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Journal of Alloys and Compounds 767 (2018) 642e650
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Melting point of Sn as the optimal growth temperature in realizing the
favored transparent conducting properties of In2O3:Sn films
Laxmikanta Karmakar, Debajyoti Das*
Nano-Science Group, Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 May 2018
Received in revised form
7 July 2018
Accepted 11 July 2018
At substrate temperature (TS) close to melting point of Sn (TSn) rapid incorporation of metallic dopants in
significant amount introduces sharp rise in mobility (m) and concentration (ne ) of charge carriers, leading
to substantial reduction in resistivity (r); simultaneous sharp widening in optical gap (Eg ) results in
optimum Figure-of-Merit (F). The Eg vs. ne 2/3 plot demonstrates two distinct regimes of TS across TSn,
leading to higher reduced effective mass of charge carriers, m*vc by 0.072m0 owing to rapid incorporation
of Sn4þ at substitutional site of In3þ in In2O3 matrix. Consequently, the self-energy due to electronimpurity scattering rises and/or the material self-converts to an alloy-like ensemble. On further increase in TS, F reduces due to enhanced optical absorption by metallic dopants and dopant induced
defects, as noted by enhanced Urbach-energy (EU ). Rather than any arbitrary TS, TSn has been demonstrated as an optimal growth temperature for ITO films grown by RF magnetron sputtering.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
ITO
Magnetron sputtering
Melting point of Sn
Burstein-Moss effect
Urbach energy
1. Introduction
Transparent conducting oxides (TCOs) belong to a special class
of semiconductor materials that can simultaneously be both optically transparent and electrically conducting. Those are used mostly
as transparent electrodes in photovoltaic devices [1e10], liquid
crystal displays (LCDs) [11,12], light emitting diodes (LEDs) [13,14],
thin film transistors [15,16] and in many other applications. TCOs
are generally based on metal oxide semiconductors such as In2O3
[17e19], SnO2 [20e26] and ZnO [27e30], and sometimes doped by
metals and halogens [31e36]. An exclusive TCO material obtained
from In2O3 on Sn doping is known as ITO (tin doped indium oxide,
In2O3:Sn) [37e39]. It is yellowish to grey in bulk form, while
transparent and colourless in thin layers, with a wide band gap
(~3.9 eV). ITO is a highly degenerate n-type semiconductor owing to
the oxygen vacancies as well as substitutional Sn dopants.
Compared to other TCO films, e.g., SnO2 and ZnO, the ITO films are
widely used because of their simultaneous low resistivity
(<103 U cm), high transmittance (~90%) in the visible region, high
reflectance in infrared region, and long-term physical stability due
to high substrate adherence, good hardness, and chemical inertness. Thin films of TCO are mostly deposited using electron beam
* Corresponding author.
E-mail address: erdd@iacs.res.in (D. Das).
https://doi.org/10.1016/j.jallcom.2018.07.130
0925-8388/© 2018 Elsevier B.V. All rights reserved.
evaporation [22,24] ion-assisted deposition [40], pulsed laser
deposition [41], sol-gel spin-coating technique [42], thermal
evaporation [43] and DC and RF magnetron sputtering
[28,30,44,45]. Among these processes RF magnetron sputtering
technique is more versatile and widely used commercially by virtue
of the involved advantages of producing high quality thin films
with superior crystalline quality due to very low contamination and
controllable deposition parameter [46e48].
In course of further development of the ITO thin films, two
issues are of concern: (i) the optical transparency and electrical
conductivity of the films hold a trade-off relation which needs to
overcome, and (ii) although a higher substrate temperature
generally facilitates the ordered crystalline orientation, a low
temperature growth of the films is always preferred in device
structures in order to make various inexpensive substrates usable
and/or to keep other component layers of the device unaffected.
Keeping all the necessary requirements in mind the ITO films in
the present investigation have been optimized at a temperature
close to the melting point of Sn, in order to utilize its best
favorable contribution to the growth of properly doped films
having a balanced combination of relevant optical, electrical and
nano-structural properties. The melting point of metallic Sn (TSn)
as a dopant to the In2O3 network in ITO films has been demonstrated to play a key role in the optimization of the TCO properties, which is the novel approach involved in the present
investigation.
L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650
2. Experimental details
The tin doped indium oxide (ITO) films were prepared using an
indigenously developed RF magnetron sputtering system, as
schematically shown in Fig. 1. The stainless steel sputtering
chamber was connected to diffusion pump and rotary pump assembly (Make: Hind High Vacuum Co. India) providing a base
vacuum ~9 10e7 Torr. A 3-inch diameter circular planar Torus®
magnetron sputter source (Make: KJ Lesker, USA) was used with
99.999% purity ITO (90% In2O3, 10% SnO2 by weight) sputtering
target (Make: Vin Karola, USA) with a Cu backing plate connected
by Indium-alloy solder bonding. High purity Ar (supplied by
Matheson, USA), in regulated flow through a mass flow controller
(Bronskhorst, Netherland), was used as the sputtering gas. Ar
plasma was produced by 13.56 MHz RF power from a regulated
power source (Comdel: CX-600S, USA) and the gas pressure in the
plasma during sputtering was measured by Baratron Guage (MKS,
USA). During deposition of the films the substrates were coupled
through a metal mask to a grounded stainless steel holder which
was heated from the back by a coil heater placed in close vicinity.
The temperature of the heater was controlled and monitored by a
K-type thermocouple directly inserted into it and the exact temperature of the substrate, placed on the substrate holder, was
calibrated at different gas pressures. At actual operating conditions,
however, the substrate holder was rotated at 10 rpm by using a
rotary driver, in order to maintain excellent uniformity of the film
growth, and the growth temperature was considered from the
calibration chart. Samples were deposited on Corning® Eagle2000™ glass substrates, properly cleaned by standard procedure
using Extran solution with distilled water (1:50) and subsequent
ultra-sonication with acetone, alcohol, DI water and dried in a
stream of hot air. The substrates were placed on the holder with
surface placed parallel to and 6 cm away from the target. Before the
actual growth of the films the target was cleaned by pre-sputtering
for 5 min, keeping the substrate covered by shutters mechanically
controlled from outside. The thickness of the films was regulated
Fig. 1. Schematic diagram of the RF sputtering system used in the present study.
643
using a quartz crystal monitor (Sycon Thickness Monitor, Model
STM-100/MF). During deposition uninterrupted chilled water
cooling of the magnetron target holder was maintained to prevent
the target from breaking. The optical transmission of the films
prepared on glass substrates was measured using a Varian Cary
5000 double beam spectrophotometer. The X-ray diffraction analysis was carried out using a conventional Cu-K X-ray radiation
(~1.5418 Å) source and Bragg diffraction setup (Seifert 3000P). The
surface morphology of the films was studied by Veeco dI CP II
(Model: 0100) atomic force microscope (AFM). Room-temperature
electrical resistivity of the films was measured by four-probe
method using Keithley 2400 source meter. The Hall mobility and
carrier concentration were measured in Hall measurement setup
using Van der Pauw configuration with 0.1 T magnetic field. A JEOLJSM2010 transmission electron microscope operating at 200 kV
was used for obtaining high-resolution micrographs (HR-TEM)
from 30 nm thick samples deposited on carbon coated copper microscope grids, supplied by Pacific Grid-Tech, USA.
3. Results and discussion
A set of films were prepared by varying the substrate temperature from 50 C to 350 C at an applied RF power of 50 W, maintaining the gas pressure in the plasma fixed at 20 mTorr arising
from 3.3 sccm Ar gas flow. Samples of 250 nm thickness were
prepared on glass substrates. The samples were used in
1.5 1.5 cm2 square shape for general characterization and in
configuration for Hall measurements in Van der Pauw technique.
Especially for transmission electron microscopic measurements
samples of ~30 nm thickness were prepared on carbon coated Cu
micro-grids.
Fig. 2(a) shows the XRD patterns of the ITO thin films deposited
on glass at different substrate temperatures (TS) varying from 50 C
to 350 C with steps of 50 C. No significant XRD peak appeared up
to TS ¼ 150 C, demonstrating the amorphous-like structure. At
TS ¼ 200 C, for the first time some crystalline peaks appeared
corresponding to the <222> and the <411> planes at 2q ¼ 30.54
and 37.48 , respectively, the <411> peak being the dominant one.
On further increase in TS to 250 C the <222> peak increased
significantly, the <411> peak reduced in intensity and in addition, a
number of peaks e.g., <211>, <400>, <440> and <622> appeared.
On continued increase in TS to 300 and 350 C the <411> peak
became gradually insignificant, the most dominant <222> peak
systematically lost its prominence and the <400> peak gained in
intensity, while the other three peaks remained virtually unchanged. It was carefully noted that during increase in TS the <222>
peak significantly increased within 200e250 C and gradually
reduced thereafter. This observation indicated some specific
chemical process in the materials growth occurring within a temperature zone between 200 and 250 C, which drew special interest
on critical investigation in the region. Accordingly, three more
samples were prepared at that temperature zone. Fig. 2(b) demonstrates a comparative study among five samples prepared at a
close variation of growth temperature. It was identified that the
<222> peak position shifted systematically towards a higher
magnitude of 2q from 30.54 to 30.72 at TS ¼ 230 C with corresponding increase in peak intensity and sharpness (peak width
reduced to a minimum of D(2q) ¼ 0.38 ); however, all such changes
occurred along the opposite direction when TS was further
increased to 240 and 250 C. The <400> peak, however, increased
gradually with increasing TS while the <411> peak became undetectable at 220 C and that again re-appeared at TS ¼ 250 C.
Since In2O3 has a cubic bixbyite structure it possesses the <111>
lowest energy plane [49]. For the present set of samples, in
particular, <222> plane demonstrates the highest intensity peaks
644
L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650
Fig. 2. (aeb) XRD patterns of the ITO thin films deposited at different substrate temperature (TS).
(c) Variation of the intensity ratio of <222> and <400> XRD peaks, I<222>/I<400>, on changes in TS, demonstrating its highest magnitude at around 230 C.
in mostly crystalline structures at TS above 200 C. Continued increase in crystallization along the <222> orientation appeared as
the regular consequence of elevated substrate temperature up to
230 C; however, the decreasing intensity of <222> peak above
230 C could be correlated to the consequence of rapid incorporation of Sn in the In2O3 matrix [50], the melting point of Sn (TSn)
being ~231.9 C. On the other hand, the gradually increasing intensity of the <400> peak might have arisen due to the elevated
oxygen vacancy in the In2O3 matrix, consequent to the increased TS
[43,51]. None of the characteristic peaks of Sn, SnO, or SnO2
appeared, indicating complete miscibility of In and Sn atoms in the
In2O3 lattice [52]. Sn being tetravalent, each Sn (IV) atom substitutionally replaced In (III) atom and thereby, donated free electrons
for pursuing elevated electrical conductivity in the carrier transport
process. So, the ITO retained the cubic In2O3 structure up to the
solid solubility limit of the SnO2 in In2O3 [53].
So, the development of <222> peak was influenced by two
competing processes: elevated substrate temperature and the
enhanced dopant incorporation; while the <400> orientation in
the material, arising out of the created oxygen vacancy, was a
consequence of only the increasing substrate temperature.
Accordingly, the intensity ratio, I<222>/I<400> versus TS plot in
Fig. 2(c) demonstrates that the critical influence of the dopant
incorporation started occurring at TS above 230 C, at the vicinity of
TSn (~231.9 C). At TS > 230 C, increasing dopant (Snþ
4 ) incorporation into the In2O3 matrix superseded the temperature effect in
controlling the crystalline structure of In2O3:Sn (ITO) films by
introducing dopant induced defects, leading to a sharp reduction in
I<222>/I<400> at elevated temperatures. Similar effect was pronounced from the position of the <222> crystalline peak in
Fig. 2(b), the continued shift of which towards increasing 2q
magnitude at elevated TS reversed back spontaneously to lower 2q
after critical influence of the dopant incorporation which started
occurring at TS > 230 C, corresponding to the melting point of Sn.
The average grain size of the ITO films was estimated from the
XRD patterns, using the Debye-Scherrer's formula [54],
D¼
0:89 l
b cosq
(1)
where l (1.5418 A) is the wavelength of X-ray beam, b is the FWHM
in radian at diffraction angleq. The grain size of the crystalline
In2O3:Sn films was found to vary only from 19 to 21 nm over the
span of TS varying from 200 to 350 C.
Fig. 3(a) presents the transmission electron micrograph of the
ITO film prepared at TS ¼ 230 C and demonstrates its significantly
crystalline structure, as was identified from the XRD studies. Each
crystallite in the micrograph was identified by the individual sharp
boundary. An average grain size of ~15 nm was estimated from the
histogram shown at the inset in Fig. 3(a), although the XRD estimate identified little larger size. The high resolution micrograph in
Fig. 3(b) clearly identified the prominent presence of <211>,
<222>, <400>, <440> and <622> crystallographic planes of In2O3,
as observed in the XRD pattern. Virtually identical crystallographic
features were obtained from the TEM studies for the film prepared
at TS ¼ 250 C. However, elemental analysis by energy dispersive
spectroscopy (EDS) identified prominent changes in the elemental
composition in the In2O3:Sn films when those were grown at
230 C and at a relatively higher TS ¼ 250 C.
Fig. 4(a) and (b) present the EDS spectra of the films prepared at
L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650
645
Fig. 3. (a) TEM micrograph of the ITO film prepared at TS ¼ 230 C. Histogram at the inset shows the average grain size ~15 nm. (b) HRTEM micrograph demonstrating the distinct
co-existence of <211>, <222>, <400>, <440> and <622> crystallographic planes.
Fig. 4. (a) & (b) EDS spectra of the In2O3:Sn films prepared at TS ¼ 230 C and 250 C. (c) & (d) Elemental analysis on relative content of In, O and Sn shown by bar diagram.
TS ¼ 230 and 250 C, respectively, and the corresponding distribution of estimated elements In, O and Sn in at.% in pie charts. The bar
diagrams in Fig. 4(c) and (d) identify the relative contents of the
individual elemental components and particularly, the changes in
the ratios of O:In and Sn:In. It has been demonstrated that the Sn
content in the In2O3:Sn matrix increased from 1.5 at.% to 3.7 at.% for
an elevation in growth temperature from 230 to 250 C [22,24]. The
typical surface morphology of the ITO film prepared at TS ¼ 230 C
is shown in Fig. 5, demonstrating an average roughness of ~1.13 nm.
Low resistivity with high transparency, particularly over the
visible light region, is a desired property in applications as transparent electrodes in optoelectronic devices. The resistivity, carrier
concentration and Hall mobility of ITO thin films prepared at
different substrate temperatures are shown in Fig. 6. The resistivity
(r) decreased from 1.42 10¡2 U cm to 1.28 10¡3 U cm with
increasing substrate temperature from 50 to 350 C. Although there
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L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650
providing higher concentration of charge carriers. For boundary
scattering, the mean free path of the free carrier is described by the
relation [38],
1
1
h 3ne 3
m ¼ 2:05258 1015 ne 3m
2e p
=
Fig. 5. AFM micrograph the ITO film prepared at TS ¼ 230 C, demonstrating typical
surface morphology with an average roughness of ~1.13 nm.
Fig. 6. Variations of the electrical resistivity (rÞ, carrier concentration (ne ) and Hall
mobility (m) of the ITO thin films prepared by RF magnetron sputtering at different
substrate temperature, exhibiting significant changes in magnitude within a narrow
temperature span, 200 < TS ( C) < 250. The dashed vertical line corresponds to the
temperature (231.9 C), melting point of Sn.
was a gross decrease in r at the initial rise in TS, the most significant
and sharp reduction in r was evident in the region 200 < TS
( C) < 250, followed by continued reduction further. On the contrary, the carrier concentration (ne ) of the films increased monotonically from 3.21 1019 cm¡3 to 1.48 1020 cm¡3 with enhanced
substrate temperature from 50 to 350 C, as obtained from Hall
measurement. The Hall mobility (m) of the charge carriers was
obtained from the relation:
m¼
1
rne e
=
l¼
(3)
where, h is the plank constant, e is the electronic charge, ne is the
carrier concentration and m is the mobility. Using this formula the
mean free path of the charge carriers in ITO samples prepared at
different substrate temperatures was estimated and plotted in
Fig. 7.
The mean free path of the charge carriers increased significantly
from 1.2 to 3.0 nm within a short span of TS from 200 to 250 C,
close to TSn. Improved crystallinity implies, in general, relatively
smaller volume of grain boundaries and the subsequently reduced
grain boundary scattering of charge carriers [55]. The mean free
path of the charge carriers is much shorter than the estimated grain
size which implies that the scattering due to grain boundary is not
dominant in the present case [56]. However, at TS close to the vicinity of TSn rapid incorporation of metallic dopants in significant
amount introduces sharp rise in mobility of the charge carriers,
leading to a significant reduction in resistivity.
During deposition of the In2O3 network growth orientation
occurs spontaneously via the lowest energy plane along <111> or
its parallel along <222> direction of the cubic bixbyite structure. Up
to TS ¼ 230 C, controlled incorporation of Sn as the dopant promotes sharp lowering in the electrical resistivity, simultaneous to
increasing intensity of the <222> orientational growth. However at
TS above a critical temperature close to the melting point of Sn (TSn
~231.9 C) uncontrolled incorporation of Sn into the In2O3 network
obstructs the spontaneous crystalline growth along <222> orientation and simultaneously, systematic promotion of the electrical
transport on increasing temperature gets obstructed due to a
constrained carrier enhancement and mobility escalation.
The optical transmission spectra of ITO thin films prepared at
different substrate temperatures are shown in Fig. 8. From the
nature of variation of the transmission spectra two distinct groups
of samples prepared at TS below and above 230 C, were identified.
All the ITO films demonstrated above 80% transmission over the
wide wavelength range 400 nme800 nm (visible range). However,
the natures of the onset of optical transmission at the lower
wavelength region were clearly separated into two different categories, along with slight differences in individual slopes. Sharp
reduction of the optical transmission at shorter wavelength
occurred due to sharp absorption at the band edges and the slope
(2)
where e is the electronic charge (1.6 10¡19 C). The Hall mobility
maintained a steady magnitude ~13.77 V¡1cm2s¡1 for increasing TS
from 50 to 200 C. However, it was carefully noted that the carrier
mobility increased very rapidly from 13.78 to ~30.28 V¡1cm2s¡1
within a very small span of TS from 200 to 250 C beyond which,
however, m attained a shallow saturation at an average magnitude
of ~32.5 V¡1cm2s¡1 above 300 C, as shown in Fig. 6.
Spontaneous reduction in the resistivity of the films with
increasing TS could be correlated to the mutually additive effects of
(i) growth temperature induced transformation of the network
from amorphous to crystalline structure and (ii) enhanced incorporation of Sn4þ as dopants at the substitutional site for In3þ,
Fig. 7. Variations of the estimated mean free path of charge carriers with substrate
temperature (TS) for ITO films prepared by RF magnetron sputtering.
L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650
647
the Burstein-Moss effect that deals with the shift of Fermi level
caused by the increased carrier concentration of the conduction
band electrons. According to Burstein-Moss effect, the band gap
changes can be accounted by Hamberg et al. [58] as:
Eg ¼ Eg0 þ ZS þ DEgBM
(6)
where Eg0 is the band gap of undoped semiconductor, ZS represent
self-energy due to electron-electron and electron-impurity scattering. Further, DEgBM , the shift in Eg due to Burstein-Moss effect, is
given by,
determined the sharpness of the band edges.
From transmission spectra the optical absorption coefficient (a)
was obtained using Lambert's formula [22],
a¼
1
T
ln
where T is the transmittance and t is the thickness of samples.
Neglecting the reflection losses and scattering effects, the variation
of absorption coefficient follows the Tauc's relation,
aE ¼ A E Eg
where ne is the carrier concentration and m*vc is the reduced
effective mass which is given as,
1
1
1
¼
þ
mvc mv mc
n
(5)
where E is the photon energy, Eg is the optical band gap, A is the
constant and n is the exponent which can be taken as 0.5, 2, 1.5 and
3 for direct allowed, indirect allowed, direct forbidden and indirect
forbidden electronic transitions, respectively. Since ITO is a direct
band gap material, n has been taken as 0.5 and Eg can be determined from the Tauc's plot as shown in Fig. 9(a). The extrapolation
of linear part of ðaEÞ2 vs E curve intersecting the energy axis at a ¼
0 gives the Tauc's band gap, Eg . Fig. 9(b) shows the variation of
Tauc's optical band gap of the ITO films at different substrate
temperatures. Eg grossly increased from 3.61 eV at TS ¼ 50 C to
4.03 eV at TS ¼ 350 C. However, a significant and sharp rise in Eg
from 3.73 to 3.99 eV was noted for increasing TS from 200 to 250 C
[57].
The sharp increase in the optical band gap can be explained by
(8)
where m*v and m*c are the effective electron mass in valence band
and conduction band respectively. Finally, the optical band gap Eg
is given by:
(4)
t
(7)
Eg ¼ Eg0 þ ZS þ
Z2 2 2 3
3p ne
2mvc
=
Fig. 8. Transmission spectra of the ITO films prepared by RF magnetron sputtering at
different TS.
Z2 2 2 3
3p ne
2mvc
=
DEgBM ¼
(9)
Fig. 10 shows the variation of Eg as a function of ne 2/3 and
demonstrates two distinct linear segments with slightly different
slopes, with a transition at ne (carrier concentration) corresponding
to TS ~230 C and above. From the slope of individual segments, m*vc ,
the reduced effective mass of the charge carriers (electron) was
estimated for two different temperature regimes. The reduced
effective mass of the charge carriers, m*vc , increased by: [m*vc ]HT e
[m*vc ]LT ¼ (0.274e0.202) m0 ¼ 0.072m0 due to an abrupt incorporation of Sn4þ at TS 230 C, around TSn [59]. Further, as a consequence of such impulsive effect, the apparently constant factor
ðEg0 þ ZSÞ in Eq. (9) for two distinct linear segments with slightly
different slopes increased by 0.15 eV across TSn. This signifies either
the increase of the self-energy (ZS) mostly due to increased
electron-impurity scattering, and/or increase of Eg0 itself by the
self-conversion of the material from doped to an alloy-like
ensemble [60].
Presence of a high concentration of impurity or defect states in
the films perturbed the band structure, resulting in a prolonged tail
extending into the energy gap. Such effect was pronounced by the
Fig. 9. (a) Tauc's plot demonstrating the direct band gap of ITO thin films prepared by RF magnetron sputtering at different TS, and (b) sharp widening in the optical band gap (Eg )
occurring at around
TS ~230 C.
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L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650
For application of TCO films in solar cells, not only high transparency over the visible region is a desired property but also high
conductivity is essential; furthermore, these two properties maintain a trade-off relation. Hence, the Figure of merit, defined by
Haacke [64], plays an important role in determining the quality of
the ITO films. Figure of merit is given by,
F¼
Fig. 10. Optical band gap (Eg ) exhibiting linear relationship with (2/3)-rd power of
carrier concentration (ne ) for ITO thin films prepared at different TS.
absorption coefficient tail, directly below the fundamental absorption edge. The experimental data were fitted to Urbach's relation [61],
a ¼ ao Exp
hn
EU
(10)
where ao is a characteristic parameter of the material, and EU is
called Urbach absorption energy, which is normally denoted as an
indicator of the structural defects. The relation between natural
logarithm of the absorption coefficient and photon energy has been
depicted in Fig. 11(a). The magnitude of the Urbach energy EU was
estimated from the slope of the linear extension of the decaying
absorption tail towards lower energy, and its changes with the
variations of the growth temperature of the ITO films are shown in
Fig. 11(b). The EU systematically reduced during initial increase in TS
up to 230 C corresponding to the improving crystalline structure
attained in the network. However, at TS > 230 C,EU increased
indeed and attained a virtual saturation at higher temperatures.
Thus, significant dopant incorporation into the film structure
plausibly occurred at and above TSn and the subsequent defect
formation at elevated temperatures superseded the temperature
effect on defect elimination and changed the ultimate nature of
variation of EU at higher TS [62,63].
T 10
RS
(11)
in which T is the transmittance at specific wavelength, normally
taken at l ¼ 550 nm where the intensity of solar spectrum is
maximum [22,64] and RS is the sheet resistance. Fig. 12 shows the
variation of F with the substrate temperature. The Figure of Merit
of the ITO films increased very rapidly at TS approaching TSn,
attained the highest magnitude at TS ¼ 240 C and then reduced
less promptly on further increase in TS. At higher TS increased
dopant incorporation although increases the carrier concentration
marginally, enhanced optical absorption by the dopants leads to the
lowering in the Figure of Merit of the ITO films.
In order to make a comparison in the variation of the Figure of
Merit of the ITO films as a function of the applied substrate temperature few results have been taken from the literature of already
published data on closely similar deposition conditions following
mostly identical growth mechanism via magnetron sputtering
[47,65,66] and also pulsed laser deposition [50]. Fig. 13 demonstrates that in terms of the magnitude of Figure of Merit, the present result does not deserve to be the superior one mainly as
because the present results are not the parametrically optimized
data. However, the objective of the present experiment was to
study the effect of substrate temperature on the characteristic
changes on the optoelectronic properties of In2O3:Sn films at the
close vicinity of the melting point of Sn. In this context it is
apparently clear that the available data on systematic analysis in
two cases [47,66] identify rather very close Figure of Merit values at
temperatures far across the melting point of Sn. The present analysis, however, identifies sharp changes in the magnitudes of
Figure of Merit across the close vicinity of the melting point of Sn,
which is the novelty of the present work that has not been carried
out earlier.
4. Conclusions
Retaining a cubic bixbyite structure, the In2O3:Sn films in mostly
crystalline phase at TS above 200 C possess the lowest energy
Fig. 11. (a) Absorption co-efficient spectra of ITO films prepared at different TS. (b) Variation of the Urbach energy (EU ) with TS exhibiting significant increase in magnitude at
TS 230 C.
L. Karmakar, D. Das / Journal of Alloys and Compounds 767 (2018) 642e650
Fig. 12. Variation of the figure of merit (F) of ITO films demonstrating a sharp increase
at TS within 230e240 C.
649
matrix. As a consequence of such impulsive effect, either the selfenergy (ZSÞ due to electron-impurity scattering rises and/or Eg0
itself increases by self-conversion of the material from a doped to
an alloy-like ensemble. The Figure of Merit of the ITO films increases very rapidly corresponding to the sharp increase in carrier
mobility at TS approaching TSn. At higher TS additional dopant
incorporation although increases the carrier concentration
marginally, enhanced optical absorption by the dopants leads to the
lowering in the Figure of Merit of the ITO films. Thus, significant
dopant incorporation into the In2O3 matrix at growth temperature
close to TSn leads to the substantial changes in most of the optoelectronic properties of ITO films as a TCO material. Thereby, the
melting point of Sn (TSn), rather than any arbitrary substrate temperature, has been identified as an optimal temperature for
growing ITO thin films, suitable for device fabrication; although
opportunities for further improvement remains open by controlling
other conventional parametric variations, as usual.
Acknowledgement
The work has been done under projects funded by Department
of Science and Technology (Nano-Mission Program) and Council of
Scientific and Industrial Research, Government of India. One of the
authors (LK) acknowledges CSIR, GoI, for providing the Senior
Research Fellowship.
References
Fig. 13. Variations in Figure of Merit of In2O3:Sn films as a function of substrate
temperature across the close vicinity of the melting point of Sn.
orientation along the <222> crystallographic plane, which normally advances at elevated growth temperature; simultaneously,
enhanced oxygen vacancy builds up the <400> orientation. On
further increase in TS > 230 C, enhanced dopant (Snþ
4 ) incorporation into the In2O3 matrix supersedes the temperature effect on
reducing the structural defects of the ITO films by introducing
dopant induced defects (as noted by the enhanced Urbach energy
EU ). This leads to a sharp reduction in I<222>/I<400> at elevated
temperatures. On increasing TS, mutually additive effects of (i)
growth temperature induced transformation of the network from
amorphous to crystalline structure and (ii) enhanced incorporation
of Sn4þ as dopants at the substitutional site for In3þ providing
higher concentration of charge carriers, progressively reduce the
resistivity of the ITO films. However, at TS close to the vicinity of the
melting point of Sn (TSn) rapid incorporation of metallic dopants in
significant amounts introduces sharp rise in the mobility of charge
carriers, leading to a sudden substantial reduction in resistivity. The
optical gap of the ITO films increases, in general, with increasing
substrate temperature. Whereas, Eg increases sharply due to the
critical influence of the dopant incorporation in abundance at
growth temperature around the melting point of Sn. Similarities in
the nature of changes in electrical and optical properties are
noteworthy. Across this temperature zone, Eg vs. ne 2/3 plot demonstrates two distinct linear segments with slightly different
slopes, demonstrating an enhancement of the reduced effective
mass of charge carriers, m*vc by 0.072m0 owing to the abrupt
incorporation of Sn4þ at the substitutional site of In3þ in the In2O3
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