Materials Sciences and Applications, 2011, 2, 1212-1218
doi:10.4236/msa.2011.29164 Published Online September 2011 (http://www.SciRP.org/journal/msa)
Effect of Annealing on the Structural and Optical
Properties of CuIn1–xAlxS2 Thin Films
Fethi Smaili*
Laboratoire de Photovoltaïque et Matériaux Semi-conducteurs, Tunis, Tunisie.
Received March 13th, 2011; revised March 29th, 2011; accepted May 31st, 2011.
ABSTRACT
The effects of annealing of CuIn1
 xAlxS2 thin films for different temperature, which were grown by thermal evaporation,
were investigated through X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS) and optical absorption
measurements. Post growth was used to modify the structural and optical properties of the CIAS thin films. Then it re-
alized in three different temperatures 250˚C, 300˚C and 400˚C. It is found that significant difference of the transparent
films in function of annealing temperature. The FWHM in the X-ray diffraction pattern is found to decrease with an
increase in annealing temperature indicating that the crystalline nature of the CIAS improves with increase in anneal-
ing temperature. The position of the (112) peak and other peaks in the X-ray diffraction pattern has been observed to
shift to higher values of 2θ with the increase of gallium concentration. We demonstrate a significant enhancement of the
inter-diffusion in the dot thin film.
Keywords: CuIn1–xAlxS2, Annealing, Structural Properties, Optical Properties
1. Introduction
Ternary and quaternary compound semiconductors of the
type I-III-VI2 have received much attention in recent
years because of their potential application in optoelec-
tronic devices [1]. Significant effort has been made to
increase the band gap in order to improve the module
performance, resulting from the trade-off between higher
voltages and lower currents at maximum power. Conver-
sion efficiencies for polycrystalline CIGS based solar
cells have been significantly improved over recent years
and the best cell is now reported at 19.5% [2]. However,
among ternary chalcopyrite semiconductors, CuInS2 may
be the most promising material for photovoltaic applica-
tions due to the band gap energy of 1.53 eV [3]. In the
search for a lattice matching between the absorbent layer
and a window buffer layer, also the Ga is a scarce and
expensive material and therefore it can be replaced by
inexpensive abundant aluminium Al. Cu(In1xAlx)S2
[CIAS] has been considered as promising alternative,
since it requires less aluminium concentration than gal-
lium to achieve a similar band gap [4]. In addition to
single junction solar cells, CIAS thin films can also find
application in tandem solar cells [5,6].
Very little is understood about the formation of intrin-
sic defects in this material, and a detailed study of these
defects and impurity levels relating to the composition of
the material is necessary in order to fabricate devices [7].
CIAS thin films have been prepared by several tech-
niques including co-evaporation [8,9], one step RF mag-
netron sputtering [10], chemical bath deposition (CBD)
[11] and sequential deposition methods [12-14]. The
various types of annealed films have a great effect on
their structural, morphological and optical properties.
The film characteristics depend on the methods and post-
growth treatments conditions while high performance
films were generally obtained with expensive techniques
[15].
In this study, we report the effects of annealed in
various temperatures on the structural, compositional and
optical properties of CIAS films.
2. Experiments
2.1. Film Preparation
We prepared by vacuum thermal evaporation CuIn1–xAlxS2
films by charging polycrystalline CuIn1xAlxS2 powders
as source materials into heated tungsten crucibles. The
base pressure during the evaporation was 106 Torr. The
glass substrate temperature was kept at 200˚C. Substrate
heating was achieved by the heater system described
elsewhere [16]. This system allowed us to insert a suit-

Effect of Annealing on the Structural and Optical Properties of CuInAl S Thin Films1213
1–xx 2
able Cu-based alloy cylinder with a diameter d ≤ D,
where D is the heater diameter (D = 100 mm), between
the heater and the substrate holder. The cylinder thick-
ness was kept constant (1 cm). The distance from cruci-
ble to sample holder was 10 cm. The substrate tempera-
ture was measured using a thermocouple in contact with
the substrate surface. After deposition the samples were
annealed in free different temperatures, in vacuum at
250˚C and 300˚C and under nitrogen atmosphere at
400˚C for 1 h. The thermal annealing was performed on
sample pieces with an area of about 2 cm2. The thin film
was placed in a quartz tube. During the annealing process,
a flow of nitrogen was passed through the tube.
2.2. Characterization of CuIn1-xAlxS2 thin Films
Optical transmittance (Texp) and reflectance were meas-
ured at normal incidence in the wavelength range 300 -
1800 nm with an UV-visible-NIR Shimadzu 3100 S
spectrophotometer equipped with an integrated sphere.
The film’s thicknesses were calculated from the positions
of the interference maxima and minima of reflectance
spectra using a standard method [17], the absorption co-
efficient was deduced [18]. The compositions of the
films were determined by EDX measurement. Whereas
the structural properties were determined by the X-ray
diffraction technique using CuK radiation (
 = 0.15418
nm).
3. Results and Discussion
3.1. Structural Properties
Figure 1 shows the X-ray diffraction pattern of the an-
nealed CuInxAl1xS2 thin films of four different composi-
tions and free different temperatures. From these patterns,
it is observed that all the films are polycrystalline in na-
ture. Peaks corresponding to metallic phase or binary
sulfed/oxide phase are not observed. This indicates that
the films are single phase. Then, for the annealing at
250˚C we can note a no improvement in crystallinity is
observed and there is no difference from the not annealed
CIAS thin films (not shown) due to the low temperature
annealing, which seems inadequate for a possible im-
provement of the disappearance of copper phase.
In addition, we note a disappearance of minor peaks
correspond with copper phase for the CIAS thin films
annealed at 300˚C. But we note a complete disappear-
ance of these peaks and an improvement of the crystal-
linity of the CIAS films, which presents small peaks
(200), (220), (116) and (224) for the CuInS2 phase. In-
deed, the presence of small peaks indicates that the small
grains were formed during annealing at temperatures
higher than 300˚C.
It is clear that the annealing under nitrogen at 400˚C
Figure 1. X-r ay dif fr a ct i on pa t t e r ns of anneale d CuIn1–xAlxS2
thin films for different composition and at various tem-
peratures (a) 250˚C, (b) 300˚C and (c) 400˚C.
which is equivalent to a vacuum annealing greatly im-
proves the crystallinity of the films.
The position of the peak corresponding to the (112)
orientation is found to shift to higher values of 2θ. This
noticeable shift of the main Bragg peak to large angles is
due to the decrease in lattice constants “a” and “c” and
therefore the “d” spacing. The shift of the diffraction peak
to higher angle and the decrease of lattice parameter with
increase of aluminium content in the studied compositions

Effect of Annealing on the Structural and Optical Properties of CuIn1–xAlxS2 Thin Films
1214
cherrer equation [19]: are in accordance to Vegards law [1]. These results indi-
cate that CIAS solid solution has been successfully ob-
tained without any phase separation into CIS or CAS or
any other secondary phases. We here with report and dis-
cuss the structural parameters of the CIAS thin films.
0.9
cos
DB


 (1)
where
 is the x-ray wavelength,  is the Bragg diffrac-
tion angle, and B is the full-width at half-maximum
(FWHM) of the peak corresponding to
 . When calcu-
lated by using the peak corresponding to the (112) plane
of CIAS. Figure 2 shows that the average grain size is
found to decrease as well as aluminium concentration
increase (Table 1).
The lattice parameters for the CIAS thin films have
been evaluated and are shown in Table 1. The lattice
parameter values “a” and “c” are found to decrease as
aluminium concentration increases. This may be due to
the fact that aluminium atom is smaller in size than in-
dium atom and hence causes shrinkage in the lattice as
they substitute indium sites in the cell. The FWHM is
found to increase as the aluminium concentration in-
creases and this can be attributed to the incorporation of
the aluminium element in the indium sites, which distorts
the normal lattice structure of CuInS2. The observed in-
crease in FWHM values corresponds to a decrease in the
effective grain size with increased aluminium incorpora-
tion into the network parameter of CuInS2.
3.2. Compositional Analysis
The chemical constituents present in the thin films have
been identified using energy dispersive X-ray analysis.
The composition of the CIAS thin films are shown in
Table 2. The composition of the CIAS films were tested
at four different regions on the film surface and the val-
ues reported in Table 2 are an average of the atomic
percentage of each element. As can be seen in Table 1,
The average crystallite size can be calculated using the
Table 1. Structural parameters of CIAS films.
Lattice parameter
Samples a (Å) c (Å) d
112 (Å) Film thickness (nm) FWHM Grain size
(Å)
CIAS: Al 00at.% 5.524 11.159 3.1997 480 0.247 320
CIAS: Al 20at.% 5.503 10.723 3.1491 320 0.986 79
CIAS: Al 40at.% 5.496 10.632 3.1375 310 0.992 75
Annealed at
250˚C
CIAS: Al 60at.% 5.474 10.421 3.1073 300 0.8232 76
CIAS: Al 00at.% 5.524 11.159 3.1997 440 0.236 340
CIAS: Al 20at.% 5.498 10.712 3.1461 450 0.937 86
CIAS: Al 40at.% 5.492 10.633 3.1361 350 0.982 77
Annealed at
300˚C
CIAS: Al 60at.% 5.480 10.348 3.1017 380 0.687 89
CIAS: Al 00at.% 5.524 11.109 3.1949 490 0.177 390
CIAS: Al 20at.% 5.502 10.771 3.1534 460 0.922 92
CIAS: Al 40at.% 5.497 10.686 3.1433 400 0.998 90
Annealed at
400˚C
CIAS: Al 60at.% 5.473 10.333 3.0974 460 0.510 102
Table 2. Thin films elemental compositions (at.%).
Samples S In Cu Al O
Cu
In Al
Al
In Al S Cu In Al Eg
CIAS: Al 00at.% 49.91 24.8625.22-- 0 1.01 0.00 1.00 1.47
CIAS: Al 20at.% 49.38 20.0625.154.85 0.111.00 0.19 0.99 1.50
CIAS: Al 40at.% 50.03 16.1724.369.73 0.130.96 0.38 1.00 1.68
Annealed
at 250˚C
CIAS: Al 60at.% 49.26 11.0525.3614.140.150.95 0.56 0.97 1.98
CIAS: Al 00at.% 50.03 25.1724.76-- 0 0.98 0.00 1.00 1.43
CIAS: Al 20at.% 49.91 19.9625.025.15 0.121.00 0.21 1.00 1.54
CIAS: Al 40at.% 49.56 15.2525.2610.040.131.00 0.40 0.98 1.81
Annealed
at 300˚C
CIAS: Al 60at.% 50.03 10.5724.3615.130.150.99 0.59 1.00 1.84
CIAS: Al 00at.% 48.91 24.8626.22-- 0 1.05 0.00 0.96 1.45
CIAS: Al 20at.% 49.26 19.2526.365.04 0.141.09 0.21 0.97 1.50
CIAS: Al 40at.% 48.93 15.6725.969.43 0.151.03 0.38 0.96 1.60
Annealed
at 400˚C
CIAS: Al 60at.% 49.06 9.54 26.1616.040.171.02 0.63 0.95 1.85

Effect of Annealing on the Structural and Optical Properties of CuInAl S Thin Films 1215
1–xx 2
Figure 2. Grain sizes of CuIn1–xAlxS2 thin films versus alu-
minum content percent for various temperatures (a) 250˚C,
(b) 300˚C and (c) 400˚C.
the ratio of Cu, In, Al and S elements in the CIAS film is
almost 1: 1: 2.
It is seen that the ratio of Cu/(In + Al) is near of the
unity and the ratio of S/(Cu + In + Al) is close to unity.
In addition, we note an increase in the Al/(In + Al) ratio
from 0 to 0.59 which is almost the composition of the
used powders.
3.3. Optical Properties
3.3.1. Transmission and Reflection Spectra
The optical transmission curves of CIAS thin films for
different annealing temperatures are depicted respec-
tively in Figure 3 and Figure 4. The samples showed a
semitransparent behaviour in the infrared region and the
thinnest and lowest Al content CIAS sample presented
also a relatively high transmission in the visible spectrum.
The transmission spectrums of the CuInS2 thin film show
interference pattern with sharp fall of transmittance at the
band edge for different annealing temperatures, which is
an indication of good crystallinity. However, the trans-
missions of other films decrease strongly when the alu-
minium percent increase in the materials; this is probably
due to the inhomogeneities of the films.
In addition, we note a decrease in the transmission
values for the CIAS thin films annealed at 250˚C and
300˚C for about 10% and 30% respectively. We explain
this decrease after annealing by the substitution of the
indium element by the aluminium and the diffused alu-
minium from the surface into the volume.
Figure 3(c) shows the optical transmission spectra of
CIAS films annealed at 400˚C. For CuInS2 film, we point
out the presence of the interference fringes due to the
multitude reflection phenomena showing homogenous
films. Moreover these films exhibit a good transparency
Figure 3. Transmission spectra of annealed CuIn1–xAlxS2
thin films for different composition and at various tem-
peratures (a) 250˚C, (b) 300˚C and (c) 400˚C.
in the visible and infrared regions. Than these interfer-
ences disappear when we increase the aluminium content
in the CIAS thin films and the transmission values de-
crease as a function of the aluminium percent and varies
from 70% to 10%, this is probably due to the inhomoge-
neities present in the volume and the perturbed relief on

Effect of Annealing on the Structural and Optical Properties of CuInAl S Thin Films
1216 1–xx 2
Figure 4. Reflection spectra of annealed CuIn1xAlxS2 thin
films for different composition and at various temperatures
(a) 250˚C, (b) 300˚C and (c) 400˚C.
the surface.
Figure 4(a) shows the optical reflection spectra of
CIAS films annealed at 400˚C. We notice an increase in
the reflection values for the CIAS films for about 70%
compared to the non alloyed films. The reason of the
increase of the reflection is that the aluminium is local-
ized near the surface rather than in the volume. Indeed,
after annealing at 300˚C and 400˚C one can note a de-
crease of the reflection values from 70% to 25% and an
improvement decrease of the interference fringes (Figure
4(c)). We explain these phenomenons by the diffusion of
aluminium from the surface into the volume and an indi-
cation of poor crystallinity.
3.3.2. Ab sorption Coefficients
The absorption coefficient has been calculated using
Equation (2) [20,21] and by means of optical transmis-
sion and reflection data taken at 300 K:

1
1R
Ln
dT








(2)
where  is the absorption coefficient, d is the thickness
of the film, T and R are the transmission and reflectance,
respectively. Figure 5 shows the absorption coefficients
as a function of the photon energy for CIAS thin films.
The main absorption appears in the high photon energy
region between 104 cm1 and 105 cm1 in the visible and
the near-IR spectral range. But additional absorption
features are found in the low energy range from 1.5 to 3
eV. This suggests that band-to-band optical transitions
occur in the high photon energy region (hν > 2 eV),
while, on the other hand, transitions between the ionized
donor and the conduction band appear in the lower en-
ergy region. The presence of a single absorption edge is
explained by the disappearance of secondary phases such
as Cu, a result confirmed by X-ray diffraction analysis of
annealing temperatures are above 300˚C. It is observed
that there is dependence between the absorption value
and the composition of the films. Then the incorporation
of aluminium atoms moves the absorption coefficients on
the left, we assume that are easily diffused in the volume
during annealing process. This result is very important
since the spectral dependence of the absorption coeffi-
cient affects the solar conversion efficiency [22]. The
absorption coefficient
 is related to the energy gap Eg
according to the equation:


n
g
hvA hvEhv   (3)
where A is a constant, h is the Planck constant and n
equal to 1/2 for direct gap and 2 for an indirect gap.
Based on the allowed direct inter-band transition, the
band-gap Eg was determined by extrapolating the straight
line of the (
 h  )2 vs. h  curve to intercept of the hori-
zontal photon energy axis. The band gap energy in-
creases with increasing the composition ratio for differ-
ent annealing temperatures (Table 2). Probably, the in-
crease of Eg may be attributed to the enhancement of

Effect of Annealing on the Structural and Optical Properties of CuInAl S Thin Films1217
1–xx 2
Figure 5. α versus the photon energy (hν) of annealed
CuIn1–xAlxS2 thin films for different composition and at
various temperatures (a) 250˚C, (b) 300˚C and (c) 400˚C.
grain size and the increase of defects in microstructure
(such as sulfur vacancies and atom displacements) and/or
deviations from stoichiometry in the films, which give
rise to defect states and thus induce smearing of absorp-
tion edge.
4. Conclusions
CIAS thin films were deposited on corning glass (7059)
substrates from CuIn1xAlxS2 powders by vacuum evapo-
ration and annealed in different temperatures and tech-
niques. X-ray diffraction analysis had shown preferred
(112) orientation and its found to shift to higher values of
2θ. Than for chemical composition we note an increase
in the Al/(In + Al) ratio from 0 to 0.59 which is almost
the composition of the used powders. In addition for the
optical properties we note that 400˚C is the best degree
of annealing temperature to obtain a material with a sin-
gle absorption edge. Also it is found that this absorption
edge shifts to higher photon energy with increasing Al
concentration.
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