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Ferroelectric thin films for microwave and photonics applications

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Ferroelectric thin films for microwave
and photonics applications
by
Ding-Yuan Chen
A dissertation submitted in partial fulfillment
o f the requirements for the degree o f
Doctor of Philosophy
(Electrical Engineering)
in The University o f Michigan
2006
Doctoral Committee:
Assistant Professor Jamie Dean Phillips, Chair
Professor Pallab K. Bhattacharya
Associate Professor Amir Mortazawi
Professor Xiaoqing Pan
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UMI Number: 3224838
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© Ding-Yuan Chen
All rights reserved
2006
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To my parents
ii
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Acknowledgements
M any people have been helping and supporting me in different ways throughout this work.
I would like to begin by thanking m y research advisor, Jamie Phillips. His knowledge,
encouragement, and patience are invaluable to m y research projects. I am grateful that he
gave me a lot o f freedom to pursue what I was interested in. It is really one o f the most
fortunate things in m y life that I have studied under his supervision during the past years.
Also, I would also like to thank Professor Pallab Bhattacharya, Professor A m ir M ortazawi,
and Professor Xiaoqing Pan fo r serving on m y thesis defense committee. I am delighted
that I have taken courses from these three good teachers and have the chance to cite their
book or papers in this dissertation.
I also thank Professor Jay Guo fo r serving as m y academic advisor and giving me useful
suggestion for taking courses in my first year. He is also generous to let me using his lab
setup several times. Special thanks must be given to Xinen Zhu in Professor M ortazaw i’ s
group and Dr. Chung-Yen Chao in Professor G uo’ s group. Xinen is our collaborator for the
BST microwave capacitor project and he helped me tremendously w ith microwave
measurement. We also spent a lo t o f time conducting experiments in E M A L and SSEL
together. Dr. Chao provided a lot o f knowledge on optical waveguiding and helped me
w ith the testing optical waveguides. Former and current fellow group members such as
Kaveh Moazzami, T im Murphy, W illie Bowen, Emine Cagin, Jeff Siddiqui, Song-Liang
iii
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Chua, W illia m Luong, Vinay Alexander helped me w ith m y research w ork in various
ways. I am grateful to all o f them.
A lot o f people in the College o f Engineering helped me and I thank them sincerely. Jihua
Chen in MSE department helped me w ith AFM . E M A L staff members including Kai Sun
and Haiping Sun helped me w ith FIB. SSEL staff members provided technical support and
administrative assistance for the cleanroom. They are: Betty Cummings, Denise Oscar,
T im Brock, Brian VanDerElzen, Jim Kulman, and Greg A llio n ....
Besides my research, I had a chance to serve as a teaching assistant fo r EECS 330
Electromagnetics, I would like to thank Professor Fawwaz Ulaby and my colleague
Tsai-W ei W u and Chuck D iv in fo r their patience and devotion. From them, I learned how
to manage student teams and balance the relationship between the students.
Lastly, I would like to thank my parents, Ching-Fu Chen and C hin-M ei Chiu and my sisters
back in Taiwan, for their support and love w hile I was finishing this program. Also I would
like to thank my friends for befriending me w hile at Michigan. They are: Prof. Shuo-hung
Hsu, Dr. Subho Chakrabarti, D r Pei-chen Yu, Dr. Y i-Y in Lin, Dr. Teh Lin, Dr. Feng-tien
Yu, Dr. Shu-jen Lee, Kuo-Cheng Kao,
Dr. Juojung Hung, Dr. Joe Lin, Dr. X ing Cheng,
Dr. Guangyuan Zhao, Dr. Guangyu L i, Dr. X in Zhu, Dr. Y um in Lu, Dr. Lihua Weng, Dr.
Yi-Chung Tung, Dr. Robert W hite, Heuy-Yi Sung, T im Lin, Bob Littrel, Jim Carlson, Dr.
Roger Verhey, Wei-Cheng Chien, M ing-Yuan Cheng, Chu-Hao Hsu, Sing-Rong L i,
Shih-Yu Chang, Chang-Hao Tsai, Cheng-Shu Kuo, Ya-Y un Su, A llen Cheng, Katharine
iv
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Chang, Yu-Shaing Lin, Jia-Y i Chen, Juang-Ying Cheueh, Tien-Ling Hsieh, Meng-Han
Chung, Yu-Chung Chang, Chu-Sheng Yang, Zong-Kwei W u, Chia-Chen Su, Dawen L i,
Bradon Lucas, Helena Chan, Tienhua Ting, Chelsea Lai, Y i-Jiunn Chien, Chi-Hung Liu,
Kai-Shiu Liao, Kai-Chung Hou, Chia-Chu Chen, Y i-H ao Chen, Jun Yang, Jihua Chen,
Yanbin Chen, A rnold A llenic, W en-Lung Huang, Sheng-Shian L i, Yu-W ei Lin, Chao-Yun
Fan , Zeying Ren, Chi-Hao Kuo, Jia-Shiang Fu, Tze-Ching Fung, Hsien-Kai Hsiao, Jianbai
Wang, Xiaochuan Bi, Zetian M i, Jonghyun Shin, Ho-Sang Lee, Sang-Hyun Lee, Michael
Holub, Swapnajit Chakravarty, Justin Sanders, Cheng Peng, Jie Yang, Zhim ei Zhu, Li-Jing
Cheng, Eunjung Cho, P hilip Choi, Yuh-Renn Wu, Xiaohua Su, Yuan Xie, Jing Wang,
Xiaoying Chen, Y ili Chen, M eng-Hung Chen, Charlene Chen, Yu-Ching Liao, Linda
Chow, Dr. Andre Lee, Dr. Andrew Fang, Id-Y en Chen,Wei-Zung Chang, Shih-Chun Lin,
Li-W en Hung, W ei-Shiang Ma, Li-W en Hong, Jerry K ao.......
Ding-Yuan Chen
A p ril 2006
v
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Table of Contents
Dedication.......................................................................................................................................ii
Acknowledgements......................................................................................................................ii i
L ist o f Figures................................................................................................................................x
L ist o f Tables............................................................................................................................. x iv
Abstract........................................................................................................................................ xv
Chapter 1 In tro d u ctio n ..............................................................................................................1
1.1 Properties o f bulk ferroelectrics................................................................................... 2
1.2 Dielectric properties o f ferroelectrics.......................................................................... 6
1.3 Properties o f ferroelectric thin film s ............................................................................ 7
1.4 Overview o f device applications.................................................................................. 9
1.5 Focus o f this study........................................................................................................ 11
1.5.1 BST tunable capacitors fo r microwave applications........................................12
1.5.2 Integration o f ferroelectric optical waveguides w ith semiconductors........... 12
Chapter 2 Properties o f ferroelectric thin film s deposited by pulsed laser deposition.. 14
2.1 Deposition o f ferroelectric thin f ilm s ........................................................................ 14
2.2 Pulsed laser deposition process.................................................................................. 16
2.3 Parameters in PLD process and their effects.............................................................19
2.3.1 Laser parameters..................................................................................................19
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2.3.2 Substrate temperature............................................................................................ 21
2.3.3 Oxygen partial pressure........................................................................................21
2.3.4 Substrate-target distance.......................................................................................23
2.3.5 Post-deposition annealing.................................................................................... 23
2.4 Crystalline properties o f PLD thin f ilm s ...................................................................23
2.5 Thin film properties o f PLD thin film s ...................................................................... 28
2.6 Electrical properties o f ferroelectric thin film s fabricated by P L D ....................... 30
Chapter 3 Voltage tunable BST capacitors fo r microwave applications........................ 32
3.1 Overview o f voltage tunable devices......................................................................... 32
3.2 M aterials fo r ferroelectric thin film tunable capacitors...........................................33
3.3 Thickness dependence o f dielectric constant o f BST film s .................................... 35
3.4 M icrowave BST capacitors on sapphire substrates................................................. 39
3.4.1 T hin film deposition and the structural properties............................................39
3.4.2 Device fabrication and low frequency electrical properties........................... 40
3.4.3 M icrowave characterization o f ferroelectric thin film s ................................... 46
3.5 The effect o f growth temperature and annealing..................................................... 47
3.5.1 Deposition temperature dependence.................................................................. 47
3.5.2 The effect o f R T A in oxygen...............................................................................50
3.5.3 Secondary hot plate annealing in nitrogen......................................................... 53
3.6 C onclusions.................................................................................................................. 58
Chapter 4 Theory o f ferroelectric electro-optic thin film waveguide interferometric
modulators............................................................................................................................... 59
4.1 Thin film optical waveguides...................................................................................59
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4.2 Fundamentals o f the electrooptic effect in ferroelectric thin f ilm s .......................62
4.2.1 Electro-optic coefficient in oxygen octahedral ferroelectrics......................... 63
4.2.2 Ferroelectric capacitor m o d e l..............................................................................65
4.2.3 Sim ulation results.................................................................................................. 66
4.3 Analysis and design optim ization o f EO interferometric modulators fo r
microphotonics applications.............................................................................................. 69
4.3.1 Intensity output characteristics o f EO M Z I m odulators.................................. 70
4.3.2 Microscale design issues...................................................................................... 76
4.3.3 EO Fabry-Perot interferometric (FPI) modulators............................................81
4.4 Integrated E O M ZM s based on P L Z T thin film s w ith tunable electro-optic
coefficients............................................................................................................................82
4.4.1 P LZT thin film s w ith tunable EO coefficient and their field-induced
birefringences...................................................................................................................82
4.4.2 Intensity output characteristics and design considerations fo r m iniaturized
M Z M s w ith tunable EO coefficients.............................................................................86
Chapter 5 Ferroelectric thin film s on semiconductors fo r integrated optics: deposition
and characterization.................................................................................................................89
5.1 Fabrication o f B a T i 03 /M g 0 /GaAs structure and its structural and electrical
properties............................................................................................................................. 90
5.2 Growth o f ferroelectric thin film s on patterned substrates..................................... 95
5.3 Pulsed laser deposited ferroelectric thin film s on Si02/Si fo r optical waveguiding
............................................................................................................................................. 105
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Chapter 6 M onolithic integration o f ferroelectric optical channel waveguides and
semiconductors.......................................................................................................................113
6.1 Fabrication challenges o f ferroelectric thin film channel waveguides................115
6.2 BaTiOs optical waveguides on GaAs fabricated by pulsed laser deposition .... 118
6.2.1 Fabrication o f B aTiO s/A lxOy/M gO /G aAs structure.......................................118
6.2.2 Characterization................................................................................................... 120
6.3 Strip-loaded BST optical waveguides on Si02/Si fabricated by pulsed laser
deposition............................................................................................................................ 122
Chapter 7 C onclusions......................................................................................................... 126
7.1 Achievements in thin film deposition and device fa b rica tio n .............................. 126
7.1.1 BST thin film microwave capacitors................................................................126
7.1.2 B a T i 03 and BST thin film waveguides integrated w ith GaAs or Si substrates
..........................................................................................................................................127
7.2 Achievements in theoretical m odeling and device design.....................................128
7.3 Future w o rk.................................................................................................................. 128
References.................................................................................................................................. 131
ix
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List of Figures
Figure 1-1 Hysteresis loop in a ferroelectric crystal.................................................................4
Figure 1-2 Paraelectric phase and ferroelectric phase in perovskite ferroelectrics.............. 5
Figure 1-3 Schematic representation o f ferroelectric domains (a) before and (b) after
poling...................................................................................................................................... 6
Figure 2-1 (a) Pulsed laser deposition setup............................................................................ 16
Figure 2-2 RBS spectrum of BST grown on Pt/Ti/Sapphire (Sample 273) by pulsed laser
deposition. The composition is Bao.57Sro.5Tio.98O2.177..................................................... 18
Figure 2-3 Plasma plume o f BST during the pulsed laser deposition under (a) 6 mTorr
and (b) 30 m T o rr................................................................................................................ 23
Figure 2-4 X R D o f (a) M gO / B aTi03 grown at 30 °C and (b) M gO grown at 350 °C and
B aTi 03 grown at 700 °C.....................................................................................................25
Figure 2-5 Plume o f BaTi03 during the deposition................................................................ 26
Figure 2-6 X -ray diffraction o f B aTi 03 grown on (a) M gO and (b) Pt/Si substrates by
pulsed laser deposition....................................................................................................... 27
Figure 2-7 Thickness measurement result using Film etrics F-20. The layer structure o f
the tested sample is BaTiOs/SiOi/Si. The dark line indicates the measured result, and
the light line indicates the simulated result..................................................................... 28
Figure 2-8 (a) SEM (b )A F M (c) optical microscopic images o f ferroelectric BST thin
film s ...................................................................................................................................... 29
Figure 2-9 (a) Capacitance- electric fie ld and (b) polarization-electric fie ld curves o f PZT
film s deposited by P L D ......................................................................................................31
Figure 3-1 X -ray diffraction o f BST film on Pt/Si substrates............................................... 36
Figure 3-2 (a) Zero-bias relative p e rm ittivity as a function o f the thickness (b) Relative
perm ittivity as a function o f the applied electric fie ld fo r t=200, 400, 600 nm ..........38
Figure 3-3 Measured dm/sm as a function o f dm (solid) and the linear fit o f the measured
data (dashed)........................................................................................................................39
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Figure 3-4 The dependence o f composition on the growth temperature............................. 41
Figure 3-5 X -ra y diffraction o f BST film s w ith the growth temperature o f (a) Tg=500 °C
and Tg=500 °C + RTA (b) Tg=600 °C and Tg=600 °C +RTA (c) Tg=700°C and
Tg=700 °C +R TA................................................................................................................ 44
Figure 3-6 (a) Cross section profile and (b) top view o f BST capacitor..............................45
Figure 3-7 T unability at different growth temperature (a) 500 °C (t=160 nm) (b) 600 °C
(53 nm) and 700 °C (38 nm )..............................................................................................46
Figure 3-8 (a) Capacitor (b) through line #1 (c) through line #2..........................................49
Figure 3-9 (a) Relative perm ittivity and (b) loss tangent o f BST film s deposited at 500
and 600 °C............................................................................................................................50
Figure 3-10 (a) Relative perm ittivity and (b) loss tangent (c) total Q factor o f BST film s
before and after R TA.......................................................................................................... 52
Figure 3-11 C -V curves before and after R T A .........................................................................52
Figure 3-12 (a) Relative perm ittivity and (b) loss tangent (c) total Q o f BST film s before
and after nitrogen annealing.............................................................................................. 55
Figure 3-13 C -V hysteresis curves before and after nitrogen annealing..............................56
Figure 3-14 C -V hysteresis curves before and after R TA...................................................... 57
Figure 4-1 Schematic o f the three-layered waveguide........................................................... 60
Figure 4-2 (a) p and (b) -An resulted from the linear and quadratic EO effect, respectively.
...............................................................................................................................................63
Figure 4-3 (a)P and Ps reversal, (b) p-E loops calculated based on P and Ps reversal.......67
Figure 4-4 Calculated p-E loops based on tunable (solid line) and fixed (dashed line)
p e rm ittivity.......................................................................................................................... 68
Figure 4-5 Calculated fie ld induced birefringence o f the PZT film based on Table 4-1.. 68
Figure 4-6 Schematic illustration o f a c-axis oriented thin film M Z I modulator w ith
lateral electric fie ld applied in arm 2. The polarization o f arm 2 rotates during
propagation w hile that o f arm 1 does not........................................................................ 71
Figure 4-7 Nonperiodic intensity-field output characteristics o f BaTiC >3 M Z I calculated
(4-29), (4-30), and (4-32) fo r Case 1................................................................................ 74
Figure 4-8 First driving electric fields fo r BaTiC >3 M Z I under different configurations .. 78
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Figure 4-9 Norm alized d riving powers per unit bandwidth fo r B a T i0 3 M Z I under
different configurations...................................................................................................... 78
Figure 4-10 Norm alized driving powers per unit bandwidth fo r B a T i0 3 M Z I under
different configurations...................................................................................................... 79
Figure 4-11 M odulation depth o f B a T i0 3 M Z I (Case I) as a function o f the ratio o f the
electrode length o f the m odulator to the optical wavelength........................................ 81
Figure 4-12 Electrooptic coefficient o f P L Z T thin film calculated versus E fie ld based on
parameters in Table 4-2...................................................................................................... 84
Figure 4-13 Field induced birefringence in P LZT thin film s when the electric fie ld (a)
normal to the optic axis and (b) along the optic axis......................................................85
Figure 4-14 Intensity outputs o f P LZT M Z M s w ith tunable EO coefficients when the
electric fie ld (a) normal to the optic axis and (b) along the optic axis........................ 87
Figure 5-1 X -ray diffraction scans o f the B a T i0 3/M gO /G aAs structure............................. 92
Figure 5-2 Polarization versus applied voltage fo r a m etal/B aTi03/M gO /G aAs capacitor
under IV , 2V, and 3V bias range.......................................................................................94
Figure 5-3 Capacitance-voltage characteristics o f a m etal/B aTi03/M gO /GaAs capacitor
demonstrating hysteresis.................................................................................................... 94
Figure 5-4 T hin film cracking o f observed -5000 nm thick B a T i0 3 on MgO/GaAs
substrates.............................................................................................................................. 95
Figure 5-5 Schematic illustration o f patterned GaAs substrates used in this w ork............ 96
Figure 5-6 (a) SEM image o f 80 nm th ick M gO film on GaAs annealed at 700 oC (b)
image o f 20 nm th ick M gO film on GaAs annealed at 600 oC .................................... 98
Figure 5-7 SEM images o f 0.55 pm B a T i0 3 thin film on MgO/GaAs (etch depth-0.5 pm)
showing (a) cracking in arbitrary directions on wide stripes and (b) predominant
cracking perpendicular to patterning on a narrow stripe............................................... 99
Figure 5-8 (a) Top view and (b) cross section SEM o f waveguides w ith inclusion o f
A lx O y layers......................................................................................................................100
Figure 5-9 (a) F ilm stress distribution on fo r ridges o f varying w idth and film thickness=
0.4pm w ith growth temperature o f 600 °C. (b) M axim um film stress versus ridge
width fo r different film thicknesses................................................................................103
Figure 5-10 (a) C ritical stresses as function o f B a T i0 3 thicknesses, (b) C ritical widths as
a function o f B a T i0 3 film thickness w ith 1 pm thick A lx O y fo r T =7 J/cm2 at the
deposition temperature 600 °C ........................................................................................104
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Figure 5-11 Optical microscopic photographs o f 4 samples under different growth
conditions. A ll are shown in the same m agnification.................................................. 107
Figure 5-12 X R D results o f four samples under different deposition conditions
108
Figure 5-13 Loss measurement setup based on prism coupling. (Courtesy o f M etricon
C o rp .)................................................................................................................................. 109
Figure 5-14 Loss measurement results fo r wavelength= 1553 nm lig h t o f four samples
under different deposition conditions. (Loss (dB/cm )=10xlogio(e'm3) ) .................... 110
Figure 5-15 A F M images o f four samples deposited under different deposition conditions.
..............................................................................................................................................I l l
Figure 6-1 B lock diagram o f optical transmitter system.............................................
114
Figure 6-2 Schematic drawing o f integrated optoelectronic system w ith the ferroelectric
thin film M Z I m odulator..................................................................................................114
Figure 6-3 Ferroelectric thin film waveguides form ed by (a) wet etching and (b) dry
etching o f the ferroelectric thin film s ............................................................................. 116
Figure 6-4 BaTiOs waveguide facets formed by (a) direct cleaving (b) diamond polishing
(c) focused ion beam etching........................................................................................... 117
Figure 6-5 Edge em itting laser output characteristics o f lasers w ith cleaved facet and
FIBed facet. [Courtesy o f Jun Yang].............................................................................. 118
Figure 6-6 (a) Schematic o f BaTiOs ridge waveguide structure, (b) simulation o f the
electric fie ld o f TE00 mode in the waveguide based on the effective index method.
120
Figure 6-7 X -ray diffraction scan o f B a T i0 3/M gO thin film onGaAs (001) demonstrating
crystalline material w ith preferred orientation.............................................................. 121
Figure 6-8 Output from a 10 pm wide B aTi 03 ridge waveguides (a) captured by a digital
camera and (b) intensity plot constructed from the digital image.............................. 122
Figure 6-9 (a) schematic o f waveguide cross section and (b) SEM o f the waveguide facets
formed by focused ion beam, (c) waveguide sim ulation based on the effective index
method................................................................................................................................ 124
Figure 6-10 (a) schematic o f waveguide testing setup and (b) CCD images o f the
waveguide output.............................................................................................................. 125
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List of Tables
Table 2-1 Benefits and drawbacks fo r different deposition methods................................... 15
Table 2-2
Lattice constants and CTEs o f b u lk BaTiC >3 and M gO. (CTE o f Pt=9xl0~6) 26
Table 3-1 Figure o f merits fo r voltage tunable capacitors..................................................... 35
Table 3-2
Comparison o f the figure o f m erit o f BST thin film s deposited by P L D ........58
Table 4-1
Parameters o f PZT film used fo r sim ulation....................................................... 66
Table 4-2 Parameters used in calculation. D efinitions o f symbols can be found in [100].
............................................................................................................................................... 84
Table 5-1. Parameters o f the oxide and semiconductor materials in b u lk form studied in
this w o rk...............................................................................................................................91
Table 5-2. Deposition parameters o f BST film s on 4 silicon substrates............................ 106
Table 5-3. Composition ratios o f BST film s on 4 silicon substrates.................................. 106
Table 5-4. Refractive indices (TE and T M ) and loss measured by the prism coupling
technique............................................................................................................................ 106
xiv
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Abstract
The goal o f this research is to im plem ent integrated ferroelectric thin film devices for
microwave and photonics applications, and to investigate the relation between ferroelectric
material parameters and device performance.
Pulsed laser deposition (PLD) was used to deposit barium strontium titanate thin film s
and parallel-plate capacitors were fabricated by standard m icrofabrication techniques. Thin
film deposition conditions, the effects o f annealing, and material properties were
investigated and correlated. Representative values obtained fo r the relative dielectric
constant in bulk film (e^) and and J/e, were 867.3 and 0.174 nm, respectively.
BST thin
film s after annealing at 300°C in N 2 exhibit loss tangent o f 0.02 at 4 GHz and voltage
tunability o f 2.2:1. The figure o f m erit ( K) is 27 at 4 GHz and 44 at 10 GHz.
Modulated optical sources are important fo r photonic circuits. The second focus of
this dissertation seeks to integrate ferroelectric optical waveguides on semiconductor
(GaAs or Si) substrates. Structural properties, electrical properties and optical properties o f
(Ba,Sr)Ti03 thin film s on Si02/Si substrates are investigated. For different deposition
conditions, the rms roughness ranges from ~3 to ~ 10 nm fo r -4 0 0 nm thick film s based on
atomic force microscopy. The scattering loss ranges from - 5 to -2 0 dB/cm. Microstructure
w ith lateral size o f - 1 pm on thin film s were suggested as the main sources o f the
scattering loss. Single-transverse-mode strip loaded ferroelectric BST waveguides on
xv
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SiCVSi were fabricated fo r 1550 nm wavelength. T hin film optical waveguides were also
demonstrated on GaAs u tilizin g A lxOy optical cladding layers obtained by the wet
oxidation
of
AlGaAs.
G rowth
on
patterned
substrates
was
utilized
fo r
B aTi0 3 /M g 0 /A lx0 y/GaAs optical waveguides to reduce thin film stress, where significant
reduction in stress induced cracking was observed fo r actual dimensions o f ridge widths
less than 6 pm. In addition to the development o f ferroelectric thin film device fabrication,
a model is presented to explain the hysteretic electrooptic response o f ferroelectric thin
film s. This model is im portant fo r electrooptic characterization o f ferroelectric thin film s,
avoiding misclassification o f the electrooptic effects in the materials. A simple, yet
accurate model is also proposed fo r extracting the electrooptic coefficient fo r varying
ferroelectric thin film crystalline orientation and applied electric fie ld direction in
electrooptic Mach-Zehnder interferometric modulators.
xvi
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Chapter 1
Introduction
Over the last several years, the bandwidth demand in communications experienced
tremendous growth. M icrowave/RF and lightwave can provide large bandwidths because
their high carrier frequencies. E lectrically tunable devices such as tunable filters and
electro-optic switches are desirable fo r signal processing at these frequencies. Ferroelectric
materials offer a wide range o f opportunities (tunable filters or matching networks) in
microwave/RF communication because o f their tunable dielectric constants. On the other
hand, the strong electro-optic effect o f ferroelectrics is currently utilized in the
state-of-the-art optical modulators in telecommunications. It is desirable to integrate thin
film ferroelectrics w ith other devices because o f low cost, compactness, and improved
performance such as high speed or low voltage. Due to the success of semiconductor
electronics and optoelectronics in wireless and optical communications, the advantage o f
integrating ferroelectric thin film s w ith semiconductor integrated circuits is apprarent.
However, ferroelectric thin film s grown on semiconductor substrates are generally o f low
material quality and have not demonstrated the desired ferroelectric properties because of
chemical and structural incompatibilities. Except fo r semiconductors, low microwave loss
substrates such as sapphire are also used in this work.
1
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This dissertation is focused on the study o f ferroelectric thin film s for tunable capacitor
and optical waveguide applications. The voltage tunable pe rm ittivity associated w ith
ferroelectric materials is attractive fo r microwave and RF tunable applications such as
microwave tunable filters, phased array antennas, and matching networks. W e w ill discuss
the pulsed-laser deposited thin film properties and their impact on the microwave
characteristics. Ferroelectric materials also exhibit large electrooptic effect which is the
underlying principle fo r optical modulators in optical communications and may find
application in optical interconnects fo r semiconductors chips. T hin film s fabricated by
pulsed laser deposition and their electrical and optical characterization are described. We
also present a model to explain the hysteretic behavior o f electrooptic coefficient- electric
field loop. Analysis and design optim ization o f electrooptic Mach-Zehnder modulator are
also investigated.
1.1
Properties of bulk ferroelectrics
The crystal structures o f materials can be classified into 32 point groups. Am ong them,
21 are noncentrosymmetric. Am ong 21 noncentrosymmetric materials, 20 are piezoelectric
except one crystal class 432 which is not piezoelectric because o f other combined
symmetry elements. From these piezoelectrics, 10 have the spontaneous polarization (Ps,
the magnitude o f polarization w ith in a single domain ferroelectrics in the absence o f an
external electric fie ld E).
Spontaneous polarization differs from the paraelectric
polarization (electronic, ionic, and orientation polarization) because it exists in the absence
o f an applied stress and are referred to as pyroelectrics [1], The spontaneous polarization
can be modulated by the application o f an electric field. Ferroelectric materials are a
subgroup o f pyroelectric materials.
2
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Ferroelectrics exhibit ferroelectricity below the Curie temperature. The physical origin
is that some atoms in noncentrosymmetric crystal structure can be displaced by an electric
field. I f this field is larger than the breakdown field, the material is only pyroelectric and
not ferroelectric [2], When the electric field is removed, an electrical polarization still
remains, called the remnant polarization ( P r). To eliminate the remnant polarization
remaining after removing the electric field, an external opposite electric field (coercive
field strength, E c) is required. Thus, a polarization-field hysteresis loop exists in a
ferroelectric crystal as shown in Figure 1-1. Using the first order approximation in the
linear region, the polarization can be written as P —Ps+(e,-l) eoE where c, and eo. are relative
perm ittivity and vacuum perm ittivity, respectively. When the applied electric field is
sufficient to saturate the polarization (usually 3 E c), the polarization remaining after
removing the field is called saturation remanent polarization
ferroelectrics, P r
=
P r.
In single-domain
P s. For practical use, the E c o f the ferroelectrics should be smaller than
its dielectric breakdown field.
Depending on the magnitude o f the coercive field,
ferroelectrics can be classified into hard ferroelectrics (large E c) and soft ferroelectrics
(small E c).
Ferroelectrics do not contain iron; they are so named because their sim ilarity to
ferromagnetics w hich contain iron. Sim ilar to ferromagnetics, a P - E (electric polarization
-electric field) hysteresis loop exists in ferroelectrics [3]. However, the physics behind
them are very different. Spontaneous magnetic polarization originates from unpaired
electron spins w hile spontaneous electric polarization is due to the noncentrosymmetric
crystal structure.
3
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In 1921, J. Valasek first discovered that Rochelle salt (NaKC 4H 406 -4 H 2 0 ) had
ferroelectricity. Other materials are discovered to have this property subsequently. There
are four types o f ferroelectrics: oxygen octahedra, compounds containing hydrogen
bonded radicals, organic polymers and ceramic polymer composites. In our study, we
60
40
CM
|
20
o
CC
N
is
0
01
-20
-40
-60
-20 -15 -10
-5
0
5
10
15
20
Electric Voltage (V)
Figure 1-1 Hysteresis loop in a ferroelectric crystal.
choose oxygen octahedra ferroelectrics due to their technological maturity. Oxygen
octahedra ferroelectrics can be divided two subgroups: normal and relaxor ferroelectrics.
Since the dielectric constant o f normal ferroelectrics exhibit weak frequency dependence,
we choose them for our study. M any o f normal ferroelectrics are o f the perovskite
structure and can be chemically expressed as A B O 3, such as (Ba,Sr)Ti03 (BST) and
4
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Pb(Zr,Ti )03 (PZT). The structure can be viewed as BO3 surrounded by A cations.
Ferroelectricity occurs in perovskites due to polarization catastrophe: i f an ion is displaced
from equilibrium slightly, the force from the local electric fie ld caused by the ionic
displacement in the crystal increases faster than the elastic restoring force [4], This leads to
the shift (-0.15 angstrom [5]) o f B cations relative to A cations (tetragonal phase,
non-centrosymmetric) as illustrated in Figure 1-2, thus creating a permanent dipole
moment. When the temperature is above Curie point ( Tc), the perovskite is in its cubic
(nonpolar) phase and becomes paraelectric. The dielectric p e rm ittivity above the Curie
point exhibits a Curie-Weiss behavior
where e0 and T0 are perm ittivity at 7=0 and the Curie-Weiss temperature, respectively.
O A2+
• B4+
Figure 1-2 Paraelectric phase and ferroelectric phase in perovskite ferroelectrics.
In polycrystalline ferroelectric materials, the whole crystal is actually made o f many
ferroelectric domains w ith random polarization directions as a result o f m inim ization o f
electrostatic and elastic energy [1], A lte r applying the poling field, the direction o f the
domains may be reoriented and some domain states are elimated (Figure 1-3).
5
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(b)
Figure 1-3 Schematic representation of ferroelectric domains (a) before and (b) after poling.
1.2 Dielectric properties of ferroelectrics
The physical origin o f the ferroelectric field-dependent p e rm ittivity can be understood
as follows: Ferroelectrics undergo a transition from the unpolarized state to the polarized
state when the temperature drops below the Curie temperature. It only requires very little
applied field to change the electrical polarization, indicating a very large dielectric
constant around the Curie temperature. On the other hand, the dielectric behavior follows
the Lyddane-Sachs-Teller (LST) relation according to the lattice dynamical theory:
_ g(°°)
col
(1-2)
£(0)
where cot,, ©l, e(00), and e(0) are transverse optical phonon frequency, longitudinal phonon
frequency, high frequency (f= o o ) perm ittivity, and static frequency perm ittivity. In the
absence of the electric field, the phonon mode softens and cut is very small, making the s(0)
very large. The static high perm ittivity in ferroelectrics is a result o f the existence the soft
phonon mode (a transverse optical phonon vibrational frequency approaches zero) [4]
6
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when the temperature is near the Curie temperature. Application o f electric field hardens
the soft phonon mode and results in the decrease o f the perm ittivity. This is the cause o f the
voltage tunability o f the perm ittivity in ferroelectrics and the electric field dependence o f
can be described by Landau theory [6]. In optical frequency, the dielectric constant
tunability can result in change o f the refractive index since the complex refractive index
and relative dielectric constant follow s this relation.
N = yfe = n + ik
^ ^
where n is the refractive index and k is the extinction coefficient, and related by the
Kram ers-Kronig relation. It shall be noted the dielectric constants o f ferroelectrics decrease
w ith increasing frequency enormously because the dipolar polarization and ionic
polarization can not fo llo w the optical frequency field, called “ dielectric dispersion” [7],
The a b ility to change the refractive index by the applied electric field is called the
electrooptic effect. From the physical point o f view, the electric fie ld o f the optical wave is
modified by the local dipole fields o f the nearby atoms. I f the dipole field reduces the
driving field and the dipole moment, the refractive index w ill be reduced.
1.3 Properties of ferroelectric thin films
Before discussing the properties o f ferroelectric thin film s, we first review that there
three forms o f atomic arrangement: single crystal, polycrystalline and amorphous. Single
crystal materials have atoms in an ordered 3D array. In amorphous materials, there is no
long-range order in the positioning o f the atoms though various short range order can exist.
Polycrystalline materials comprise many crystalline grains, not aligned in the same
7
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direction. In oriented polycrystalline materials, there is one preferred orientation inside
those grains.
It is desirable to have ferroelectrics in the form o f thin film s microscale devices and
low voltage operation [8], Before the 1980s, ferroelectric materials were used in bulk form
because o f the in ability to produce high quality thin film s. Later, various methods have
been developed to fabricate ferroelectric thin film s as discussed in [9]. The availability o f
ferroelectric thin film deposition techniques enables integration w ith semiconductor device
technologies. Although exceptions may exist [10], the properties o f thin film s are usually
inferior those o f bulk ferroelectrics. For example, the magnitudes o f spontaneous
polarization and the perm ittivity are usually low er due to the “ dead layer” (nonideal layer)
between the thin film and the substrate. The dead layer is usually amorphous and has no
spontaneous polarization. The reduced dielectric constant that is typically observed in
ferroelectric thin film s is also a consequence o f the hardening o f lowest soft phonon mode
[ 11].
In this study, BST and BaTiC >3 are the major materials because o f their chemical
stability. The major disadvantages o f Pb(Zr,Ti)C >3 and SrBiaTaaOg are the v o la tility o f lead
and bismuth
constituents
and their complicated
introduction
into
semiconductor
fabrication facilities! 10], Their properties o f BST and BaTiCL in bulk and thin film form
are summarized here:
•
A t -193 K, bulk BTO changes from rhombohedral to orthorhombic structure; at -280
K, it changes from orthorhombic to tetragonal structure; at -395 K (Curie temperature
o f BT), it changes from tetragonal to cubic structure; it also loses its ferroelectricity and
8
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becomes paraelectric.
•
The Curie temperature o f b u lk BTO crystal is -3 9 5 K and that o f bulk STO crystal is
~30K. On the other hand, thin film BaTi03 and BST may not have a w ell defined Curie
temperature as bulk materials [12]. For a bulk BST to be ferroelectric at room
temperature, Ba content x should be larger than 0.7. However, thin film BST could be
ferroelectric at room temperature when Ba content x>0.44 and its ferroelectricity is
believed to be induced by a two-dimension compressive stress [13]. BST at its Curie
temperature has a higher p e rm ittivity and dielectric tunability than what BTO has [14]
, and thus BST is useful in many applications.
•
The P-E hysteresis for BST (Ba:Sr=75:25) thin film was not observed below the Curie
point o f corresponding bulk BST. A thorough understanding o f properties o f BST film
is still lacking [15].
W hile polycrystalline oxide film s
may have sufficient properties for some
applications such as microwave tunable capacitors and memory storage capacitors, the
superior properties o f highly crystalline epitaxial film s attractive fo r ferroelectric field
effect transistors because defects at the interface seriously degrade device performance
[16, 17]. For electrooptic applications, epitaxial growth is also preferred because the
improved surface roughness reduces optical scattering [18].
1.4 Overview of device applications
Ferroelectric-thin-film-based technology has made significant progress in various
research fields and the performance continue to improve. The unique properties o f
ferroelectrics
include
pyroelectricity,
piezoelectricity,
ferroelectricity
9
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and
high
perm ittivity. Ferroelectric materials find applications in many areas thanks to these unique
properties.
In microwave applications, tunable capacitors and phase shifters can be obtained by
tuning the p e rm ittivity o f ferroelectric materials and they are important in making the
tunable filters and phased array antennas. In electrooptic (EO) applications, the large
spontaneous polarization o f ferroelectrics is the cause fo r the large linear electrooptic
coefficient [19]. Ferroelectric optical waveguide devices can be used in EO modulators in
telecommunications and EO tunable filters fo r wavelength-division-m ultiplexing systems
[20]. In dynamic random access memories (D R A M s), the feature of high dielectric
perm ittivity is utilized to fabricate high perm ittivity capacitors to reduce the area or keep a
reasonable
thickness
to
reduce
tunneling
problem
through
the
capacitor.
In
micro-electro-mechanical system (M E M S ) and wireless system, microactuators and
surface acoustic wave (SAW ) and film bulk acoustic resonators (FBARs) devices can be
made o f the ferroelectric material using its piezoelectricity [21]. In infrared detectors, the
pyroelectric property is utilized [22], In particular, we focus on are the ferroelectric thin
film microwave tunable capacitors and electrooptic waveguides for integrated optics.
Currently, the semiconductor industry is interested in ferroelectrics due to the
potential applications in D R A M and ferroelectric random access memories (FeRAMs). For
storage capacitors in Gbit density D R A M , high perm ittivity dielectrics are necessary to
meet the demand o f a m inim um value o f capacitance (-2 5 fF per cell.) The (Ba,Sr)TiC >3
(BST) system is the most promising material as alternative dielectrics due to its high
dielectric constant, lead-free nature, low dielectric loss and lack o f fatigue problem.
10
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In FeRAM s, ferroelectricity (spontaneous and reorientable polarization) is used as a
memory state [23]. Compared to current nonvolatile memories, FeR AM consumes less
power and less write-access time w ith its polarization hysteresis and low switching
voltages [2]. The FeRAMs can be classified into two types: 1 transistor/1 capacitor, metal
ferroelectric semiconductor field effect transistor (MFSFET), metal ferroelectric metal
insulator semiconductor FET (M FM IS FE T). The first one has destructive read-out w hile
the other two have non-destructive read-out and smaller cell size (higher density). They are
nondestructive because the sign o f polarization direction can be determined by reading the
conductance o f semiconductor channel [24], Research on F R A M has been extensively
done on the basis o f the Pb(ZrTi )03 (PZT) system and the Srl^TaaO g (SBT) system. The
reason why BST is not appropriate for nonvolatile memory applications is that the
asymmetric switching o f its polarization and its low Curie temperature despite its chemical
stability [13].
1.5 Focus of this study
Despite o f many unique properties, there are several challenges:
1. The microstructure, grain boundaries and stochiometry are d iffic u lt to control but
they affect the electrical and/or optical properties.
2. M icrofabrication technology is not mature such as etching.
3. It is d iffic u lt to integrate ferroelectric thin film s w ith semiconductor circuits because
their growth usually require very high temperature. In this dissertation, we w ill focus our
research in the follow ing two fields:
11
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1.5.1 BST tunable capacitors for microwave applications
BST tunable capacitors can offer benefits such as compactness, high tunability at room
temperature, high power handling capability, continuous tuning range, and easy packaging
at the same time [25], However, high tunability and sufficient low microwave loss is
challenging to achieve simultaneously. In our study, the goal is to find the lin k between the
material properties and microwave characteristics. We use pulsed laser deposition to grow
BST film s. X -ray diffraction and Rutherford backscattering spectroscopy have been used
for thin film characterization. A fte r device fabrication, lo w frequency (1 M H z) C -V
measurements and two port microwave measurement are conducted for electrical
properties. Rapid thermal annealing is proposed as a way to reduce the dielectric dispersion
and microwave loss.
1.5.2 Integration of ferroelectric optical waveguides with
semiconductors
W ith the scaling o f semiconductor devices, denser integrated circuits are fabricated.
However, the electrical interconnects do not adequately scale in size due to RC lim its o f
electrical wires and electromagnetic interference. Optical interconnects have been
proposed as a solution for interconnect scaling. We w ill explore the possibility o f
integrating ferroelectric optical waveguides w ith semiconductors as the enabling
technology for optical interconnects.
In addition, external optical modulation in telecommunication can avoid the chirping
problem, relaxation oscillation and mode-hopping and the d iffic u lty o f maintaining single
mode in direct modulation o f lasers [26], Currently electroaborption modulators are
common
fo r
semiconductor
optoelectronics.
Electroabsorption
modulators
12
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have
difficulties including large insertion loss, unstable chirp versus drive voltage, and high
cost. Compared to electroabsorption modulators (E A M s), conventional bulk LiNbCb
electrooptic Mach-Zehnder interferometric modulators (EO M ZIs) have the benefits o f
direct phase control, lower cost, lower chirping and higher extinction ratio. Nevertheless,
bulk EO M ZIs
are typically
long (several centimeters compared to hundreds o f
micrometers o f E A M ) to reduce the driving voltage. Integrating thin film EO M ZIs w ith
semiconductor lasers and photodetectors could reduce the driving voltage and may realize
high performace optoelectronic integrated circuits (OEIC). Nertheless, it is d iffic u lt to
integrate crystal L iN b 03 modulators w ith semiconductor lasers due to the vo la tility o f L i
ions. On the other hand, oxide thin film epitaxy can provide the possibility o f m onolithic
integration o f ferroelectric EO B aTi03 waveguides w ith semiconductors [27]. Pulsed laser
deposition w ill be used to fabricate thin film waveguides on semiconductors in our study.
13
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Chapter 2
Properties of ferroelectric thin films deposited
by pulsed laser deposition
In this work, pulsed laser deposition is utilized to grow ferroelectric thin film s. The
growth process, deposition parameters, structural and electrical properties o f pulsed-lased
deposited ferroelectric B a T i0 3 thin film s w ill be presented.
2.1 Deposition of ferroelectric thin films
Current methods o f fabricating ferroelectric thin film s include: RF sputtering [28],
sol-gel [29], molecular beam epitaxy (M B E ) [30], metal-organic chemical vapor
deposition (M O C V D ) [31], and pulsed laser deposition (PLD ) [32]. The first two provide
polycrystalline or amorphous thin film s, w h ile the others can provide highly oriented thin
film s. In general, it is desired to grow film at comparatively lo w temperature to be
compatible w ith silicon processing. The m ajor benefits and drawbacks o f these techniques
are listed in Table 2 - 1.
PLD offers a number o f advantages over M B E and M O C V D used for the preparation
o f thin film s o f ferroelectrics; in particular, high quality film s can be deposited at lo w
substrate temperatures and high deposition rates. Since ferroelectric ceramics are
multi-component oxides, it is d iffic u lt to grow stoichiom etric film b y M O C V D and M B E .
14
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Comparatively, PLD has the advantages o f easy preservation o f the stochiometric ratio o f
the target material and relatively lo w cost.
Techniques
PLD
MBE
benefits
Good m ultielem ent control,
versatile, >10 m Torr
reactive gas, lo w substrate
temperature
H ig h ly expitaxial
M OCVD
H ig h ly expitaxial
Reative sputtering
Good m ultielem ent control,
versatile
drawbacks
Small area, particulates
H igh substrate temperature,
high cost
H igh substrate temperature,
high cost
Im purities; d iffic u lt to
m aintain a high sputtering
rate due to reaction between
target and reactive gas
Table 2-1 Benefits and drawbacks for different deposition methods.
The main advantages and disadvantages o f PLD are listed below:
Advantages:
•
Thin film s can have the same stoichiom etry as the target -a result o f the high heating
o
rate o f the target surface (10 K/s) due to pulsed laser irradiation
•
high deposition rate (- 1 0 nm /m in) possible
•
generally o f lo w growth temperature compared to other techniques
•
simple and flexible setup
•
clean process w ithout filaments
•
can obtain semi-epitaxial quality
Disadvantages:
•
small particulates on the surface (maybe from target inhomogeneity)
•
small area, and nonuniform ity across the wafer
•
target surface m odification
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The first disadvantage is an intrinsic problem w hich hampers the applications o f PLD
in device fabrication because it may generate large defects in electronic devices or induce
large scattering loss in optical devices. This certainly reduces the yield in production. Some
methods such as mechanical particle filte r and target polishing are proposed to reduce the
problem [32]. The second disadvantage can be solved by rastering the laser and rotating the
substrate. Despite o f the above drawbacks, pulsed laser deposition (PLD ) is an effective
research tool because o f its versatile deposition capability and the ease o f stoichiometry
control. Moreover, PLD can easily grow hig h ly oriented film s w ith a higher deposition rate
compared to other deposition techniques and its result can be easily scaled up by using RF
sputtering or m olecular beam epitaxy.
2.2 Pulsed laser deposition process
Pulsed laser deposition (PLD) is a versatile technique that has been used in growing
hard and m ulti-com ponent materials in the past decade. A n illustration o f a typical PLD
system is shown in Figure 2-1.
Substrate
SubstrateV Assy
Target /
i
L
Target
Assy
Figure 2-1 (a) Pulsed laser deposition setup.
16
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In contrast to its simple setup, the physical processes and principles are complex and
are still not w e ll understood [15]. PLD is a process in w hich a focused high energy (~1
J/cm ) laser ablates a rotating target to form a fu lly ionized plasma plume, and then
component materials in the plume such as electrons, ions, neutrals, and ionized species are
deposited onto a substrate. The stoichiometry o f the material is typ ica lly preserved during
the interaction. The ambient environment is typ ica lly in vacuum or w ith a reactive
background gas to enhance the deposition rate or to m aintain the stoichiometry.
In Rutherford backscattering spectroscopy (RBS), positively charged He ions o f a
high kinetic energy (typically l-3 M e V ) are directed at the sample. The incident ions are
scattered elastically from the atoms in the sample and the number o f scattered ions and
their energy is measured. RBS provides inform ation on the composition o f the sample, the
distribution o f those components and the thickness o f the sample. Figure 2-2 shows the
RBS spectrum o f BaTiCb film s grown b y PLD, indicating fa irly good composition control.
The major growth mechanism in excimer PLD can be divided into periodic bursts o f highly
driven growth and relatively long periods o f uninterrupted surface relaxation.
The thin
film form ation process in PLD generally can be divided into the fo llo w in g stages.
•
Laser radiation interacts w ith the target.
•
Electromagnetic energy is converted to electronic excitation and then to thermal,
chemical and mechanical energies and causes ablation and evaporation processes.
•
Formation o f the Knudsen layer and the absorption by the plasma: When strong
evaporation occurs, the gas near the phase interface is not in translational equilibrium
and the translational equilibrium is achieved w ith in a few mean free paths by collisions
between particles in a thin region. This region is called Knudsen layer.
17
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•
Plasma expansion and deposition o f the ablation materials w ith the substrate
•
Nucleation and grow th o f a thin film on the substrate surface
3500
3000
2500
Measured
Simulated
| 2000
Q1500
Ba
1000
500
0
100 200 300 400 500 600 700
Channel
Figure 2-2 RBS spectrum o f BST grown on Pt/Ti/Sapphire (Sample 273) by pulsed laser
deposition. The composition is Bao.57Sro.5Tio.98O2.177.
The plasma processes include plasma production, plasma plume interaction w ith the
laser and plasma expansion. The laser-induced plasma plume characteristic is susceptible
to the laser parameters or the target materials. Thin film growth b y PLD is a process that
depends on many factors, such as particle density, ionization degree, and type o f
condensing particles, as w ell as the temperatures. A difference in plume profiles or
densities also results in a difference in film quality. It has been shown that the plasma
characteristic would affect the optical properties o f thin film s like B aTi 03 [33]. The exact
18
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model for the plume does not exist since the physics is not w e ll understood. A thermal
model and a hydrodynam ic model are usually utilized to analyze the plasma dynamics.
2.3 Parameters in PLD process and their effects
2.3.1 Laser parameters
Laser parameters are im portant to induce different ablation mechanisms. We discuss
them in terms o f wavelength, laser fluence and pulse duration. The prim ary factors in the
PLD process that influence thin film properties are described in the follow ing.
Laser wavelength: L ig h t in the infrared range w ould excite electrons w ith in the conduction
band (intraband transitions) and vibrations. On the other hand, a U V laser can produce
single-photon or m ulti-photon interband transitions [34], U V lasers can m inim ize the
particulate density since the U V lig h t ablates in large part b y photon sputtering instead o f
thermalization.
Laser fluence or electric field amplitude: The laser fluence effect can be divided into two
regimes: (a) A t low laser fluence, the vapor produced by the leading edge o f the laser pulse
behaves like a thin and transparent medium, (b) A t high laser fluence, the vapor temperature
is high enough to cause appreciable atomic excitation and ionization and the vapor can be
described as a plasma. Then the vapor begins to absorb the incident laser radiation leading to
vapor breakdown and plasma form ation [35], The electric field o f a electromagnetic wave is
given by [36]
E =
f 20) ^
1/2
(2- 1)
\ c n s aJ
where 0> is the power density and s {) is the vacuum p erm ittivity. The electric fie ld is usually
larger than the dielectric breakdown field o f the ablated materials. The laser fluence w ill
19
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affect the stoichiometry o f the thin film . For example, the lower laser fluence gives the higher
concentration o f Pb during deposition because Pb is volatile and can be evaporated from the
target even the ablation is below the nonthermal threshold [37],
Pulse Duration: It is interesting to note that the C W laser deposition does not produce
energetic plasma. In contrast, pulsed laser can produce energetic species and m obile adatoms.
I f the laser pulse is short enough (fs or ns), laser interaction w ith the desorbed plasma can be
neglected [38]. The short pulse duration is required fo r spatially well-defined and chem ically
stoichiom etric ablation w ith lo w damage o f the surrounding material. Due to the short laser
pulsed duration (- 1 0 ns) and the small temporal spread (<=10 ms) o f the ablated species, the
deposition rate is very high and a layer-by-layer nucleation is favored.
Currently, K rF excimer lasers have been used w idely for PLD processing, although other
laser sources such as A rF or Ti:Sapphire lasers are also used. For thin film deposition, we w ill
focus on K rF laser w hich is an almost ideal source. This is associated to its high photon
energy (wavelength: 248 nm), its short pulse length (typically 10-40 ns), and its relatively
poor coherence (an excimer laser’ s output is hig h ly m ultim ode) [34], The high photon energy
allows fo r direct photo-dissociation o f materials and short optical penetration depth in many
solids. The short pulse duration is necessary fo r spatially well-defined and chem ically
stoichiometric ablation w ith negligible damage o f the surrounding regions, especially for
heat sensitive substrates. The poor spatial coherence diminishes interference effects. Am ong
the disadvantages o f excimer lasers are their lo w efficiency, their relatively lo w repetition
rates, the poor pulse-to-pulse stability, and the relatively high operating costs.
Pulse repetition rate: The repetition rate o f the pulsed laser can affect the kinetic energy o f
the ejected species, and thus the particulate density. The trend is that a higher repetition rate
20
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could generate higher particulate density [38].
2.3.2 Substrate temperature
The substrate temperature determines the adatom’ s m o b ility and affects the
crystalline quality directly. Pulsed laser deposition system can grow amorphous,
polycrystalline and h ig h ly oriented film depending on the substrate temperature. Generally,
high temperature favors defect free growth and lo w temperature favors polycrystalline or
amorphous growth due to the energetic particle impingement. In PLD, the energetic
ablation species helps to raise the substrate surface temperature. Consequently, P LD tends
to demand a lower substrate temperature for crystalline film growth compared to other
techniques. Another im portant effect o f the substrate temperature is related to the thermal
expansion mismatch between the film and the substrate. I f the thermal expansion mismatch
is too large, cracks and/or peeling could happen during the cool down after the PLD
process.
2.3.3 Oxygen partial pressure
Since the laser is decoupled from the chamber, the interaction between laser and
target is very weak and the oxygen pressure can range from 1 atm to very high vacuum.
Oxygen partial pressure is a very complex parameter. It can affect the crystalline structure,
lattice parameter, surface roughness, oxygen deficiency, or the phase o f the crystal
(orthogonal or tetragonal). I f the oxygen pressure is too low, the film would have oxygen
vacancies. I f it is too high, the surface would be very rough [39],
Although PLD can often achieve congruent evaporation, some volatile elements (Pb,
Bi, K , or L i) may be lost during the travel from the target to the substrate [32], Oxygen
fille d environment is to ensure m inim um oxygen deficiency and volatile element
21
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deficiency [40], For example, high oxygen pressure reduces Pb deficiency during PZT
deposition because PbO has a lower vapor pressure (higher b o ilin g point or less volatile)
than Pb does [40].
On the other hand, the collision between plume particles and the ambient oxygen w ill
increase i f the oxygen pressure is high and the angular distribution o f the plume w ill be
broadened [32] as manifested in Figure 2-3 and the deposition rate is reduced. Typically,
the distribution is expressed by cosn0 where 0 is the angle from the surface normal o f the
target and n is a function o f the oxygen pressure [36].
(a)
22
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(a)
Figure 2-3 Plasma plume o f BST during the pulsed laser deposition under (a) 6 mTorr and (b) 30
mTorr.
2.3.4 Substrate-target distance
The target-substrate distance (D) can affect the stoichiometry o f the film and the film
uniform ity. Its effect on film properties is closely related to the oxygen pressure (P ). A
scaling law o f PD n= constant (n~ 2-3) is obtained for optimized conditions [41], [42], A n
increase o f the substrate-target distance w ould im prove the film unifo rm ity at the cost o f
reducing film deposition rate [43], Also, lost volatile elements increase during the traveling
from the target to the substrate i f the distance increases
2.3.5 Post-deposition annealing
The post-annealing after PLD is often conducted to reduce the oxygen vacancies and
reduce the porosity o f oxide thin film s [32].
2.4 Crystalline properties of PLD thin films
The crystal structures o f the thin film are greatly affected b y the substrate/film lattice
mismatch, the coefficient o f thermal expansion (CTE) mismatch, the film thickness and the
23
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growth condition. PLD is versatile in making thin film s w ith different grades o f crystalline
structures. Epitaxial grade film s usually are grown at high temperature and the lattice
constant mismatch between the film and the crystalline substrate is small. The growth
condition for polycrystalline materials is less stringent. The substrate does not need to be
crystalline and the growth temperature can be low er than what is required for epitaxy.
Amorphous film s can be grown at room temperature as shown in Figure 2-4 (a) as opposed
to Figure 2-4 (b) in w hich M gO was grown at 350 °C and BaTiC >3 grown at 700 °C.
The crystalline quality o f BaTiCh thin film s grown by P LD can depend on the substrate
temperature and the substrate used. BaTiC f deposition on M gO crystal was carried out
under an oxygen pressure o f 10 m T orr at a substrate temperature o f 700 °C using KrF laser
'y
(fluence: 5 J/cm ) and the resulting plume is shown in Figure 2-5. Since B a T i 03 has a
lattice constant and a coefficient o f thermal expansion sim ilar to those o f M gO (Table 2-2),
B aTi 03 thin film is very h ighly oriented on M gO substrates as can be seen in Figure 2-6
(a). However, B a T i 03 thin film grown on polycrystalline Pt/Si substrate w ith 10m Torr
oxygen pressure and 600 °C is polycrystalline (Figure 2-6 (b)) because no crystalline
registration exists in this structure. It should be noted that 700 °C is too high for Pt-Si
because o f the thermal expansion mismatch between Pt and Si. The crystallinity could also
depend on the growth temperature as w ill be discussed in the next chapter.
24
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15
25
35
45
55
65
75
26 (Degrees)
15
,
,
25
35
(a)
!
1
45
55
---------1
65
75
20(degrees)
(b)
Figure 2-4 XRD o f (a) MgO/ BaTi03 grown at 30 °C and (b) MgO grown at 350 °C and BaTi03
grown at 700 °C.
25
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2-5 Plume of BaTi03 during the deposition.
Lattice constants at room
temperature (nm)
CTE at 700 K
MgO(cubic)
0.421
14xl0"6
B aT i 0 3(tetragonal)
a=b=0.3992
c=0.4032
12.1xl0'6
Table 2-2 Lattice constants and CTEs o f bulk B aT i03 and MgO. (CTE o f Pt=9xl0"6)
26
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B a T i0 3 on MgO
w1
-t—
"c
1000
.d
-4—'
</)
c
(D
10
20
30
40
50
60
70
80
90
20(degrees)
(a)
BaTiO on Pt-Si
< 1000
TO Q-
W 100
10
20
30
40
50
60
70
80
90
20 (Degrees)
(b)
Figure 2-6 X-ray diffraction of B aT i03grown on (a) MgO and (b) Pt/Si substrates by pulsed laser
deposition.
27
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2.5 Thin film properties of PLD thin films
The thickness can be measured by a variety o f methods. One convenient way to
measure m ultilayer thicknesses at a tim e is optical spectral reflectance method. The
method examines the interference o f reflected lig h t from the layer using white light which
is incident norm ally on the sample. The interference pattern whichs depend on the
thickness, optical constants and surface roughness is measured b y the detector. Then, the
computer softwave finds the best fit o f thicknesses for the measured interference pattern.
For example, a sample w ith BaTiOs thin film on top o f Si02/Si sample is measured using a
commercial equipment Film etrics F-20 and the result is shown in Figure 2-7. The surface
m orphology can be examined by atomic force m icroscopy (A F M ) and scanning electron
microscopy (SEM ) as shown in Figure 2-8 (a) and (b). The surface roughness is usually
good (RMS Roughness < 10 nm). However, micron-sized particulates are usually
embedded inside the film as shown in the optical microscopic photograph, Figure 2-8 (c).
SarfielD. -
Operstor: [
FILMETRICS'
Cursor IrtnveMnglh (nm* B7B.92 ‘ V:f.0138
631.33 nm
j: M e a su re j
R.&natyia
SJrueture:jBT0Sio2Si
Er* Structure
DtSfaey; |R«uK« Summery
BaTl03=631.33lun
[8102=1990 0 nm
[Goodness of 11= 0.85626
Measurement fr|8
Show Itble ! Sww Stststics i
Figure 2-7 Thickness measurement result using Filmetrics F-20. The layer structure o f the tested
sample is BaTiCh/SiCVSi. The dark line indicates the measured result, and the light line indicates
the simulated result.
28
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
R ou gh ne vi Audi;
Figure 2-8 (a) SEM (b)AFM (c) optical microscopic images o f ferroelectric BST thin films.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.6 Electrical properties of ferroelectric thin films fabricated by
PLD
Electric polarization in ferroeletrics arises from spontaneous polarization in addition to
electronic, ionic, orientational, space charge polarization [5]. The electrically switchable
spontaneous polarization is the origin o f the P -V hysteresis curve. It is generally accepted
that the C -V curve o f a ferroelectric capacitor is often different from the electric field
derivative o f it P -V curve. Typically, the value o f the former is smaller than the latter. The
prim ary reason fo r this behavior is that the P -V hysteresis loop is measured w ith a strong
A C field, larger than the coercive field, w h ile C- V is measured w ith a high frequency A C
field, smaller than the coercive field, superimposed on the D C bias [1]. Electrical
properties o f ferroelectric PZT thin film s (#281, ~330 nm) fabricated b y PLD are shown in
Figure 2-9.
6000 ---- r
(dP/dE)/e
.CXpermittivity
°
-400
-200
0
200
400
Electrical dc field (kV/cm)
(a)
30
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40
E
o
lz
o
I
-20
-40
-400 -200
200
400
Electrical dc field (kV/cm)
(b)
Figure 2-9 (a) Capacitance- electric field and (b) polarization-electric field curves o f PZT films
deposited by PLD.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3
Voltage tunable BST capacitors for microwave
applications
3.1 Overview of voltage tunable devices
M icrowave and RF tunable devices that can tune over a wide frequency range are
important for m icrowave phase shifters, tunable filters and impedance matching networks.
For wireless communications applications, the tunability can offer some fle x ib ility to adapt
to operating conditions, such as frequency or RF drive level changes. M icrowave tunable
devices include GaAs PIN diodes [44], micromachined devices [45], high temperature
superconductors (HTS) [46], ferrite devices [47] and ferroelectric devices [25], Compared
to semiconductor p-i-n phase shifters, the ferroelectric phase shifter has several
advantages, such as higher power handling capability, higher quality factor (low er loss)
and higher tuning speed. In addition, micromachined devices usually require d iffic u lt
biasing and stringent packaging and HTS devices usually require cryogenic operation. In
comparison w ith ferrites, ferroelectric devices are smaller, lighter and consume less power.
Thus, there is a lo t o f research focusing on the frequency agile RF and microwave device
applications o f ferroelectric thin film s, such tunable filters [48], tunable phase shifters [49]
and tunable matching networks in communication applications.
32
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Compared to interdigitated capacitors, the parallel plate capacitor is easy to analyze
and has larger tunability and low er tuning voltage. On the other hand, the interdigitated
capactiors offe r easy fabrication (1 mask process) and requires low er capacitance values
for operation at higher m icrowave frequencies. In our study, the parallel plate structure is
chosen.
Rapid thermal annealing has been used to recrystallize the BST thin film s improve
the dielectric properties at lo w frequency (10 kH z) [50], W e w ill investigate its effect at
microwave frequency. W e also found nitrogen annealing at 300 °C can enhance the total Q
factor below 4 GHz.
3.2 Materials for ferroelectric thin film tunable capacitors
As mentioned in Chapter 1, ferroelectric thin film s exhibit tunable dielectric constants.
The major candidates fo r m icrowave tunable thin film capacitors are KTaC >3 (KTO ),
SrTiC>3 (STO) and BaxSri_xT i 03 (BST). K T O and STO require cryogenic temperature or
high electric field to have reasonable tunability. For example, STO achieves 1.5:1 tunabiliy
when the electric field > 30 kV /cm at 77 K or the electric field > 400 kV /cm at room
temperature [6], The most common material for m icrowave tunable applications is
BaxSri.xT i 03 because its loss is relatively lo w and its tunability is large at room
temperature. The barium concentration affects the tunability and the loss tangent. The
tunability increases w ith the increasing Ba concentration w hile the loss tangent also
increases [12]. To have a reasonably high tunability and lo w loss, the Ba concentration is
chosen to be 0.5 in our study.
Theory o f dielectric losses in ferroelectric thin film s was summarizeded by Tagantsev
[6]. Both the intrinsic (fundamental phonon loss mechanisms) and the extrinsic losses (due
33
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to the defects) o f the thin film need to be considered. The extrinsic microwave losses in
ferroelectric film s may result from the charged defects and grain boundaries. The intrinsic
contribution dominates the loss under an electric field whereas the extrinsic one dominates
w ithout the field.
For microwave tunable capacitors, it is generally believed that ferroelectric film s in
the paraelectric phase can avoid piezoelectric transformation, thus reducing microwave
loss [51]. However, microwave loss from 10-20 GHz is lo w fo r polar materials since the
domain w a ll m otion is frozen and no piezoelectric transformation in this frequency range
[52], It is still debated whether or not the ferroelectric thin film should only be in the
paraelectric phase.
For integrated circuits applications in microwave and m illim eter frequency range,
using low loss substrates is very critical to the circuit and whole system performance. As a
result, we use sapphire substrates because o f their low loss compared w ith the silicon
substrates. The tunability is usually m axim ized near the Curie temperature. However, the
loss tangent is usually larger around the Curie temperature. A figure o f m erit K is defined
as:
K _ e (E * 0) - e (E = 0)
( 3 - 1)
s ( E = 0)tan(£)
where tan(5) is the loss tangent and e(E) is the equivalent dielectric constant o f the thin film
under an applied electric field E. A summary o f the figure o f merits for voltage tunable
capacitor is shown in Table 3-1.
34
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Definition
Comments
Tunability
The ratio o f maximum to minimum
dielectric constant: e(0)/ e(E)
Usually maximized near the Curie
point; depend on crystallinity and
orientation [53]
Q
factor=l/tan(S)
The inverse o f the dielectric loss
tangent
Usually small near the Curie point
Linearity
The behavior o f the capacitor in
which the capacitance varies in
direct proportion to the applied
voltage
Nonlinear tuning curve lim it the
dynamic range [54]
Table 3-1 Figure o f merits for voltage tunable capacitors.
3.3 Thickness dependence of dielectric constant of BST films
The metal-ferroelectric-metal capacitor is the building block for microwave tunable
circuits and current nonvolatile memories. W e w ill discuss the fabrication and its thickness
dependence in this section. For tunable BST capacitors, it is know n that thicker BST film s
give higher zero-fteld pe rm ittivity and higher tunability [55]. To design a capacitor w ith a
designated zero-fteld capacitance, the zero-field perm ittivity-thickness data are required.
In
this
section,
we
study
the
dielectric
properties
o f metal-ferroelectric-metal
(metal/BST/Pt-Si) structures.
BaxSri.xTiC >3 (x=0, 0.5) thin film s were grown on Pt/TiCb/SiCb/Si substrates (Pt 150
nm /TiCb 40 nm/SiCb 500 nm/(100) Si, Radiant Technologies) b y pulsed laser deposition
(PLD). A K rF laser (248 nm, pulse width: 25 ns, fluence: 3 J/cm ) is employed to ablate the
BST target and generate a plasma plume for thin film deposition. The deposition was
carried out under an oxygen pressure o f 20 m Torr at a substrate temperature o f 500 °C. The
film thickness was determined using a shadow mask and Dektek profilometer. Then the top
35
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electrodes (Pt 100 nm) were fabricated using an e-beam evaporator and a shadow mask. Pt
is used due to its high w o rk function to produce a large Schottky barrier height. From the
X -ray diffraction, the BST film is polycrystalline as shown in Figure 3-1 since there is no
crystalline registration.
BaTiO on Pt-Si
3
</)
-i—i
o
o
'c
D
o
o
co
CO
ro<N
_Q
L—
<
o
o
coo
o
1000
-i—i
'w
c
100
0
c
10
20
30
40
50
60
70
80
90
20 (Degrees)
Figure 3-1 X-ray diffraction o f BST film on Pt/Si substrates.
The C -V measurement was done by the Boonton capacitance meter w ith 1 M H z test
signal externally biased b y Keithley source measure unit. The zero field pe rm ittivity and
tunability o f the p e rm ittivity were then extracted from the C -V measurement. As shown in
Figure 3 -l(a ), the zero-bias relative p e rm ittivity o f the BST film s increases as the thickness
increases. This is consistent w ith the “ dead layer” theory, i.e. a low relative dielectric
constant (~10) surface layer existing between the BST film and Pt-Si substrate. The effect
o f the dead layer on reducing the total dielectric constant is stronger for thinner film s [11].
On the other hand, the tunability for different thickness is shown in Figure 3-1 (b). The
36
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tunability o f the relative p e rm ittivity is higher for a thicker film . Due to the strong
thickness dependence o f dielectric properties, it is im portant to include its effect when
comparing samples undergoing different growth or annealing conditions.
The dead layer thickness can be obtained using the “ series capacitor model” [56].
Assume dm, dt, di, em, £&, e, are film thickness, b u lk film thickness, dead layer thickness,
measured dielectric constant, dielectric constant o f the bulk film , and dielectric constant o f
the dead layer, respectively. Based on the equation for the series capacitors o f the bulk film
and dead layer [57], we have
dm
Sm
Thus, there is
y-intercept o f d/Si.
dh , dj
£b
£i
d m - d i , d i .. d m , d i
Sb
£i
£b
(3-2)
£i
a linear relationship between dm/em and dmw ith agradient o f 1kb and
From our zero-bias data in Figure 3-2 and
theaboveequation,we plot
dm/em as a function o f dm as shown in Figure 3-3. B y fittin g the plot, the relative dielectric
constant in bulk film (£b) is 867.3 and d/a, is 0.174 nm. Compared w ith other studies, ct, for
BST on Pt-Si is slightly smaller than the value (-10 00) o f PLD Bao.sSro.sTiCb on A u
bottom electrodes [56] but is much larger than the value (-5 5 0 ) o f sputtered Bao.sSro.sTiCb
onP t/Si [57]. The d/cj ratio (0.174 nm), however, slightly is larger than the ratio (-0.1 nm)
o f sputtered Bao.sSro.sTiCh on Pt/Si [57] or PLD Bao.sSro.sTiCb on SrRuCb /LaAlCb [58]
but is much smaller than that (0.4 nm) on S rR u03/M gO [59] and the value o f [56],
37
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>*»
>
E
a)
a.
a)
>
ro
<d
-4— '
0.2
0.3
0.4
0.5
0.6
0.7
Thickness (nm)
(a)
800
t=0.6 (urn
700
>»
H
—
1
>
CD
600
Q.
500
0
>
TO
0
q:
400
’■4—>
t=0.2 jam
300
1
200
Field(V/cm)
(b)
Figure 3-2 (a) Zero-bias relative permittivity as a function o f the thickness (b) Relative
permittivity as a function o f the applied electric field for t=200, 400, 600 nm.
38
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1
0.8
g*
0.6
c
?
0.4
0.2
0
0
100
200
300
400
500
600
700
d (nm)
m'
Figure 3-3 Measured dm/emas a function o f dm(solid) and the linear fit o f the measured data
(dashed).
3.4 Microwave BST capacitors on sapphire substrates
3.4.1 Thin film deposition and the structural properties
The BST thin film was deposited b y pulsed laser deposition (PLD ) using an excimer
laser (k = 248 nm, 25 ns pulse width, 10 Hz, -3 .5 J/cm2). Due to the requirement o f lo w
microwave loss, we choose sapphire as the substrate material. The substrate-to-target
distance is set to 10 cm to get good u n ifo rm ity on - 3 cm x 3 cm substrates. The substrate
temperature is maintained at 500 °C, 600 °C and 700 °C and oxygen partial pressure at 30
mTorr. The oxygen pressure is relatively lo w compared to other studies such as [60]
because o f their relatively short substrate-target distance (5 cm) and the relationship
between the oxygen partial pression and the distance (discussed in Section 2.3.4). A fte r
deposition, the sample was annealed at the growth temperature for 30 minutes in an oxygen
39
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environment. The film thickness was determined using a shadow mask and the Dektek
profilometer. Each o f three samples was then divided into two pieces and rapid thermal
annealing (R T A ) was conducted fo r each o f the three samples h a lf at 700°C for 3 m in and
not for the other half. The chemical composition o f BST film s was characterized by
Rutherford backscattering spectroscopy (RBS). The temperature dependence o f the
composition in BST film s at Tg=600 °C are shown in Figure 3-4.
Generally, we observed metal deficiency, in particular Sr deficiency. W e w ill discuss
the relation between the chemical composition and microwave loss later. W e then carried
out x-ray diffraction studies on them and the results are shown in Figure 3-5. The substrate
temperature could affect the crystalline quality o f the BST film . A lthough there was no
preferred orientation in all o f the samples due to the polycrystalline Pt platforms, we found
that the crystalline quality improves i f the growth temperature increases. There were only
some weak peaks fo r Tg=500°C and BST ( O il) intensity increased significantly after R TA.
A t Tg=600°C, BST (001) and BST ( O il) appeared though the peak intensity o f BST (001)
enhanced further after R TA. The peak intensity o f BST (001) o f Tg=700°C was the highest
among all samples but it decreased after RTA.
3.4.2 Device fabrication and low frequency electrical properties
Using
conventional
photolithography
and
electron-beam
evaporator,
bottom
electrodes (Ti/A u/P t 50/200/100 nm) were formed on a 500 pm thick sapphire substrate by
lift- o ff technique. The BST thin film was deposited by pulsed laser deposition (PLD) using
an excimer laser (X = 248 nm, 25 ns pulse width, 10 Hz, -3 .5 J/cm ) at a substrate
temperature o f 500 °C, 600 °C and 700 °C and oxygen partial pressure o f 30 mTorr. A fte r
deposition, the sample was annealed at the growth temperature fo r 30 minutes in an oxygen
40
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environment. Top electrodes (T i/P t/A u 30/100/200 nm) were formed b y the same w ay as
bottom electrodes. The cross section p ro file is shown in Figure 3-6. Low frequency (1
M H z) C -V measurement results (Figure 3-7) show that the dielectric constant o f film s
grown at is barely tunable for 500°C, but is tunable after R T A due to better crystallinity.
This may be attributed to the crystallinity improves w ith increasing growth temperature.
2
__ _ _ -o
o -----------------------------------< y -
"
1.5
o
-e— (Ba+Sr)/Ti
-a - Ba/Sr
-o - 0/(Ba+Sr+Ti)
0.5
0
450
500
550
600
650
700
750
Temperature(°C)
Figure 3-4 The dependence of composition on the growth temperature.
41
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CD
O
O
O
T =500 °C ~
g
CO
CM
CNI
H
Q-
&
CM
CM
O
O
O
_
CM
O
O
O
O
CO
F“
_
CM
CM
O
W
DO
E=r v> ^
CO
CN
CD
CO
o
o
o
T =500 °C + RTA
CM
CM
CM
00
CM
CM
CM
CM
CM
O
O
O
CM
30 40 50 60
20 (Degrees)
80 90
(a)
42
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
T =600 °C
g
0
0
o
(/)
CD
i
W O
w p—
0
O
(fi
-4—»
'c
3
_Q
<
T =600 °C + RTA
-i—<
0
C
0
-f-j
C
10 20 30 40 50 60 70 80 90
20 (Degrees)
(b)
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
T =700 X
g
_Q)
CD
O
CO
CD
_o
(fi
"c
3
T =700 UC + RTA
_ci
<
'co
c
0
10 20
30 40 50 60 70 80
20 (Degrees)
90
(C )
Figure 3-5 X-ray diffraction o f BST films with the growth temperature of (a) Tg=500 °C and
Tg=500 °C + RTA (b) Tg=600 °C and Tg=600 °C +RTA (c) Tg=700°C and Tg=700 °C +RTA.
44
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Metal
Figure 3-6 (a) Cross section profile and (b) top view o f BST capacitor
45
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
60
>>
I 50
>
40
<
CD
l
<D
>
'■+
—
1 30
CD
Qt
20
10
0
■80 -60
-40
-20
0
20
40
60
80
Electric Field (MV/m)
(a)
900
800
&
700
E
600
>
T =700°C
Q.
Q)
500
TO
CD
m
£
400
300
200
-400 -300 -200 -100
0
100 200 300 400
Electric Field (MV/m)
(b)
Figure 3-7 Tunability at different growth temperature (a) 500 °C ( t= l60 nm) (b) 600 °C (53 nm)
and 700 °C (38 nm).
3.4.3 Microwave characterization of ferroelectric thin films
M icrowave frequency dependent real and im aginary parts o f p e rm ittivity in
ferroelectric thin film s can be described b y Cole-Cole function [61]
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
where s*r s, e r tK, and x represent static, high frequency p e rm ittivity and relaxation time. P
is a coefficient and equals to one in for simple Debye-type relaxation. The origin o f
microwave loss and dispersion remains unknown and their extraction is not easy due to the
d iffic u lty to de-embed the capacitor from the peripheral circuit and parasitics.
One characterization technique is based on the tw o-port measurement method
proposed b y M ortazaw i’ s group in the U niversity o f M ichigan [62]. Three structures
(capacitor, through line #1 and through line #2) as shown in Figure 3-8 are used to
determine the loss contribution due to the BST film and conductor separately. Their
dimensions
are
designed
in
coplanar
waveguide
(CPW )
structure
to
fit
the
ground-signal-ground (GSG) probe w ith 150 pm pitch size fo r the measurement. The size
o f probe pads is 50 pm x 50 pm, the top electrodes are 10 pm wide and w ith 3 different sets
o f lengths (4 pm, 8 pm and 16 pm), and there are tapered transition lines to connect these
electrodes in order to eliminate the sharp discontinuity and thus the resultant radiation loss.
The gap between signal line and grounds is 100 pm in width. The bottom electrodes are
designed to be slightly bigger than the top electrodes to ensure the parallel plate formation.
3.5 The effect of growth temperature and annealing
3.5.1 Deposition temperature dependence
It would be useful to see how the the substrate temperature affects the microwave
performance. S-parameters o f the capacitor, through line #1 and through line #2 were
measured by a vector network analyzer (V N A ). These S-parameters were converted to
ABCD-parameter by software (AD S) individually. Figure 3-9 shows microwave dielectric
properties o f samples grown at 500 °C and 600 °C. The sample grown at 600°C possessed a
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
higher dielectric constant than that grown at 500 °C did, w hich maybe a result o f higher
crystallinity. It shall be pointed out that the one at 500 °C in Figure 3-9 had a much higher
dielectric constant than the one at 500 °C in Figure 3-7. The reason may be that different
substrate-target distances used (Figure 3-7: 10 cm; Figure 3-9: 7 cm) and/or the film in
Figure 3-9 was thicker. The one at 600 °C also had a higher loss w hich may be attributed to
more metal deficiency as discussed previously.
(a)
2 a .,+ 2 a 2 + a3
(b)
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2 3 -1 + 2 3 2
(C )
Figure 3-8 (a) Capacitor (b) through line #1 (c) through line #2.
700
650
600
&
•|
'E
&
T= 600 °C; t=220 nm
550
500
>
(D
450
400
T= 500 °C; t=200 nm
350
300
0
2
4
6
8
10
12
Frequency (GHz)
(a)
49
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0.08
0.07
0.06
c 0.05
0)
o>
J§ 0.04
w
T= 600 °C; t=220 nm
CO
° 0.03
0.02
T= 500 °C; t=200 nm
0.01
0
2
4
6
8
10
12
Frequency (GHz)
(b)
Figure 3-9 (a) Relative permittivity and (b) loss tangent o f BST films deposited at 500 and 600 °C.
3.5.2 The effect of RTA in oxygen
BST film s were deposited on sapphire substrates w ith bottom electrodes at 600 °C
and 30 m Torr oxygen pressure. The top electrodes were formed using standard
photolithography and lift- o ff process. M icrowave measurement was taken first and then
the sample (#331) was annealed at 700 °C for 10 sec. The dielectric constant and loss
tangent o f thin film before and after R T A are shown in Figure 3-10.
As shown there, R T A
can effectively reduce the dielectric dispersion and the loss tangent. The dielectric constant
ranges from 640 to 660 and the loss tangent is less than 0.02 after R TA. The reason may be
the reduction o f the interface states between the electrodes and BST film s. The tunability
drops a little b it after R TA based on the 1-M Hz C -V measurement setup (Figure 3-11).
However, the tunability is a little b it larger after R T A at m icrowave frequency. Although
the film Q factor is higher after R TA , the measured total Q factor is lower (Figure 3-10)
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
probably due to the observed metal roughening after annealing [63] and the destroying o f
the capacitors.
650
& 600
A fte r R T A
£ 550
® 500
Before RTA
$ 450
400
0
1
2
3
4
5
F reqency (G H z)
6
7
(a)
0.08
0.07
0.06
? 0.05
Before RTA
03
w 0.04
w
■2 0.03
I
A fter RTA
0.02
0.01
Freqency (G H z)
51
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
40
Before RTA
1-20
10
After RTA
0
0
2
4
6
8
Freqency (GHz)
10
12
(c)
Figure 3-10 (a) Relative permittivity and (b) loss tangent (c) total Q factor o f BST films before
and after RTA.
60 0
>
As deposited
£ 400
E
After RTA
(2 30 0
/
L—
0
■ M
_co
0
200
x 100
-30
V o lta g e (V )
Figure 3-11 C-V curves before and after RTA.
52
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.5.3 Secondary hot plate annealing in nitrogen
Nitrogen annealing was used to remove the dielectric relaxation below 1 M H z [64],
W e here w ill investigate the effect o f nitrogen annealing at m icrowave frequencies. BST
film s were deposited on sapphire substrates w ith bottom electrodes at 500 °C and 30 m Torr
oxygen pressure. It was found that film s grown at 500 °C exhibit poor adhesion on sapphire
and hard fo r later processing. R T A was used to im prove the adhesion. A fter annealing the
sample (#346, BST film s~210 nm thick) b y R T A at 700 °C fo r 10 sec, the top electrodes
were
formed
using
standard
photolithography
and
lift - o ff process.
M icrowave
measurement was taken first and then the sample was put on a hotplate set at 300 °C in
nitrogen ambient for 30 min. The second m icrowave measurement was conducted after the
nitrogen annealing. The microwave measurement results before and after the nitrogen
annealing are shown in Figure 3-12. W e also observe that the C -V hysteresis is reduced
significantly after nitrogen annealing (Figure 3-13) in contrast to the R T A effect (Figure
3-14), possibly due to the reduction o f the interface trap density [50],
The interface trap density can be calculated based on the C -V measurement [50]
(3-4)
where A is the effective capacitor area and C(V/) and C(V;) are defined in Figure 3-13 (a).
W e can calculate the interface trap density drops from 1 .1 9 x l0 13 to 7.39x10 12 C/cm2 after
nitrogen annealing and increases from 2.30x1013 to 2.46x10 13 C/cm2 after rapid thermal
annealing. The difference o f the interface trap density may account the difference o f the
behaviour o f the microwave loss. It should be noted that the C -V hystereis arises from the
interface traps, not the P-V hystereis because no P -V hysteresis is observed.
53
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
500
Before annealing
400
]>
”
l
300
.
o>
a
a>
>
A fte r annealing
200
_ re
©
o'
100
o
_ l________________L
0
2
4
6
8
10
12
Frequency (GHz)
(a)
0.05
Before annealing
0.04
C
a>
a>
c
re
H
V)
(/)
o
0.03
0.02
0.01
o
After annealing
o
2
4
6
8
10
12
Frequency (GHz)
(b)
54
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
60
After N anneal
50
40
Before N anneal
30
20
10
0
0
2
4
6
8
10
12
Freqency (GHz)
(c)
Figure 3-12 (a) Relative permittivity and (b) loss tangent (c) total Q of BST films before and after
nitrogen annealing.
Another interesting phenomenon is that the dielectric contant decreases after R T A in
the low frequency (1 M H z, Figure 3-14) but increases at the microwave frequency (Figure
3-10) w hile the dielectric constant o f the RTA-treated sample decreases fo r both low and
microwave frequency (Figure 3-12 and Figure 3-13). This probably occurs because the
dielectric dispersion was very severe before R T A but was m ostly removed after R TA.
For the N 2 annealed sample, the dielectric loss is about 0.02 at 4 GHz and the tunability
is 2.2:1. Using (3-1), the figure o f m erit is 27 at 4 GHz and 44 at 10 GHz. This result is
better than those reported results o f BST thin film s deposited b y PLD, as shown in Table
3-2.
55
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7 10'13
CI(V1)
Cu(V1)
/ a Cu(V2)
CI(V2)
1.19x10
■20 -15
-10 -5
0
5
10
Applied Voltage (V)
15
20
(a)
Cu(V2)
Ci(V1)
LL
5 10' 13
7.39x10
-20
-15
-10 -5
0
5
10
Applied Voltage (V)
G
15
20
(b)
Figure 3-13 C-V hysteresis curves before and after nitrogen annealing.
56
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Cu(V2)
CI(V1)
2.30x101J C/cm
0
•20
-15
-10 -5
0
5
10
Applied Voltage (V)
15
20
(a)
i-12
Cu(V2)
CI(V1)
IL.
<
D
O
c
CO
-4—'
o. 4 10'13
to
O
2.46x10
-20
C/cm'
-15
Applied Voltage (V)
(b)
Figure 3-14 C-V hysteresis curves before and after RTA.
57
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Authors
F ilm
Substrate
Measurement
Tunability
Dielectric
Method
measured
Q
K
from C -V
(%)
L im et al.
Ba0.6Sr0.4TiO3
M gO (100)
[65]
Chong et al.
Bao.5Sr0.5T i 0 3
L a A lO j
22
15
Dual resonator
22
33.3
7.33
62
15
9
54
50
-2 7
54
83
-4 4
method at ~7.7 GHz
Bao.5Sro.5Ti03
N b :S rT i0 3
[67]
Ours
68
at 10 GHz
[66]
Cao et al.
Interdigital capacitors
Parallel plate
capacitors at 100 kHz
Bao.5Sr0.5Ti03
Pt bottom on
Parallel plate
sapphire
capacitors : two port
at 4 GHz
Ours
Bao.5Sr0.5T i 0 3
Pt bottom on
Parallel plate
sapphire
capacitors: two port at
10 GHz
Table 3-2 Comparison o f the figure o f merit o f BST thin films deposited by PLD.
3.6 Conclusions
BST thin film microwave tunable capacitors have been fabricated and their thickness
dependence has been studied. The dielectric constant increases gradually as the film
thickness increases. Two port measurement method and three kinds o f structures have been
used to determine the microwave loss contributions from the film and the metal electrodes.
It is found that the both rapid thermal annealing and nitrogen annealing could improve the
dielectric dispersion and dielectric loss. However, the total Q factor may drop after
annealing except for the case o f nitrogen annealing below ~ 4 GHz.
58
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Chapter 4
Theory of ferroelectric electro-optic thin film waveguide
interferometric modulators
Design and m odeling o f electrooptic waveguide for integrated optics would require the
knowledge o f the relation between the applied electric field and the refractive index
change. However, the electro-optic properties o f ferroelectric thin film s depend greatly on
the growth conditions. Moreover, the electrooptic response could be very complex, even
hysteretic. Therefore, it is desirable fo r electro-optic thin film engineers to have a fast and
simple method to analyze the EO properties as a feedback to future development.
In this chapter, we investigate the voltage tuable electrooptic coefficient o f
ferroelectrics, the extraction o f EO coefficient from EO M Z I under different electric field
configurations and the design optim ization o f EO M Z I. A t last we incorporate the tunable
EO coefficient into the EO M Z I analysis.
4.1 Thin film optical waveguides
Thin film optical waveguides are the enabling technology fo r optical integrated
circuits. The concept o f optical waveguides can be easily described b y ray optics picture
and total internal reflection. Waveguides consist a core region (high refractive index) and a
cladding region (low
refractive index) w hich
surrounds the core region.
In
a
two-dimensional asymmetric slab waveguide, the simplest structure is composed o f the
59
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three-layered structure in Figure 4-1. The refractive index tij and the w idth dj are assigned
to regions j , j = 1 to 3. W hen the lig h t w ith wavelength X opropagates in the waveguide, the
transverse attenuation constant
a j and transverse propagation constant kj in the
waveguide are given as
= 1,3
(4' 1)
and
£2= A 2”2 - a 2
where
(4' 2)
k 0= I n / A0 is the ffee-space propagation constant and j3 i is the propagation
constant along the z
direction o f the guided mode. The transverse field distribution F(x)
is depicted as follow s:
ru
-► z
clad d in g
layer
g u id in g
layer
WG
substrate
layer
ni
Figure 4-1 Schematic of the three-layered waveguide.
60
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x <0
F ( x ) = ^ Be~JklX + C e ik'x
0 < x < d2
(4-3)
x > d 2.
B y matching the boundary condition o f the interface, we can obtain the transverse
resonance condition below:
(4-4)
k2
" k2
w ith £21 = <^23 = 1 fo r a TE mode and S,2X = n \ l n x and
£23 = n 2 In ] for a T M mode.
Solving (4-4), we can get j3 i, a j, and ki. I f the waveguide is symmetric, a dimensionless
parameter E-number defined as V = k()d 2■yjnl - n 2 can be seen as a measure o f the field
confinement.
The above discussion for slab waveguides is straightforward. However, design o f
rectangle dielectric waveguides is generally very complicated because it is almost
impossible to get a closed-form analytical solution, and an interactive method is required
[68]. To model guided waves accurately, complex numerical methods that include the
vector properties o f electromagnetic waves need to be employed [69],
A much simpler method which is called “ effective index approximation” decomposes
the rectangle waveguide into two vertical waveguide and one horizontal waveguide and
finds the effective index and propagation constant [70]. One classification o f optical
waveguides is the mode structure: single mode or m ultimode. Single mode waveguides are
desired because no modal dispersion exists and Mach-Zehnder type switches can be
formed based on them. Due to the sim plicity o f the effective index method, we w ill use it to
design single-mode waveguides. However, one should keep in m ind that single-mode
61
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waveguide design is not so straightforward and is s till an attractive academic topic and
large core (~10 pm) could be single-mode i f the ridge height and etch depth are designed
carefully [71]. The materials can be dielectrics, semiconductor, or polymer. In our work,
we study waveguides made o f ferroelectric core and dielectric (M gO , SiC>2, air) cladding.
4.2 Fundamentals of the electrooptic effect in ferroelectric thin
films
The electrooptic effect is the dependence o f the refractive index o f a material on an
applied electric field. Commercial LiNbOs modulators are the most successful example o f
the applications o f this effect. For single crystal materials, the EO effect can be described
by the change o f the index ellipsoid or the change o f the optical im perm eability tensor. I f
the change o f the im perm eability tensor is proportional to the applied electric field, it is
called the linear electrooptic (Pockels) effect and the proportionality factor is called the
Pockels coefficient (r). I f the change o f the im perm eability tensor is proportional to the
square o f the applied electric field, it is called the quadratic electrooptic (Kerr) effect and
the proportionality factor is called the K err coefficient (s). Here, we define the total EO
coefficient (p) as the derivative o f the im perm eability change w ith respect to the applied
electric field and may be represented by
c/An”2
p = ----------« r + sE
dE
(4-5)
i f the total EO effect is a superposition o f the Pockels effect and the Kerr effect. Both the
Pockels effect and the K err effect contribute to p and the refractive index change as shown
in Figure 4-2 respectively.
62
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p
sE
(a)
Figure 4-2 (a) p and (b) -An resulted from the linear and quadratic EO effect, respectively.
In some cases, the dependence o f the im perm eability tensor on the electric field is
not linear, quadratic, or their superposition. It was observed that the linear EO coefficient
o f P LZT changes its sign when the electric field change from positive to negative and
surpasses a threshold fie ld [72], Hystersis loops o f EO coefficient-the electric field(r-E ) in
perovskite oxides have been measured[73-76]. However, no clear explanation was given.
Moreover, the EO coefficient o f B aTi 03 was observed to increase w ith the electric field at
low field range and reduce again at a high field [77, 80, 81]. The behavior is attributed to
the domain reorientation, the poling o f the film or the electrical leakage. The objective o f
this w ork is to establish a model for the electrooptic hysteresis characteristics o f
ferroelectrics based on parameters that may be obtained from electrical measurements.
4.2.1 Electro-optic coefficient in oxygen octahedral ferroelectrics
DiDom enico et al. proposed to regard the linear EO effect in ferroelectrics as a
quadratic EO effect biased by the spontaneous polarization [19], The electrooptic effect in
oxygen-octahedra ferroelectrics maybe expressed by a fourth rank tensor gyk relating the
change o f the im perm eability tensor to the polarization vectors Pk and Pi as [82]
63
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For sim p licity, P is considered to be along a fo u r-fo ld octahedral axis o f ferroelectrics. The
change o f ordinary refractive index can be expressed as
A
<4-7>
E m ploying the “ hard ferroelectric approxim ation” , the polarization under an electric fie ld
w hich is parallel the spontaneous polarization can be w ritte n as [78]:
P (E ) = PM+ e 0(e r - l ) E .
(4-8)
Inserting (4-8) in to (4-7), we get
ta , = - ^ g P l - n l g s a( s , - \ ) P sE.
< 4 ' 9 )
where the term containing E2 is neglected assuming eo(sr- l ) E « P s. Therefore, the linear
EO (Pockels) coefficient fo r lig h t polarized parallel to P can be expressed as:
r33
=
S
1 2
= 2 g iiF ik _ l K .
(4_10)
T -,
- n 0E
2
S im ila rly, fo r lig h t polarized perpendicular to P, the EO coefficie nt is
r \l
=
2 & 1 2 ^0 ( £ r
~
l ) P s-
(' 4 _ 1 1 ')
It should be noted that the above derivation o f the linear EO coefficie nt (r) is based on
the “ hard ferroelectric approxim ation” in D iD om enico’ s sim p lifie d theory. A lthough this
approach is good fo r estim ating the EO coefficient, it is in su fficie n t to explain the
64
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hysteretic shape o f the r-E loop. Here, we attem pt to generalize the theory to the total EO
coefficient (p). The total EO coefficie nt is defined as:
dAn
P =
(4-12)
dE
The measurement o f the EO coefficie nt is measured as a function o f an applied dc fie ld
w ith a sm all ac fie ld . Thus, the polarization P m ay be divided in to a dc term Pdc and an ac
term Pac. A ccording to(4-12), the EO coefficie nt can be expressed as:
dAn
P =
-2
dE
~ 2S(Pdc + Pac)
d(P dc+ P ac)
dE
= 2 g P s 0 ( s r -1 ).
(4-13)
Compared (4-13) w ith (4-10), r is replaced b y p and P s is replaced b y P. M oreover, the
dielectric constant is tunable in ferroelectrics.
4.2.2 Ferroelectric capacitor model
A m odel described b y Ogata and Ishiw ara [79] is used in this w ork to describe the
polarization and capacitance characteristics o f ferroelectric th in film capacitors. The
forw ard and backward electrical polarizations are expressed
P ^ = Ppara + Pje,-ro
where
(forw ard) and
P'~ = Ppara + P}erro
(backward)
Ppara ~ ef s oE + PsPtanh (— )
o
P;m (E ) = P „ { 2 f H E ) - l ) andP/: ro(iT) = ^ ( 2 /9(£„, - £ ) - l ) .
The fie ld dependence o f p is described by
65
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(4-14)
(4-15)
(4' 16)
dj3(E)
dE
In Eq. (4-10)
( i- p m
E of
1
(1
-)■
1 + e x p (- E - E
c
-5 {
) / E of)
1- P ( E ) \
(4 -1 7 )
and E q .(4 -ll), p e rm ittiv ity is considered to be a constant. In
ferroelectric m aterials, the high
frequency capacitance deviates from
the direct
differentiatio n o f its lo w frequency P-E measurement [79], The m agnitudes o f forw ard and
backward p e rm ittiv ity are
dPp a ra (E)
\
J
dE
s f.{E) =
'
dPp a ra (E)
V
/
dE
■+ a
dPLAE)
+a
(4-18)
fe rro \ E )
(4-19)
4.2.3 Simulation results
The th in film sim ulated is PZT film fo r the purpose o f com parison w ith Ref. [75]. To get
the EO coefficient, we need to substitute Eq. (4-14)-(4-19) in to (4-13). Table 4-1 shows the
parameters used in this sim ulation. M ost values were taken from the extracted result from
Ogata [79].
Sf
df
PSD
a
o
>
A
1020
260 nm
4.5 |xC/cm2
IV
0.1
Psf
Vc
Vof
A
G
35 nC/cm2
0.2 V
0.25 V
0.07
0.01 m4/C2
Table 4-1 Parameters of PZT film used for simulation.
W e show both P-reversal and Ps-reversal o f the PZT film in Figure 4-3(a). Their
corresponding p-E loops are shown in Figure 4-3 (b). As can be seen from Figure 4-3 (b),
there are tw o “ humps” w hich were also observed in Ref. [75]. p-E loop considering the
abrupt spontaneous polarization reversal and tu n a b ility o f p e rm ittiv ity (dashed lin e in
Figure 4-3 (b) )also has the “ humps” but the shape o f p-E loop does not look sim ila r to that
o f Ref. [75],
66
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0.4
0.2
-
0.2
-0.4
■8 106-6 106-4 106-2 106 0
2 1064 1066 106 8 106
Electric field (V/m)
(a)
3 10'10
N X
4-1
c
<D
’o
r 10
o
o
o
4-1
Q_
O
O
r 10
,*10
-2 10'
-3 10',*10
■8 106-6 106-4 106-2 106 0 2 1064 1066 1068 106
Electric field (V/m)
(b)
Figure 4-3 (a)P and Ps reversal, (b) p-E loops calculated based on P and Ps reversal.
The tunable p e rm ittiv ity was not taken in to account in previous studies [80]. The p-E
loops w ith tunable p e rm ittiv ity and fixed p e rm ittiv ity are shown in Figure 4-4. It can be
seen that the EO coefficient w ith o u t considering the tu n a b ility o f p e rm ittiv ity increases
m onotonically w ith the increasing applied fie ld and does not have peaks.
67
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,-10
e
5 10
*/ /[ / / {
o
% -5 10
o
Electric field (V/m )
Figure 4-4 Calculated p-E loops based on tunable (solid line) and fixed (dashed line) perm ittivity.
0.005
0.004
/__ „
c
< 0.003
0
o
c 0.002
0
O)
CO
’ L_
H—
0
0.001
L00
o-
vyy
-0 001
1
1
1
1
------------- 1
1
-1
--------8 106-6 106-4 106-2 106 0 2 1064 106 6 106 8 106
Electric field (V/m)
Figure 4-5 Calculated field induced birefringence o f the PZT film based on Table 4-1.
The EO th in film is often characterized b y m easuring the electrooptic coefficient
d ire ctly [75] or m easuring the field-induced birefringence and calculating the EO
coefficient based on the birefringence [83, 84], Authors using the second m ethod often
claim ed that th e ir
th in film s exhibited quadratic EO effect. W e w ill prove that the second
method requires more attention when it is used to fin d the EO coefficient. The measured
68
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bireffigence has the contribution from the linear, quadratic, or higher order EO effect and is
given by:
- A (Sn) = - A (ne - n 0) = HeP33 ~ n°P u E.
(4-20)
and
3
3
Ane = ~ P i i E M 0 =
( 4 ' 2 1 )
A lthough the shape o f the birefringence (Figure 4-5) bears some resemblance to the
quadratic term in Figure 4-2(b), it is im proper to say that the quadratic term is dominant.
The shape looks lik e a parabola because the linea r EO co e fficie n t switches its sign due to
the reversible spontaneous polarization.
4.3 Analysis and design optimization of EO interferometric
modulators for microphotonics applications
Understanding o f the relation between the output characteristics and the device
parameters (device dimensions, the electric fie ld direction) is crucial during design process
o f m icroscale EO M ZIs. In Section 4.3.1, the exact expression and tw o approxim ate
expressions o f the output intensity characteristics o f M ach-Zehnder interferom etric (M Z I)
m odulators under different schemes o f electric fie ld configuration are compared. The
v a lid ity o f approxim ate expressions is discussed. The result o f our analysis reveals that a
higher EO coefficient does not necessarily im p ly a low er sw itching voltage. In fact, the
sw itching voltage is strongly associated to the electric fie ld direction w ith respect to the
optic axis (c-axis) o f the device crystal. For example, it is natural to desire to use rsi o f
B aTi0 3 , i.e. the highest EO coefficient (r5i=820 pm /V compared w ith r33=28 pm /V ) to
make EOMs. However, rsi is only active when the electric fie ld applied norm al to the optic
axis and its corresponding sw itching voltage is larger that that o f r 33 when the electrode
69
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length I is greater than -6 5 0 0 X. Therefore, the advantage o f lo w sw itching voltage in using
rsi o f BaTiC >3 o n ly emerges when I is around or less than hundreds o f m icrom eters. Since
the goal o f this w o rk is to design m icroscale m odulators, it maybe proper to u tiliz e rsi in
photonics applications. In Section 4.3.2, other design issues w ill be discussed in terms o f
polarization sensitivity, d rivin g pow er and extinctio n ratio. These parameters also strongly
depend on the electric fie ld configuration and the m agnitude o f the acting EO coefficient.
In the electric fie ld configuration where rsi o f B aT iO j is u tilize d , the EO M is very
polarization insensitive. However, the extinction ratio is a function o f the electrode length.
We develop an approach to m axim ize the extinction ratio b y choosing an appropriate
electrode length based on the phase retardation expression. In Section 4.3.3, an analog o f
the firs t h a lf period voltages is drawn between M Z I and Fabry-Perot interferom etric (FP I)
modulators. In the fo llo w in g analysis, we o n ly consider the linear EO effect (Pockels
effect), and assume that the optical confinem ent factor [85] is unity.
4.3.1 Intensity output characteristics of EO MZI modulators
For M Z I m odulator composed tw o single-m ode waveguides, usually one arm (arm 1)
is under no electric fie ld and the other arm (arm 2) is under an electric fie ld (Figure 4-6).
The optical wave is separated b y a 3-dB Y ju n ctio n . Intensity m odulation can be achieved
after the tw o waves combines at the output. I f the tw o waves are in phase, the output w ill be
m axim ized; whereas i f the tw o waves are out o f phase, the output w ill be zero due to the
single-mode nature o f the output waveguide [86]. The sw itching voltage depends on the
length o f the electrode and the strength o f the EO coefficient.
E lectric fie ld is often applied in the transverse direction because the longitudinal
electric fie ld requires much higher m agnitude to achieve the sw itching operation. O w ing to
70
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the absorption loss o f m etal electrodes, lateral electrodes are preferred over the vertical
electrodes. U sually, it is desirable to u tiliz e the highest EO coefficie nt in the crystals, such
as rsi o f B a T i03 when the electric fie ld is norm al the c-axis as shown in Figure 4-6.
However, we w ill show that rsi only produces the low est sw itching voltage when the
electrode length is sm aller than -6 500 wavelengths. The intensity-voltage output
characteristics o f linear EO M Z I m aybe periodic or non-periodic [81], but th e ir analysis is
not entirely proper fo r a ll types o f crystal classes. W e w ill give a detailed analysis o f output
characteristics fo r 4m m , 6mm, 4 3 m and 3m crystal classes in the fo llo w in g .
W aveguide
losses are ignored fo r sim p licity, but im portant in form ation o f m odulation behaviors is
preserved.
light output
TE light input
Figure 4-6 Schematic illustration o f a c-axis oriented thin film M Z I modulator w ith lateral electric
field applied in arm 2. The polarization o f arm 2 rotates during propagation while that o f arm 1
does not.
4.3.1.1 For 4mm (BaTiOa) or 6mm (ZnO) classes
For arm 1 where no electric fie ld is applied, the principal refractive indices along the a-, b-,
and c-axis are
71
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1) Case I (E p a ra llel to a or b-axis):
The case where E is parallel to a-axis should be the same as the case where E is
parallel to b-axis because the crystals are uniaxial. The EO tensor o f 4mm is w ritte n as
0
0
rn
0
0
ri3
0
0
r33
0
rSl
V0
(4-23)
0
0
0
0
0
The principal refractive indices in arm 2 where E is parallel to a-axis are
n 30r5lE ta n #
n x2
=
n o ~
n \r iXE tan <9
-,n y2 = n 0 ,n z2 = ne + ■
tan 20 =
where
2rslE
U nl-U nf
(4-24)
(4-25)
Since the optical waves in tw o arms see diffe re n t orthogonal sets o f eigenstates, they
need to be decomposed to their respective sets o f eigenstates w hich have different
propagation speeds. The decomposed components then propagate along the arms and
interfere at the output. Em ploying the Jones m a trix calculation, the m agnitude o f exact
output intensity can be obtained. Assum ing the x-polarized lig h t w ith input intensity
m agnitude o f u n ity travels along y direction fo r z-cut EO M Z I, the Jones vectors fo r tw o
arms at the output (denoted by J / and J?) are given by
2K
exP| - X
J, =
+ sin2#exp
cos 0exp - j
(
(4-26)
0
\
sin2#
— l
, 2m A \ sin2#
'exP | - J — —
\
CXP
. 2m z2l
A
2m ,2l
A
72
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(4-27)
, and their sum is
J —J i + J ■
(4-28)
\
2 J
'
The exact norm alized output intensity (the output intensity divided b y the m axim um output
intensity) is
/ , = w 2+ K r
•
(4-29)
Assum ing 0 can be approxim ated as zero, no extraordinary wave is excited in both
arms when the input is x-polarized. The in ten sity can be given b y the phase delay o f tw o
ordinary waves:
I 2 = cos
= co s2{ m 3
0r5iE l tan6,/(2 Z ))
(4-30)
where A, <f>x-i,</>xi, and / represent the optical wavelength, the phase delay in a rm l, the
phase delay in arm 2 and the electrode length, respectively.
A ccording to (4-30), the firs t intensity m inim um occurs when <f>i0- §2 0= 71 or klAn= n
(An is defined as the amount o f change after the application o f the electric fie ld ). The above
leads to
(4-31)
where Ejhp represents the firs t h a lf period electric field. It is noted that n0 and ne need to be
switched in i f n0<ne. For B aT iC f M Z I, the in ten sity output calculated b y (4-29) and (4-30)
are shown in Figure 4-7. A closer lo ok at Figure 4-7 reveals that the intensity m inim um s o f
(4-29) are not zero. A better approxim ation m ay be necessary. Since nX2+n Z2 is close to
nxi+ n zi in this case, the norm alized output intensity (4-29) fo r x-polarized input can be
sim p lifie d to
73
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(4 -3 2 )
where T i and To are the phase retardation in arm 1 and arm 2, respectively.
Equation (4-32), named “ phase retardation expression” b y us, is also shown in Figure
4-7. A lthough the deviation from the exact solution predicted b y (4-29) is not sm aller than
that b y (4-30) in Figure 4-7, we w ill show how (4-32) helps to m axim ize the m odulation
depth in m icroscale design in Section 4.3.2.
phase difference, neglecting
rotation of principal axes
exact
phase retardation method
1
0
0
1
2
3
4
5
6
Electric field (V/|um)
Figure 4-7 Nonperiodic intensity-field output characteristics o f B aTi03 M Z I calculated (4-29),
(4-30), and (4-32) for Case I.
2) Case I I (E parallel to c-axis):
Petraru et al. [81] analyze this case based on the phase retardation expression.
However, the phase retardation method shall not be used because nX2+nZ9 is not close to
nxi+ n zi here. The im perm eability tensor is diagonal before and after the application o f the
74
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electric fie ld here, so there is no angular rota tion o f the p rin cip a l axes (0=0), indicating that
the phase difference expression is appropriate. The p rin cip a l refractive indices o f arm 2
are:
nx2 ~ nyi = no ~ n 0
3rl3E/2.
(4-33)
For o- and e-wave, the phase differences between tw o arms are
& 2 - & 1 = ~ m lr u E l/A .
(4-34)
Thus the h a lf wave electric fields can be w ritte n as
E,
A
,£
=
A
(4-35)
” e*33
» o l r \3
4.3.1.2 For 43m (GaAs, GaP,CdTe) class:
A lthough the 4 3 m crystal class is o p tic a lly isotropic, it is noncentrosym m etic and
the EO tensor can be w ritte n as
0
0
0
0
0
0
0
0
0
0
0
r 41
0
r41
0
0
0
r,
(4-36)
A fte r the electric fie ld is applied, the angular rotation 0 is independent o f the electric fie ld
and is always 7i/4 (45°). Since n /4 is nonnegligible, the phase retardation method is in valid.
S im ilar to the previous analysis, the output Jones vector and output intensity fo r
x-polarized lig h t at the input are given by:
75
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(
J'=-
2 v°y
1
+—
2
f
cos
1+ cos
v
f
2 sin
v
f
m r4lEl
3
r A
v
j sin
m rAlEl
X
jm r.,El
3
T - lX
JJ
I = |v4,|2 + | ^ | ' = —+ —cos(m ir/UEl/x).
and
(4 -3 7 )
(4-38)
I f 0 is carelessly ignored, the output Jones vector is
t
1
i ■
r\
(4-39)
C oincidentally, the norm alized in ten sity expressions derived fro m both methods are
the same. Further, expressions o f firs t h a lf period electric fie ld fo r x-polarized and
z-polarized inputs are the same in GaAs m odulators since no birefringence exists before
applying an electric fie ld . This result is different from (4-35) where birefringence exists
before the application o f the electric fie ld .
4.3.1.3 For 3m (LiNb0 3 , LiTa03) class:
The form o f electrooptic tensors o f the 3m crystals is sim ilar to that o f the 4m m or
6mm crystals except the additional term r 22, w hich can be ignored since its effect is
negligible in the diagonalization o f the im perm eability tensor [87], This, the analysis o f 3m
crystal class is the same as that o f 4m m crystal.
4.3.2 Microscale design issues
Since the analysis o f d rivin g voltage and d rivin g pow er in 4 3 m crystal class under
d ifferent schemes o f electric fie ld directions is very direct, we o n ly discuss that in 4mm
(and 3m) crystal class. For B a T i0 3 M Z I (n0=2.437, ne=2.365, r i3=8 pm /V , r33=28 pm /V ,
r 51=820 pm /V ), the firs t h a lf period electric fields fo r both x-polarized and z-polarized
waves in case I and II are calculated based on (4-31) and (4-35) and are plotted in Figure
76
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4-8 as a function o f the electrode length. It is found that fo r BaTiCE M Z I, the sw itching
voltage em ploying rsi is the sm allest except fo r l>6500Z. W e can conclude that different
elements in the EO tensor have diffe re n t degrees o f im pact o f the m odulator perform ance
not on ly because o f th e ir m agnitude but also their positions in the tensor form .
The lumped m odulator d rivin g pow er is per u n it bandw idth defined in [88]. The power
per u n it bandw idth (bandw idth=A v) norm alized to the waveguide cross section is given by:
The anisotropic dielectric constants (e ) along diffe re n t axes in BaTiCE [89] are used.
M agnitudes o f the norm alized d rivin g pow er per u n it bandw idth o f different schemes o f
electric fie ld configuration are shown in Figure 4-9. It is noted that the d rivin g pow er fo r
the configuration where the electric fie ld is norm al to the optic axis is almost independent
o f the electrode length, especially when / is large compared to X. T his can be understood
from (4-31). B y expressing n0 = n +(|n0-ne|)/2 and ne= n -(|n0-ne|)/2 and assuming 4|n0-ne
| » XII, (4-31) can be expanded and sim p lifie d to
(4-41)
77
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1000
100
E//c-axis,o-wave(r
a.
Ella
or b-axis,o or e-wave(r )
4000
2000
0
8000
6000
l/X
Figure 4-8 First driving electric fields for BaTiO j M ZI under different configurations
1000
. £ "o
I I
100
E//c-axis,o-wave(r )
■o .i=:
0 c
N 3
i—
0
Q.
.
E//a or b-axis,o or e-wave(r )
51
-
0
200
400
E//c-axis,e-wave(r
600
800
1000
t/X
Figure 4-9 Normalized driving powers per unit bandwidth for BaTiCb M Z I under different
configurations.
78
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The equation enables us to extract the EO coefficient from fin d in g the refractive
indices and firs t h a lf electric fie ld . In addition, this equation im plies that the firs t sw itching
electric fie ld (voltage) is inversely proportional to the square root o f the electrode length,
m aking the d rivin g pow er nearly independent o f the electrode length. The accuracy o f
(4-41) has been discussed in detail in Ref. [87] and is shown in Figure 4-10. For example,
the deviation is less than 8% as long as the electrode length is m ore than 100 wavelength
long.
(4-31)
c
o
’w
(4-41)
-2 0
>
0
■o
-40
0
O)
B
-60
0
(4-35)
o
s -80
Q_
-100
0
200
400
600
800
1000
Length(^)
Figure 4-10 Normalized driving powers per unit bandwidth for B aTi03 M Z I under different
configurations.
M odulation depth w hich can be defined as
( I m a x - I m in )
/(Im a x + Im in )
is an im portant
figure o f m erit fo r intensity m odulators. The o n -o ff extinction ratio fo r electric fie ld norm al
to the optic axis is actually a function o f the electrode length though it is not so obvious. It
is d iffic u lt to fin d a way to m axim ize the m odulation depth from (4-29) based on the exact
79
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intensity expression or (4-29) based on the phase difference m ethod neglecting the
eigenstate rotation. Therefore, we examine (4-30) m ore carefully. The second term in
(4-32) needs to be -0.5 to have zero transm ission (off-state). This happens when T i=2N7i
(N is a positive integer) and r 2=F 1+271 (at Ejhp). For T i= 2 N ti, we have the condition fo r
m axim um m odulation depth:
NX
/ = - ----------------------------------------------------------------- (4-42)
0 0 ~ n e)
, where N is a positive integer. In Figure 4-11, the m odulation depth never reaches 1
because the electrode length is not w e ll designed as (4-42).
A lthough (4-42) is derived from the approxim ate expression (4-32), the condition o f
m axim um m odulation depth using (4-42), i.e. /=13.89 NX fo r BaTiCb M Z I, is quite
consistent w ith the result from the exact expression (4-29) as shown in Figure 4-11. The
usefulness o f the “ phase retardation expression” or (4-32) can be seen here.
0.95
q_
0
"O
c
0.9
0.85
O
J5
0.8
o
0.75
:3
■o
0.7
0.65
10
15
20
25
30
35
40
45
50
l/X
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Figure 4-11 Modulation depth o f B aT i03 M ZI (Case I) as a function o f the ratio o f the electrode
length o f the modulator to the optical wavelength.
There are tw o other benefits when the electric fie ld is norm al to the optic axis: less
therm al d rift and polarization in sensitivity. The therm al d rift due to the pyroelectric charge
buildup is expected to be sm all when the electric fie ld is not along the optic axis [91].
E O M Z I generally requires special electrode layout to reduce the high se nsitivity to the
optical polarization [90]. The polarization in se n sitivity is explained as follow s. S im ila r to
where the input is x-polarized, the output intensity fo r the z-polarized input is given b y
/ 4 = cos2((^zl - <f>z2) l 2) = cos2[ m lr ^ E l tan< 9/(2/l))
(4-43)
Since the difference between n0 and ne is sm all fo r m ost m aterials, the intensity-voltage
output characteristics are almost the same fo r x-polarized and z-polarized input waves.
Hence, the curves o f the d rivin g electric fie ld (voltage) and pow er fo r tw o polarizations (oand e- waves) are indistinguishable on Figure 4-8 and Figure 4-11.
4.3.3 EO Fabry-Perot interferometric (FPI) modulators
EO FPI can be used as tunable filte rs [92] or spatial lig h t m odulators [93], and it can
also be used to determ ine EO coefficients [94], E iN b 03 channel waveguide FPI
m odulators (length= 0.7 cm) w ith 4V half-w ave voltage have been fabricated [95].
Intensity-voltage output characteristic can be m odeled using the transm ission fu nction o f
FPIs.
The transm ission o f FPI can be expressed as
T =
<4' 44>
where R is the re fle c tiv ity o f the m irrors.
The above function has a m inim um whenever the phase term in the denom inator is
(2N-1)tc. Hence, the firs t m inim um o f FPI occurs when 2kl/Sn= n. On the other hand, the
81
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intensity output o f M Z I has the firs t m inim um when k/An= n. Therefore, the expressions
fo r firs t h a lf period voltage fo r M Z Is derived above are s till va lid to EO FPIs except that I
needs to be replaced b y 21.
4.4 Integrated EOMZMs based on PLZT thin films with tunable
electro-optic coefficients
In this section, we incorporate the effects o f tunable electro-optic coefficient and the
orientational relation between the electric fie ld and the optic axis (fie ld and optic axis
configuration). W e found those effects are not negligible in P LZ T devices.
Am ong ferroelectric m aterials, P LZ T electro-optic (EO ) ceramics has been o f
particular interest because its EO coefficient is greater than that o f L iN b 03 [72]. The
applications
o f PLZT
EO
th in
film s
include EO
deflector
switches
[96]
and
Mach-Zehnder m odulators (M Z M s) [97], [98]. Instead o f being a sim ply linear or
quadratic E O M ZM , the intensity output o f P LZ T th in film
M Z M e xh ib itin g the
behaviour o f “ chirping” at lo w fie ld and “ unchirping” at high field. T hapliya et al.
attempted to explain those phenomena b y using a m odel w hich assumes the presence o f
the depletion layer thickness in conjunction w ith a fixe d K e rr coefficient [97], However,
that m odel failed to explain the asym m etry the intensity-voltage characteristics and the
drop o f m odulation depth at higher electric fields. The incom plete understanding o f the
output characteristics m ay create challenges fo r further applications o f P LZT devices.
4.4.1 PLZT thin films with tunable EO coefficient and their field-induced
birefringences
The electro-optic coefficients o f ferroelectric P LZT th in film s are dependent upon the
applied fie ld [7 3 ],[9 9 ],
To account fo r this fact, the electro-optic coefficient can be
expressed as Equation (4-13). The acting electro-optic coefficient is not o n ly dependent
82
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on the optical polarization o f the in put lig h t but also dependent on orientational relation
between the electric fie ld and the o p tic axis [87]. In this paper, however, we assume that
the electro-optic response fo r the electric fie ld along the optic axis is the same as that fo r
the electric fie ld norm al to the optic axis. This assumption w ill help us to evaluate the
importance o f the orientational relation between the electric fie ld and the optic axis. W e
have chosen a particular p-E relationship (electro-optic response) o f P LZT film fo r the
m odulators. The parameters used in the calculation are listed in Table 4-2 and the
resultant p-E relationship is shown in Figure 4-12. For cla rity, o n ly the forw ard direction
o f the electro-optic response is considered.
The birefringences in the cases o f the fie ld norm al to and along the optic axis are
respectively expressed as
dn± =
2
n'pE\w\9
dnll= ——r?pE
(4-45)
2
>
where p ,E and 0 represent the electro-optic coefficient, the applied fie ld , and the rotation
angle o f the p rin cip a l axes. Since p is linear w ith the applied electric fie ld , the
birefringence is approxim ately proportional to the square o f the applied electric fie ld . As
a result, they m ay lo ok lik e quadratic in the birefring ence-field curves as shown in Figure
4-13. It can be noted that the different birefringences in d iffe re n t “ fie ld and optic axis
configurations” maybe an account fo r the divergent measurement results o f quadratic EO
(K err) coefficients [101]-[104], The difference is ty p ic a lly around 100 tim es measured in
different groups and tanO is around the order o f 0.01 at lo w fie ld (< 1 .5 xl0 6 V /m ).
83
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g
Ec (V /m )
Eop(V /m )
8
£r
0.02
2 x l0 6
3 .8 x l0 /
1
220
a
Psp (p C /c n r)
Psf(pC /cm 2)
Eof(V/m )
n0
ne
0.4
12.5
30
3 x l0 b
2.5
2.49
Table 4-2 Parameters used in calculation. Definitions o f symbols can be found in [100].
1.2 107 -8 106 -4 106
0
4 106 8 106 1.2 10
Electric field (V/m)
Figure 4-12 Electrooptic coefficient o f PLZT thin film calculated versus E field based on
parameters in Table 4-2.
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-
0.001
-
0.002
D)
-0.003
td -0.004
I
I -0.005
*o
-0.006
-0.007 -----------'---------- '---------- L
-1.5 107 -1 107 -5 106
0
5 106
1 107 1.5 10
Electric field (V/m)
(a)
0.002
0
_
c
<,
8
c
-
0.002
CD
D)
| -0.004
.8
■O -0.006
0
1■ -0.008
CD
-
-
0.01
0.012
-1.5 107 -1 107 -5 106
0
5 106
1 107 1.5 107
Electric field (V/m)
(b)
F igu re 4-13 F ie ld ind uce d b ire frin g e n ce in P L Z T th in film s w hen the e le c tric fie ld (a) n o rm a l to
the o p tic axis and (b) along the o p tic axis.
85
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4.4.2 Intensity output characteristics and design considerations for
miniaturized MZMs with tunable EO coefficients
Here we calculate the intensity outputs o f 5.3 pm th ick P L Z T th in film M Z M s w ith
electrode length (L ) o f 3500 pm operating at x=i.5 nm. For the case where the electric fie ld
norm al to the optic axis, we have proposed a form ula (4-28) fo r the intensity output. For
the case, where the electric fie ld along the optic axis, the in te n sity is sim ply cos (0 /2 )
where O is the phase difference between tw o arms. The in ten sity outputs o f P LZ T M Z M s
fo r the tunable EO coefficients are shown in Figure 4-14. In Figure 4-14 (a), the
“ chirping” and “ unchirping” phenomena are also observed. M oreover, it exhibits the
asym metry o f the intensity-voltage characteristics w ith respect to the central peak
position and the drop o f the m odulation depth at high fields w hich the previous
researchers failed to explain using a fixe d K e rr coefficie nt and the presence o f depletion
layer thickness [97]. The fla t top o f the central peak is actually a result o f “ pseudo-quartic
EO effect” (AnocE4), since tanG in Eq. (2) is proportional to pE at lo w fie ld [87]. Since the
sw itching slope is ve ry steep, it m ay be useful in designing M Z M s w ith u ltra -lo w d rivin g
voltage.
For the case where the direction o f the electric fie ld along optic axis, there is a
huge drop o f peak intensity as shown in Figure 4-14 (b). This can be understood from the
fact that the field-induced birefringence w ill be a positive fin ite value at its peak. I f this
value is not (2N+1)7t, the peak in ten sity w ould be less than one. A lthough the reduction
o f peak intensity has not been observed, it should be perceivable since the observation o f
a positive fin ite value in birefringence is not rare [104], M Z M s should be operated at a dc
bias away from it.
86
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0.8
</>
c
CD
-t—*
C
0.6
0.4
03
E
o
z
0.2
-60
-40
-20
20
0
40
60
Applied voltage (V)
(a)
0.8
’(/>
c
CD
C
3
Q.
•4
-1
3
O
T3
03
N
0.6
0.4
03
E
o
z
0.2
■20 -15
-10
-5
0
5
10
15
20
Applied voltage (V)
(b)
Figure 4-14 Intensity outputs of PLZT MZMs with tunable EO coefficients when the electric
field (a) normal to the optic axis and (b) along the optic axis.
87
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W e have exam ined the im pact o f the tunable EO coefficients o f P LZ T th in film s on
compact E O M ZM s fo r integrated optics. Inte nsity outputs are analyzed and compared
w ith other investigations. Design considerations are discussed in m iniaturized M ZM s.
W e also provide a possible explanation fo r the divergent measurement results from
different researchers.
88
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Chapter 5
Ferroelectric thin films on semiconductors for integrated
optics: deposition and characterization
O ptical integrated circuits w hich compose several m iniaturized devices on a single
substrate m ay o ffe r functionalities, such as filte rin g , sw itching, com bining/branching or
am plifying. It is foreseen that signal processing using optical circuits maybe required fo r
future V L S I circuits. Compared to b u lk devices, optical th in film waveguide devices are
usually cheaper, sm aller and possess w ider bandw idth [88], Ferroelectric th in film device
is one im portant category in the integrated optical components and its integration w ith
sem iconductor is interesting to scientists and engineers. The developm ent o f th in film
optical devices based on ferroelectric/sem iconductor integration is hampered b y several
w ave-guiding and structural requirements: 1. a chem ically stable b u ffe r to prevent As
in terdiffusion 2. su fficie n tly th ick crack-free film s 3. the achievement o f low -loss ridge
waveguides through etching or other techniques [97] 4. the achievement o f the proper
refractive index contrast fo r w aveguiding and to prevent leakage to the substrate and [105]
5. a highly-oriented ferroelectric film is needed to obtain large electro-optic response. We
w ill address these d iffic u ltie s in the fo llo w in g and Chapter 6.
In this chapter, we choose GaAs and Si as the sem iconductor substrates: the form er
currently provide high perform ance lasers and photodetectors w h ile the latter is com patible
89
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to CMOS technology w h ile silico n photonics is prom ising fo r the next generation
chip-to-chip interconnects. BaTiOs and BST th in film s w ould be studied because they have
electro-optic effects [81]. In Section 5.1, we firs t examine the structural and electrical
properties o f a prom ising structure (BaTiO s/M gO /G aAs) and p o in t out its problem ,
cracking in the case o f th ic k M gO . Then, we discuss one solution to the cracking b y grow th
on patterned substrates in Section 5.2. A t last, we examine the optical properties and
surface m orphology o f BST th in film s grow n on SiCh/Si and relate them to the grow th
condition in Section 5.3.
5.1 Fabrication of B aTi03/MgO/GaAs structure and its structural
and electrical properties
Ferroelectric oxides such as LiNbC >3 and BaTiCb are transparent in the visib le and
infrared [81] and possess strong electro-optic coefficients m aking them attractive fo r active
and passive optical components. LiNbCb is usually chosen as the w aveguiding m aterial
due to its technological m atu rity and lo w cost. However, it is d iffic u lt to integrate high
speed electronics and LiNbCb optical m odulators on the same sem iconductor substrate
because the v o la tility o f L i during the grow th [106].
BaTiCb has a nearly-cubic crystal structure com patible fo r GaAs integration.
However, the integration o f perovskite oxides w ith GaAs is s till challenging due to the
requirements o f elevated grow th temperatures, significant la ttice m ism atch,
and
in co m p a tib ility o f arsenic w ith the oxides and oxygen w ith GaAs. M gO provides a
potential means o f fa cilita tin g the integration o f perovskite oxides on GaAs, where there is
a near 4:3 commensurate lattice match, lo w M gO deposition temperatures, and an
excellent in te rd iffu sio n barrier in M gO. Previously, high q u a lity cube-on-cube oriented
(001) M gO film s on (001) GaAs at low grow th temperatures (~350°C) have been achieved
90
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
by pulsed laser deposition [107], electron beam evaporation [108], and molecular beam
epitaxy using molecular oxygen [109]. Requirements o f thin layers and control over the
oxygen sensitive GaAs/MgO interface suggests that growth techniques such as molecular
beam epitaxy are best suited to provide the desired properties fo r M gO buffer layers.
Parameters o f these materials in the bulk form are given in Table 5-1.
Refractive
index at 1 pm
GaAs
MgO
3.503
1.723
BaTiO j
2.332
Lattice constants
at room
temperature (nm)
0.5653
0.421
a=b=0.3992
c=0.4032
Coefficents o f
thermal expansion
(CTE) at 700 K
6.9x1 O'6
14x106
E (Young’s
modulus)
GPa
86
317
12.1x106
100
Poisson’ s
ratio
0.31
0.29
0.29
T able 5-1. Parameters o f the o xid e and sem iconductor m aterials in b u lk fo rm studied in this w ork.
MgO was grown on GaAs by molecular beam epitaxy using an elemental
magnesium effusion cell and an electron cyclotron resonance (ECR) source for oxygen
(2.45 GHz, 250 W ). Previous experiments have reported that a high oxygen flu x is required
to prevent the growth o f three-dimensional features from M g-GaAs reaction and
subsequent polycrystalline growth o f M gO w ith rough surface m orphology [110], where
the usage o f atomic oxygen here is expected to provide greater benefit in this respect than
molecular oxygen. After growth o f the M gO layer, h a lf o f the sample was loaded into a
vacuum chamber for the deposition o f BaTiO^. BaTiC >3 was deposited by pulsed laser
deposition using an excimer laser (X=248nm, 25ns pulse width, 10Hz, ~2 J/cm2) at a
substrate temperature o f 600°C. The thickness o f the B aTi 03 layer was determined to be
150nm from spectroscopic reflectance measurements. X -ray diffraction measurements
(0-20 scan) indicate predominantly c-axis oriented thin film s, as indicated in Figure 5-1.
For the M gO/GaAs sample, a weak (002) M gO reflection is observed, where the low
intensity is believed to be prim arily a result o f the small thickness o f the MgO layer.
91
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20
30
40
50
60
70
80
20 (Degrees)
Figure 5-1 X-ray diffraction scans of the BaTi03/MgO/GaAs structure.
Metal-oxide-semiconductor capacitors were fabricated
on BaTiCb/MgO/GaAs
samples b y depositing T i/P t/A u (20/100/200 nm) electrodes w ith a diameter o f 250 pm
using a shadow mask. The metal/BaTiCE/MgO/GaAs capacitors show insulating behavior
w ith leakage currents less than In A . Leakage current fo r these capacitors becomes
significant above +4V. The polarization versus applied voltage (P-V) characteristics was
measured using a Radiant Technologies 66A Ferroelectric Test System. The P-E
characteristics for varying applied voltage are shown in Figure 5-2 fo r applied voltages o f
IV , 2V, and 3V, where P -V curves become distorted at and above 4V due to leakage
current. A non-saturating P -V loop is observed w ith a remanent polarization o f ~0.4
92
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pC/cm2 for a 3V loop. The P -V characteristics are sim ilar in shape and magnitude to
previous m etal-ferroelectric-insulator-semiconductor reports o f SrBi2Ta 2C>9 on silicon
[111] and SrBi 2Ta 209 on GaAs [112]. The reduced polarization may be the result o f
nonoptimal deposition conditions or interdiffusion problem. The capacitance-voltage
characteristics o f the metal/BaTiCh/M gO/GaAs capacitors are shown in Figure 5-3. C -V
characteristics were measured in the forw ard and reverse direction after holding the sample
at -5V and +5V , respectively, for several minutes. The oxide capacitance is determined to
be 119 pF, resulting in an effective dielectric constant o f A5.5s0 fo r the BaTiC^/M gO
dielectric stack. The values obtained outside o f the range o f - 4 V to + 4 V were determined
to be a rtific ia lly large due to leakage current, and are therefore not shown. A n
anti-clockwise hysteresis is clearly observed in the C -V characteristics w ith a voltage shift
or “ memory w indow ” o f approximately 2 V . The anti-clockwise direction o f the hysteresis
is consistent w ith switching polarization in a ferroelectric, as opposed to slow trap states at
an interface. The existence o f such a sizable memory w indow given a small measured
remanent polarization is consistent w ith earlier theoretical prediction that the w indow is
prim arily determined b y the coercive field [113].
93
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2.0
BaTiO /MgO/GaAs
CM
|
0.5
o
0.0
s:
- o .5
-
2.0
2
1
0
2
1
3
Voltage (V)
Figure 5-2 Polarization versus applied voltage for a metal/BaTi03/MgO/GaAs capacitor under
IV, 2V, and 3V bias range.
120
118
BaTi03/MgO/GaAs
f = 1 MHz
q.
J
116
CD
O
c
CD
114
110
108
■ 4 - 3 - 2 - 1 0
1
Voltage
2
3
4
Figure 5-3 Capacitance-voltage characteristics of a metal/BaTiOa/MgO/GaAs capacitor
demonstrating hysteresis.
94
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5.2 Growth of ferroelectric thin films on patterned substrates
In the previous section, the M gO and B a T i 03 thicknesses are very thin (M gO : 16 nm;
BaTi 0 3 : 150 nm). T hick (~ 0.5 pm) B a T i 03 thin film s on GaAs substrates usually crack or
peel as shown in Figure 5-4. The cracking or peeling o f thin film s is form idable to the
making o f optical thin film waveguides w hich usually require -0 .5 thick film s.
Figure 5-4 Thin film cracking of observed -5000 nm thick BaTi03 on MgO/GaAs substrates.
In this section, we study the thin film stress in the integration o f B a T i 03 w ith GaAs
substrates. GaAs (001) substrates were patterned p rior to deposition (Figure 5-5) into long
(~1 cm) ridges w ith widths from 3 to 60 pm and depths ranging from 80nm to 1pm by
conventional photolithography and reactive ion etching (RIE). M gO and B a T i0 3 thin film s
were deposited b y pulsed laser deposition (PLD ) using a a K rF excimer laser (wavelength=
248 nm, pulse w idth= 25 ns, fluence -3 -5 J/cm ) laser and a vacuum chamber. M gO thin
film s were deposited b y a M g target under an oxygen pressure o f 1.5x1 O'4 T orr at a
substrate temperature o f 350-450 °C. B aTi 03 thin film s were deposited using a B a T i0 3
target, an oxygen partial pressure o f 10 m Torr, and a substrate temperature o f -600-700 °C.
95
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X -ray diffraction data indicates h ig h ly c-axis oriented polycrystalline thin film s. The
refractive indices o f the M gO and B a T i 03 thin film s were measured to be 1.71 and 2.42
(near the b u lk value) b y ellipsometry.
Figure 5-5 Schematic illustration of patterned GaAs substrates used in this work.
The effects o f thermal expansion mismatch for BaTiOs deposition and M gO deposition
were investigated separately.
M gO/GaAs layers o f thickness 20 nm and 80 nm were
examined b y scanning electron microscopy (SEM) before and after a 2 hr thermal
annealing between 600 °C -700 °C to simulate the thermal conditions o f B aTi 03 deposition.
SEM images (Figure 5-6(a)) showed that wider (60 pm ) ridges were peeled for the 80
nm -thick film annealed at 700 °C, but the w ider ridges were not peeled for 20 nm annealed
at 600 °C(Figure 5-6 (b)). These results suggest that BaTiCb deposition on these layers
would lik e ly result in peeling, and that thinner M gO and/or a reduced temperature
difference between M gO and BaTiOs deposition w ould be needed. To achieve this, a
thinner M gO thin film (20nm) deposited at 450 °C and B a T i 03 deposition at 600 °C were
investigated.
96
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B a T i 03 thin film s were deposited on M gO/G aAs thin film s using the process defined
above (20 nm M gO deposited at 450 °C, B a T iO j deposited at 600 °C). N o peeling is
observed on w ide and narrow ridges but significant cracking is observed on wide ridges as
illustrated in Figure 5-7(a). Cracking is observed in all directions in-plane for the wide
ridges, but is greatly reduced and confined to the direction perpendicular to the patterned
ridge for narrow patterns (in Figure 5-7 (b)). The reduced cracking is attributed to strain
re lie f in the direction perpendicular to the patterned ridge, where the strain p rofile becomes
more uniaxial. The strain profile perpendicular to the patterned features, and corresponding
cracking, w ill be described in more detail in the follow ing. The cracking does not appear to
have a strong dependence on etch depth for the dimensions studied, but peeling appears
more prevalent for wide ridges w ith deeper etches. The mechanism is not understood at this
time.
M gO buffer deposition is usually very slow so it is d iffic u lt to obtain a thick layer
which is required for low loss waveguides.
The refractive index o f A lxOy is about 1.7 and
can be formed b y the wet oxidation o f AlGaAs. To incorporate thick layers o f A lxOy,
oxidized G aAs/AlGaAs heterostructures (1 pm thick AlG aAs) were used in place o f GaAs
substrates. The heterostructures are patterned b y R IE and oxidized at 420 °C for 2 hrs prior
to deposition o f M gO and B aTi03 layers.
Narrow ridges appear crack-free after SEM
analysis (Figure 5-8 (a)), follow ing sim ilar behavior to experiments on GaAs substrates.
The stress in thin film s may be caused by thermal stress and intrinsic stress. Since the
deposition temperature is high, intrinsic stress is assumed to be negligible. Cracking
97
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(b)
Figure 5-6 (a) SEM image of 80 nm thick MgO film on GaAs annealed at 700 oC (b) image of 20
nm thick MgO film on GaAs annealed at 600 oC.
98
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narrow ridge
* .j j
-*V
» *
w
' flr',
^
V
~
^
r
>
:
y . r
■~ *:?
. . . .
-- -
f/»
.
■MMl
■• • . > , —
-•‘t i,-: ..
£ ;y
* £
/
.
'« ’ ,*
-
'4
■ 2 |jm
1
(b)
Figure 5-7 SEM images of 0.55 pm BaTiCf thin film on MgO/GaAs (etch depth~0.5 pm)
showing (a) cracking in arbitrary directions on wide stripes and (b) predominant cracking
perpendicular to patterning on a narrow stripe.
99
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(b)
Figure 5-8 (a) Top view and (b) cross section SEM of waveguides with inclusion of AlxOy layers.
100
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perpendicular to the patterned ridges is observed fo r all cases. Cracking w ith a component
parallel to the ridges w ill be investigated here. The x-component thermal stress is analyzed
using a sim plified 1-D model developed b y E. Suhir [114]. The variables G, d0, Td, To, a/,
as, hf, hs, Ef, Es, Vf, and vs denote shear modulus o f the buffer (124 GPa for A lxOy), buffer
(adhesive) thickness, deposition temperature, room temperature, thermal expansion
coefficient, thickness, Y oung’ s modulus and Poisson’s ratio o f the top layer film (subscript
f) and the substrate (subscript s), respectively. The film stress is symmetric on the ridge and
the stress on the left-hand side (Figure 5-5) induced by thermal expansion mismatch can be
expressed b y [115]
p m
er(x) =
(5-1)
hf
where interfacial shear stress o f the trilayer system is
t(x)
G ( a f - a.)(T . - T n) sinh Bx
= — - 1------— ----- ---------—
d0f3 cosh(2/?/)
(5-2)
^
’
and the parameter (3 is
-,1/2
P =
(5-3)
kEX
E f hf J
In our analysis, the 20 nm thick M gO layer is neglected because it is much thinner
compared to the A lxOy layer. The film stress p rofile for a 0.4 pm thick layer on a 500 pm
thick substrate w ith 1 pm thick A lxOy buffer is shown in Figure 5-9(a) fo r varying ridge
w idth. From these calculations, the maximum film stress (occurring at the m iddle o f the
ridge) is obtained, as shown in Figure 5-9 (b). The critical strain energy release rate T is
defined as
101
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2<j2h f
r = — -^ -(J /c m
)
(5-4)
Ef
where o c is the thickness-dependent critical stress. Given different critical strain energy
release rate, the critical stresses may be determined as a function o f the film thickness as
shown in Figure 5-10 (a). Moreover, by comparing the m axim um film stress (Figure 5-9
(b)) on ridges on different widths and critical stress Figure 5-10 (a)) fo r a particular layer
thickness, we can determine “ critical w idth” beyond w hich the film w ould crack parallel to
the ridge. In Eq. (5-2), the shear stress is independent o f the CTE o f the buffer layer
(adhesive) [115]. Assuming the critical strain energy release rate=7 J/cm2, we obtain the
critical w idth as a function o f the B aTi 03 thickness Figure 5-10 (b)). Since the film
thickness for a particular ridge always increases during the growth, the slight upward
bending o f the curve is not physical and should be discarded in real applications. The
experimental crack spacing is around 7 pm, suggesting that the cracks negligibly interact
[116]. The calculated value o f the critical w idth is 7-8 pm which is close to the
experimental crack spacing (~7pm ) in the longitudinal direction. The experimental critical
w idth in the transverse direction is 10-20 pm. The model presented here is only
considering one dimension, and the estimation o f the material parameters and critical strain
energy release rate is probably not h ighly accurate, but the trend is consistent to the
experimental data presented here. The cracking depends on ridge w idth and can be reduced
by narrow ridges. Second, the peeling effect should be avoided w ith a lower deposition
temperature. The cracking does not appear to have a strong dependence on etch depth for
the dimensions studied, though a few scattered ridges exhibited peeling that appeared to be
more prevalent fo r wide ridges w ith deeper etches. This behavior is not understood at this
time.
102
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7 10
(0
(/)
0)
/=30 /an
</)
1=20 /an
1=10 /Mi
1=3 /an
■L_
*-«
E
0
0
5
10
15
20
25
30
35
Distance from the edge( jim)
(a)
6 109
i
i
(0
't/T
</>
hf -0 .2 fjm
5 109
— hf =0.4 ftm
— hf =0.6 /urn
+-> 4 109
(0
E
E
3
3 109
hf =0.8 fun
hf -1 .0 /an
-
2 109
E
*
re
/ / ' ' ' '
1 10°
/ 'S < ■
0
0
'
I
I
I
5
10
15
20
25
30
35
40
Ridge width(^m)
(b)
Figure 5-9 (a) Film stress distribution on for ridges o f varying width and film thickness= 0.4pm
with growth temperature o f 600 °C. (b) Maximum film stress versus ridge width for different film
thicknesses.
103
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2 10
r = 0.05 J/m
r - 0.1 J/m2
r = 0.5 J/m2
r = 1 J/m2
'
w
r - 5 J/m2
ft
« 5 108
0
0
0.2
0.4
0.6
0.8
1
Film thickness(jam)
(a )
7.68
7.66
%
7.64
“D
7.62
7.6
7.56
7.54
0
0.2
0.4
0.6
0.8
1
Film thickness(ium)
(b)
Figure 5-10 (a) Critical stresses as function of BaTi03 thicknesses, (b) Critical widths as a
function of BaTi03 film thickness with 1 gm thick AlxOy for f =7 J/cm2 at the deposition
temperature 600 °C
104
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5.3 Pulsed laser deposited ferroelectric thin films on S i0 2/Si for
optical waveguiding
Silicon photonics is prom ising fo r future optical interconnects [122], However, Si
crystal is centrosymmetic and does not show Pockels effect as GaAs. To achieve Pockels
effect for silicon photonics, ferroelectric thin film s could be fabricated on SiC>2 (2000
nm )/Si substrates where the SiC>2 layer acts as an optical buffer to ensure waveguiding. In
this section, we study the relationship between the optical film properties and the surface
morphology. W e pick BST instead o f B aT iC f because BST thin film s usually have less
particulates in our PLD system and it also possesses the EO effect[117]. Moreover, the
hysteretic EO effect is much smaller compared to B a T i 0 3 . However, it should be
remembered that the crystal quality on Si02 is inferio r to the M gO/GaAs and it may
indicate a weaker EO effect.
First, BST thin film s were prepared by pulsed laser deposition on Si02 (2000 nm)/Si
substrates under four different conditions. It should be noted that the Si02 buffer thickness
should be thick enough to prevent leakage to the silicon substrates. The deposition
parameters are listed in Table 5-2. Optical microscopic photographs o f the film s are shown
in Figure 5-11. A ll the film s have some particulates. In addition, sample 396 has cracks on
the film . Figure 5-12 shows the X -ray diffraction pattern. A ll o f them are poly crystalline. It
can be found that higher temperature and low er oxygen pressure promote (100) oriented
growth. Rutherford backscattering is used to examine the chemical composition. The
composition ratios o f Ba, Sr and T i are shown in Table 5-3. W e can find that 393 (high T,
high P) and 394 (low T, lo w P) are o f the most accurate stochiometry w hile 392 (low T,
high P) is oxygen-rich and 396 (high T, lo w P) is Ti-rich. Am ong them, we expect that
sample 393 has the largest EO effect because film s grown at low er growth temperature and
105
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T i-ric h film s usually possess low er dielectric constants [118] and the EO effect is roughly
proportional to the dielectric constant [19].
Sample number
Growth temperature(°C)
392
393
394
396
550
650
550
650
Oxygen
Pressure
(m Torr)
30
30
6
6
Laser repetition rate
(Hz)
10
10
10
10
Table 5-2. Deposition parameters o f BST films on 4 silicon substrates.
sample
Ba
Sr
Ti
O
392
0.100
0.090
0.190
0.620
393
0.100
0.100
0.200
0.600
394
0.100
0.100
0.200
0.600
396
0.090
0.090
0.220
0.600
Table 5-3. Composition ratios o f BST films on 4 silicon substrates.
Thickness
(T,P)
njE
n-[M
dn
njE
nTM
dn
Loss
633nm
633nm
633nm
1550nm
1550nm
1550nm
(dB/cm)
392
.3628
L,H
2.1868
2.2342
.0474
2.1030
2.1543
.0513
6.63
393
.4343
H,H
2.1996
2.2578
.0582
2.1905
2.1181
.0724
22.83
394
.3265
L ,L
2.3293
2.3310
.0017
2.2575
2.2383
.0192
5.61
396
.3601 (crack)
H ,L
2.3340
2.3258
.0082
2.2363
2.2273
.009
"
Table 5-4. Refractive indices (TE and TM) and loss measured by the prism coupling technique.
106
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(a) 392: 550 °C, 30mTorr
(b) 393: 650 °C, 30 mTorr
I
I
100 |Jin
(c) 394: 550 °C, 6 mTorr
(d) 396: 650 °C, 6 mTorr
Figure 5-11 Optical microscopic photographs of 4 samples under different growth conditions. A ll
are shown in the same magnification.
107
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Intensity (arb. units)
:
1000
.n
1000
c6
Wo
DQ CN
20
30
40
50
60
70
20
30
40
20 (Degrees)
60
20 (Degrees)
(b) 393: 650 °C, 30 mTorr
(a) 392: 550 °C, 30 mTorr
Intensity (arb. units)
50
1000
C
D
1000
_Q
k_
03
CO
c
a>
40
50
60
40
20 (Degrees)
50
60
20 (Degrees)
(c) 394: 550 °C, 6 mTorr
(d) 396: 650 °C, 6 mTorr
Figure 5-12 XRD results o f four samples under different deposition conditions.
108
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70
80
Then optical measurement using prism coupling was conducted on four samples to
obtain the refractive index and the loss. The loss measurement setup based on the m oving
fiber probe approach is shown in Figure 5-13 and the result fo r X= 1553 nm is shown in
Figure 5-14. W e can see that sample 393 is the most lossy one w hile sample 392 or sample
394 exhibit 6.63 and 5.61 dB/cm. It should be noted that all samples except sample 396, we
used m inim um gain. For sample 396, we had to use m axim um gain and 396 had an
intensity o f about 4% o f the other samples. However, the intensity from sample 396 did not
change much when the angle o f incidence on the prism changed so most o f the intensity
was not due to the guiding mode. Thus, we must ignore sample 396 when discussing the
loss.
Propagating Mode
Laser Beam
Fiber Probe
x
to Detector
Figure 5-13 Loss measurement setup based on prism coupling. (Courtesy o f Metricon Corp.)
To find the relationship between loss and the growth condition, we can start w ith the
chemical composition. Based on Table 5-3, we know sample 393 and sample 394 have
sim ilar composition ratio but very different loss values. It may be concluded that the
correlation between the composition and the loss is weak. W e also conducted atomic force
microscopy (A F M ) on four samples. A F M images and the values o f rms roughness are
109
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shown in Figure 5-15. A lthough there is an approximate theory relating surface roughness
and scattering loss [119], we can not find the direct relationship between the roughness and
loss for our samples. This is probably because their roughness values are much smaller
than the wavelength in the loss measurement (A,= 1553 nm). However, one interesting point
is that sample 393 has some large size (~1 pm) “ h ills ” and they are about the length o f the
optical wavelength ((X= 1553 nm). This may explain w hy sample 393 is the most lossy
sample.
300
350
E 300
£250
V a lu e
< 250
Va lu e
Error
5.2 5 7 2
0 .0 1 6 3 4 5
245.31
200
C hisq
150
Chisq
NA
o 150
®100
®
0
0.5
1.5
1
100
2.5
2
0
0.5
P ro p a g a tin g d is ta n c e (c m )
1
1.5
2
2.5
3
3.5
P ro p a g a tin g d is ta n c e (c m )
(a) 392: 550 °C, 3 0m T orr
(b) 393: 650 °C, 30 m Torr
300
250
c 250
=>200
Chisq
Value
Error
1.2924
0 .0 4 3 3 8 3
75 62 3
NA
150
NA
® 100
50
0
0
0.5
1
1.5
2
2.5
P ro p a g a tin g d is ta n c e (c m )
0
0.2
0.4
0.6
0.8
1
1.2
P ro p a g a tin g d is ta n c e (c m )
(d) 396: 650 °C, 6 m Torr
(c) 394: 550 °C, 6 mTorr
Figure 5-14 Loss measurement results for wavelength= 1553 nm light o f four samples under
different deposition conditions. (Loss (dB/cm)=10xlogio(e'm3))
110
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(c)
394: 550 oC, 6 mTorr,, rms roughness=3.4 nm
(d) 396: 650 oC, 6 mTorr, rms roughness=10.7 nm
Figure 5-15 AFM images of four samples deposited under different deposition conditions.
A short summary o f this study is listed below:
• Low pressure-> high refractive index (which im plies high packing densities) and this was
also observed for other film s [120]; lo w index anisotropy
• Low temperature -> lo w loss
• (Low temperature, high pressure) -> worst crystallinity
111
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• (High temperature, lo w pressure) -> may induce cracks
• (H igh temperature, high pressure) and (low temperature, lo w pressre)-> better
stochiometry
• Also, it is expected that (650 °C, 30 m Torr) has the largest EO effect because film s grown
at lower growth temperature and T i-ric h film s (650 °C, 6 m Torr) usually possess lower
dielectric constants [118], Unfortunately, film s grown at this condition (650 °C, 30 m Torr)
also exhibit very high loss.
In conclusion, the best condition fo r lo w loss and high quality BST film s probably is
low temperature and lo w pressure (550 °C, 6 mTorr). I f one pursue the largest EO effect,
probably (650 °C, 30 m Torr) is the best but the loss w ould be very high.
In this chapter, we began w ith the BaTiO i/M gO /G aAs structure and its thin film
properties. Then, we present “ the growth-on-pattemed-substrates method” as a way to
solve the cracking issue and analyze the problem using a stress model. A t last, we used
PLD to fabricate BST/Si02/Si structures under four conditions and found the condition for
the lowest loss and the condition for the highest EO effect.
112
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Chapter 6
Monolithic integration of ferroelectric optical channel
waveguides and semiconductors
Currently, electrooptic modulation by ferroelectric waveguide modulators is favored in
optical communications and many sensing applications and it may offer potential
applications in optical interconnects for integrated circuits. Figure 6-1 shows the block
diagram o f modern fiber-optic transmission system. Integration o f ferroelectric EO
waveguide modulators w ith semiconductor active optoelectronic devices as shown in
Figure 6-2 may provide an enabling technology fo r optoelectronic integrated circuits
(OEIC) and optical interconnections in semiconductor V L S I circuits. Compared to other
OEIC candidates such as photonic crystal modulators [121] or m icroring modulators [122],
ferroelectric optical waveguides on semiconductors could offer the benefits o f low cost and
easy fabrication.
For the waveguide design, we use the effective index method to design our waveguides.
For our material structure, the ridge height is usually 0.1-0.5 pm and the width is -3 pm.
The optical buffer should be around one optical wavelength thick [68],
In this chapter, we begin w ith the challenges fo r fabricating channel waveguides on
semiconductors in Section 6.1. In Section 6.2, we w ill present wide (-1 0 pm ) waveguides
113
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based B a T i 03 /M g 0 /G a A s/A lx0 y/GaAs. In Section 6.3, we present results for nearly
single-mode (~3 pm wide) BST strip-loaded waveguides integrated w ith silicon dioxide/
silicon substrates.
Laser
D iode
M ultiplexer
D river
A m p lifie r
MZI m od ulator
V
O utpu t
Figure 6-1 Block diagram o f optical transmitter system.
Detector
Integrated Mach-Zehnder type
EO modulator
Laser
Figure 6-2 Schematic drawing o f integrated optoelectronic system with the ferroelectric thin film
M Z I modulator.
114
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6.1 Fabrication challenges of ferroelectric thin film channel
waveguides
The BaTiCh/MgO/GaAs structures studied in the last chapter is problematic for integrated
optics applications due to the in a b ility to obtain sufficiently th ick M gO cladding layers for
optical waveguiding and the presence o f cracking in the thin film s that are lik e ly a result o f
thermal expansion mismatch. In this session, the use o f 1 pm thick A lxOy buffer is
presented. Also, the technique o f deposition on pre-pattemed substrates relieve strain and
to reduce the need fo r post-deposition patterning.
The second challenge to form channel waveguides is the d iffic u lty o f etching
ferroelectric thin film s due to the redeposition and the need o f very high power plasma.
Figure 6-3 shows ferroelectric thin film waveguides fabricated b y wet BHF etching and
C^-based reactive ion etching o f the ferroelectric thin film . As can be seen, the sidewalls
are extremely rough and may create a lo t o f loss or even hamper the waveguiding. Other
methods are required to form channel waveguides. In the fo llo w in g two sections, we w ill
discuss two possibilities: growth on patterned substrates and strip-loaded waveguides.
These two methods can provide suitable optical waveguiding.
Another challenge is the facet formation. Mechanical cleaving o f semiconductor
substrates is the usual way to define the at the cavity ends o f edge-emitting lasers.
However, ferroelectric thin film s usually very brittle and direct cleaving may generate
cracked edges as shown in Figure 6-4.
115
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(b)
Figure 6-3 Ferroelectric thin film waveguides formed by (a) wet etching and (b) dry etching of the
ferroelectric thin films.
116
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(c)
Figure 6-4 BaTi03waveguide facets formed by (a) direct cleaving (b) diamond polishing (c)
focused ion beam etching.
The last challenge is that EO channel waveguiding devices are usually long (typically
several m illim eters long) comparable to electronic devices. Defect densities need to be
m inim ized to ensure a reasonable yield because any m icron sized defect may k ill the whole
waveguide i f it is single-mode. In Section 6.2, the narrowest waveguide is —10 pm wide
because defect densities were high due to the wet oxidation process. In Section 6.3, single
mode waveguides w ill be presented.
117
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35
30
T=288K
600x8p.m
CL
10
cleaved, lth=7.2mA
5
0,
0
FIBed, I =8.8mA
200
100
300
Current (mA)
Figure 6-5 Edge emitting laser output characteristics of lasers with cleaved facet and FIBed facet.
[Courtesy of Jun Yang]
6.2 BaTi03 optical waveguides on GaAs fabricated by pulsed
laser deposition
6.2.1 Fabrication of BaTi0 3 /AlxOy/MgO/GaAs structure
The structural design for optical waveguiding in BaTiCF thin film s on GaAs is shown
schematically in Figure 6-6 (a). Design features the incorporation o f A lxOy buffer layers
obtained through the wet oxidation o f AlG aAs [123]. This technique provides a means o f
achieving a thick buffer layer w ith low refractive index (n ~ l .7), w h ile m aintaining a thin
GaAs layer fo r the growth o f M gO buffer layers and the BaTiC >3 thin film o f interest.
O ptical confinement in the lateral direction is achieved in this design through
photolithographic patterning. Simulation using commercial software (A pollo Photonics)
118
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based on the effective index method shows that optical waveguiding can be achieved as
shown in Figure 6-6 (b).
The fabrication procedure for achieving the designed ridge waveguide structures is
outlined in the follow ing. First, GaAs/Alo.98Gao.o2As/GaAs layers (30nm/500nm/200nm)
were grown b y m olecular beam epitaxy on GaAs (001) substrates. Ridge waveguide
patterns o f w id th ranging from 3pm to 60pm were defined b y standard photolithography
and transferred using reactive ion etching using a B C f/A r etch chemistry. Samples were
etched to depths ranging from 80nm (just through the GaAs cap) to 1pm (completely
through the AlG aAs). A wet oxidation step was then performed at a temperature o f 420 °C
for 1 hr to oxidize the AlG aAs material under the ridges. A fte r oxidation, M gO and B a T iO j
thin film s were deposited by pulsed laser deposition using an excimer laser (X=248nm,
25ns pulse w idth, 10Hz, ~2 J/cm2). The M gO buffer layers were deposited at a substrate
temperature o f 450°C and oxygen partial pressure o f 1.5x1 O'4 using a M g target. The
B aTi 03 thin film s were deposited at a substrate temperature o f 600°C and oxygen partial
pressure o f lO m T using a B a T iO j target. The estimated thickness o f M gO and B a T i 03
were determined to be 20nm and 1000 nm, respectively.
ridge width
MgO buffer
GaAs cap v
etch depth
BaTiO
IALO
GaAs (001)
(a)
119
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i (\/
i'
5?
f
.
/ \'
rv r.. :......
:
\\h[
5
6
Y(urri)
7
:
i
' ............ I-
1
:
2
3
4
6
9
(b)
Figure 6-6 (a) Schematic of BaTi03ridge waveguide structure, (b) simulation of the electric field
of TEOO mode in the waveguide based on the effective index method.
6.2.2 Characterization
X -ray diffraction scans ( 6-29) were used to determine the crystalline characteristics o f
the thin film s. A m ajority o f the surface area o f the patterned samples contains regions
outside o f the ridges, where x-ray diffraction scans w ill probe material deposited on the
etched and oxidized amorphous A lxO y surface. The thin film materials deposited on the
GaAs surface on top o f the ridges are o f higher quality, but cannot be probed directly by
x-ray diffraction in the absence o f advanced focusing optics. B aTiC VM gO thin film
deposition on GaAs substrates was used as an indicator o f material quality deposited on top
o f ridges, where an x-ray diffraction scan o f B aTiC VM gO thin film structure deposited on
GaAs under sim ilar conditions is shown in Figure 6-7.
120
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“ i—
i— i—
r-
- I—
I—
I—
T1r
f—
■ 't
CM
o
O
</)
-t—«
o
'c
w
X!
<ro
o
o
O
O
I—
03
CM
<ro
O
CD
o
I—
CD
CD
'w
c
o
c
O
0
CD
u
20
1
30
i
40
50
20 (degrees)
i__
60
70
Figure 6-7 X-ray diffraction scan of BaTiC^/MgO thin film on GaAs (001) demonstrating
crystalline material with preferred orientation.
A selected sample w ith 1pm thick B a T i 0 3 , 20 nm thick M gO , and an etch depth o f
100 nm was studied for optical waveguiding. This sample was thinned mechanically to
200pm, scribed, and cleaved to provide facets fo r optical coupling. V isible light was
coupled into the B aTi 03 ridges by end-firing w ith a frequency doubled N d :Y A G laser
(X=532nm). A n elliptical output beam was observed when coupling lig h t into the
waveguides, where the output captured by a digital camera fo r a 10 pm wide ridge o f
length 1mm is shown in Figure 6-8(a). The output beam is m ultim ode, where numerous
guided modes are estimated using the effective index approximation. A n intensity plot
constructed from the digital image is shown in Figure 6-8 (b). The output light shows
additional optical guiding adjacent to the elliptical output beam. The additional lig h t output
may be attributed to guiding in the A lxOy layer or BaTiC >3 material on the sidewalls o f the
ridges. A measurement o f optical power travelling through coupling and collection optics
w ith and w ithout the waveguide present resulted in values o f 110 m W and 6 m W . The
121
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observed efficiency o f 5.45% represents coupling loss, reflection and losses at facets, and
waveguide losses. L ight scattering in the waveguide material was observed at defects
present in the BaTiCb material produced b y the pulsed laser deposition process.
(b)
Figure 6-8 Output from a 10 pm wide B aTi03 ridge waveguides (a) captured by a digital camera
and (b) intensity plot constructed from the digital image.
6.3 Strip-loaded BST optical waveguides on S i0 2/Si fabricated by
pulsed laser deposition
In the previous section, we explored the possibility o f A lxOy using wet oxidation.
However, this method usually generates a lo t o f micro-sized defects on the sample and is
undesirable fo r single-mode waveguides (~3 pm wide). In this section, we use thick SiC>2
122
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buffer and strip-loaded structure to form channel waveguides w h ile avoiding the etching o f
ferroelectric thin film s. It should be noted that the crystalline quality is o f course worse
than the BaTiCVAlxOy /M gO /G aAs structure.
BST was chosen here because the particulates were usually much less than those on
BaTiCb thin film s. BST film s were grown on top o f 2 pm thermal SiCh/Si substrates at 30
m Torr and 550 °C by PLD. A 500 nm thick layer o f SiC>2 was deposited by plasma
enhanced chemical vapor deposition (PEC VD ) at 200 °C. Then 3, 5 and 7 w ide ridges are
formed by plasma etching o f SiC>2 w hich the BST layer acts as an etch stop. Then the
sample is cut into small pieces w ith the waveguide length o f ~ 3 mm. The waveguide cross
section schematic, the SEM picture after FIBed-facet form ation and the sim ulation result
are shown in Figure 6-9.
BST waveguides are then tested based on the tapered fiber coupling. The setup as
shown in Figure 6-10 (a) consists a laser diode (Santee TSL-220), a tapered fiber, a
microscope objective and a CCD camera. L ight is coupled into waveguides using the
tapered fiber (OZ Optics, TP M J-3A-1550-8/125-1-10-2.5-14-1-AR). The output imaged
b y the CCD camera as shown in Figure 6-10 (b) is single mode. The waveguide (~ 3mm
long) output power is ~ 7 pW w hile the laser output is 2 m W .
123
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*— Width=3 pm— ►
S i0 2 0.5 |jm
BST 0.5 um
S i0 2 2 |jm
Si
(a)
(c)\
Figure 6-9 (a) schematic of waveguide cross section and (b) SEM of the waveguide facets formed
by focused ion beam, (c) waveguide simulation based on the effective index method
124
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CCD camera
Objective Waveguide
Tapered fiber
______
(b)
Figure 6-10 (a) schematic o f waveguide testing setup and (b) CCD images o f the waveguide
output.
In conclusion, novel ridge waveguides consisting o f B aTiC V M gO /G aA s/A lxOy/GaAs
layer structures were investigated. The use o f A lxOy layers and thin film deposition on
patterned substrates provide a means o f obtaining BaTiC >3 thin film s and optical
confinement layers o f suitable thickness fo r optical waveguiding on GaAs substrates. Also,
single-mode strip-loaded BST waveguides are investigated. Optical waveguiding was
observed in these structures showing prim ary optical confinement in the B aTi 03 thin film .
Further study o f optical losses and electro-optic properties o f these structures are desired to
determine their suitability for integrated optoelectronics.
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Chapter 7
Conclusions
7.1 Achievements in thin film deposition and device fabrication
7.1.1 BST thin film microwave capacitors
In the study, we have shown the association o f growth conditions and electrical
properties. BST thin film s were fabricated using PLD under different growth conditions
and annealing conditions. M icrowave tw o-port measurement was employed to separate the
loss from the metal and the film s. The loss tangent o f 0.2 pm thick BST film deposited at
500 °C is lower than 0.012 up to 10 GHz. Tunability o f 2.4:1 is achieved at the bias voltage
o f 15 V . Dielectric loss o f ~0.01 is close to the value in single crystal BST [6].
It is found that the growth temperature affects the crystallinity o f BST thin film s and
the dielectric constants and losses. The low er the growth temperature is, the low er the
dielectric constant and the loss are. However, lo w growth temperature usually produces
small tunabilty. Rapid thermal annealing in oxygen was found to reduce the film loss but
the metal loss increases considerably. On the other hand, nitrogen annealing at 300°C
increases the Q factor below 4 GHz and reduces the dielectric dispersion. The dispersion
improvement maybe beneficial for wideband microwave system design. For the N 2
annealed sample, the figure o f m erit is 27 at 4 GHz and 44 at 10 GHz. This result is better
than those reported results o f BST thin film s deposited by PLD.
126
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7.1.2 BaTi03 and BST thin film waveguides integrated with GaAs or Si
substrates
In this study, we successfully integrate ferroelectric optical waveguides onto
semiconductors w ithout etching the ferroelectric thin film s by two methods: 1. growth on
patterned substrates, and 2. strip-loaded waveguides. W e proved that the stress could be
relieved by growing ferroelectrics on patterned substrates though their thermal expansion
coefficients are very different. U tiliz in g the wet-oxidized thick A lxOy buffer, optical
waveguiding has been demonstrated fo r the BaTiCVM gO/GaAs/AlxOy/GaAs structure.
W e also proposed use focused ion beam (F IB ) to form waveguide facets. The facets
have been demonstrated to be optically flat and FIB is useful fo r waveguide facet
formation, especially for heterogeneous material integration where cleaved facets are not
easy to obtain.
W e also study the dependence o f optical losses o f BST thin film s grown on SiCVSi
under different PLD conditions. D ifferent characterizations tools such as A F M , X R D and
RBS were employed to examine the thin film properties and the prism coupling technique
was used to measure the loss. The roughness (measured by A F M ) is not directly linked to
the optical propagating loss since the roughness o f these film s is sim ilar (~ 5 nm).
However, their lateral dimensions o f microstructures are very different on these film s (>10
times difference). It may be concluded that micron-sized microstructures increase the
scattering loss significantly. Strip-loaded BST single-mode waveguides on SiC^/Si were
also demonstrated. W e prove that ferroelectric optical interconnect is a viable technology
for silicon photonics in future optical interconnects.
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7.2 Achievements in theoretical modeling and device design
In the theoretical modeling, we propose a way to explain the hysteretic and peaked
electrooptic response in ferroelectric thin film s. The nonlinear susceptibility and hysteretic
electrical polarization behavior can explain the tunable electrooptic characteristics. Our
model can predict the hysteretic electrooptic response once the P - V and C -V relationships
are known.
W e also point out that the switching voltages o f EO interferom etric modulators not
only depend on the EO strength but also the electric field direction w ith respect to the
crystal axis in uniaxial crystals. W e propose a simple yet accurate form ula to extract the
linear EO coefficients w hile the electric field is normal to the crystal axis o f the M Z
modulators. For m inim izing the switching voltages, this kind o f analysis should be carried
out to determine the electric field direction w ith respect to the crystal axis because a large
EO strength does not always im p ly a small switching voltage.
W e also find a way to m aximize the extinction ratio fo r the E field normal to the crystal
axis b y choosing a suitable electrode length. Finally, we combine the tunable EO properties
in the analysis o f interferometric modulator analysis. The sim ulation result agrees w ith the
experimental result very well.
7.3 Future work
In the microwave tunable BST capacitors project, we have shown the association o f
annealing and electrical properties. Further optim ization should be continued to improve
the microwave characteristics o f BST capacitors. For example, it was proved that T i-rich
BST film s usually produces lower dielectric loss.
D ifferent target-substrate distances and
oxygen pressures should be tested to see how the T i-rich film s can be achieved and the
128
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relationship between the chemical composition and dielectric properties should be studied.
Also, the interface roughening after annealing could increase the dielectric loss. W e should
study the annealing effect on the interface roughening. In the future, tunable filters and
matching networks based on BST thin film s w ill be fabricated using optimized capacitors.
In the ferroelectric optical waveguide project, the ultim ate goal is to make an EO
switch or EO filter. Optical clarity is one demanding factor in optical waveguide
fabrication. It was found in our study that the optical loss could be very high even the film
looks shiny by naked eyes. Currentlly, our best waveguides shows ~5dB/cm loss. However,
losses smaller than 2 dB / cm are probably required for practical applications [124].
The
prism coupling technique could provide an easy w ay to tell i f the optical scattering loss is
too high and/or the film structure is suitable (cu t-o ff or not, roughness) for optical
waveguiding. So we suggest checking i f film s are suitable fo r fabricating waveguides to
save time. W e should continue to optimize our growth condition to obtain lo w loss
waveguides. One direction is to reduce the oxygen pressure or change the the target
substrate distance.
W e w ill also explore more materials such as silicon nitride and zinc oxide materials
as the buffer layers between ferroelectrics and semiconductors. Although the idea o f wet
oxidized A lxOy is feasible, the yield is actually very lo w due to the generation o f defects
during wet oxidation, especially on long, narrow single mode waveguides. A prelim inary
study has shown PECVD Si02 is not suitable to act as an optical buffer for GaAs substrates
since the large thermal mismatch results in serious cracking and peeling. Silicon nitride is
more prom ising because the thermal mismatch between SiNx and GaAs is much less. On
the other hand, zinc oxide as the optical buffer is also prom ising because o f its low
129
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refractive index and electrical conductivity. The electrical conductivity o f ZnO may lead to
M Z I device w ith vertical electrodes in w hich ZnO serves as the optical buffer and the
bottom electrodes.
130
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