. Van Keuls, F.A. Miranda, and R.S.
K atiyar,/. Appl. Phys., 90 (2001) 896.
[16]
M. Jain, S.B. Majumder, R.R. Romanofsky, F.W. Van Keuls, F.A. Miranda, and
R.S. Katiyar, Mat. Res. Soc. Symp. Proc., 720 (2002) H.2.2.1; M. Jain, S.B.
Majumder, R.R. Romanofsky, F.W. Van Keuls, F.A. Miranda, and R.S. Katiyar,
Integrated Ferroelectrics, 42 (200) 343.
[17]
111 and 116 series Microcaps®, American Technical Ceramics.
[18]
B.H. Hoerman, G.M. Ford, L.D. Kaufman, and B.W. Wessels, Appl. Phys.
Lett.,13 (1998) 2248.
191
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[19]
C.M. Carlson, T.V. Rivkin, P.A. Parilla, J.D. Perkins, D.S. Ginley, A.B.
Kozyrev, V.N. Oshadchy, and A.S. Pavlov, Appl. Phys. Lett., 76 (2000) 1920.
[20]
S.Y. Lee, and T.Y. Tseng, Appl. Phys. Lett., 80 (2002) 1797.
192
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CHAPTER 5
Studies Of Microwave Dielectric Properties Of BST
Thin Films Heterostructured With Low Loss Dielectrics
This chapter is divided into two sections. In the first section (5.1), studies o f barium
strontium titanate thin films heterostructured with low loss dielectric, MgO is described.
Section 5.2 describes the BST thin films heterostructured with MgTiOs and its effect on
dielectric properties o f BST thin films.
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5.1 Heterostructured Bao,5Sro.sTi0 3 :MgO Thin Films
BaxSri-xTiOa is attractive for microwave device applications due to its non-linear
dielectric response in d.c. bias fields. For the microwave transmission, the insertion loss
in the device has to he minimized. In an effort to bring down the insertion losses, the
heterostructured Bao.sSro.sTiOs {BST50} thin films with intermediate low loss MgO
layers using sol-gel technique were synthesized and characterized. Three different
BST50:MgO hetrostructured films (type I, II, and HI) with different BST/MgO layer
sequences and thicknesses were deposited in order to characterize these for microwave
properties. The correlation between the structure/microstructure and the dielectric
properties are presented in this section. Eight element coupled microstrip phase-shifters
were fabricated on these films and the performance o f those at microwave frequencies in
the 15-17 GHz frequency range was evaluated. The high frequency figure o f merit
(k ),
dramatically improved to > 87 °/dB in the optimized heterostructured composite thin film
and is the highest known reported value measured in the Ku hand region for BST based
materials. These results represent the current state o f the art technology.
5.1.1 Introduction
In general, deposition parameters (deposition temperature, post deposition
annealing temperature, annealing ambient etc.) and suitable doping helps in increasing
the tunability and reducing the dielectric loss tangent or leakage current densities o f BST
thin films [1-10].
194
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A d d ition o f lo w d ielectric constant and lo w lo ss m aterials in syn th esizin g the
co m p o site B S T is also an attractive approach to a ch iev e the desirab le d ielectric properties
suitable
for m icro w a v e d ev ices.
There are several reports
for syn th esizin g bulk
co m p o sites b y adding M gO , Z rO i, A I2O 3, M gT iO s, X i0 2 , and/or Z nO into B S T to reduce
the d ielectric lo s se s [1 1 -1 4 ], T he d ielectric constant o f th ese o x id e s is typ ically in the
range o f 1 0 -2 0 and lo ss e s in the range o f <10'^. T herefore, after m ix in g th ese o x id e s w ith
h ig h K B S T m aterial, dep en din g upon the m eth od o f m ix in g and their quantity, the
d ielectric constant o f co m p o site m aterials is ex p ected to d ecrease proportionality as
dictated b y the m issin g rules. In the p ro cessin g o f co m p o site and m u ltilayer film s, the
exp erim en tal con d ition s h ave to b e op tim ized
for obtaining the enhancem ent o f
properties. T i0 2 addition w a s found to reduce the d ielectric lo s se s and tunability [14];
h o w ever, it d o es not reduce the d ielectric constant o f the co m p o site as upon addition o f
alternative o x id e s su ch as M gO and A I2O 3. B y increasin g the T i0 2 concentration m ore
than 10 wt% , B S T X -ray diffraction peak s disappear and the 5 0 wt% T i0 2 added B S T
sh o w ed no d etectab le tunability. C hang
et.al.
reported the preparation o f M g O -m ix ed
B ST :b u lk ceram ics and P L D thin film s [1 5 ], w h ere a n on-un iform ch em ical distribution
w ith respect to the film th ick n ess w a s ob served in 4 0 -6 0 wt% M gO m ix ed B S T film s. M g
content in th e film s w a s different w ith respect to th e M g con tent o f the target due to the
d ifferen ces in the vapor pressure b etw een M g and other contents. M g p hase segregation
w a s ob served in 1 wt% and ab ove M gO m ix ed B S T . T he so l-g e l tech niq ue has also b een
u sed b y so m e researchers for the preparation o f 0-3
ceram ic/ceram ic co m p o site
ferroelectric film s [1 6 -1 9 ]. B S T co m p o sites w ith o x id e s, such as AI2O 3 and M gO h ave
a lso b een studied for their poten tial ap plications as p h ase shifters in m icro w a v e tunable
195
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devices [20]. The approach has been well justified in the bulk composite system [13,21]
but good results in the thin film form have not been obtained so far. Joshi and Cole [22]
prepared (l-y)BST.yM gO composite films by sol-gel method. A few attempts have also
been made by other workers to prepare thin films [11,23,24] but the large surface
roughness (~2 pm) hampered the possibilities o f depositing finely spaced ID electrode
structure desirable for designing the phase shifters.
A common approach to synthesize MgO:BST thin films by pulsed laser or sol-gel
deposition has been to add extra MgO (up to 60 wt%) in stoichiometric BST target (in
case o f PLD deposition) [15] or BST precursor sol (in case o f sol-gel deposition) [23].
For PLD deposition the exact stoichiometry o f MgO does not translate into thin films due
to the difference in the vapor pressures o f Mg and other cations in BST. For sol-gel
deposition, by using the mixed MgO/BST sol, the differences in the crystallization
kinetics and chemical forces restrict MgO to be homogeneously distributed in BST
matrix. In the present study to synthesize multi-layers o f BST50:MgO heterostructured
thin films by sol-gel technique, a different deposition approach is undertaken.
This novel approach showed the potential o f synthesizing high quality films and
appreciable enhancement o f BST thin film dielectric properties for tunable microwave
device applications. MgO was chosen primarily because o f its low dielectric constant and
low loss tangent. In the case o f composite structure o f high dielectric constant BST with
low dielectric constant material, lowering o f the dielectric constant o f the composite and
the uniformity o f distribution o f phases highly depends upon the method o f mixing and
the quantity o f material (MgO) added. As a result the dielectric properties and tunability
o f the composites can be tailored. The results o f the systematic studies o f the structural.
196
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microstructural, and dielectric properties o f these films are presented here. The
optimization o f the BST:MgO by changing the thickness and sequences o f the two
individual chemical (BST and MgO) layers in the heterostructured film with the aim to
achieve the optimum dielectric properties and figure o f merit for tunable microwave
device applications, is studied.
5.1.2 Experimental Details
BST50 and chemically individually deposited heterostructured BSTiMgO films
were prepared using sol-gel technique. BST50 and MgO sols were prepared using barium
acetate, strontium acetate, Ti-IV-isopropoxide (as described in chapter 2, section 2.1.2.1),
and magnesium ethoxide as precursor materials and acetic acid and 2-methoxy ethanol as
solvents. Thin films o f pure BST50 were deposited by the sol-gel technique on (100)
LaAlOs by spin coating technique. Three types o f BST50/MgO layer sequences were
followed for the preparation o f heterostructured BST film (Table 5.1.1).
Table 5.1.1: Sample identification
Sample
Box No.
First layer
Terminating layer
BSTzMgO
Type I
24/23/37
BST
BST
80:20
Type II
40/36
MgO
BST
62:38
Type n i
41/42
MgO
BST
68:32
In type I film, thick coating o f each BST and MgO were deposited alternately on
the LAO substrate, with BST 50 as the first and the terminating layers to get the
197
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BST:MgO volume ratio o f ~ 80:20. For type II film, coatings o f both BST50 and MgO o f
different thicknesses were deposited alternatively with MgO as the first and BST as the
terminating layers to get the BST:MgO volume ratio o f ~ 68:32. For Type Eli film, thin
coatings o f both BST50 and MgO were deposited alternatively with MgO as the first and
BST as the terminating layers to get the BST:MgO volume ratio o f ~ 62:38 (more MgO
in the film as compared to Type I and II films). After the deposition o f each coating, the
films were immediately thermally treated in a preheated furnace at 600 °C for five
minutes. On finishing the final coating procedure, all the films were annealed at 1100 °C
for 6 h. The structural characteristics o f the films were determined from the X-ray
diffraction studies in the range o f 20 = 20-105°. Atomic force microscopy (AFM) was
used to examine the surface morphology and roughness o f the thin films. A planar
capacitor structure, which simulates the structure o f tunable devices, was employed to
investigate the dielectric properties o f these thin films. Capacitor structure consisted o f
fifty fingers each 7 mm long, 20 pm wide and with gaps o f 15 pm (as mentioned in
Chapter 2, section 2.2.5.1). The dielectric dispersion (1 kHz-1 MHz) and capacitancevoltage (C-V) (1 MHz) characteristics o f the films were measured by an impedance
analyzer interfaced with a computer controlled thermal stage (70 - 700 K) probe station.
An eight element coupled microstrip phase shifter (CMPS) shifters as described in
Chapter 2, section 2.2.6 was deposited on the films and the performance o f these was
evaluated by measuring the transmission (S 21) scattering parameters between 15 to 17
GHz using an HP 85IOC network analyzer.
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5.1.3 Results and Discussions
5.1.3.1 Structural and microstructural characterizations
X-ray diffraction studies
BST50:MgO films deposited on LaAlOa substrate were highly (100) oriented as
in case o f pure BST films (Figure 5.1.1 (a)) deposited by sol-gel method. However there
were a few very low intensity peaks indicating a few misaligned planes in the films
(Figure 5.1.1(b)), which increased in intensity when more volume% o f MgO was
included in the composite film structure. Type II film had higher intensity for the peaks
corresponding to misaligned planes as compared to the type I film. In addition, the
BST50:MgO films showed a peak at 2 0 = 43.01°, which corresponded to MgO in the
heterostructured films. The lattice parameters o f the pure and heterostructured BST50
thin films were taken as the average o f the lattice parameters calculated from the (hOO)
lines in the diffraction pattern. The lattice parameter o f the pure BST50 film was found to
be 0.3933 nm, which was lower than that o f the bulk BST50 (0.3947 nm). This
observation is in agreement with other reports on BST50 films deposited on LAO
substrates where lattice parameter (of BST) decreased due to the built on compressive
strain [25].
The lattice mismatch o f ~3 % in the twinned LAO substrate, and the
difference in the thermal expansion coefficients o f the film and the substrate are believed
to give rise to strain in these films. The lattice parameters for type I, II and HI
BST50:MgO heterostructured films were calculated as 0.3934, 0.39497, and 0.3929 nm
respectively. No appreciable change in the lattice parameter values were observed for the
heterostructured films from the pure BST50 film, which indicated that MgO did not go
into the BST lattice site in detectable amount.
199
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BST50 film on LaAIO,
o
o
(a)
CM
1100 °C/6h
3
ffi
V)
o
o
c
0)
fO
o
o
1
CO
-
20
35
50
65
80
95
110
20 (Degree)
(b)
o
M (M
O
o
CO
O)
3
d
(0
c
0)
c
20 (Degree)
o
o
CO
-X.
20
30
40
50
60
70
80
90 100 110
26 (Degree)
Figure 5,1.1
X-ray diffractograms o f (a) pure BST50 film and (b)
Heterostructured BST50:MgO film s.
200
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R B S Analysis
The RBS channeling spectram o f the Type III heterostructured film (Fig. 5.1.2)
showed that the deposited layered nature o f the MgO films is lost after annealing and it
may be distributed in the BST layers as MgO crystallites.
Energy (MeV)
100
100
0.6
200
300
400
500
Channel
S rT iO ,
Figure 5.1.2 RBS channeling spectrum o f the Type III BST50:MgO film.
201
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AFM analysis
Surface morphology of the BST50:MgO composites has appreeiahly ehanged as
eompared to the pure BST50 film as shown in Fig 5.1.3. Pure BST50 film was found to
have large grains of ~30 nm in size. In the Type I and II films, stepped structures was
observed. It was believed that BST grains grow up to a certain limit and then due to the
hindrance caused by MgO, the new two dimensional nuclei of BST forms on top of a
growing face before the latter is completed so that several mono-layers grow
simultaneously giving rise to a stepped structure [2]. Surface steps were also observed on
the (100) oriented MgO substrates annealed above 1000 °C [27,28]. For higher armealing
temperatures, the steps heeame progressively more pronounced and higher with a
maximum at around 1145 °C [29]. These surface steps were thought to form nucleation
for the growth and the long step lengths were thought to allow the growth of the BST thin
films. Type III film showed smaller grains as compared to other BST films in the present
study. It was believed that in the type IE film, since more volume ratio of MgO is present
and the individual thicknesses of the BST and MgO layers were lower and the phases are
homogeneously mixed in the film, which reduced the grain growth of BST. These surfaee
steps were thought to form nucleation for the growth and the long step lengths were
thought to allow the growth of the BST thin films. Type HI film showed smaller grains as
compared to other BST films in the present study. It was believed that in the type III film,
since more volume ratio of MgO is present and the individual thicknesses of the BST and
MgO layers are lower and the phases are homogeneously mixed in the film, which could
have redueed the grain growth of BST.
202
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a)
(b)
(c)
d
1.0
0.0
1.0
2 . 0^'^
um
0.0
Figure 5.1.3 AFM micrographs of the (a) pure BST50, (b) Type I, (c) Type II, and (d)
Type m BST50:MgO films.
203
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Cross-sectional SEM analysis
Figures 5.1.4 (a) and (b) show the cross-sectional SEM images of the pure and
Type I BST50:MgO heterostructured films. Dense microstructure was observed in Type I
BSTSOiMgO film as compared to the pure BST50 film. No separate layers were observed
in type I BST50:MgO film, which indicated that after annealing, BST50 and MgO did not
maintain the continuous layer form, which may be due to the thin layers of BST50 and
MgO in the case o f type I film.
5.1.3.2 Dielectric and electrical properties
Phase transition studies
Figure 5.1.5 shows the dielectric constants and loss tangents of pure BST50 and
BST50:MgO (Type I, II, and HI) films as a function of temperature measured at 1 MHz.
All of the films showed a dielectric maximum around 255-260 K corresponding to the
transition from ferroelectric to paraelectric (tetragonal to cubic) phase for the
composition. The second broad-hump around 180 K in case of pure BST50 film was
believed to be due to the orthorhombic to tetragonal phase transition.
The feature was suppressed in the BST50:MgO films. The dielectric constant and
loss tangent o f the heterostructured films (type I, II, and IB) were less than that of the
pure BST50 film. It was believed that the low dielectric constant of non-timable MgO
grains intermix with BST and diluted the effect of the BST phase on the overall dielectric
constant and loss o f the composite film.
204
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217:16
I'odutii
(b)
1 5 .0 k V 1 0 .6 in m x 4 0 ,1 k S E (U ) 9 /13/02 18:06
'
'
'
'
• ' - , ' 0 0 ^ ,^ *
Cross sectional SEM of (a) pure BST50 and (b) Type I
Figure 5.1.4
BST50:MgO heterostructured film.
205
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4500
Pure BST50
Type I
Type II
Type III
4000
■g
3500
w
3000
8
2500
'E
4-*
OJ 2000
0>
E 1500
o
1000
500
75
125
175
225
275
325
375
425
Temperature (K)
0.20
—
pure BST50
—o— Type I
—★—Type II
Type III
<s 0.10
0.00
75
125
175
225
275
325
375
425
Temperature (K)
Figure 5.1.5 Temperature dependent dielectric constant (a) and loss tangents (b)
o f the pure BST50 and BST50;MgO heterostructured films.
206
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Capacitor-voltage characteristics: Tunability and K factor
Figure 5.1.6 shows the variation in the K factor of the pure and heterostructured
BST50 films as a function of temperature. Room temperature dielectric properties
(dielectric constant, dielectric loss, tunahility, and K factor) of the pure, type I, type II,
and type III BST50 films measured at 1 MHz are presented in Table 5.1.1. The figure of
merit of the heterostructured films was higher as compared to the pure/doped/composite
BST films deposited hy other techniques and reported in the literature [15,28,30,31]. As
listed in Table 5.1.11, depending on MgO content there was a considerable reduction in
the tunahility of the heterostructured films as compared to the pure BST50 films. The
reduction in the tunahility and loss values was observed for all the heterostructured films
as compared to that of pure BST50 films. The improvement of the figure of merit in the
case of BST50:MgO hetero-structured thin films was due to the reduction of insertion
loss (or dielectric loss) in the heterostructured BST50 films.
There are reports that in the BST:MgO composites, with the phase transition of
BST through the Curie temperature, a volume expansion occurs, which is resisted by the
MgO grains, creating a compressive stress state at the interface [32], which subsequently
results in a depolarization force that reduces the tunability of the material. In Type II
film, the tunability improved and hence the K factor of the film improved to ~ 55. In
Type III film, with more MgO volume ratio, the tunability further decreased and the loss
tangent increased, and hence the figure of merit (K factor) of the films decreased. This
could also be due to the increase in the connectivity of the BST phase (higher dielectric
constant) in the composite. Nevertheless, the K values improved subsequently over that
of pure BST films and stayed in the range 45-55 in the films studied.
207
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60
50
-o— Pure BST
^^ T y p e I
Type II
40
o
o
30
20
10
0
75
125
175
225
275
325
375
Tem perature (K)
Figure 5.1.6 Temperatiure dependent K factor of the pure BST50 and BST50:MgO
heterostructured films.
TABLE 5.1.II: Room temperature dielectric properties of the pure and heterostructured
BST50 films measured at 1 MHz and applied electric field of 25.3 kV/cm.
Dielectric
constant
tanS
Tunability
(% )
K factor
2714
0.0215
51.87
24.08
1729
0.0049
25.55
51.42
Type n BST50:MgO
1277
0.0052
28.48
55.12
TypeinBST50:M gO
1541
0.0070
31.37
44.90
Film
Pure BST50
Type I BST:MgO
208
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Dielectric Breakdown
Current-voltage characteristics of the BST:MgO heterostructured films were
performed at room temperature. As shown in Fig. 5.1.7, leakage current of Type II
BST:MgO film was lowest among the three composite films, which suggested that
dielectric breakdown strength of the Type II film should he higher as compared to the
other two films. This added feature also made it more useful for the tunahle dielectric
devices.
5.1.3.3 High frequency Phase shifter studies
The heterostructured films were evaluated at microwave frequencies (13-15 GHz)
by measuring the degree of phase shift (S21) and insertion loss (S n ) of an eight element
coupled phase shifter fabricated (at NASA Glenn Research Center) on the films. The
phase shift and insertion loss characteristics of the pure BST50 and heterostructured
BST50:MgO films are shown in figures 5.1.8 and 5.1.9 respectively. The detailed data is
presented in Table 5.1.III. The observed values at microwave frequencies showed the
improvement in the figure o f merit
(k
= phase shift/insertion loss) for the Type I film,
which was 72 7dB (at 13 GHz) as compared to that of pure BST50 film (29 °/dB) (at 13.9
GHz) measured at 333 kV/cm. The figures of merit of the phase shifters were found to he
77 °/dB (at 16.25 GHz), 48 7dB (at 16.25 GHz) for Type II and Type IE films as
compared to -29 7dB (at 15.2 GHz) for the pure BST50 film measured at 400 kV/cm.
The figure of merit of this heterostructured film was better than most of the existing
literature reports on BST thin films prepared hy sol-gel or other deposition techniques
[28,33,34].
209
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a.
T yp ell
,-10
,-11
100
200
Voltage (V)
Figure 5.1.7 Cuirent-voltage characteristics of the Type I, II, and III BST50:MgO
films at room temperature.
210
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350
OT 200
S
100
50
100
150
200
»
Ph
Ph
Ph
*— Ph
(BST50)
(Type I film )
(Type II film)
(Type III film )
250
300
350
400
Bias voltage (V)
Figure 5.1.8 Phase shift of the pure BST and BST50:MgO films as a function
of bias voltage.
GQ
T3
(0
o
■03a)
C
D)
(0
-10
o - Mag
Mag
^3— Mag
'iSr— Mag
S
-12
(B ST 50)
(T ype I film )
(T ype II film ) "
(T ype III film ) ■
-1 4
0
50
100
150
200
250
300
350
400
Bias voltage (V)
Figure 5.1.9 Insertion loss values of the pure BST and BST50:MgO films as a
function of bias voltage.
211
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The range of
k
(°/dB) for BST films grown by PLD technique are -26-50 (7dB) under
the similar measurement conditions. As summarized in Table 5.1.Ill, the figure of merit
o f heterostructured films increased to 87 °/dB for Type II film (at 533 kV/cm), which is
the highest known value so far reported in the open literature. In general, under the high
voltage, the reflection and other device losses of the circuit are picked up; hence the
increase in losses with higher bias voltage was observed (Figure 5.1.9). The lowest
insertion losses for Type II and Type III films at 16 GHz were 3.96 dB and 3.15 dB
respectively. This yielded a figure of merit of - 6 6 °/dB and 96 °/dB for Type II and Type
III films respectively. Type III film showed highest figure of merit of 96 °/dB, which is
the highest among the reported value in the open literature for the devices in the Ku band
region
5.1.3.4 Depth profile XPS studies
To understand the distribution of BST and MgO within the bulk of the film, the
compositional depth profile XPS measurements were done on Type II heterostructured
film (with the best results). The sample was sputtered for 1 minute with Ar ions, prior to
record the XPS spectra, in order to remove the carbon contaminations from the film
surface. Figure 5.1.10 shows the depth profile spectra (in terms of intensities) of Ba, Sr,
Ti, and Mg elements present in the film. The films showed the diffused interface between
BST and MgO layers, but the layers (BST and MgO) maintained their identity. These
observations support for the low coimectivity of BST layers in case of Type II film.
212
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TABLE 5.1.III: Detailed phase shifter data of the pure BST50 and heterostructured
BST50;MgO films.
Film
Frequency
(GHz)
Average
loss
(dB)
E. Field
(V)
(kV/cm)
Phase shift
(degree)
Figure of
merit
(degree/dB)
Pure
BST50
13.9
15.0
15.2
-13.01
-10.71
-10.49
(300)
400
385.7
316.6
308.3
29.6
29.6
29.4
Film
Frequency
(GHz)
Average
loss
(dB)
E. Field
(V)
(kV/cm)
13.0
15.0
-3.62
-4.18
(250)
333
262.3
2 2 0 .1
72.5
52.6
16.00
16.25
-3.54
-3.37
(300)
400
268.1
260.6
75.7
77.3
16.00
16.25
-3.54
-3.37
(400)
533
301.4
293.9
85.2
87.2
16.00
16.25
-5.06
-4.43
(300)
400
227.42
212.06
44.9
47.8
16.00
16.25
-5.06
-4.43
(400)
533
260.79
240.38
51.6
54.2
BST:MgO
Type I
BST:MgO
Type II
BST:MgO
Type III
Phase shift
0
213
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Figure of
merit
(VdB)
Substrate
0)
10
0.0
5.0x10^
1.0x10''
1.5x10''
2.0x10''
Sputter time (sec)
Figure 5.1.10 Depth profile XPS studies of the Type II BST50:MgO film.
214
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5.1.4 Conclusions
In conclusion, BST:MgO composite thin films were prepared by multilayer
deposition of BST50 and MgO layers using sol-gel technique. The sequence and
thickness of BST and MgO layers were found to affect microstructure and the dielectric
properties o f the films. The heterostructured films showed improved loss tangent and
hence K factor was considerably increased as compared to the pure BST films. For type
n film optimum improvement was observed both at low and high frequencies as
compared to the other BSTiMgO composite films in the present studies and those
reported in the previous studies. The significantly improved figure of merit for such a
film is -8 7 °/dB at 533 kV/cm. 87 7dB was obtained for phase shifter deposited on Type
III BST:MgO film. By taking the minimum insertion losses of the film, the figure of
merit of 96 7dB was obtained at 16 GHz. To our knowledge this is the highest figure of
merit and field tolerance (for type II film) measured in the Ku hand region [33, 34] for
BST based phase shifters. The comparison of figure of merit for various BSTiMgO
heterostructured films clearly indicates that the sequence of BST and MgO layers and
their thicknesses have considerable effect on the dielectric properties at microwave
frequencies.
5.1.5 References
[1]
L.A. Kjiauss, J.M. Pond, J.S. Horwitz, D.B. Chrisey, C.H. Mueller, and R. Treece,
Appl. Phys. Lett, 69, 25 (1996).
[2]
W.J. Kim, H.D. Wu, W. Chang, S.B. Qadri, J.M. Pond, S.W. Kirchoefer, D.B.
Chrisey, and J.S. Horwitz, J. Appl. Phys., 8 8 , 5448 (2000).
215
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3]
W. Chang, J.S. Horwitz, A.C. Carter, J.M. Pond, S.W. Kirchoefer, C. M. Gilmore,
and D.B. Chrisey, A/?/?/. Phys. Lett., 74, 1033 (1999).
4]
M. Jain, S.B. Majumder, A. Martinez, R.S. Katiyar, F.W. Van Keuls, R.R.
Romanofsky, and F.A. Miranda, Integrated Ferroelectrics, 42 (2002) 343.
5]
J.S. Horwitz, W. Chang, A.C. Carter, J.M. Pond, S.W. Kirchoefer, D.B. Chrisey,
J. Levy, and C. Hebert, Integrated Ferroelectrics, 22 (1998) 279.
6]
M.W. Cole, W.D. Nothwang, C. Hubbard, E. Ngo, and M. Ervin, J. Appl. Phys.,
93 (2003) 9218.
7]
M. Jain, S.B. Majumder, and R.S. Katiyar, Appl. Phys. Lett., 82 (2003) 1911.
8]
I. Takeuchi, H. Chang, C. Gao, P.G. Schultz, X.D. Xiang, R.P. Sharma, M.J.
Downes, and T. Venkateshan, Appl. Phys. Lett., 73 (1998) 894.
9]
H.D. Wu, and F.S. Bames, Integr. Ferroelectics, 22 (1998) 811.
10]
H. Chang, I. Takeuchi, and X.D. Xiang, Appl. Phys. Lett., 74 (1999) 1165.
11]
E. Ngo, P.C. Joshi, M.W. Cole, and C.W. Hubbard, Appl. Phys. Lett., 79 (2001)
248.
12]
L.C. Sengupta and S. Sengupta, IEEE Trans. Ultrason. Ferroelectr. Freq.
Control, 44 (1997) 792.
13]
E.F. Alberta, R. Guo, and A.S. Bhalla, Ferroelectrics, 268 (2002) 169.
14]
Q.X. Jia, B.H. Park, B.J. Gibbons, Y.J. Huang, and P. Lu, Appl. Phys. Lett., 81
(2002) 114.
15]
W. Chang and L. Sengupta, J. Appl. Phys., 92 (2002) 3941.
16]
D.A. Barrow, T.E. Petroff, R.P. Tandon, and M. Sayer, J. Appl. Phys., 81 (1997)
876.
17]
Q.F. Zhou, H.L. W. Chan, and C.L. Choy, Thin Solid Films, 375 (2000) 95.
18]
M.C. Cheung, H.L.W. Chan, Q.F. Zhou, and C.L. Choy, Nanostruct. Mater., 11
(1999) 837.
19]
M. Jain, S.B. Majumder, R.S. Katiyar, and A.S. Bhalla, Materials Letters, 57
(2003) 4232.
20]
P.K. Sharma, K.A. Jose, V.V. Varadan, Mater. Res. Soc. Symp. Proc., 606 (2002)
175.
21]
L.C. Sengupta, and J. Synowcznski, Integrated Ferroelectrics, 17 (1997) 287.
216
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[22]
M.W. Cole, P.C. Joshi, M.H. Ervin, M.C. Wood, and R.L. Rfeffer, Thin Solid
Films, 374 (2000) 34.
[23]
P.C. Joshi, and M.W. Cole, Appl. Phys. Lett., 77 (2000) 289.
[24]
T.V. Rivkin, J.D. Perkins, P.A. Parilla, D.S. Ginley, C.M. Carlson, L.C. Sengupta,
L. Chiu, X. Zhang, Y. Zhu, and S. Sengupta, Mater. Res. Soc. Symp. Proc., 656E
(2001) DD 5.7.1.
[25]
S. Hyun, J.H. Lee, S.S. Kim, K. Char, S.J. Park, J. Sok, and E.H. Lee, Appl. Phys.
Lett., 11 (2000) 3084.
[26]
A.A. Chernov, Contemporary Physics, 30 (1987) 251; I.V. Markov, “Crystal
Growth for Beginners”, World Scientific Publishing Co. Singapore, (1995) p 196.
[27]
E.J. Cukauskas, S.W. Kirchoefer, and W. Chang, J. Crystal Growth, 236 (2002)
239.
[28]
C.M. Carlson, T.V. Rivkin, P.A. Parilla, J.D. Perkins, D.S. Ginley, A.B. Kozyrev,
V.N. Oshadchy, and A.S. Pavlov, Appl. Phys. Lett., 76 (2000) 1920.
[29]
E. J. Cukauskas, S.W. Kirchoefer, and W. Chang, J. Crystal Growth, 236 (2002)
239.
[30]
B.H. Park, Y. Gim, Y. Fan, Q.X. Jia, and P. Lu, Appl. Phys. Lett., 11 (2000) 2587.
[31]
M.W. Cole, P.C. Joshi, and M.H. Ervin, J. Appl. Phys., 89 (2001) 6336.
[32]
S.C. Tidrow, E. Adler, T. Anthony, W. Wiebach, and J. Synowczynski, Integrated
Ferroelectrics, 28 (2000) 151.
[33]
C.L. Chen, J. Shen, S.Y. Chen, G.P. Luo, C.W. Chu, F.A. Miranda, F.W. Van
Keuls, J.C. Jiang, E.I. Meletis, J.C. Jiang, E.I. Meletis, and H.Y. Chang, Appl.
Phys. Lett., 78 (2001) 652.
[34]
F.W. Van Keuls, C.H. Mueller, R.R. Romanofsky, J.D. Warner, F.A. Miranda,
S.B. Majumder, M. Jain, A. Martinez, R.S. Katiyar, and H. Jiang, Integrated
Ferroelectrics, 42 (2002) 207.
217
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5.2 Heterostructured Bao sSro sTiOaiMgTiOa Thin Films
It is well known as well as inferred from the studies done so far on BST:MgO
film that the highly hygroscopic nature of MgO makes it very difficult to keep the sample
in condition and to have the reproducible measurements. Several new low K additives
(AI2O3, Zr0 2 , MgTiOs) were considered to replace MgO in the composites. After a few
considerations (chemistry, structure, and hygroscopic nature of the material), MgTiOa
was considered a suitable candidate to study the new BST;MgTi0 3 composites.
In this section, the preparation and studies on the BST: MgTi0 3 heterostructures
have been presented. An approach is undertaken to use the low loss, non-hygroscopie
MgTi0 3
(MX) for the
fabrication of the Bao.5Sro.5Ti0 3 :MgTi0 3
(BST50:MT)
heterostructured composite thin films by depositing multilayers of BST50 and MX hy the
sol gel technique. This studies research focus on measuring and understanding the
properties of the composite films, with perspective of their application to microwave
devices. Various characterization tools such as XRD, Raman spectroscopy, and AFM,
were used to gather the structural and microstructural characteristics of the films. It is
shown that the level of dielectric constant, dielectric loss, tunahility, and K factor suitable
for designing microwave devices are achievable. In the composite films several order of
magnitude reduction in leakage currents are also expected.
5.2.1 Introduction
(Bai-xSrx)Xi0 3 (BSX) is a suitable material for microelectronic device applications
due to its high response of the dielectric permittivity to an applied electric field.
However, the large dielectric constants found in this system limit its usefulness at
218
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microwave frequencies. In addition, reported values of tan5 for BST thin films from
different research groups are varied. In general, the values of tan5 of the BST are too high
to be of great practical use in microwave tunable devices. Recently, significant
improvements in reducing the dielectric losses have been reported in BST bulk ceramics
using various types of ferroelectric composite materials. Typically, the low dielectric loss
and low dielectric constant material MgO has been used as the secondary phase in the
composites [1-4]. The solubility limit of Mg in BST was found to be ~2 mol % [5], and it
was believed that above 2 mol%, the effect of Mg doping is more like of a eomposite
effect. With respect to Mg doping BST, it has been shown that BST:MgO composites
have reduced dielectric constants and loss tangents (over a very wide frequency range
from 100 Hz-10 GHz) and thereby, they are expected to be good for high frequency
device applications [3,4]. Another low loss materials that have been used in the
composites are MgTiOa [6 ], AI2 O3 [7], Zr0 2 [8,9].
In the thin film form, MgO [3], Ti0 2 [10], AI2O3 [II] doped BST thin films have
been prepared by PLD teehnique. The films showed the reduction in the losses of the
BST films. In the previous section (section 5.1), the improvement of dielectric properties
have been demonstrated in the thin films of Bao.sSro sTiOsiMgO prepared by the sol-gel
technique. An improvements in the figure of merit (tunability/loss) of the BST50:MgO
heterostructured films compared to pure BST50 films was clearly demonstrated.
However, the leakage characteristics of these films did not improve significantly.
Moreover, due to the hygroscopic nature of MgO, the BST:MgO heterostructured films
may suffer with longer period of time instability. MgTi0 3 , on the other hand, has low loss
and is non-hygroscopie in nature, which makes it a attractive candidate for making the
219
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composite structure with BST. In addition, there is no report on the preparation of BST
composite with low loss MgTiOa (MT) material in thin film form and thus it is also
interesting to know the effect of MT on the dielectric properties of BST thin films. We
know that Mg in the illmenite MT structure is surrounded by thin layer of the TiOe
octahedra, therefore we expect that MT will have less reactivity with the free OH ions as
compared to MgO.
In this section, we used the low loss and non-hygroscopic MgTiOs for the
fabrication o f the Bao.sSro.sTiOsiMgTiOs (BST50:MT) heterostructured thin films by solgel technique and studied the effects of MT layers on the structural, physical, dielectric
properties, and leakage current behavior of the BST:MT films.
5.2.2 Experimental Details
Preparation of BST50 sol for the thin film deposition can be found in Chapter 2.
Magnesium ethoxide and Ti-IV-isopropoxide were the precursor materials used for
preparing the MgTiOs sol. BST50:MT heterostructured thin films were prepared by
alternate deposition of MT and BST50 layers. In Type A BST50:MT film, single coatings
of BSTSOand MT were deposited alternately. For Type B film, thicker BST and MT
layers were deposited, where thicker layers were achieved by multiple coatings. After
each coating, intermediate heating process was done at 600 °C for five minutes in order to
drive off the organics and subsequently form the desired compound. Various structures
and deposition sequences are possible to adapt in such an approach. In the present
approach, in order to achieve the best connectivity in the ferroelectric phase, altemating
BST50 and MT layers with BST50 as the first and the terminating layers were used to
220
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synthesize the composite multilayer thin film structure. All the pure BST and composite
films were finally annealed at 1100 °C in air for
6
hours. Details of the films and their
identification numbers are given in Table 5.2.1 below.
Table 5.2.1: Sample identification
Film Name
Sample
Sample box no.
First layer
Terminating
layer
69/70/44/45/81
MT
BST50
67/68/78/79
MT
BST50
BST50:MT-thin
Type A
(Thin layers)
BST50:MT-thick
T ypeB
(Thick layers)
X-ray diffraction technique was used to identify the phase formation and
crystallographic orientation of the pure and composite BST films. The surface
morphology of the post-annealed films was observed by atomic force microscopy. The
Raman measurements were performed using JY T64000 spectrometer equipped with a
charge coupled device (CCD) detection system. The 514.5 nm line of an Inova argon ion
laser (Coherent Inc.) was used as an excitation source. XPS depth profile was used to
understand the distribution of individual elements within the film structure in the
BST:MT heterostructured films. The low frequency (IkHz-lMHz) dielectric response of
the films was obtained using impedance analyzer (Agilent Tech. Inc., HP4294A)
interfaced with a computer controlled thermal stage (MMR Inc.). The dielectric tunability
and K factor were deduced from the capacitance vs. voltage (C-V) characteristics.
Current-voltage (TV) characteristics of the BST50:MT (Type A) films were studied and
the leakage current behavior was compared with the other heterostructured film. An eight
221
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element coupled microstrip phase shifter (CMPS) was deposited on the films and the
characteristics of this was measured in terms of phase shift & insertion loss with the
application of electric field and between 15 to 17 GHz using an HP 85IOC network
analyzer.
5.2.3 Results and Discussions
5.2.3.1 structural and microstructural characterizations
X-ray diffraction analysis
X-ray diffraction analysis of the pure and BST50:MT films deposited on LAO
substrates (Figure 5.2.1) showed that both the films exhibit (100) orientation with respect
to (100) oriented LAO substrate. Some weak intensity peaks corresponding to other
misaligned planes were also observed in both the films. Lattice parameters of both the
films were calculated by taking the average of lattice parameter calculated from all (hOO)
peaks corresponding to BST in the diffraction pattern. The lattice parameter of BST50
and BST50:MT (Type A) films were 3.93
A and 3.92 A respectively. The diffraction
pattem confirms that no detectable reaction occurred between the BST50 and MT
components on armealing at 1100 °C for
6
hours. This indicates that MT is phase
compatible with BST as reported in case of ceramics annealed below 1400 °C [6 ]. Thus
the inclusion of the MT layers does not deteriorate the orientation of the BST50 thin
films and it was believed that MT maintains its identity in the BST structure.
222
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o
o
CM
tf)
C
o
oo
(C)
J?L
-j-J L
20
35
50
65
80
95
110
20 (degree)
Figure 5.2.1. X-ray diffractograms of the pure BST50 (a) and Type A (b) and Type
B (c) BST50:MT films.
223
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AFM analysis
Figures 5.2.2 (a), (b), and (c) show the surface morphology of the pure BST50
and BST50: MT films (Type A and Type B). The surface morphology of the two films
was significantly different. Pure BST thin film was textured having grains of -400 nm in
size, while in the BST50:MT heterostructured film, smaller grains of size -200 nm in
Type A and -140 nm in Type B films were observed. Like MgO, MT was also found to
suppress the exaggerated grain growth and thus narrowing the grain size distribution.
This resulted in more uniform microstructure. In the heterostructured film, MT may be
present in the BST matrix and hindered the growth of the continuous BST film. There is
no apparent distinction in the shape and size of the grains on the films that would allow
distinguishing between BST and MT grains in the AFM images.
S.2.3.2 Ram an spectroscopic studies
Raman spectra were obtained from the side view of the cut sample area at
different locations, chosen randomly. Room temperature Raman spectra of the pure
BST50, BST50:MT, and pure MT thin films on LAO substrates are shown in Figure
5.2.3. Both BST50 (Fig. 5.2.3(a)) and BST50:MT (Type A, (Fig. 5.2.3(b))) films showed
characteristic Raman broadband corresponding to cubic BST structure, and the peaks
corresponding to LAO (substrate) at around 125, 152, and 486 cm'' (wavenumber).
Raman spectra of very thin MT film (Fig. 5.2.3(c)) and BST50:MT film (Fig. 5.2.3(b))
showed peaks at aroxmd 224, 281, 305, 325, 396, 485, 502, 641, and 714 cm"', which
correspond to illmenite form of MT aecording to the previous report [12].
224
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a)
(c)
Figure 5.2.2. AFM micrographs of the BST50 (a). Type A (b), and Type B (c)
BST50:MT films.
225
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3
(0
0)
c
0)
c
9o
200
400
600
Raman shift (cm ')
800
1000
Figure 5.2.3. Room temperature Raman spectra of the pure BST50 (a), BST%):MT
(Type A) (b), and pure MT (c) thin films on LAO substrate. Peaks corresponding to
substrate are shown with the asterisk (*) mark and those to MT are shown with circle (o).
226
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The peak at 77 cm'^ in case of the BST50:MT film is the plasma line from the argon
laser. Raman spectra in conjunction with the X-ray results showed no additional peak
corresponding to mixed phases, which suggested the presence of two distinct and
unreacted phases (BST and MT) in the BST50:MT film.
S.2.3.3 Depth profile XPS studies
The XPS depth profile studies were performed on the Type A and Type B
BST50:MT films. The samples were sputtered for 1 minute with Ar ions prior to
recording the XPS spectra, in order to remove the carbon contaminations from the
surfaces. Depth profile was conducted to understand the distribution of elements of the
layers (BST and MT) in the composite films. Depth profile spectra of each individual
element were recorded. After background subtraction, the peaks in the XPS data for each
element were fitted with the Gaussian function. Figure 5.2.4(a) and (b) shows the plots of
intensities of peaks corresponding to various elements present in the Type A and Type B
layered composite films. It was observed that there is a significant interdiffusion between
the BST and MT layers in case of Type A film, thus no drastic change in the intensity of
the elements were observed within the film structure. The thickness of the constituent
layers were small in case of Type A film, which may interdiffuse while annealing at high
temperatures and formed a homogeneous distribution of the elements within the film
structure. Whereas in the Type B film (Figure 5.2.4(b)), intensities of the Ba and Sr peaks
were found to reduce significantly as we reach the MT layer while sputtering. Similarly,
intensity of the Mg element was found to reduce significantly when it reached BST layer.
Thus, in ease of the Type B film, layered structure was maintained with some diffusion at
227
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30
—
Ti
- A- Mg
—
Ba
—
Sr
25
20
n
15
(0
c
a> 10
5
A▲
0
0.0
5.0x10
1.0x10
1.5x10'
S puttering (sec)
- A
5.0x10"
1.0x10'
1.5x10'
-
Mg
2.0x10'
S p u tte r tim e (sec)
Figure 5.2.4. XPS depth profile spectra of the (a) Type A and (b) Type B BST50:MT
films.
228
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the interfaces even after annealing, which could he due to the greater thickness of the
constituent layers in case of Type B film as compared to the Type A film.
S.2.3.4 Dielectric and electrical properties
Phase transition studies
Figure 5.2.5 (a) and (h) shows the temperature dependence of dielectric constant
and loss tangent respectively of the pure and Type A BST50:MT thin films at room
temperature measured at 1 MHz. The composite film showed a broad temperature
maximum in the temperature dependent dielectric constant as compared to that of pure
BST50 film. The dielectric constant 892 and dielectric loss tangent 0.0048 of BST50:MT
(at room temperature) were lower as compared to the corresponding values 2714 and
0.014 measured on pure BST50 film.
Capacitor-voltage characteristics: Tunability and K factor
The dielectric tunability and K-factor (tunahility/loss tangent) were calculated
from the C-V characteristics by applying 25.3 kV/cm electric field. The temperature
dependent tenahility and K-factor of the BST and type I BST50:MT films are presented
in Fig 5.2.6. The tunability 15.28 % of the BST50:MT composite thin film was smaller
than that for the BST50 film (43.47 %). However, since the dielectric loss tangent in the
composite film is considerably reduced, the K-factor of BST50:MT thin films was found
to he similar i.e. 31.43 vs 30.03 of pure BST50 film at 1 MHz. The decrease in the
tangent loss was attributed to the low loss MT (-10'"^) phase, enhancement in the physical
quality o f BST component, and the clean interfaces at the substrate in the composite film.
229
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4500
-■— Pure BST50
^ BST50:MgTiO,
(thin)
4000
3500
jn 3000
c
8
2500
O
'C
4-I 2000
o
o 1500
0)
Q 1000
500
80
130
180
230
280
330
380
430
Temperature (K)
0.18
Pure BST50
BST50:MgTIO,
(thin)
0.16
0.14
0.12
CO
c
re
'4-1
0.10
0.08
0.06
0.04
0.02
0.00
80
130
180
230
280
330
380
430
Temperature (K)
Figure 5.2.5. Temperature dependent dielectric constants (a) and loss tangents (b) of
the pure BST and Type A BST50:MT films.
230
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70
60
-^ T (% )B S T 5 0
-* -T (% )B S T 5 0 ;M T
- n - K factor(BST50)
- A - K factor {BST50:MT)
50
40
A
n
c 30
u
.to
20
A'- — ^
10
0
75
150
225
300
375
Temperature (K)
Figure 5.2.6. Temperature dependent tunability and K factor o f the pure BST50 and
type A BST50:MT film.
a
E
<
BST:MgO-Type III
BST:MgO-Type II
3
o
BST:MT, thi
,-10
,-11
10
100
200
Voltage (V)
Figure 5.2.7. Room temperature current-voltage characteristics o f BST :MT Type A
film compared with BST50:MgO (Type II and Type HI) film.
231
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It is also believed that MgTiOs being non-hygroscopic in nature, the composite films
were least disturbed by the ambient moisture. As a result overall loss characteristics o f
such engineered composite films were also improved as in the case o f BST50:MgO
(Section 5.1) composite films.
Dielectric breakdown
The dependence o f the leakage current on the dc electric field for BST50:MT thin
films as compared with BST50:MgO films (Type II and III) is illustrated in Figure 5.2.7.
At an electric field o f 100 kV/cm, which corresponds to ~5.2 V applied on a sandwitchconfigured capacitor with a 520 nm thick dielectric layer, the leakage current for
BST50:MT was about 1.03 nA (in the present configuration), which is equivalent to a
leakage current density o f about 5.7 x 10'^ A/cm^. This value is better than that o f the
leakage currents o f the well-prepared sandwitch structure due to different transportation
path o f the charges. In the planar structure, the charges flow along the surface where the
higher defect concentration causes an increase in the mobility o f charges and thus higher
leakage currents [13]. In the similar electrode configuration, the BST50:MT composite
film show improved leakage characteristics compared to the pure BST50 and
BST50:MgO composite films, which were reported to show improved dielectric
properties at low frequencies.
5.2.3.S High frequency phase shifter characteristics
The films were evaluated at microwave frequencies by measuring the degree o f
phase shift and insertion loss o f an eight element coupled phase shifter fabricated (at
232
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(a)
t
BST50:MT (thin), Box 69
150
•V. 100
o
corrected 16.00
corrected 16.25
corrected 16.50
corrected 16.75
C orrected 17.00
400
Bias voltage (V)
-
2.0
(b)
BST50:MT (thin), Box 69
-2.5
CD
-3.0
vT
>»o
-3.5
c -4.0
D)
n
S -4.5
-5.0
•— M IA 16.00
■ -M 2 A 16.25
M3A16.50
*— M4A 16.75
* - M5A 17.00
100
200
300
40G
Bias voltage (V)
Figure 5.2.8.
Bias voltage dependent phase shift (a) and insertion loss (b)
characteristics o f the phase shifter fabricated on Type A BST50;MT film, at different
frequencies.
233
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200
a) BST:MT-thick, Box 67
1 6 .25
17.00
400
Bias voltage (V)
(b) BST:MT-thick, Box 67
S
-2.0
5 -3.0
16.25
16.75
17.00
400
Bias voltage (V)
Figure 5.2.9. Bias voltage dependent phase shift (a) and insertion loss (b) characteristics
o f the phase shifter fabricated on Type B BST50:MT film, at different frequencies.
234
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NASA Glenn Research Center). The phase shifter characteristics o f the Type A and B
films at various frequencies are shown in Figure 5.2.8 and Figure 5.2.9 respectively. The
observed values at microwave frequencies showed the improvement in the figure o f merit
(K=Phase shift/insertion loss) for the BST50:MT heterostructured films. Values o f figure
o f merit o f all the films in this study are listed in Tables 5.2.II, 5.2.1II, and 5.2.1V. The
k
value for Type A film was ~59.8 °/dB at 16 GHz, while for Type B film, it was 72 °/dB at
16.25 GHz measured with applied electric field o f 533 kV/cm. In general, under the high
voltage, the reflection and other device losses o f the circuit are picked up; hence the
increase in losses with higher bias voltage was observed (Figures 5.2.8 and 5.2.9). The
lowest insertion losses for Type A and Type B films were 2.6649 dB (at 16 GHz) and
2.0222 dB (at 16.25 GHz) respectively. This yielded a figure o f merit o f ~ 72.55 °/dB (at
16 GHz) and 92.44 °/dB (at 16.25 GHz) for Type A and Type B films respectively. The
figure o f merit o f these heterostructured films was better than the BST thin films prepared
by other techniques [14-16]. Figure o f merit o f Type B BST50:MT film is very attractive
for the tunable microwave devices in the Ku band region
Table 5.2.II: High frequency phase-shifter characteristics o f the pure BST50 film.
Film
Frequency
(GHz)
Average loss
(dB)
E. Field
(V)
(kV/cm)
Phase shift
(degree)
Figure of merit
(7dB)
Pure BST50
13.9
15.0
15.2
-13.01
-10.71
-10.49
(300)
400
385.7
316.6
308.3
29.6
29.6
29.4
235
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T able 5.2.III: Phase shifter characteristics o f the Type A BST50:MT film.
Film
Frequency
(GHz)
Average loss
(dB)
Type A
BST50:M T
(69)
16.00
16.25
17.00
-3.24
-3.21
-3.23
E. Field
(V)
(kV/cm)
/inn
)
Phase shift
(degree)
Figure of merit
(7dB)
193.3
181.1
166.6
59.8
56.5
51.5
Table 5.2.IV: Phase shifter characteristics o f the Type B BST50:MT thin films.
Film
Frequency
(GHz)
Average loss
(dB)
E. Field
(V)
(kV/cm)
Phase shift
(degree)
Fignre of merit
(7dB)
Type B
BST50:M T
(67)
16.25
17.00
-2.59
-2.39
(400)
533
186.9
150.5
72.11
62.77
5.2.4 Conclusions
In this section, multilayer deposition o f BST50 and MgTiOs thin films using solgel technique is demonstrated to be a novel approach to synthesize homogeneous
BST50:MT composite thin films with very low leakage currents. To our knowledge this
is the first approach o f making BST composite with MT in thin film form.
Microstructural features were significantly modified by the MT layers insertion. The
insertion o f MT layers in the BST film resulted in more uniform microstructure with
smaller grains, which improved the thermal stability, decreased the dielectric constant but
reduced the dielectric losses significantly. Introduction o f MT layers in the BST film
reduced the tunability and losses without sacrificing the K facto r (measured at IMHz).
236
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For the Type A BST50:MT composite film, considerable reduction in the dielectric loss,
and better leakage current values (several orders o f magnitude) compared to pure BST50
and BSTiMgO (Type II and Type HI) composite films were obtained. High frequency
phase shifter characteristics o f the BST50:MT composite films resulted in the figure o f
merit o f 60 °/dB for Type A and 72 °/dB for Type B film at 16 GHz and 16.25 GHz
respectively. Taking the lowest insertion losses, figure o f merit was -7 2 7dB (at 16 GHz)
and 92.44 7dB (at 16.25 GHz) for Type A and Type B films respectively. High figure o f
merit o f the BST50:MT films are very attractive for the tunable microwave devices in the
Ku band region
5.2.5 References:
[1]
E. NgO, P.C. Joshi, M.W. Cole, and C.W. Hubbard, Appl. Phys. Lett., 19 (2001)
248.
[2]
J. Synowczynski, L.C. Sengupta, and L.H. Chiu, Integrated Ferroelectrics, 22
(1998) 341.
[3]
W. Chang and L. Sengupta, J. Appl. Phys., 92 (2002) 3941.
[4]
M. Jain, S.B. Majumder, R.S. Katiyar, D.C. Agrawal, and A.S. Bhalla, Appl.
Phys. Lett., 81, (2002) 3212.
[5]
B. Su and T.W. Button, J. Appl. Phys., 95 (2004) 1382.
[6]
E.F. Alberta, R. Guo, and A.S. Bhalla, Ferroelectrics, 268 (2002) 169.
[7]
X. Liang, W. Wu, Zhongyan, Materials Science and Engineering B, 99 (2003)
366.
[8]
L.C. Sengupta and S. Sengupta, IEEE Transactions on Ultrasonics, Ferroelectric,
and Frequency Control, 44 (1997) 792.
[9]
Sung-Gap Lee and Dae-Seok Kang, Materials Letters, 57 (2003) 1629.
237
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[10]
Q.X. Jia, B.H. Park, B.J. Gibbons, Y.J. Huang, and P. Lu, Appl. Phys. Lett., 81
(2002) 114.
[11]
K.B. Chong, L.B. Kong, L. Chen, L. Yan, C.Y. Tan, T. Yang, C.K. Ong, and T.
Osipowicz, J. Appl. Phys., 95 (2004) 1416.
[12]
J.A. Linton, Y. Fei, A. Navrotsky, American mineralogist, 84 (1999) 1595.
[13]
F. Wang, A. Uusimaki, S. Leppavuori, S.F. Karmenenko, A.I. Dedyk, V.l.
Sakharov, IT . Serenkov, /. Mat. Res., 13 (1998) 1243.
[14]
C.M.Carlson, T.V. Rivkin, P.A. Parilla, J.D. Perkins, D.S. Ginley, A.B. Kozyrev,
V.N. Oshadchy, and A.S. Pavlov, Appl. Phys. Lett., 76 (2000) 1920.
[15]
C.L. Chen, J. Shen, S.Y. Chen, G.P. Luo, C.W. Chu, F.A. Miranda, F.W. Van
Keuls, J.C. Jiang, E.I. Meletis, J.C. Jiang, E.I. Meletis, and H.Y. Chang, Appl.
Phys. Lett., 78 (2001) 652.
[16]
F.W. Van Keuls, C.H. Mueller, R.R. Romanofsky, J.D. Warner, F.A. Miranda,
S.B. Majumder, M. Jain, A. Martinez, R.S. Katiyar, and H. Jiang, Integrated
Ferroelectrics, 42 (2002) 207.
238
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CHAPTER 6
Lead Strontium Titanate Thin Film
Frequency Phase Shifter Applications
For
High
Lead strontium titanate thin films were synthesized on platinized silicon and
LaAlOa (LAO) substrates using sol-gel technique. The films on LAO substrates were
highly (100) oriented and showed the same transition temperature value as reported in the
case o f bulk, whereas the films on Pt/Si substrate were polycrystalline and showed
considerably lower transition temperature. The low dielectric loss o f the PST30/LAO
films makes them attractive for fabricating tunable dielectric devices. Accordingly, eightelement coupled microstrip phase shifter were fabricated on PST30/LAO film and were
tested in the frequency range ~15-17 GHz. The maximum phase shift obtained at 15.75
GHz was 271° and the corresponding insertion loss was -4.84 dB. The maximum
k
value
(phase shift per dB loss) for PST30 film was -5 6 °/dB, which is better than commonly
observed value in the pure BST films and that makes PST30 a potential candidate
material for further investigations for microwave applications.
6.1 Introduction
There have been numerous efforts to integrate thin films o f high dielectric
constant materials into electronic devices, by academic groups that focus on the
exploration o f new material systems and industrial groups that highlight the integration
and reliability issues. In recent years, considerable research effort is being directed
towards the development o f thin ferroelectric films for applications in computer
239
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memories (both non-volatile and dynamic random access memories, NVRAM and
DRAMs), tunable capacitors for high frequency microwave applications, un-cooled infra­
red detectors, micro-electromechanical systems (MEMs), and electro-optic modulators
[1-4]. The ferroelectric perovskite thin films o f materials, such as BuxSri.xTiOs (BST)
have been considered as the potential candidates for the development o f frequency agile
microwave devices [5,6]. The research in the present thesis was focused on the
development o f thin film perovskite materials for microwave devices. As mentioned in
the earlier chapters, the electric field dependent dielectric constant (known as tunability),
in such materials makes them attractive for radio frequency (rf) and microwave
electronics, which include the field dependent capacitors, tunable resonators, filters,
variable frequency dividers and phase shifters [5-9]. The transition temperature o f
materials for such applications is helow the device operating temperature, since the
dielectric losses are high in the ferroelectric region. The perovskite materials such as
SrTiOs and BaxSri.xTiOs (BST) have been studied extensively for such applications.
SrTiOs is a good material to work at low temperatures. BST is an attractive material for
room temperature mw application because its Curie point can he shifted to just below
room temperature to get maximum tenability and low loss in the paraelectric region.
However, the relatively high loss tangents in BST films, warranted the search o f
alternative materials for room temperature applications.
Nomura and Sawada have studied SrTiOaiPbTiOs solid solution systems and
found easy formation o f good homogeneous compositions [9]. The Curie temperature o f
PbxSri-xTiOs (PST) system was found to vary linearly with composition. The PST
compositions are well behaved and fairly reproducible compared to the widely studied
240
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(Bs,Sr)Ti 03 (BST) system. An important advantage is that only one phase transition
occurs in the PbTiOs-SrTiOs system [9,10] as compared to three transitions in the case o f
BaTiOa-SrTiOs solid solutions. Moreover the processing temperature is lower in lead
based materials compared to BST, which offers additional advantage over BST thin
films. As a result, PST has been considered as a potential candidate material for the
future tunable microwave device components such as resonators, filters etc. when used in
paraelectric region [11-13]; and for ultra-large-scale integration (ULSI) dynamic random
access memory (DRAM) capacitor in the ferroelectric region [14-16]. In the past few
years, the temperature and the de bias dependent dielectric constant and losses o f PST
system have been studied in thin film and bulk forms at low frequencies (up to IMHz).
The compositions o f PbxSri.xTiOa with x<0.3 have Curie temperature below room
temperature [9], therefore, it is possible to expect that the PST films will show the
properties similar to other paraelectric materials (such as BST). The transition
temperature o f PST30 bulk ceramics has been reported just below the room temperature
(-1 0 °C), thus it is quite suitable for the room temperature microwave applications [9,
17].
PST thin films have been deposited by various techniques, such as misted
chemical deposition [14], pulsed laser deposition [16,18], chemical solution deposition
[15], etc. However, limited attempt has so far been made to synthesize PST thin films by
sol-gel technique [15], and characterize them for tunable microwave device applications.
A few reports on the polycrystalline PST films in the paraelectric region have been
reported with the possibilities o f using them for tunable microwave devices. The
dielectric studies were presented in the low frequency region only [13]. Substrates like
241
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platinized-silicon have a drawback that the deposited films are mostly polycrystalline,
which have higher losses. The well-textured films with transition temperature just below
room temperature have been reported to show better dielectric properties for room
temperature microwave applications [5,19,20], Thus, single crystal substrates, such as
AI2O 3, MgO, LaAlOs, are used to achieve epitaxial thin films with low losses.
For the work presented in this chapter, thin films o f PbxSri.xTiOa with x=0.3 were
fabricated on platinized silicon (Pt/Si) and lanthanum aluminate (LAO) substrates using
sol-gel technique. The structural, microstructural, and the dielectric studies o f these films
were carried out. Finally, eight element coupled microstrip phase shifters were fabricated
on the PST film (on LAO) and characterized at high frequencies to test thier suitability
for microwave applications.
6.2 Experimental Details
Lead acetate, strontium acetate, and Ti-IV isopropoxides were used as precursor
materials for the preparation o f PST sol. The details o f the calculation and preparation o f
the sol is mentioned in chapter 2, section 2.1.2.4. Thin films o f PST30 were deposited by
spin coating the PST30 sol onto clean platinized silicon (Pt/Si) and lanthanum aluminate
(LAO) substrates at a 2500 rpm for 10 s. Multilayer coating approach was used to get
film thickness o f ~500nm. The heating schedule o f the film was decided on the basis o f
the thermal analysis o f PbojSro.yTiOs sol-gel derived powder. The crystallinity and
degree o f orientation were studied using the X-ray diffraction technique. The surface
morphology o f the films was studied using an atomic force microscope (AFM)
(Nanoscope nia multimode AFM Digital Instruments). The circular gold electrodes o f
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100 |am radius were DC sputtered on the PST30/Pt/Si film. Whereas, the interdigitated
capacitor consisted o f 50 fingers that were 7 mm long, 20 pm wide, and spaced 15 pm
apart were fabricated by lift off technique on the PST30/LAO film. The dielectric
properties (at 1 MHz) o f both PST30 films were measured by an impedance analyzer (HP
4294A). A computer controlled thermal stage (MMR Technologies) was used to measure
the temperature dependent dielectric properties. An eight element coupled microstrip
phase shifter (CMPS) was deposited on the PST30/LAO film and the performance o f
these CMPS at MW frequencies (Ku band) was evaluated by measuring the transmission
(S 21) scattering parameters between 15 to 17 GHz using an HP 8510C network analyzer.
6.3 Results And Discussions
6.3.1 Thermal Analysis of sol-gel derived powder
Figure 6.1 shows the plots o f (a) the differential scaiming calorimetric (DSC) and
(b) thermo gravimetric analysis (TGA) for Pbo.sSro.vTiOs gel powders taken in alumina
crucible with heating rate o f 7 °C/min in air. The endothermic peak at around 50 °C and
the corresponding weight loss was evident from the TGA curve due to the evaporation o f
solvents. In the temperature range 200-400 °C, the sample showed several large
exothermic peaks in the DTA curve, which were attributed to the decomposition o f the
organic species present in the as-derived sol-gel powder. Major weight loss (-40% ) was
also observed in this temperature range. On raising the temperature further, the
exothermic peak due to perovskite phase crystallization had resulted in the temperature
range o f 550-650 °C. On the basis o f the thermal studies o f the powder it was concluded
that the intermediate firing temperature o f 400 °C was good enough for the removal o f
243
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25
P S T 30/70 gel pow der
Heating rate=7 °C/min
20
15
Exo
10
Endo
M
‘S
5
0
■5
20
120
220
320
420
520
620
50
720
Temperature ( C)
Figure 6.1 DTA and TGA plots o f the sol-gel derived PST30 powder.
244
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organics and the annealing temperature should be above 600 °C for the proper formation
and crystallization o f PST.
6.3.2 Structural and microstructural studies of thin films
X-ray diffraction studies
Figure 6.2 (a) shows the X-ray diffractogram o f the PST (30/70) film on Pt/Si
annealed at 700°C for 1 hr (-500 nm). The film was found to be polycrystalline in nature.
However, the PST30 film grown on (100) LAO substrate (-500 nm) and annealed at 900
°C for 3 h was highly (100) oriented (Figure 6.2(b)). Both the films were phase pure and
all the peaks in the X-ray diffraction pattern correspond to either PST or the substrate.
A F M Analysis
Figure 6.3 (a) and (b) show the 2D AFM images o f the surface morphology o f
spin coated PST30 films on Pt/Si and LAO substrates respectively. Both the films had
different surface morphology. The PST30 film on Pt/Si was found to have small uniform
size grains (-60 nm), whereas the film on LAO had bigger grains o f -2 4 0 nm. The bigger
grain size o f PST film on LAO could be due to the higher annealing temperature used as
compared to that for film on Pt/Si.
245
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d
(Q
C
4)
C
20
30
40
20 (degree)
50
60
* LAO peak
c
0)
40
50
60
20 (degree)
70
F ig u r e 6.2 X-ray diffraction patterns o f (a) PST30/Pt/Si and (b) PST30/LAO
films.
246
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1.0
. 0.5
0.0
0.0
0.0
0.5
1.0
Figure 6.3 AFM micrographs (1x1 pm) o f the spin coated PST30 films on the
(a) Pt/Si and (h) LAO substrates.
247
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6.3.3 Dielectric studies o f thin films
For dielectric studies at low frequencies (IkH z-lM H z), top circular gold
electrodes o f 100 pm radius were deposited on the PST30 film on Pt/Si substrate,
whereas the film on LAO substrate was evaluated using 50 finger interdigitated
electrodes. Figure 6.4 shows temperature dependent dielectric constants and dielectric
loss behaviour o f the two films at 1 MHz. In both cases, the temperature dependence o f
the dielectric constant was significantly broader as compared to the PST30 ceramic
(Figure 6.5) [9,11]. The dielectric constant was much higher for PST30 film on LAO at
all temperatmes than that on Pt/Si, which could be due to bigger grain size in PST30 film
on LAO. Grain size has influence on the dielectric properties as bigger grain sized film
was found to have higher dielectric constant as compared to the smaller grain sized film
[20].
In the PST30 film on Pt/Si, the maximum o f the dielectric transition was found at
about 180 K, however for film on LAO the transition temperature
shifted to 280 K.
The value o f the phase transition temperature o f PST30 in bulk iss reported to be 279-283
K. The film on Pt/Si had phase transition much lower (-100 K) than that o f the bulk. The
origin o f the shift o f transition temperature in the film as compared to the bulk could be
related to the nature o f strain in the film. Several possible factors including the deposition
process, and the lattice and thermal expansion coefficient mismatch between the film and
the substrate could contribute to the strain in the thin film. As the transition temperature
o f the PST30 film on LAO was very close to that in the bulk thus it was believed that the
film on LAO had very little or no stress. Dielectric constant and loss tangents o f the PST
film on LAO were 1158 and 0.019 respectively, as compared to 178 and 0.03 in case o f
248
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0.12
1200
0.10
1000
0.08
tn
I
u
800
g
200
s (film on Pt/Si)
E (film on LAO)
tans (film on Pt/Si)
tans (fiim on LAO)
eo
0.06 n
0.04
160
0.02
140
120
75
125
175
225
275
325
375
0.00
425
Temperature (K)
Figure 6.4 Temperature dependent dielectric constant and loss tangents o f the
PST30 films on Pt/Si and LAO.
35000
Pb„,Sr„-TiO,
0.3
0.7
3
30000
> 25000
20000
o>
a
o> 15000
>
0 10000
0
0.00
50 100 150 200 250 300 350 400 450
Temperature (K)
Figure 6.5 Temperature dependent dielectric constant and loss tangents o f the
PST30 ceramic (prepared by solid state reaction method) [11].
249
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PST30 film on Pt/Si. The room temperature tunability and loss tangent at low voltages
(-38 V) o f PST30 films on Pt/Si and LAO were 31 % & 0.035, and -1 9 % & 0.012
respectively. As the PST30 film on LAO showed good dieleetrie properties at low
frequencies, it was interesting to test the film at high frequency for tunable device
applications.
6.3.4 High Frequency Phase Shifter Studies of Thin Films
Figure 6.6 shows the room temperature phase shift and insertion loss
characteristics o f phase shifter fabricated on PST30/LAO film measured in the frequency
range 15.75-17.25 GHz. The insertion losses shown in the figure represents a device
characteristie o f the entire circuit, which includes reflection losses, mismatch losses, and
other power loss o f the entire circuit. In general, under the high voltage, the circuit losses
are picked up hence the increase in losses with higher bias voltage was observed. As seen
in the Figure 6.6(b), insertion loss increased at higher voltages, which were due to
increase in the reflection and other device losses. Highest phase shift at different
frequencies and corresponding minimum losses are mentioned in the Table I. The
maximum phase shift obtained at 15.75 GHz was 271° and the corresponding lowest
insertion loss was 4.84 dB. This yielded a figure o f merit (K=degree o f phase shift per dB
loss) o f 56 °/dB for the PST30 film, which is the first reported high frequency result for
PST thin films. The figure o f merit o f pure BST films deposited by sol-gel technique and
tested in the similar device structure was found to be 34-45 °/dB [5]. As shown in Table
6.1, it was encouraging that the sol-gel derived PST films exhibited good phase shifter
characteristics comparable to the sol-gel derived BST films, and further improvement in
250
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300
_
250 -
0)
0L>.
D) 200
0
■o
-
CO 150 -
o
w
100
1 5 .7 5
1 6 .0 0
1 6.25
1 7.00
1 7 .2 5
£
Q.
100
200
300
Bias voltage (V)
m
(tT
o
0
■o
3
»
o>
n
*
100
1 6 .0 0
16.25
1 7 .0 0
1 7 .2 5
200
300
400
Bias voltage (V)
Figure 6.6 Phase shifter characteristics o f the PST30 film on LAO in terms of
(a) Phase shift and (b) insertion losses at different frequencies.
251
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the insertion loss values can be done by taking care o f the losses due to improved design
(which is not the main theme o f this thesis). I also believe that making the composite o f
these films with low loss materials will further improve the figure o f merit by reducing
the losses as observed in PST ceramics [11] and BST composite thin films [21,22].
Table 6.1: High frequency characteristics o f the eight element coupled phase shifter
fabricated on PST30 film on LAO at 533kV/cm.
Frequency
Phase shift S21
A verage loss
tCav
(GHz)
(at 533 kV/cm)
(dB)
(7dB )
15.75
271.12
5.523
49.1
16.00
250.946
5.248
47.8
16.25
236.982
5.134
46.2
17.00
207.484
5.308
39.1
17.25
200.9
5.566
36.1
6.4 Conclusions
PST30 films were successfully grown on Pt/Si and LAO substrates by sol-gel
technique. The films on LAO substrate were highly (100) oriented and had transition
temperature very close the bulk value. However, PST30 film on Pt/Si substrate were
polycrystalline in nature and the transition temperature was shifted to ~100 K lower with
respect to the bulk data. The phase shifter was fabricated on the PST30/LAO film and its
characteristics were evaluated in terms o f the degree o f phase shift and the insertion
losses. Maximum phase shift and corresponding minimum insertion losses were 271° and
4.84 dB respectively, resulted in the figure o f merit o f 56 °/dB at 15.75 GHz, which
makes PST30 films a potential candidate material for microwave device applications.
252
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6.5 References
[1]
O. Auciello, C.M. Foster, and R. Ramesh, Annu. Rev. Mater. Sci., 28 (1998) 501.
[2]
D.L. Polla, L.F. Francis, Annu. Rev. Mater. Sci., 28 (1998) 563.
[3]
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CHAPTER 7
Conclusions And Future Work
7.1 Conclusions
The sol-gel process has been demonstrated as an economic way to synthesize device
quality BST, composite and related thin films for microwave device applications. Several
variable parameters like precursor, substrate, coating layer thickness, intermediate heating
and annealing temperatures, annealing ambient, which generally influence the epitaxial and
composite natvue o f the BST films were studied. Some o f the main findings are listed below:
(i)
Highly oriented BST films were grown on LaAlOs substrates using Sol-gel technique.
The epitaxy o f BST (50/50) thin film was markedly improved with the increase in
annealing temperature. The Grain size o f the BST films was found to increase with
the increase in annealing temperature. Such treatments also showed the effect on the
dielectric characteristics o f the films.
(ii)
On the well-characterized films, eight element coupled microwave phase shifters
were designed using the standard lithographic method and phase shift & loss
characteristics were measured between 12-18 GHz range. The degree o f phase shift
was increased from 221 to 308° (measured at 14.5 GHz in an electric field ranging
from zero to 30V/pm) with the improvement o f the epitaxial quality o f the films.
However, the insertion loss was also increased with the increase in annealing
temperature (and hence the grain size) conditions and it resulted in the effective
figure o f merit,
k
(phase shift per dB loss), relatively low -3 2 °/dB {for the film
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annealed at higher temperature (>1050 °C) and time} versus 43.84 °/dB for the film
annealed at 1050 °C for 2h.
(iii)
As the losses were lowered in the film annealed at 1050 °C/2h, the Mn doping was
tried in the BST films to improve the phase shift and loss characteristcics. Thin films
o f BSMnT were successfully grown on LAO substrates. It was found that Mn doping
has a strong influence on the growth characteristics, microstructure, and electrical
properties o f BST thin films. The degree o f (100) texturing improved up to 3 at % Mn
doping. Pole-figure analysis showed the improvement o f in-plane epitaxy with and
upto 3 at % M n doping. The grain size also increased up to 3 at % Mn doping. The
dielectric constant o f the BST film increased (up to 3 at % M n doping) while
maintaining the low dielectric loss values (tan5 ~ 10'^). Phase shifter measurements
showed the increase in degree o f phase shift from 239° to 337° with 0 to 3 at% Mn
doping. The insertion loss also increased from 5.4 dB (undoped) to 9.9 dB (3 at % Mn
doping content). As a result, no effective improvement in the
k
factor (which
remained in the range o f 33 - 44°/dB) was noticed. The change o f the degree o f phase
shift could have been due to the better (100) texturing o f BST with Mn doping (up to
3 at%) as revealed by the XRD analysis. The higher insertion loss with the increase in
Mn content could be either due to the higher surface roughness (which was noticed by
the AFM studies) or the creation o f excess oxygen vacancies due to the acceptor
nature o f Mn doping in the BST lattice.
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(iv)
Since it is important in microwave devices to have desirable dielectric constant o f the
resonator material while having at the same time low loss & high tuning and figure o f
merit, an approach to dilute the dielectric constant o f BST with low K, low loss
material was made. Multilayer and special sequential deposition scheme o f BST50
and low loss materials like MgO are demonstrated to be the most desirable and novel
approach to synthesize BST heterostructured (or composite) thin films. In the
BST50:MgO heterostructured films, the effects o f the BST and MgO layers
thicknesses were studied. It was observed that the layer thicknesses play an important
role in the dieleetrie, tunable, electrical, and phase shifter properties. The signifieantly
improved figure o f merit o f 87 °/dB was obtained at 16.25 GHz (at an applied voltage
o f 400 V) for the BSTiMgO o f both thieker layers deposition type films (optimizing
the BST and MgO layer thicknesses) as compared to 29.34 °/dB (at 15.2 GHz) for the
best pure BST50 film deposited by the same sol-gel technique. Taking the lowest
insertion loss the figure o f merit o f the BST:MgO film (Type HI film) improved to
~96 7dB at 16.25 GHz. To my knowledge this is so far the best-reported figure o f
merit value and the field tolerance measured in the Ku hand region o f the BST based
phase shifters, in the literature. Also, the novel sequential deposition scheme was the
first reported study on the BSTiMgO composite films, in the literature.
(v)
Some moisture attributed effeets on the BST:MgO composite films were noticed in
the dielectric measurements. Attempts were made to reduce such effects by replacing
MgO by a suitable &less moisture prone material like MgTiOs (MT). Attempts were
made to synthesize BST composite with MgTiOs in thin film form. Following the
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same approach as described in the case o f BST:MgO films, BST50:MT composite
thin films were fabricated by the multilayer deposition o f BST and MgTiOa layers
using sol-gel technique. The leakage current characteristics were measured and it
definitely showed an improved I-V behavior supporting the hypothesis & the current
approach. The typical leakage current on the BST50:MT film was an improvement by
a factor o f two on the best BST50:MgO film. The figure o f merit improved to ~72
7dB with increasing BST and MT thicknesses (Type B film) as compared to 59 °/dB
in the case o f films with thinner BST and MT layers (Type A film). Taking the lowest
insertion losses, figure o f merit was -7 2 7dB (at 16 GHz) and 92.44 7dB (at 16.25
GHz) for Type A and Type B films respectively.
(vi)
The high reproducibility o f BST stochiometry has been questioned in the literature
when its use in the device has been considered. In search o f a possible alternative
material, this thesis research has also considered a brief study o f this topic and
touched some aspects o f a new candidate material lead strontium titanate (PbSrTiOs,
or PST). So far no PST thin film approach for the phase shifter applications has been
cited in the literature prior to this study. Thin films o f PST were deposited on
platinized silicon (Pt/Si) and lanthanum aluminate (LAO) substrates using the
chemical solution deposition technique. The PbojSro yTiOa (PST30) film on LAO was
highly (100) oriented and showed transition temperature as in the case o f hulk,
whereas the film on Pt/Si was polycrystalline and showed lower transition
temperature. PST30 film deposited on LAO substrate was tested in terms o f their
degree o f phase shift and insertion loss characteristics at high frequencies. The
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preliminary results gave the phase shift o f 271° at 15.75 GHz and the best
eorresponding value o f insertion loss o f 4.843 dB. The calculated
k
value (phase shift
per dB loss) o f PST30 film was 56 °/dB.
In brief, the BST films prepared in this research exhibited an overall improvement in
the dielectric properties at low and high frequencies over the fihns prepared by the sol-gel
technique and reported (by the other workers) in the literature. To the best o f my knowledge,
from the synthesis approach taken in this thesis work, BST50:MgO heterostructured films
(produced in this thesis project) showed the highest figure o f merit and field tolerance
measured in the Ku band region on the phase shifters designed from these films. These films
have demonstrated a great promise for application in the tunable microwave devices and also
have room for further enhancement o f the charaeteristics. In addition, lead strontium titanate
thin film studied in this thesis, will provide a new direction in an important emerging area o f
microwave & related applications.
7.2 Future work:
This thesis while tried to accomplish the goals o f the projected work, has opened up
several directions & avenues where the materials can be improved and also more work can
be carried out in order to enhance the importance o f the present work in the microwave and
related fields o f research. Some topics are described briefly below:
1.
The current research is carried out by using the chemieals with easily available
purities. The chemical properties play a very important role in significantly reducing
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the losses in materials at MW frequencies. Therefore, some o f the processing should
be repeated by considering the highest possible purity chemicals, measuring the
synthesized samples, and then the role and importance o f the optimum purity
chemicals used should be established.
2.
Spectral photosynthesis (Dr. A.S. Bhalla ''Effect o f spectral wavelength on the
synthesis o f materials'" Private communication) has noticeable effects on the resulted
sintered materials, grown crystals, and thin films. Such studies should be undertaken
in order to see the possible reduction o f the dielectric losses in the BST and doped
films.
3.
Special sequential depositions o f MgO & BST and MgTiOs & BST for synthesizing
the composite films have been demonstrated and improvement in the MW phase
shifter characteristics are obtained. By no means the current results are the ultimate
results. There may be plenty o f room to further optimize these parameters. Further
work is needed to pin those parameters down where the best composition, layer
thickness & processing conditions can be identified for the best films.
4.
So far the present composite films have the microstructure, which is dictated by the
two constituents (e.g. BST and MgO) and the processing parameters. There are
several processing routes where these phases can be more orderly and spatially
connected (e.g. equally spaced on a submicron or nano scale and three dimensionally)
in the films. Such processing should be tried and measurements should be done.
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Effect o f the individual sizes o f the two phases should be studied and the role o f
interfaces should be analyzed.
5.
The losses in the PST thin films are still moderate than the desirable losses for the
good microwave devices at high frequencies and also are much higher than the
corresponding best ceramic composition samples. The use o f high purity chemicals
will definitely improve these properties. Also the composite o f PST films with low
loss materials like AI2O 3, MgO, or MgTiOs should be tried which will further
improve the losses o f the films (as observed in PST ceramics and BST composite thin
films). The processing conditions should be further optimized to improve the MW
dielectric characteristics o f these films.
6.
The approach o f making composites using low dielectric constant and low loss
material presented in the present thesis can be applied to other materials (like
PbZrTiOs, BaZrTiOa etc.), where the specific needs o f tailoring the losses and
dielectric constants are desirable. This approach o f layer deposition for making
composites could be highly beneficial.
7.
The multilayer deposition technique presented in this thesis will definitely be useful
for many other applications (such as in ferroics and anitferroics), where properties o f
the two phases are antagonist and the phases maintain their separate identity
(chemistry must be adjusted).
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