Effect of substrate temperature on the physical

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Apr 30, 2016 - Tin chalcogenide thin films (SnS, SnS2 and Sn2S3) belong to the family of IV-VI ... Vapor Deposition [17] and chemical bath deposition [18].
Ceramics International 42 (2016) 12262–12269

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Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Effect of substrate temperature on the physical properties of co-evaporated Sn2S3 thin films T. Srinivasa Reddy, M.C. Santhosh Kumar n Optoelectronic Materials and Devices Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, Tamil Nadu 620015, India

art ic l e i nf o

a b s t r a c t

Article history: Received 6 April 2016 Received in revised form 28 April 2016 Accepted 29 April 2016 Available online 30 April 2016

We report the deposition of tin sulfide (Sn2S3) thin films by co-evaporation technique at different substrate temperatures. The influence of substrate temperature on the structural and optical properties of the thin films is investigated. X- ray diffraction (XRD) analysis and Micro-Raman studies confirm the formation of Sn2S3 phase. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to examine the surface morphology. The transmission spectra of the deposited Sn2S3 thin films have been recorded in the wavelength range of 200–3000 nm using UV–vis-NIR spectrometer. Film thickness (d) and optical constants such as refractive index (n), extinction coefficient (k), real (ε1) and imaginary (ε2) parts of the dielectric constants of thin films are estimated from the optical transmittance. The optical band gaps of the deposited films at different substrate temperatures are in the range of 1.46– 1.64 eV. Hall effect measurements confirm the n-type nature of the as-prepared Sn2S3 thin films. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Co-evaporation Sn2S3 Thin films Raman analysis Optical properties Refractive index

1. Introduction In recent years, binary and ternary semiconducting chalcogenide thin films such as CuS [1], SnxSy [2], Cu2SnS3 [3], AgInSe2 [4] have been attracting much interest due to their potential application in optoelectronic devices and solar cells. Dittrich et al. [5] have observed the importance of sulfo-salts materials in the field of optoelectronic applications. The optoelectronic properties of Sn –S based compounds are suitable for building photovoltaic p-n or p-i-n structures with high conversion efficiency of the order of 25% [6]. Tin chalcogenide thin films (SnS, SnS2 and Sn2S3) belong to the family of IV-VI group semiconductor materials. These semiconductor thin films are made up of inexpensive, non-toxic and earth abundant [7] constituents. SnS and SnS2 have been used for many applications such as absorbers [8] and window layers [9] in thin film solar cells because of their suitable optical and electrical properties. Among the tin chalcogenides, Sn2S3 is a semiconductor having layered structure and is a type I mixed valance compound. The optical and electrical property of Sn2S3 thin films depends on the crystalline structure and stoichiometry. These films are suitable for the fabrication of near lattice-matched hetero junctions like Sn2S3/CdTe, Sn2S3/GaSb, Sn2S3/AlS, which find applications in the detection and generation of infrared radiation [10]. Different direct band gap values such as 0.95 eV [11], 1.16 [10] and 2.0 eV n

Corresponding author. E-mail addresses: [email protected] (T.S. Reddy), [email protected] (M.C.S. Kumar). http://dx.doi.org/10.1016/j.ceramint.2016.04.172 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

[12] are reported for the of Sn2S3 thin films with highly anisotropic conduction nature [13]. Sn2S3 thin films have been prepared using different methods such as spray pyrolysis [2,10,12,14,15], potentiostatic electrodeposition [16], Plasma-Enhanced Chemical Vapor Deposition [17] and chemical bath deposition [18]. In the present work, the deposition of Sn2S3 thin films has been carried out by co-evaporation technique, for the first time. In this investigation Sn2S3 thin films are deposited on soda lime glass substrates at different substrate temperatures from 150 °C to 300 °C. The structural, morphological and electrical properties of the co-evaporated Sn2S3 thin films are studied and reported here. The optical properties such as refractive index n (λ), extinction coefficient k (λ) and dielectric constant (ε) of the deposited films studied and reported for the first time using the interference phenomena in transmission spectra data.

2. Experimental Tin sulfide (Sn2S3) thin films have been deposited on glass substrates at different substrate temperatures from 150 °C to 300 °C at intervals of 50 °C using co-evaporation technique. Tin wire (Sigma-Aldrich, 99.999%) and Sulfur powder (SigmaAldrich) were taken as source materials. Tin wire evaporated from molybdenum boat by applying a constant power and sulfur powder was evaporated from a glass crucible kept in a tungsten basket. In this technique, the source to substrate distance was fixed at 22 cm. The evaporation was carried out in a high vacuum chamber with ultimate vacuum of the order of 3.5  10  6 Torr. After

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reaching the evaporation temperature of source materials, shutter over the boats was moved to sideways for the deposition of the Sn2S3 thin film on the glass substrates [3,19]. The rate of deposition and thickness of the films was determined using SQM 160 quartz crystal thickness monitor. Prior to the deposition process, the glass slides were first cleaned with detergent solution and then thoroughly washed in running water. These slides were cleaned with Isopropyl alcohol, double distilled water and followed by ultrasonic cleaning for 30 min. These substrates are dried under infrared (IR) lamp (200 W) and cleaned with acetone. Finally, these cleaned substrates were loaded into the substrate holder of vacuum chamber. The substrates were further cleaned by ion-bombardment (HT) in vacuum chamber for ten minutes, prior to the deposition of the film. A 1KW heater with PID controller was used as the substrate heater. X- ray diffractometer (RIGAKU ULTIMA III) with a CuKα radiation source (λ ¼1.5406 Å), was used to confirm the structure and phases of the deposited films. Raman spectroscopy was carried out to identify the phases present in the Sn2S3 thin films using Horiba labRAM HR evolution micro Raman spectrometer. The Scanning electron microscopy (Zeiss Ultra 55 FE-SEM) and Energy dispersive spectroscopy (EDX) were used to analyze the morphology and composition of the films deposited at different substrate temperature. The atomic force microscopy (AFM- Park NX10) was used to study the morphology of the films. The optical transmittances versus wavelength measurements were carried out using UV–visNIR spectrometer (JASCO V-670) in the range of wavelength 200– 3000 nm. Hall measurement technique (ECOPIA HMS-5000) was employed for electrical measurements using vander pauw configuration.

deposition temperature beyond 150 °C; it is observed that the preferential orientation is along (130) plane of Sn2S3. It indicates that maximum number of crystallites have a growth orientation along (130) direction. Guneri et al. [18] also observed a similar (130) preferred orientation for Sn2S3 thin films grown by chemical deposition. From the XRD data lattice parameters are calculated using the relation

1 d2(hkl)

3.1. Structural analysis Fig. 1 shows the X-ray diffraction patterns of the as-deposited Sn2S3 thin films at different substrate temperatures on glass substrate. The diffraction peaks observed for the as-deposited samples are at 2θ values of 15.24°, 21.33°, 23.60°, 26.41°, 27.57°, 30.94° and 31.93°, which represent (120), (130), (220), (111), (140), (310) and (211) planes respectively. These values are in close agreement with the standard orthorhombic phase of the Sn2S3 (JCPDS 14-0169). At lower deposition temperature (150 °C) the films shows preferential orientation along (120) plane and upon increasing the

=

h2 k2 l2 + + a2 b2 c2

(1)

where d is the inter planar distance and (hkl) are the miller indices. The calculated lattice parameter values are a ¼8.77 Å, b ¼14.23 Å, and c ¼3.77 Å. These values are closer to the orthorhombic system of Sn2S3 (JCPDS 14-0169). The average crystallite size (D) of the films are determined by Debye- Scherer formula [20] given by

D=

0. 94λ β cos θ

(2)

where λ is the wavelength of the X-ray radiation (1.54 Å ), θ is the Bragg angle and β is the full width at the half maximum (FWHM) of the (130) peak. The FWHM and average crystallite size values of as-deposited Sn2S3 thin films are tabulated in Table 1. These values are very closer to the crystallite values reported by Guneri et al. [18] using chemical deposition. The instrumental broadening and physical factors such as crystallite size, micro strain and dislocation density are found to influence the width of the XRD peak of thin films. The micro stain (ε) and dislocation density (δ) are calculated using the formulae.

ε= 3. Results and discussion

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β cos θ 4

(3)

and

δ=

15ε aD

(4)

where a is the lattice constant and D is the average crystallite size of the films. The values of micro strain (ε) and dislocation density (δ) of Sn2S3 thin films are tabulated in Table 1. From the table it is observed that the average crystallite size of the films increases up to 250 °C beyond that decrease with increase of substrate temperature. The decrease in the crystallite size and increase in micro strain and dislocation density of the film deposited at the substrate temperature of 300 °C can be understood from the following: at higher temperatures scattering of the atoms from heated substrate surface might restrict the formation of clusters of crystallites and might lead to low grain size [21]. Reddy et al. [7] also observed that the variation of the micro strain, dislocation density of the films decreased proportionately and correspondingly the crystallite size increased with the increase of the substrate temperature. 3.2. Raman analysis The Raman spectra of the as-deposited Sn2S3 thin films in the Table 1 Structural parameters of the Sn2S3 thin films at different substrate temperatures.

Fig. 1. X-ray diffraction patterns of Sn2S3 thin films at different substrate temperatures.

Substrate temperature (°C)

FWHM (β) (radians)

Crystallite size Micro Dislocation den(D) (nm) strain (ε) sity (δ)  1016 (m  2)

150 200 250 300

– 0.4762 0.4366 0.4514

– 17 20 18

– 0.117 0.107 0.110

– 11 9.46 10.06

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low intensity broad Raman peak of Sn2S3 phase observed at 179 cm  1 is closer to the value reported by Barone et al. [23]. Thus Raman studies along with XRD confirm the formation of single phase Sn2S3 thin films by co-evaporation technique. 3.3. Morphological studies

Fig. 2. Raman spectra of Sn2S3 thin films at different substrate temperatures.

wavelength range of 50–600 cm  1 at room temperature are shown in Fig. 2. It can be seen that for all films exhibits Raman peaks corresponds to Sn2S3. The observed optical Raman phonon vibrations are at 61 cm  1, 91 cm  1, 179 cm  1, 220 cm  1 and 307 cm  1. The Raman mode observed at 91 cm  1 is attribute to Sn2S3 phase, which is close agreement with reported Raman data by Chandrasekhar et al. [22]. The observed Raman modes at 61 cm  1 [22] and 307 cm  1 [18] are belongs to the Sn2S3 phase. The Raman mode at 220 cm  1 is assigned to Sn2S3 phase as reported earlier by Guneri et al. [18]. The slight variation in the Raman frequency might be due to the film growth condition. It is observed that the intensity of Raman mode at 91 cm  1 and 307 cm  1 increases with increase of substrate temperature. The

Fig. 3(a)–(c) depicts SEM micrographs of the Sn2S3 thin films deposited at substrate temperature of 200 °C, 250 °C and 300 °C. All deposited films exhibit poly crystalline nature and orthorhombic petal like crystallite morphology. Fig. 3(a - c) shows densely packed petal like crystallites distributed uniformly throughout the sample. These crystallites are directed randomly on the surface of the films without any visible cracks and holes. Similar type of petal-like morphology was observed in SnS thin films using different methods [24,25]. The average grain size of the films deposited at substrate temperatures of 200 °C, 250 °C and 300 °C are 30 nm, 109 nm and 70 nm respectively. Fig. 3(d) shows, the cross sectional view of the Sn2S3 thin films deposited at a substrate temperature of 250 °C. The thickness of the films on glass substrate is estimated as 415 nm. The elemental composition of the Sn2S3 thin films have been studied with substrate temperature. Fig. 3(e), shows the typical EDX spectra of the film deposited at a substrate temperature of 250 °C. From this figure, it is clear that, the films contain peaks corresponding Sn and S, which are nearly in stoichiometric ratio. The peak corresponds to Si is from the glass substrate. The chemical composition values are tabulated in Table 2. From the elemental composition analysis, it is observed that the Sn/S ratio of the sample decrease from 0.92 to 0.81 with increase of substrate temperature up to 250 °C, further increases to 0.90 with increase of substrate temperature. At lower substrate temperature, it is observed that the sulfur content in the layer is low compared to the film deposited at 250 °C. The variation of the

Fig. 3. SEM micrographs of Sn2S3 thin films at substrate temperatures of (a) 200 °C (b) 250 °C (c) 300 °C (d) Thickness of the Sn2S3 thin film at substrate temperature of 250 °C (e) EDX spectrum of the film at substrate temperature of 250 °C.

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Table 2 Elemental composition and band gap values of the Sn2S3 thin films at different substrate temperatures. Substrate temperature (°C)

150 200 250 300

Composition results (at%)

Ratio

Sn

S

Sn/S

48.08 46.87 44.99 47.61

51.92 53.13 55.01 52.39

0.92 0.88 0.81 0.90

Band gap Eg (eV)

1.45 1.48 1.64 1.46

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243 nm with substrate temperature. The growth of grains depends up on the availability of thermal energy on the substrate surface during the deposition process. At lower substrate temperatures the available thermal energy is low for the nucleation and growth of crystallites on the substrate, which leads to the lower grain size and higher average surface roughness. At higher substrate temperatures, the availability of thermal energy is sufficient to have coalescence of the grains on the substrate surface, which leads to the formation of larger grains and lower surface roughness. 3.4. Optical properties

compositional ratio of the deposited films depends on the substrate temperature. In general, the evaporated tin and sulfur atoms might reach the substrate surface with different energies due to their different vapor pressures. Normally, sulfur atoms reach the substrate with higher speed than tin atoms. Among these, a few sulfur atoms might be scattered back due to either adatom mobility or substrate temperature, which leads S deficiency in the layers at lower film thicknesses. The film deposited at 300 °C shows increase in Sn/S ratio compared to the other samples might be due to the increase in thickness of the films. Devika et al. [26] observed that Sn/S ratio increases with increase of thickness of the SnS thin film and also noticed that the composition of the films depends on the film thickness or substrate temperature. The AFM 3D micrographs of Sn2S3 thin films are recorded with a scan area of 5 mmx5mm and shown in Fig. 4(a–d). The surface of the films consists of hill like structure with sharp peaks. The sharpness and height of the hills are found to decrease and the width is found to increases with the increase in substrate temperature. The surface roughness (rms) and grain size of the films are estimated from AFM. The average rms values vary from 24 nm to 4 nm and the average grain size increases from 100 nm to

Fig. 5 shows the optical transmittance spectra of the Sn2S3 thin films prepared at different substrate temperatures in the wavelength range of 200–3000 nm. The transmission spectra can be roughly divided into four regions [27–29]: transparent region (3000–1550 nm, α ¼0), weak absorption region (1550–1050 nm, α is small), medium absorption region (1050–850 nm, α is large) and strong absorption region (500–850 nm, transmission decreases drastically). A sharp fall in transmittance is observed for all films, which is related to the fundamental absorption edge or critical wavelength (λc). This sharp fall in transmittance of the films also can be attributed to the single phase of the material in the films. All the grown films exhibit an optical transmittance approximately 60–80% above 900 nm. From the figure, it is observed that a small variation in transmittance and a shift in the fundamental absorption edge of the films. This can be correlated to the variation of the film thickness or change in the stoichiometry of the deposited films. Devika et al. [30] also observed a similar shift in the fundamental absorption edge (λc) because of the variation in stoichiometry of the single phase of SnS thin films with substrate temperature.

Fig. 4. Atomic force micrographs (AFM) of the Sn2S3 film at substrate temperatures of (a) 150 °C (b) 200 °C (c) 250 °C (d) 300 °C.

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Fig. 5. Optical transmission spectra of the Sn2S3 thin films at different substrate temperatures.

3.4.1. Refractive index Transmission spectrum of Sn2S3 thin films exhibit interference fringes from 800 nm to 3000 nm. The refractive index (n) of the samples in the spectral region of transparent and weak absorption can be estimated using method of envelop developed by Swanepoel [27] based on the idea of Manifacier et al. [28].

⎡ ⎤1/2 n=⎣ N+ N2 − n20 n2s ⎦ if λ > λc

(5)

Where

N=

n20 +n2s T −T +2n0 ns M m 2 TMTm

where n0 is the refractive index of the surrounding medium (air), which is taken as1 and ns is the refractive index of the glass substrate, which is taken as 1.51, TM is the transmittance maxima, Tm is the transmittance minima, λ and λ c are incidental and critical wavelengths. The variation of refractive index of Sn2S3 thin films with wavelength is shown in Fig. 6. It is observed that the refractive indices of all the samples vary in the weak and medium absorption region with wavelength. Generally the refractive index depends up on the preparation conditions, growth of crystal grains and film thickness. In this study, the effect of crystalline quality on the refractive index is marginal. However, the variation of the

refractive index values depends on the thickness of the deposited films. The figure depicts that the refractive index of all the samples attains a sharp peak at a point of critical wave length (λc) below 900 nm, beyond, which the refractive index values for all films have a sharp decreases with increase in wavelength and again a marginal increase with wavelength and finally become constant above 1450 nm, showing normal dispersion behavior. Khadraoui et al. [12] reported similar behavior of refractive index variation with wavelength for Sn2S3 thin films. Similar variation of refractive index has also been observed in SnS thin films [8,31]. The refractive index of the film deposited at 300 °C varies between 2.77 and 3.16. These values are lower than the reported refractive index values of chemically deposited Sn2S3 thin films [18]. It is observed that the position of the peak shifts towards higher wavelengths with increase of substrate temperature up to 250 °C, beyond which the peak shifts towards lower wavelength. The peak position again shift towards higher wavelength for the substrate temperature of a 300 °C. The observed shift in position of peak for all the films can be correlated to the variation in film thickness. Selim et al. [31] noticed that the refractive index attains a peak in the absorption region, which is shifted towards higher wave length with increase of film thickness. The sharp increase in the refractive index values at the fundamental edge is due to sharp change in the value of absorption coefficient below the critical wavelength. From this discussion it is clear that the refractive index and extinction coefficient of the deposited films depends on the substrate temperature. But also it is seen that the film thickness varies with substrate temperature, which can be attributed to the growth of the films at different conditions and there by the Sn/S ratio in the films. The dielectric constants of the as-deposited Sn2S3 thin films have been calculated in the above critical wavelength range using the relation

ε = ε1+iε2

(6)

where ε1 and ε2 are the real and imaginary parts of the dielectric constant (ε). The real and imaginary parts of dielectric constant values of the films are calculated using the following equations:

ε1=n2 −k2

(7)

ε2=2nk Fig. 7(a–b) depicted the variation of real and imaginary parts of the Sn2S3 thin films with wavelengths. It can be seen that the ε1 follows a similar trend in nature with that of the refractive indices patterns. However, ε2 values of all the samples increases with increase of wavelength. 3.4.2. Thickness of Sn2S3 thin films Sn2S3 thin film samples exhibit well defined interference fringes in the visible and near IR range of the transmittance spectrum. If thickness of the films is not uniform or slightly tapered all interference fringes might have been disappeared and transmittance curves look like a smooth curve. The thicknesses of the deposited Sn2S3 thin films are calculated using two maxima or two minima as proposed by Swanepoel [27].

d=

Fig. 6. The variation of refractive index and extinction coefficient (inset figure) of Sn2S3 thin films with different substrate temperatures.

λ1λ2 2 ( λ1n2 −λ2n1)

(8)

where n1 and n2 are indices of refraction for two adjacent transmittance maxima or minima of the interference fringes and λ1, λ2 are corresponding wavelengths. The thickness of Sn2S3 thin films deposited at substrate temperature of 150 °C, 200 °C, 250 °C and 300 °C are estimated as 415 nm, 553 nm, 525 nm and 580 nm respectively. The variation in thickness of the films with different

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Fig. 7. Variation in (a) real (ε1) and (b) imaginary parts (ε2) of dielectric constant of the Sn2S3 thin films at different substrate temperature with wavelength.

substrate temperature might be due to the change in composition of the film. The thickness value of the film is closer to the value obtained from cross sectional SEM images. 3.4.3. Extinction coefficient Optical absorption values (A) can be determined from the interference free-transmission curve (Tα) of transmission spectra using envelop method [27].

A=

⎫ 1⎧ ⎨P + P2+2QTα (1−R2 R3) ⎬ ⎭ Q⎩

(9)

Where

P = ( R1−1)( R2−1)( R3−1)

Q = 2Tα ( R1R2+R1R3−2R1R2 R3)

⎡ (1 − n) ⎤2 R1=⎢ ⎥ ⎣ (1 + n) ⎦

⎡ (n−ns) ⎤2 R2=⎢ ⎥ ⎣ (n+ns ) ⎦

different for films grown at different substrate temperatures. The energy band gap (E g) of the deposited Sn2S3 thin films are evaluated using the following equation

α=

⎡ ⎢⎣ ln

( ) ⎤⎥⎦ 1 T

d

if λλ c 4π

(10)

Inset in Fig. 6 shows extinction coefficient of all Sn2S3 thin films at different substrate temperatures. It can be seen that the extinction coefficient of all the samples increases with increase of wavelength at above critical wavelength (λc). This value of λc was

Fig. 8. Variation of (αhν)2 vs. hν of the Sn2S3 thin films with various substrate temperatures.

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Table 3 Electrical properties of the Sn2S3 thin films at different substrate temperatures. Sample substrate temperature (°C)

Carrier concentration no (cm  3)

Resistivity (Ω cm)

Mobility cm2/VS

Carrier type

150 200 250 300

1.56  1019 1.83  1019 3.46  1019 5.20  1019

2.27  10  1 2.24  10  1 2.12  10  1 2.01  10  1

1.76 1.52 0.852 0.598

n n n n

absorption coefficient (α), extinction coefficient (k) and real (ε1) and imaginary (ε2) parts of the dielectric constants are calculated from transmission spectra using envelop method and reported. The Hall effect measurements showed that the films are n-type in nature. From Hall effect data, it is observed that all the films possess higher carrier concentration in the order of 10 19 cm  3, and low electrical resistivity in the order of 10  1 Ω.cm.

References sulfur content of the films increases with increase of substrate temperature up to 250 °C, beyond which it is found to decrease. The variation in band gap also follows the similar behavior. Therefore, the band gap of the Sn2S3 thin films depends on the sulfur content in the films. Devika et al. [26] also observed a higher band gap in a slightly sulfur rich SnS thin films.

4. Electrical properties Electrical properties of the Sn2S3 thin films deposited on glass substrates at different substrate temperatures are studied using Hall measurement technique at room temperature. The measured carrier concentration, resistivity and mobility of the as-deposited films are tabulated in Table 3. It is observed that all the films exhibit n- type behavior. Khadraoui et al. [12] also observed n-type behavior of Sn2S3 thin films deposited by spray pyrolysis. The variation in the electrical resistivity of the deposited films depends on thickness, grain size, and presence of binary phases of the films. In the present investigation the effect of film thickness on the electrical properties is marginal. In this study, there is no chance to the effect of binary phases on the electrical properties of the films because no binary peaks are traced in the deposited films. Thus, the variation in electrical conductivity and resistivity of the films depends mainly on the grain size of the films. In this study, observed that the average grain size of the deposited films increase with increase of substrate temperature. From the table, observed that the increase in carrier concentration and decrease in resistivity of the deposited films with increase of substrate temperature. The high resistivity of the films at lower substrate temperatures might be due to the availability of many intercrystalline regions so the grains cannot grow sufficiently large. At higher temperatures, the grains are comparatively larger and allow the carriers freely in the lattice leading a reduction in resistivity. Devika et al. [21] also observed that the higher resistivity of SnS films at lower substrate temperature might be due to the lower grain size and also the presence of highly resistivity binary phases (SnS2) (Table 3).

5. Conclusion Sn2S3 thin films have been successfully prepared on glass substrates using co-evaporation technique for the first time. From XRD patterns of the film deposited at 200 °C, 250 °C and 300 °C showed polycrystalline Sn2S3 with preferred orientation along (130) plane. Raman studies confirmed the formation of Sn2S3 phase. SEM studies showed the densely packed petal like crystallites, which are distributed uniformly throughout the sample without any visible cracks or holes. The AFM studies reveled that the grain size of the Sn2S3thin films increases with increase of substrate temperature. The optical band gap of the films deposited at substrate temperature from 150 °C, 200 °C, 250 °C, and 300 °C are estimated as 1.45 eV, 1.48 eV, 1.64 eV and 1.46 eV respectively. The optical constants such as thickness (d), optical absorption (A),

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