Materials Science in Semiconductor Processing 40 (2015) 26–34

Contents lists available at ScienceDirect

Materials Science in Semiconductor Processing

journal homepage: www.elsevier.com/locate/mssp

Effect of thermal annealing on physical properties of vacuum

evaporated In2S3 buffer layer for eco-friendly photovoltaic applications
S.P. Nehra a, S. Chander b,n, Anshu Sharma c, M.S. Dhaka b
a
Centre of Excellence for Energy and Environmental Studies, Deenbandhu Chhotu Ram University of Science and Technology, Murthal, Sonepat 131039, India
b
Department of Physics, Mohanlal Sukhadia University, Udaipur 313001, India
c
Department of Physics, Deenbandhu Chhotu Ram University of Science and Technology, Murthal, Sonepat 131039, India

art ic l e i nf o a b s t r a c t

Article history: The present communication reports the effect of thermal annealing on the physical properties of In2S3
Received 24 March 2015 thin films for eco-friendly buffer layer photovoltaic applications. The thin films of thickness 150 nm were
Received in revised form deposited on glass and indium tin oxide (ITO) coated glass substrates employing thermal vacuum eva-
15 June 2015
poration technique followed by post-deposition thermal annealing in air atmosphere within a low
Accepted 17 June 2015
temperature range 150–450 °C. These as-deposited and annealed films were subjected to the X-ray
Available online 3 July 2015
diffraction (XRD), UV–vis spectrophotometer, current–voltage tests and scanning electron microscopy
Keywords: (SEM) for structural, optical, electrical and surface morphological analysis respectively. The composi-
In2S3 thin films tional analysis of as-deposited film is also carried out using energy dispersive spectroscopy (EDS). The
Vacuum evaporation
XRD patterns reveal that the as-deposited and annealed films ( r 300 °C) have amorphous nature while
Thermal annealing
films annealed at 450 °C show tetragonal phase of β-In2S3 with preferred orientation (109) and poly-
XRD
Optical properties crystalline in nature. The crystallographic parameters like lattice constant, inter-planner spacing, grain
SEM size, internal strain, dislocation density and number of crystallites per unit area are calculated for
thermally annealed (450 °C) thin films. The optical band gap was found in the range 2.84–3.04 eV and
observed to increase with annealing temperature. The current–voltage characteristics show that the as-
deposited and annealed films exhibit linear ohmic behavior. The SEM studies show that the as-deposited
and annealed films are uniform, homogeneous and free from crystal defects and voids. The grains in the
thin films are similar in size and densely packed and observed to increase with thermal annealing. The
experimental results reveal that the thermal annealing play significant role in the structural, optical,
electrical and morphological properties of deposited In2S3 thin films and may be used as cadmium-free
eco-friendly buffer layer for thin films solar cells applications.
& 2015 Elsevier Ltd. All rights reserved.

1. Introduction absorbing materials is under progress. A number of compound

semiconducting materials like In2S3, In2Se3, InZnSex, ZnO, ZnS,
In last few years, the thin film solar cells based on heterojun- ZnSe are studied as a substitute to the traditional CdS buffer layer
tions using CdS thin film as window layer have been paid more [4]. Among these, indium sulfide (In2S3) is an important semi-
attention due to their promising conversion efficiencies, but the conductor material for optoelectronic and photovoltaic applica-
growth of CdS window layer in large area production of thin film tions due to its chemical stability, wider optical band gap (2.0–
solar cells has serious environmental problems owing to great 2.75 eV), photoconductive behavior, high optical transmittance
amount of cadmium containing waste and the utilization of am- (480%) and controllable electrical properties [5–7]. It is a com-
monia in the deposition process [1,2]. The eco-friendly cadmium- pound semiconductor of III–VI group and exits in three crystal-
lographic forms in nature α-In2S3 (cubic structure, stable upto
free alternative buffer layers are yet to match the efficiency of thin
693 K), β-In2S3 (spinel structure, stable upto 1027 K) and γ-In2S3
film solar cell with CdS buffer layer and the reported world-record
(layered structure, stable above 1027 K). The β-In2S3 is found in
efficiency of CIGS solar cells with Cd-based as well as Cd-free buffer
most stable form at temperature below 673 K with tetragonal
layers is found to be 21.7% and 19.7% respectively [3]. The research
structure and it is a non toxic compound as well as exhibits n-type
work concerned to replace CdS buffer layer by other nontoxic low- electrical conductivity. The internal strain affects the crystal
structure of In2S3 thin films due to thermal expansion mismatch
n
Corresponding author. Fax: þ91 294 2423641. between film and substrate surface, variation in inter-atomic
E-mail address: sckhurdra@gmail.com (S. Chander). spacing and recrystallization processes [8–11]. It has potential

http://dx.doi.org/10.1016/j.mssp.2015.06.049
1369-8001/& 2015 Elsevier Ltd. All rights reserved.
 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34 27

applications to manufacture the picture tubes of color television solar cell applications, therefore, a study on post-deposition ther-
and fabrication of green as well as red phosphors [12]. The In2S3 mal annealing treatment on the physical properties of In2S3 thin
thin films can be used as window and buffer layer in heterojunc- films is undertaken in this paper. The thin films of thickness
tion solar cell applications due to wide optical energy band gap 150 nm were deposited on glass and ITO coated glass substrates
and eliminating the toxicity of cadmium. So, In2S3 films is a most employing thermal vacuum evaporation deposition technique.
promising replacement for cadmium sulfide in Cu(In,Ga)Se2 (CIGS) These as-deposited films were subjected to thermal annealing in
based photovoltaic cells and optical buffer layer in solar cells such air atmosphere within temperature range 150–450 °C. The physi-
as In2S3/CuInX2 (S, Se) because it prevents health risks to operators cal properties analysis of these as-deposited and annealed films
and also decreases environmental pollution [2,10,13]. have been carried out employing X-ray diffraction, UV–vis spec-
The In2S3 thin films can be prepared using a number of physical trophotometer, current–voltage tests and scanning electron mi-
and chemical deposition techniques such as atomic layer deposi- croscopy. The compositional analysis of as-deposited film is also
tion, chemical bath deposition, magnetron sputtering, thermal undertaken using EDS. The crystallographic parameters are cal-
evaporation, close-spaced evaporation, spray pyrolysis, ion layer culated for thermally annealed (450 °C) thin films. The optical
gas reaction, modulated flux deposition and wet chemical method parameters like extinction coefficient, optical energy band gap,
etc. [14–22]. Among these techniques, thermal vacuum evapora- absorption coefficient and refractive index of as-deposited and
tion is one of the most promising deposition technique and has a annealed films are also calculated.
number of advantages like most productive, very high deposition
rate, low material consumption and low cost of operation.
Calderon et al. [23] fabricated CuInS2 thin film solar cell with 2. Experimental details
structures Mo/CuInS2/ZnS/ZnO and Mo/CuInS2/ In2S3/ZnO using
ZnS and In2S3 as buffer layers and concluded that the former 2.1. Growth of In2S3 thin films
structure have best efficiency of 7.8% as compared to later struc-
ture's efficiency of 6.5%. The effect of sulfurization temperature The In2S3 powder of purity more than 99.99% was procured
and time on the optical, electrical and photoelectrical properties of from Alfa Aesar, USA. The In2S3 thin films were deposited on glass
β-In2S3 thin films was investigated by Yoosuf et al. [24] employing and ITO coated glass substrates at ambient temperature employing
two-stage process with rapid heating in H2S atmosphere. They thermal vacuum evaporation. The glass substrates were used to
found that the films have n-type conductivity and optical band gap carry out the structural, optical and surface morphological analysis
was increased with sulfurization temperature and time. The role of while ITO coated glass substrates for electrical analysis. These
chlorine doping on the opto-electronic properties of spray pyr- substrates were cleaned before deposition with acetone followed
olysed β-In2S3 thin films is studied by Cherian et al. [25] who by isopropyl alcohol and then rinsed in deionized water and fixed
observed that the photosensitivity and crystallinity were increased at the substrate holder. The base pressure, deposition pressure and
with optimum chlorine doping. Meng et al. [26] reported the deposition time were set as 3  10  3 mbar, 5  10  6 mbar and
single phase cubic structure of chemical bath deposited nanos- 20 min respectively. The deposition rate varied from 0.7 Å/s to
tructured indium sulfide thin films. Yahmadi et al. [27] in- 1.7 Å/s. The thickness of deposited films was verified by quartz
vestigated that the crystallinity of the indium sulfide thin films thickness monitor and found to be 150 nm.
was strongly affected by the annealing treatment and deposition
parameters like deposition time, pH-solution and thioacetamide. A 2.2. Thermal annealing
study of electrochemically deposited In2S3 thin films is undertaken
by Mari et al. [28] and found that the films have tetragonal phase To obtain homogeneous and uniform structure of In2S3 thin
in the absence of oxide. They also concluded that these films may films, the as-deposited films were subjected to the post-deposition
be applied onto CISe2 substrates for performance measurement of thermal annealing in the air atmosphere within the temperature
a solar cell. Hsiao et al. [29] reported n-type In2S3 nanoflake based range 150–450 °C for a period of one hour. The annealing tem-
thin films and observed a unique cross-linked network structure perature was maintained with the help of digital microprocessor
and optical band gap was found to be 2.5 eV. The tetragonal of the automatic controlled furnace (Metrex Muffle).
structure of β-In2S3 is reported by Sall et al. [30] and they con-
cluded that the crystallinity was improved with substrate tem- 2.3. Characterizations of In2S3 thin films
perature. They also observed that the surface roughness decreased
with temperature. Recently, the effect of film thickness on physical 2.3.1. Structural analysis
properties of In2S3 thin films is reported by Chander et al. [31] The crystal structure of the thin films was analyzed by X-ray
employing thermal evaporation technique. They found that the diffractometer (Panalytical X Pert Pro) using Cu Kα radiation with
films have amorphous nature and optical band gap decreased with λ ¼1.540598 Ǻ in the range 20–70° at scan speed of 0.5 °/min.
film thickness. So, the physical and chemical properties of the
films are strongly dependent upon the deposition techniques, film 2.3.2. Optical analysis
thickness, annealing treatment, substrate, doping and substrate The optical absorbance and transmittance measurements were
temperature. The annealing treatment may be undertaken in va- carried out employing a UV–vis spectrophotometer (Shimadzu
cuum, air and gaseous medium like Ar, N2, H2 etc. Generally, the UV-2450) at room temperature in the wavelength range 200–
post-deposition annealing treatment is used to improve structural 800 nm.
and optoelectronic properties, therefore, studies concerning the
effect of annealing on physical properties are very important 2.3.3. Electrical analysis
which play a significant role in efficiency improvement of optoe- The transverse current–voltage (I–V) measurements of as-de-
lecronic devices. The suitable thickness for In2S3 thin film as buffer posited and annealed In2S3 thin films were performed employing
layer lies in the range 50–150 nm and the study on post-deposi- source-meter (Keithley-2450). The electrical contacts for I–V
tion thermal annealing treatment of these films as buffer layer is measurements have been made on ITO coated glass samples using
still pending. silver conductive paste. The measurements were performed with
Thorough literature survey reveals that there is a need to study increasing step 0.01 V within the voltage range from 2.0 V to
non-toxic In2S3 thin films to use it as buffer layer in eco-friendly þ2.0 V. The current–voltage characteristics were monitored with
28 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34

Fig. 1. The X-ray diffraction patterns of as-deposited and annealed In2S3 thin films.

the help of Kickstart computer software and measurements were The diffraction peaks are observed in XRD pattern of In2S3 thin
performed at room temperature. films annealed at 450 °C at angular positions 27.402°, 30.701°,
33.288°, 43.749° and 47.830° corresponding to orientations (109),
2.3.4. Surface morphological and compositional analysis (215), (0012), (1015) and (2212) respectively which are well in-
The surface topography of as-deposited and annealed In2S3 thin dexed with the JCPDS data file 25–390 [33]. It reveals that these
films was taken by a scanning electron microscopy (Zeiss EVO 18). films show tetragonal phase of β-In2S3 with preferred orientation
The compositional analysis of as-deposited film was also under- (109) and polycrystalline in nature. No peaks corresponding to
taken employing EDS (Acquisition 116). oxide phases are observed which confirmed the absence of any
oxidation when the films were deposited onto glass substrate and
annealed at temperature 450 °C. The intensity of predominant
3. Results and discussion
peak (109) is observed to be relatively high as compared to the
other peaks which confirm the polycrystalline nature with tetra-
3.1. X-ray diffraction analysis
gonal phase of the films annealed at 450 °C. This result is well
supported with earlier reported work of Sall et al. [30], Loredo
The X-ray diffraction patterns of as-deposited and annealed
magnetron sputtered In2S3 thin film of thickness 150 nm are et al. [34] and Barreau et al. [35].
presented in Fig.1. The crystallographic parameters like lattice constants (a and c),
The XRD patterns reveal that the as-deposited and annealed inter planner spacing (d), full width at half maxima (FWHM), grain
In2S3 thin films ( r300 °C) are amorphous in nature. It is observed size (D), internal strain (ε), dislocation density (δ) and number of
that the these films have poor crystallinity and low intensity crystallites per unit area (N) were calculated for thermally an-
which may be attributed to the more probability of interaction of nealed (450 °C) thin films using relations concerned and tabulated
the material with the substrate and consistent of the deposited in Tables 1 and 2.
material stacked in random fashion [30], therefore, these films The lattice parameter (a) and (c) of tetragonal phase were
showed amorphous nature. The low annealing temperature evaluated from the relation concerned and inter-planner spacing
(r300 °C) of In2S3 films is not sufficient to provoke significant (d) was calculated using Bragg's diffraction law [10,36].
crystallization. The growth of thin film is taken place via nuclea-
1 h2 + k 2 l
tion, coalescence, channels and holes while these are not well = + 2
2
connected in amorphous films. A random arrangement of atoms d a2 c (1)
into volume leads the amorphous structure. Zhong et al. [32] also and
found that the In2S3 buffer layers have amorphous nature at dif-
ferent substrate temperatures. nλ = 2d sin θ (2)
 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34 29

Table 1 the other peaks. The grain size is found 11.38 nm corresponding to
The lattice parameters (a and c) and inter-planner distance (d) of 450 °C thermally preferred orientaion (109). The grain size in amorphous thin films
annealed In2S3 thin films.
is very fine and low which revealed to the very high full width at
Position 2θ (deg) (hkl) d (Ǻ) a (Ǻ) c (Ǻ) half maxima and may be attributed to the non visibility of any
reflection in the X-ray diffraction pattern. Generally, increase in
Obs. Std. Obs. Std. Obs. Std. film thickness and annealing temperature responsible for the
sharper and intense preferred orientation which may be attributed
27.402 (109) 3.252 3.269 7.608 7.619 32.312 32.329
to the larger grains. The variation in grain size can be explained by
the crystal growth and formation of nuclides [12]. The improve-
ment in crystallinity of In2S3 thin films may provide the activation
Table 2 energy to occupy the minimum energy position of the adsorbed
The crystallographic parameters of 450 °C thermally annealed In2S3 thin films. atoms and enhance the recrystallization due to the coalescence of
the islands by increasing surface diffusions and volume [10]. The
Position 2θ (deg) FWHM D (nm) ε  10  3 δ  1015 m  2 N  1015 m  2
results are in good agreement with the earlier reported work of
27.402 0.7508 11.38 13.43 7.72 33.92 Revathi et al. [18].
30.701 0.4776 18.02 7.57 3.07 8.53 The internal strain (ε) in thin films is defined as the disar-
33.288 0.5722 15.14 8.34 4.36 14.40 rangement of lattice created during the deposition process and
43.749 0.5432 16.46 5.90 3.68 11.19
47.830 0.4546 19.97 4.47 2.50 6.27
was calculated by relation concerned [38].
β
ε=
4 tan θ (4)
Here, h, k, l are Miller indices of the lattice plane, n is the in-
teger, λ is wavelength of incident radiation and θ is Bragg's angle. The imperfection in crystal is known as dislocation and the
It is visible in Table 1 that the calculated inter planner spacing, length of dislocation lines per unit volume of the crystal is defined
lattice parameters a and c are found 3.252 Ǻ, 7.608 Ǻ and 32.312 Ǻ as the dislocation density (δ). It was calculated using Williamson–
corresponding to predominant peak (109). The parameters a and c Smallman relation.
are observed to be decreased for other low intensity peaks which 1
δ=
may be attributed to the variation of angular position of the other D2 (5)
peaks [37]. Generally, increase and decrease in lattice parameters
The number of crystallites per unit area (N) was calculated
may be due to the change in density and nature of native im-
using following relation [38].
perfections. The change in lattice parameters is connected with
the growth parameters like annealing temperature, substrate t
N=
temperature, film thickness etc. and also on the mismatch of D3 (6)
thermal expansion coefficient of the film and the substrate. The
Here, t is the thickness of the thin films.
grain size (D) of the annealed films was calculated using Debye– It can be seen from Table 2 that the internal strain, dislocation
Scherrer formula [18]. density and number of crystallites per unit area are found
kλ 13.43  10  3, 7.52  1015 m  2 and 33.92  1015 m  2 respectively
D= corresponding to the predominant (109) orientation for the film
β Cos θ (3)
annealed at 450 °C.
Here, k is the Scherrer constant having value 0.94 and β is the
full width at half maxima (FWHM). 3.2. Optical analysis
The FWHM is calculted corresponding to the all observed dif-
fraction peaks and found in the range 0.4546–0.7508. It is ob- The optical absorbance and transmittance spectra of as-de-
served from Table 2 that the FWHM coresponding to predominant posited and annealed In2S3 thin films were measured in the wa-
peak (109) is found 0.7508 which is relatively high as compared to velength range 200–800 nm and presented in Fig. 2.

Fig. 2. The (a) absorbance and (b) transmittance spectra of as-deposited and annealed In2S3 thin films.
30 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34

Fig. 3. The variation of (a) absorbance coefficient with photon energy and (b) Tauc plot (αhν)2 v/s hν of as-grown and annealed In2S3 thin films.

It is visible in Fig. 2a that the In2S3 thin films have low absor- the electronic transition between valence band and conduction
bance in the visible region but high in ultraviolet region. The ab- band is found to be direct due to linear nature of the Tauc plot at
sorbance is found in the range 1.75–2.65% for as-deposited and higher photon energy region. The optical energy band gap of In2S3
annealed thin films in lower wavelength range and decreased with thin films is varied in the range 2.84–3.04 eV and found to be in-
wavelength. It is also observed that the absorbance is found to be creased with thermal annealing which may be attributed to the
decreased with thermal annealing in air atmosphere due to sulfide more reallignment in orientation and strong interaction between
vacancy (S-vacancy) which revealed the band to band transition the substrate and vapor atoms [37]. Generally, in polycrystalline
occurred between conduction band and ionized donor. The ab- compound semiconductor, the optical energy band gap can be
sorption edge is found to be shifted towards lower wavelength affected by the stoichiometric deviations, change in preferred or-
with annealing and blue shift is observed. The absorbance spectra ientation, dislocation density, disorder at the grain boundaries and
are sensitive to the variation of annealing temperature on the quantum size effect [18]. In the present study, the increase in
surface of the layers and distribution of grains which indicated the optical energy band gap with annealing may be due to the for-
semiconducting nature of films. The optical transmittance spectra mation of lacalized states in the high photon energy region. It is
of In2S3 thin films (Fig. 2b) show that the transmittance is in- also visible in Tauc plot that the optical absorption edge shows
creased in the lower wavelength range and found more than 65% blue shifts with annealing temperature. The variation of optical
for as-deposited and annealed films in the visible range. It is also band gap with annealing temperature is also shown in Fig. 4b. The
observed that the transmittance is found to be increased with obtained results of optical band gap are in good agreement with
thermal annealing. earlier reported work of Sall et al. [30], Barreau et al. [35] and
The optical band gap was calculated using the Tauc relation Methew et al. [40].
[39]. The refractive index (n) was calculated using Herve–Van-
n damme formula [31].
(
αhν = A hν − Eg ) (7)
⎛ A ⎞2
Here, α absorption coefficient, h is the plank constant, ν is the n2 = 1 + ⎜⎜ ⎟⎟
⎝ Eg + B ⎠ (9)
frequency of light, A is a constant, Eg is the optical energy band gap
and n is the integer which depends on the nature of the optical Here, A and B are constants having values of 13.6 eV and 3.4 eV
transition. It has value n¼ 1/2 and 2 for allowed direct and indirect respectively and Eg is the optical band gap. The variation of re-
band gap transition respectively. The absorption coefficient (α) fractive index with annealing temperature is presented in Fig. 4b.
was calculated from the relation concerned [38]. The theory of reflectivity of light has been used to calculate the
2.303 A extinction coefficient (k) from the absorption coefficient using
α= relation concerned [41].
t (8)
λα
Here, A is the absorbance and t is the film thickness. The var- k=
4π (10)
iation in absorption coefficient with photon energy and Tauc plot
(αhν)2 v/s hν of as-deposited and annealed In2S3 thin films is The graphical representation of extinction coefficient of as-
presented in Fig. 3. grown and annealed In2S3 thin films with photon energy is shown
It is clearly visible in Fig. 3a that the absorption coefficient is in Fig. 4a.
observed to be increased with photon energy and found maximum It is clearly visible in Fig. 4a that the extinction coefficient is
at higher photon energy region which indicated the band to band found to be increased with photon energy and found maximum at
transition occurred in the high photon energy region (hν 42.8 eV) higher photon energy (  4 eV), thereafter it is decreased con-
while transitions between conduction band and ionized donor tinuously. It is also found to be decreased with post-deposition
occurred in lower energy region. It is also observed that the ab- annealing which revealed to the dominance in density tempera-
sorption coefficient is found to be decreased with thermal an- ture dependence [42,43]. The refractive index is related to the
nealing. The optical band gap is evaluated by extrapolating the electronic polarizability of ions and the local field inside the op-
straight line towards hν-axis of Tauc plot (Fig. 3b) for zero ab- tical materials. It is clearly visible in Fig. 4b that the refractive
sorption coefficients. It is clearly visible in Fig. 3b that the nature of index of as-grown and annealed In2S3 thin films is varied from
 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34 31

Fig. 4. The variation of (a) extinction coefficient (k) with incident photon energy and (b) optical band gap and refractive index with annealing temperature.

Fig. 5. The transverse current–voltage plot of as-deposited and annealed In2S3 thin films.

2.293 to 2.367 and observed to decrease with post-deposition annealing temperature which indicates that the low annealing
thermal annealing which may be attributed to the variation in temperature creates the minor crystal growth. The resistivity of the
packing density of the films. The results are well agreed with films is found to be increased and the electrical conductivity de-
earlier reported work of Chander et al. [31] and El-Nahass et al. creased with annealing temperature owing to the inverse relation
[44]. with carrier concentration [43]. The increment in resistivity may be
attributed to the contamination from furnace, grain boundary oxi-
3.3. Electrical analysis dation, sulfur loss due to leaching and elemental diffusion from the
substrate into the thin film. Generally, the resistivity is found to de-
The transverse current–voltage measurements were performed crease with post-deposition annealing treatment because the grain
for as-deposited and annealed In2S3 thin films employing a boundaries have created the potential barriers to enhance the elec-
source-meter with increasing step 0.01 V and are presented in trical resistivity of thin films. The presence of oxygen in the thin films
Fig. 5. can also affects the properties of grain boundaries which cause a
It is visible in Fig. 5 that the variation in current with voltage decrement in the resistivity of the film [5]. The results show opposite
for as-deposited and annealed In2S3 thin films thin films is found behavior of resistivity with annealing from the earlier reported works
to be linear. The current is observed to be decreased with of John et al. [5,41].
32 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34

Fig.6. The SEM image of In2S3 thin films (a) as-deposited and (b–d) annealed films 150 °C to 450 °C.

3.4. Surface morphological and compositional analysis The compositional analysis of as-deposited film was carried out
employing EDS and shown in Fig. 7and Table 3.
The surface topography was analyzed using scanning electron The EDS pattern of as-deposited film shows the presence of indium
microscopy SEM images of as-deposited and annealed In2S3 thin and sulfur elements with silicon as major content due to glass sub-
films are presented in Fig. 6. strate which indicate the amorphous nature of the films with pre-
The high resolution SEM images show that the as-deposited sence of the oxygen. The In:S ratio is found to be 2/3 for as-deposited
In2S3 thin films are smooth, uniform, homogeneous, continuous, In2S3 thin film and the average atomic percentage of indium and
fully covered and free from defects like pin holes and cracks. The sulfur are found to be 4.54% and 6.63% respectively. As stated earlier,
grains in the thin films are similar in size and densely packed. No the growth of thin film is taken place via nucleation, coalescences,
voids and inclusions are found. The surface is appeared to be channels and holes which are not connected to each other in the
regular and after annealing at 150 °C a slight roughness is also amorphous thin films and same effect of the glass substrate is clearly
observed which indicates to the more reallignment in orientation visible in the EDS pattern owing to the presence of various consistent
of the deposited atoms of film [45]. Generally, the grains in the of the used glass substrate in the deposition process.
films depended on the annealing temperature, substrate tem-
perature and film thickness. The growth of thin film is based on
nucleation, coalescence, channels, holes and continuous film. In a 4. Conclusion
perfect continuous film, cracks, pin holes and defects are absent
but practically it is not possible [46]. These defects are so mini- In this paper, a study on effect of post-deposition thermal an-
mum in a good thin film. Fig. 6(c and d) shows that the surface nealing on the physical properties of In2S3 thin films for eco-
roughness of In2S3thin films is also observed to increase with thermal friendly buffer layer photovoltaic applications is reported. The thin
annealing due to different shape and size of surface feature for films films of thickness 150 nm were deposited on glass and ITO coated
annealed at 300 °C and 450 °C which indicate phase change at higher glass substrates employing thermal vacuum evaporation deposi-
annealing temperature from amorphous to tetragonal and improve- tion technique followed by annealing within a low temperature
ment in crystallinity as confirmed by XRD patterns. It is also cleared range 150–450 °C. The XRD patterns reveal that the as-deposited
that the surface of annealed films become dense and grains are found and annealed films (r300 °C) have amorphous nature while films
uniform in size as compared to as-deposited film. The surface mor- annealed at 450 °C show tetragonal phase of β-In2S3 with pre-
phology results are well supported by earlier reported work of Sall ferred orientation (109) and polycrystalline in nature. The crys-
et al. [30], Otto et al. [39] and Asenjo et al. [47]. tallographic parameters like lattice constant, inter-planner
 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34 33

Fig. 7. The EDS pattern of as-deposited In2S3 thin films.

thankful to the UGC, New Delhi for providing fellowship under

Table 3
The compositional elemental analysis of as-deposited In2S3 thin film.
Dr. D.S. Kothari Postdoctoral Fellowship scheme.

Element (s) Series Atomic %

References
Indium (In) L 4.54
Sulfur (S) K 6.63
Silicon (Si) K 24.92 [1] M.G. Sandoval-Paz, M. Sotelo-Lerma, J.J. Valenzuela-Jáuregui, M. Flores-Acosta,
Oxygen (O) K 34.28 R. Ramírez-Bon, Thin Solid Films 472 (2005) 5–10.
Carbon (C) K 22.20 [2] V.G. Rajeshmon, N. Poornima, C.S. Kartha, K.P. Vijayakumar, J. Alloy. Compd.
553 (2013) 239–244.
Sodium (Na) K 7.43
[3] M.A. Green, K. Emery, Y. Yoshihiro, W. Warta, E.W. Dunlop, Prog. Photovolt.:
Total 100
Res. App. 23 (2015) 1–9.
[4] D. Hariskos, S. Spiering, M. Powalla, Thin Solid Films 480–481 (2005) 99–109.
[5] T.T. John, S. Bini, Y. Kashiwaba, T. Abe, Y. Yasuhiro, C.S. Kartha, K.
P. Vijayakumar, Semicond. Sci. Technol. 18 (2003) 491–500.
[6] G.R. Gopinath, R.W. Miles, K.T.R. Reddy, Energy Procedia 34 (2013) 399–406.
spacing, grain size, internal strain, dislocation density and number
[7] S.S. Wang, F.J. Shiou, C.C. Tsao, S.W. Huang, C.Y. Hsu, Mater. Sci. Semicond. Proc.
of crystallites per unit area are calculated for thermally annealed 16 (2013) 1879–1887.
(450 °C) thin films. The optical parameters like extinction coeffi- [8] L. Liu, W. Xiang, J. Zhong, X. Yang, X. Liang, H. Liu, W. Cai, J. Alloy. Compd. 493
(2010) 309–313.
cient, optical energy band gap, absorption coefficient and re- [9] R. Ranjith, T.T. John, C.S. Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Mater.
fractive index are also calculated. The optical band gap was found Sci. Semicond. Proc 10 (2007) 49–55.
in the range 2.84–3.04 eV and observed to increase with annealing [10] N. Revathi, P. Prathap, K.T.R. Reddy, Solid State Sci. 11 (2009) 1288–1296.
[11] P. Prathap, N. Revathi, Y.P.V. Subbaiah, K.T.R. Reddy, J. Phys.: Condens. Matter
temperature. The current–voltage characteristics show that the as- 20 (2008) 035205.
deposited and annealed films exhibit linear ohmic behavior and [12] N. Revathi, P. Prathap, Y.P.V. Subbaiah, K.T.R. Reddy, J. Phys. D: Appl. Phys. 41
the electrical resistivity was found to be increased with annealing. (2008) 155404.
[13] A. Timoumi, H. Bouzouita., M. Kanzari, B. Rezig, Thin Solid Films 480-481
The SEM studies show that the as-deposited and annealed films (2005) 124–128.
are uniform, homogeneous and free from defects and voids. The [14] C. Bugot, N. Schneider, D. Lincot, F. Donsanti, J. Nanotechnol. 4 (2013) 750–757.
[15] J.F. Trigo, B. Asenjo, J. Herrero, M.T. Gutierrez, Sol. Energy Mater. Sol. Cells 92
grains in the thin films are similar in size and densely packed and
(2008) 1145–1148.
observed to be increased with thermal annealing. The experi- [16] D.H. Hwang, S. Cho, K.N. Hui, Y.G. Son, J. Nanosci. Nanotechnol. 14 (2014)
mental results revealed that the thermal annealing in air atmo- 8978–8981.
[17] H. Izadneshan, V.F. Gremenok, J. Appl. Spectrosc. 81 (2014) 293–296.
sphere played significant role in the structural, optical, electrical [18] N. Revathi, P. Prathap, R.W. Miles, K.T.R. Reddy, Sol. Energy Mater. Sol. Cells 94
and morphological properties of deposited In2S3 thin films and (2010) 1487–1491.
may be used as cadmium-free eco-friendly buffer layer for thin [19] R. Jayakrishnan, V. Ganeshan, Mater. Sci. Semicond. Proc. 24 (2014) 220–224.
[20] J. Qiu, Z. Jin, W.B. Wu, L.X. Xiao, Thin Solid Films 510 (2006) 1–5.
films solar cell applications. [21] F. Rahman, J. Podder, M. Ichimura, Surf. Rev. Lett. 20 (2013) 1350014.
[22] W. Lee, S. Baek, R.S. Mane, V.V. Todkar, W.S. Olga Egorova, S.H. Chae, Lee, S.
H. Han, Curr. App. Phys. 9 (2009) S62–S64.
[23] C. Calderon, J.S. Oyola, P.B. Perez, G. Gordillo, Mater. Sci. Semicond. Process. 16
Acknowledgments (2013) 1382–1387.
[24] R. Yoosuf, M.K. Jayaraj, Sol. Energy Mater. Sol. Cells 89 (2005) 85–94.
The authors are highly thankful to the University Grants [25] A.S. Cherian, M. Methew, C.S. Kartha, K.P. Vijayakumar, Thin Solid Films 518
(2010) 1779–1783.
Commission, New Delhi for providing financial support under the [26] X. Meng, Y. Lu, X. Zhang, B. Yang, G. Yi, J. Jia, Appl. Surf. Sci. 258 (2011)
Major Research Project Scheme (F. No. 42-802/2013 (SR)). AS is 649–656.
34 S.P. Nehra et al. / Materials Science in Semiconductor Processing 40 (2015) 26–34

[27] B. Yahmadi, N. Kamoun, R. Bennaceur, M. Mnari, M. Dachraoui, K. Abdelkrim, [39] K. Otto, A. Katerski, A. Mere, O. Volobujeva, M. Krunks, Thin Solid Films 519
Thin Solid Films 473 (2005) 201–207. (2011) 3055–3060.
[28] B. Mari, M. Mollar, D. Soro, R. Henriquez, R. Schrebler, H. Gomez, Int. J. Elec- [40] M. Methew, M. Gopinath, C.S. Kartha, K.P. Vijayakumar, Y. Kashiwaba, T. Abe,
trochem. Sci. 8 (2013) 3510. Sol. Energy 84 (2010) 888–897.
[29] Y.J. Hsiao, C.H. Lu, L.W. Ji, T.H. Meen, Y.L. Chen, H.P. Chi, Nanoscale Res. Lett. 9 [41] T.T. John, C.S. Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Vacuum 80
(32) (2014) 1–7. (2006) 870–875.
[30] T. Sall, B.M. Soucase, M. Mollar, B. Hartitti, M. Fahoume, J. Phys. Chem. Solids [42] A.A. Al-Ghamdi, S.A. Khan, A. Nagat, M.S.A. El-Sadek, Synthesis and optical
76 (2015) 100–104. characterization of nanocrystalline CdTe thin films, Opt. Laser Technol. 42
[31] S. Chander, S. Choudhary, A. Purohit, N. Kumari, S.P. Nehra, M.S. Dhaka, Mater. (2010) 1181–1186.
Focus (2015), http://dx.doi.org/10.1166/mat.2015.1236. [43] S. Chander, M.S. Dhaka, Physica E 73 (2015) 34–39.
[32] Z.Y. Zhong, E.S. Cho, S.J. Kwon, Thin Solid Films 547 (2013) 22–27. [44] M.M. El-Nahass, B.A. Khalifa, H.S. Soliman, M.A.M. Seyam, Thin Solid Films 515
[33] Powder Diffraction Data File, Joint Committee of Powder Diffraction Standard, (2006) 1796–1801.
International Centre for Diffraction Data, USA Card No. 25-390. [45] K. Bouabid, A. Ihlal, Y. Amira, A. Sqaq, A. Outzourhit, G. Nouet, Eur. Phys. J.
[34] S.L. Loredo, Y.P. Mendez, M.C. Rodriguez, S.M. Fernandez, A.A. Gallegos, A. Appl. Phys. 40 (2007) 149–154.
V. Dimas, T.H. Garcia, Thin Solid Films 550 (2014) 110–113. [46] M.S. Dhaka, Recent Trends in Thin Film Technology, Kalpana Publication, Jai-
[35] N. barreau, S. Marsillac, J.C. Bernede, A. Barreau, Appl. Surf. Sci. 161 (2000) pur, 2015.
20–26. [47] B. Asenjo, C. Guillen, A.M. Chaparro, E. Saucedo, V. Bermudez, D. Lincot,
[36] P. Rao, S. Kumar, Thin Solid Films 524 (2012) 93–99. J. Herrero, M.T. Gutierrez, J. Phys. Chem. Solids 71 (2010) 1629–1633.
[37] S. Lalitha, R. Sathyamoorthy, S. Senthilarasu, A. Subbarayan, K. Natarajan, Sol.
Energy Mater. Sol. Cells 82 (2004) 187–199.
[38] A. Purohit, S. Chander, S.P. Nehra, M.S. Dhaka, Physica E 69 (2015) 342–348.