Journal of Modern Physics, 2010, 1, 360-363
doi:10.4236/jmp.2010.16051 Published Online December 2010 (http://www.SciRP.org/journal/jmp)
A Dependence of Crystallinity of In2O3 Thin Films by a
Two-Step Heat Treatment of Indium Films on the
Heating Atmosphere
Yuichi Sato, Fumito Otake, Hirotoshi Hatori
Graduate School of Engineering and Resource Science, Akita University, 1-1, Tegata, Akita, Japan
Received July 25, 201 0; revised August 28, 2010; September 3, 2010
Abstract
A difference in crystallinity of In2O3 thin films on sapphire substrates by heat treatment of indium films was
reported. Indium films were heated in an inert atmosphere or in air until they reached a specified temperature
and then oxidized in air at much higher temperatures. Crystallinity of the In2O3 thin film which was
heat-treated in air from room temperature was quite poor. On the other hand, narrow X-ray rocking curves of
the In2O3 films were obtained when the temperature was increased in an inert atmosphere to a specified tem-
perature.
Keywords: Solid Phase Epitaxy, Oxides, Sapphire, Semiconducting Materials
1. Introduction
Recently, single-crystal sapphire wafers have been used
widely as substrates for heteroepitaxial growth of semi-
conductor thin films such as gallium nitride (GaN) [1].
Sapphire is a transparent material, therefore, practical
applications of it to such as transparent substrates for
single-crystalline thin film photovoltaic cells have been
also expected. However, it is relatively difficu lt to collect
electric currents effectively in the thickness direction of
the semiconductor thin films because sapphire is also an
insulating material. Therefore, to allow electric currents
to flow effectively to the thickness direction of the
semiconductor thin film some conductive layers should
be formed between the sapphire substrate and the semi-
conductor thin film.
Indium oxide (In2O3), which is a wide band gap mate-
rial, is widely used as a transparent conducting film. It is
usually prepared by sputtering techniques [2] and het-
eroepitaxy of In2O3 on sapphire substrates have been also
investigated by using various techniques such as atomic
layer deposition [3], pulsed laser deposition [4] and reac-
tive magnetron sputtering [5] methods. As In2O3 has a
cubic crystal structure [6], it can has a heteroepitaxial
relationship between [111] In2O3 and [0001] sapphire. In
such case, In2O3 film will grow epitaxially on the sap-
phire substrate. In addition, another semiconductor thin
film which will be formed on the In2O3 film will also
grow epitaxially on it if the semiconductor has a het-
eroepitaxial relationship to In2O3. Therefore, the semi-
conductor thin film will have better properties than films
grown on polycrystalline or amorphous underlayers. As
an example for it, we have epitaxially grown In2O3 thin
films on sapphire substrates by a sputtering method and
grown Cadmium Telluride (CdTe) thin films on the un-
derlayers [7]. In the experiment, the CdTe thin films
could be epitaxially grown on the In2O3 thin films with-
out deterioration compared with CdTe thin films grown
directly on sapphire substrates.
Metal indium thin films can be quite easily deposited
using simple and inexpensive vacuum evaporation tech-
niques. In addition, indium can be readily oxidized by
heating at a low temperature in an atmosphere containing
oxygen. Therefore, it may be possible to easily produce
indium oxide thin films using simple evaporation and
oxidization processes. Furthermore, performing this
processing by laser radiation heating will permit local
oxidization of indium thin films, enabling the transpar-
ency and conductivity of the films to be locally con-
trolled. From the easiness and possibility for expansion
of the process utility, we have considered that it is worth
to develop this processing method.
The ultimate goal of our research is to obtain het-
eroepitaxially grown In2O3 thin films on single-crystal

Y. SATO ET AL.361
sapphire substrates by using the above-mentioned simple
techniques. In the pursuit of this goal, we observed dif-
ferent crystallinities of In2O3 thin films that had been
heat treated by a two-step heat treatment. This difference
depended on the atmosphere used in the two-step heat
treatment. In this paper, we report the effect of the heat-
treatment atmosphere on the crystallinity of In2O3 thin
films obtained by the two-step heat treatment of metal
indium thin films.
2. Experiments
Indium thin films were deposited by using a vacuum
evaporation system with a resistive heater for evaporat-
ing indium shots (6 N). Sapphire (0001) single-crystal
wafers and synthetic quartz glass plates for comparison
were used as substrates. The indium thin films were
about 100 nm thick. After deposition, the indium thin
films were heat treated in an infrared heating furnace and
oxidized to form In2O3 thin films. The heat treatment
patterns are schematically shown in Figure 1. In the first
step of the heat treatment, the temperature was increased
from room temperature to 500℃ in an inert N2 atmos-
phere or air. In the second step, the inert atmosphere
was replaced with air when the temperature reached to
500℃, then the temperature was increased to 900℃ and
maintained for 1 h. Crystallinities of the obtained thin
films were evaluated by X-ray diffraction (XRD).
3. Results and Discussion
First, indium thin films that had been deposited on quartz
glass substrates were conventionally heat treated in air
for 1 h at various temperatures. Figure 2 shows the XRD
patterns of the obtained thin films. In this case, the tem-
perature was increased from room temperature in air (i.e.,
not in an inert atmosphere). The indium thin films gradu-
ally become oxidized with increasing heat-treatment tem-
perature. The diffraction peaks for metal indium are not
visible when the heat-treatment temperature exceeds
500℃. Thus, In2O3 thin films are obtained when the
heat-treatment temperature exceeds 500℃. In addition, it
is considered that crystallization of the In2O3 thin films
progresses with increasing heat-treatment temperature as
the X-ray intensity of In2O3 increases by increasing the
temperature.
Figure 3 shows X-ray rocking curves (XRCs) of the
In2O3 (222) peaks of films prepared on sapph ire and quart z
glass substrates. The films were produced by a conven-
tional single-step heat treatment in air at 900℃ for 1 h
with no inert atmosphere. The XRC of the In2O3 thin
film obtained on sapphire substrate is broad and almost
(a)
(b)
Figure 1. Heat treatment schematics of (a) the two-step heat
treatment using inert atmosphere tried in this work and (b)
conventional single-step heat treatment with no inert at-
mosphere. In the two-step treatment, the temperature was
increased in an inert atmosphere to 500℃ and then the
inert atmosphere was replaced with air.
same with that obtained on quartz glass substrate, that is,
crystallinity of the film is quite poor. There is no differ-
ence in the XRCs of the films prepared on the sin-
gle-crystal sapphire and quartz glass substrates despite
In2O3 having a heteroepitaxial relationship with the sap-
phire substrate. This implies that there is no epitaxial
effect from the single-crystal sapphire substrate to the
In2O3 thin film. This is considered to be because the
melting point of indium is about 157℃, so that indium
thin films may be oxidized at this low temperature,
which causes their recrystallization temperature to in-
crease abruptly. Therefore, films that have been oxidized
at low temperatures are not able to recrystallize when
heat treated in an atmosphere containing oxygen. Con-
sequently, In2O3 films prepared on a single-crystal sap-

Y. SATO ET AL. 362
Figure 2. XRD patterns of indium thin films deposited on
quartz glass substrates that were heated using a conven-
tional single-step heat treatment in air at five different
maximum temperatures with no inert atmosphere.
phire substrate have similar poor crystallinities to In2O3
thin films prepared on amorphous quartz substrates.
Next, we tried heat treatments of indium thin films
from room temperature to a specified temperature in an
inert atmosphere in the first step. In the second step, the
inert atmosphere was replaced with air and the tempera-
ture was increased to a specified maximum temperature.
In this study, N2 gas was used as the inert atmosphere at
the low temperatures because N2 gas does not react di-
rectly with metal indium [8]. Indium thin films that had
been deposited on the sapphire and quartz substrates
were heat treated in the N2 gas atmosphere until the
temperature reached 500℃. The atmosphere was then
replaced with air and maximum temperature was main-
tained for 1 h.
Figure 4 shows the XRC profile about In2O3 (222)
peak for the In2O3 thin film produced by the two-step
heat treatment on sapphire substrate. As a comparison,
the XRC profile for the film prepared on same sapphire
substrate by the conventional single-step heat-treatment
is also shown in it. Both films were heat-tertated by same
conditions except for the atmosphere until the tempera-
ture reached 500℃. The film prepared by the two-step
heat treatment obviously has a narrower XRC profile
than that of the film by the single-step heat treatment
Figure 3. XRC profiles showing the In2O3 (222) peak of
In2O3 thin films produced by conventional single-step heat
treatment in air at 900℃ with no inert atmosphere.
with no inert atmosphere. Similar results for the XRC
profiles were reproducibly observed even when the heat
treatment was performed at various maximum tempera-
tures.
It is considered to be difficult for atoms to migrate in
films that had been oxidized at low temperatures when
the films are heated from room temperature in an oxygen
containing atmosphere. In contrast, atom migration oc-
curs relatively easily in films that had been heat treated
in an inert atmosphere at low temperatures and oxidized
at higher temperatures. By adjusting the heat-treatment
atmosphere, it should be possible to prevent indium thin
films from oxidizing at low temperatures at which In2O3
does not have a high crystallinity. In such cases, indium
thin films are oxidized at much higher temperatures, so
that the rearrangement and crystallization occur effec-
tively on single-crystal sapphir e substrates.
4. Summary
Indium thin films that had been deposited on sin-
gle-crystal sapphire substrates by vacuum evaporation
were heat treated using the two-step method to obtain
In2O3 thin films. Crystallinity of obtained In2O3 thin
films was poor when the heat treatment was not initially

Y. SATO ET AL. 363
(a) (b)
Figure 4. XRC profiles about the In2O3 (222) peak of In2O3 thin films obtained on sapphire substrates by (a) the two-step heat
treatment and (b) conventional single-step treatment. In the two-step treatment, the temperature was increased in an inert
atmosphere to 500℃ and then the inert atmosphere was replaced with air.
performed in an inert atmosphere. In contrast, a clear
difference and improvement in the crystallinity was ob-
served when the temperature was increased from room
temperature to a specified temperature in an inert at-
mosphere in the first step prior to the second step of the
heat treatment in an oxygen-containing atmosphere.
Therefore, this kind of two-step heat treatment will be-
come an effective method for producing epitaxially
grown In2O3 thin films easily in the simple oxidation
method of metal indium thin films.
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