Effects of rapid thermal annealing on the
morphology and electrical
properties of ZnO/In films
Tae Young Ma, Dae Keun Shim
Department of Electrical Engineering & Research Institute of Industrial
Technology, Gyeongsang National University, 900 Gazwa-dong,
Chinju, Gyeongnam 660-701, South Korea
Thin Solid Films 410 (2002) 8–13
Results and discussion
Zinc oxide (ZnO) films were prepared by ultrasonic spray
pyrolysis on indium (In) films deposited by evaporation and
subsequently subjected to rapid thermal annealing (RTA) in air
Auger electron spectroscopy (AES) was carried out to
determine the distribution of indium atoms in the ZnO films.
The resistivity of the ZnO on In (ZnO/In) films decreased to
2 ×10-3 Ω cm by diffusion of the In.
The results of depth profiling by AES showed a hump of In
atoms around ZnO/In interface after RTA at 800 ℃,which
disappeared on RTA at 1000 ℃.
ZnO films, in particular, are attracting more attention as
transparent conductive films and gas sensors because of their
amenability to doping , their low cost, non-toxicity and
stability in hydrogen atmospheres
Recently, several researchers have reported that ZnO films
can be easily grown by ultrasonic spray pyrolysis (USP). The
USP technique is considered to be a very useful method due
to the simplicity of the apparatus and the low cost of the raw
In USP, zinc acetate and indium chloride diluted in an
alcoholic solution have previously been used as sources for
However, we found that the preparation of ZnO:In films from
In-containing solution is not effective from the viewpoint of
reproducibility and uniformity of the dopant distribution.
In this paper, we propose a new process for the preparation
of ZnO:In films. The merit of this new method is in the
simplicity of the process. We can easily modulate the
electrical and morphological properties of ZnO/In films by
controlling the RTA treatment conditions.
Silicon dioxide-coated Si wafers were used as substrates.
A ;10-nm-thick In film was deposited on the substrates by a
thermal evaporation method. A ;300-nm-thick ZnO film was
subsequently grown on the In film by the USP method.
In the USP process, zinc acetate diluted in methanol was
selected as the zinc precursor at a concentration of 0.03
Aerosols produced by pulverizing the solution are conveyed
to the reactor by nitrogen gas at a flow rate of 1 1min-1. The
substrate temperature was 230 ℃.
We annealed the ZnO/In film in a RTA apparatus(MILA-3000,
ULVAC) to diffuse In atoms into the ZnO film. The annealing
temperatures were 600, 800 and 1000 ℃ and the annealing
time varied between 10 s and 4 min.
Results and discussion
Fig. 1. XRD spectra of ZnO films
annealed at: (a) 800; (b) 1000 ℃
for 30 s in air; and (c) as-deposited.
Fig. 1 shows the XRD spectra of ZnO films on SiO2 RTA-
treated at various temperatures. The XRD patterns show that
as-deposited ZnO films have a poor polycrystalline structure .
RTA at 800 ℃ for 30 s leads to an increase in preferred
orientation, with the (002) axis perpendicular to the substrate.
The peak intensities of the
(100) plane parallel to the
substrate and of the (002)
plane normal to the substrate
are notable in Fig. 2a. It seems
that annealing temperature
higher than 800 ℃ for 30 s
deteriorates the crystallites of
the ZnO/In films.
The XRD peaks of these ZnO/In films are shown in Fig. 2b. From
the XRD peaks, it is believed that the lack of oxygen and a fast
diffusion process in vacuum deter the In-doped ZnO films from
The highest peaks of In2O3 powder (222 and 400 planes)
are found at 2θ=29.88 and 34.78. However, oxygen in the
ZnO film is not sufficient to form In2O3 film and the RTA
time is too short for oxidation of In. We believe that In2O3
cannot be formed during this RTA treatment.
The mean grain size of the film annealed at 1000 ℃ for 30s
was found to be 220 nm, with standard deviation of 78 nm.
Fig. 3 shows the SEM images of
the ZnO films corresponding to
Fig. 1. The as-grown ZnO film
appears to be an amorphous
Fig. 3. SEM images of ZnO films annealed at: (a) room
temperature;(b) 800; and (c) 1000 ℃ for 30 s in air.
The SEM images of the ZnO/In
films rapidly annealed at various
temperatures are shown in Fig.
The film annealed at 600 ℃ (Fig.
4a) shows fine grains. The grain
size increases with increasing
RTA temperature (Fig. 4b,c).
Fig. 4. SEM images of ZnO/In films annealed at: (a) 600; (b) 800;
and (c) 1000 ℃for 30 s in air.
Comparing Fig. 3 and Fig. 4, it is evident that In diffusion into the ZnO film
deters grains from growing in a hexagonal shape, which roughens the
surface morphology of the films. 11
Fig. 5 shows SEM images of
the ZnO/In films annealed in
vacuum at 800 and 1000 ℃ for
30 s. No clear grain
boundaries are evident in Fig.
5, which is consistent with the
XRD results of Fig. 2b.
Fig. 5. SEM images of ZnO/In films annealed at: (a) 800; and (b)
1000 ℃ for 30 s in vacuum.
Fig. 6. Resistivity variation of
ZnO/In films with RTA time. The
samples were annealed at (a) 600
(▲), 800 (■) and 1000 ℃ (●) in
air; and (b) 800 (■) and 1000 ℃
(●) in vacuum.
Fig. 7a shows that a hump
of In atoms is found at the ZnO/SiO2
interface after annealing at 800 ℃ for 10 s;
Fig. 7b shows that the hump diminishes
with increasing annealing time. Fig. 7c
shows that the hump of In atoms has
disappeared on RTA at 1000 ℃ for 10 s.
Into the vacuum, which increases the
resistivity of the films annealed at 1000 ℃.
Fig. 7. Auger depth profiles for ZnO/In
films annealed in vacuum at (a) 800 ℃
for 10 s; (b) 800 ℃ for 30 s; and (c) 1000
℃ for 10 s.
The peak at approximately 0.076
keV in Fig. 8 may be discarded as
noise. If this peak were not from
noise, but from Si, another peak
should be detected at
approximately 1.6 keV. However,
no notable peak was found at this
value. Therefore, the Si
concentration in the ZnO region in
Fig. 7, mostly obtained from the
peak at 0.076 keV, is negligible.
Fig. 8. AES spectrum of a ZnO film annealed at 1000 ℃ for 10 s.
Zinc oxide films were prepared by USP on ;10-nm thick indium
films, followed by RTA in air or vacuum.
However, the XRD peak of the (100) and (002) directions became
pronounced after the RTA treatment in air.
Recrystallization of the Indoped ZnO films in vacuum was
suppressed by a lack of oxygen and a fast diffusion process.
The In diffusion into ZnO film deters grains from growing in a
hexagonal shape, which roughens the surface of the films.
Anneal at 1000 ℃ for 10s and the resistivity to 2 ×10-3 Ωcm.
We can easily change the surface morphology and resistivity of
ZnO films by In doping with the RTA process, which will widen the
applicability of the ZnO films. 16
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