Available online at ScienceDirect

ScienceDirectJ. Mater. Sci. Technol., 2014, 30(10), 1006e1011

Structural Morphological and Optical Properties of SnSb2S4

Thin Films Grown by Vacuum Evaporation Method

N. Khedmi1), M. Ben Rabeh1)*, M. Kanzari2)

1) Laboratoire de Photovoltaïques et Matériaux de Semi-conducteurs-ENIT-Université de Tunis el Manar,BP 37, le belvedere, 1002 Tunis, Tunisie

2) Laboratoire de Photovoltaïques et Matériaux de Semi-conducteurs-ENIT-IPEITunis Montfleury-Universitéde Tunis, Tunisie[Manuscript received October 21, 2013, in revised form January 6, 2014, Available online 2 April 2014]

* Corresp92 15(M. Ben1005-03JournalLimited.http://dx

SnSb2S4 thin filmswere prepared frompowder by thermal evaporation under vacuum of 1.33�10�4 Pa (10�6 Torr)on unheated glass substrates. The effect of thickness on the structural, morphological and optical properties ofSnSb2S4 thin films was investigated. Films thickness measured by interference fringes method varied from 50to 700 nm. X-ray diffraction analysis revealed that all the SnSb2S4 films were polycrystalline in spite withoutheating the substrates and the crystallinity was improved with increasing film thickness. The microstructureparameters: crystallite size, strain and dislocation density were calculated. It was observed that the crystallitesize increased and the crystal defects decreased with increasing film thickness. In addition, by increasing thefilm thickness, an enhancement in the surface roughness root-mean-square (RMS) increased from 2.0 to6.6 nm. The fundamental optical parameters like band gap, absorption and extinction coefficient werecalculated in the strong absorption region of transmittance and reflectance spectrum. The optical absorptionmeasurements indicated that the band (Eg) gap of the thin films decreased from 2.10 to 1.65 eVwith increasingfilm thickness. The refractive indexes were evaluated in transparent region in terms of envelope method, whichwas suggested by Swanepoul. It was observed that the refractive index increased with increasing film thickness.

KEY WORDS: Ternary system; Thermal evaporation technique; Thin films; Thickness; Atomic force microscopy (AFM)

1. Introduction

Sulfosalts or thiosalts family, created by chemistry during theXIXth century[1], belong to a class of complex sulfides mineralswith the general formula AmBnXp, where A stands for metallicelements like Cu1þ, Ag1þ, Pb2þ, Sn2þ, Fe2þ, Mn2þ, Hg2þ.; Brepresents semi-metallic elements such as As3þ, Sb3þ, Bi3þ andX can be either sulfur S or selenium Se[2]. Study of the phaseequilibrium and crystal chemistry of the systems SneSbeSsulfosalt given in literature[3] shows that these compounds arecharacterized by complex chemistry and complex crystal struc-tures. Recently, sulfosalts semiconductors thin films have gainedconsiderable interest because of their potential applications inmany technological fields such as: thermoelectric energy

onding author. Prof.; Tel.: þ216 97 44 23 62; Fax: þ216 7198; E-mail address: mohamedbenrabeh@yahoo.fr

Rabeh).02/$e see front matter Copyright� 2014, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2014.03.019

conversion application[4], phase change memory devices[5], X-rays detectors[6], and thin film solar cells[7]. Dittrich et al.[8]

studied different materials for photovoltaics and suggested“sulfosalt” to be a new absorber for solar cell applications.Several methods have been used to prepare sulfosalt layers suchas: PVD[9], RF sputtering[10], spray pyrolysis[11], pulsed laserdeposition[12,13], and sonochemical synthesis[14]. A great deal ofinterest has been focused on the growth of the promising ma-terials of thiosalts tin antimony sulfur which may be used as anabsorber material in photovoltaic solar cells. The ternary com-pound SnSb2S4 has a high optical absorption coefficient ofalmost 105 cm�1[15] and optical band gap varying from 1 to2.75 eV depending on the method preparation[16,17]. In addition,as part of study of the section SnSeSb2S3, Smith and Parise[18]

determined the structure of SnSb2S4 by high-resolution trans-mission electron microscopy (HRTEM). They have demon-strated that the SnSb2S4 structure consists of ribbons of edge-sharing semi-octahedra MX5 (M ¼ Sn, Sb) with space groupPnnm and lattice parameters a ¼ 2.564, b ¼ 2.038 andc ¼ 0.390 nm[18]. In order to exploit SnSb2S4 for large-scaleelectronic and optoelectronic device applications, it is essentialto have a better understanding of the factors that control the

N. Khedmi et al.: J. Mater. Sci. Technol., 2014, 30(10), 1006e1011 1007

properties of SnSb2S4 thin films. However, the effect of thick-ness on the properties of tin antimony sulfide thin films wasseldom reported. In this paper, we investigated the structural,morphological and optical properties of the SnSb2S4 thin filmsgrown by vacuum evaporation method.

2. Experimental

2.1. Synthesis of SnSb2S4

The initial ingot of SnSb2S4 material was grown using thehorizontal Bridgman method. Stoichiometric amount of 99.99%pure Sn, Sb, and S were used. The growth of crystals was carriedout in quartz ampoule, which was pre-cleaned by chemicaletching in concentrated acid HF, washed in distilled water thenwith acetone, and finally, dried in oven at 150 �C for 30 min. Theampoule containing the pure elements were sealed under vacuumof 1.33 � 10�4 Pa (10�6 Torr), transferred to a programmablefurnace (Nabertherm-Allemagne) and then heated slowly (10 �C/h) up to 600 �C. A complete homogenization could be obtainedby keeping the melt at this temperature for about 51 h. To avoidcracking due to the thermal expansion of the melt upon solidi-fication the ampoule was cooled at a rate of 10 �C/h. Thecompound obtained by this process is black and this form ofingot surface dotted with many shiny crystals of small sizes(Fig. 1), which will be crushed into fine powder to use as a fillerin the evaporator for the development of thin films.

2.2. Film preparation

Thin films of SnSb2S4 were prepared by thermal evaporationfrom a molybdenum boat on non-heated glass substrates ofrectangular shape (2.5 cm � 1.5 cm) under vacuum of1.33 � 10�4 Pa (10�6 Torr) using a high vacuum coating unitAlcatel. The substrates were placed directly above the source at adistance of 15 cm. Glass substrates were previously cleaned withwashing agents (commercial detergent, acetone, ethanol anddeionized water) before being introduced into the vacuum sys-tem. Film thickness was measured by interference fringesmethod[19] and varied from 50 to 700 nm.

2.3. Characterization of powders and thin films

Crystallite phases and crystal orientations of the powders andthe prepared films were analyzed using a Philips PW 3710 withCuKa1 radiation (l ¼ 0.15406 nm). The surface morphology of

Fig. 1 Ingot of SnSb2S4 crystal.

the films was probed with the atomic force microscopy (Veeco D3100 AFM). The optical transmittance and reflectance wererecorded at normal incidence in the wavelength range of 300e1800 nm using a Shimadzu UV-visible-NIR spectrophotometer.

3. Results and Discussion

3.1. Structural study

X-ray diffraction (XRD) patterns were used to study thestructural properties of SnSb2S4 powder. Fig. 2 shows the typicalXRD pattern of the SnSb2S4 powder. This figure indicates thatthe material is polycrystalline with (621) preferred orientation.The diffraction peaks in the XRD pattern could be indexed topure orthorhombic crystal structure of SnSb2S4 with space groupof Pnnm and no diffraction peaks corresponding to impuritieslike Sb2O3 and SnO2 were observed, which confirms that the as-synthesized product was single phase of pure SnSb2S4. Given theBragg relation 2d sin q ¼ l and the inter-planar spacing for anorthorhombic system, the lattice constants a, b and c of theSnSb2S4 compound were determined and the values are 2.549,2.048 and 0.383 nm, respectively. These values are in goodagreement with standard (JCPDS) database with card number35-1496[18].Fig. 3 displays XRD patterns of SnSb2S4 films grown at room

temperature with different thickness in the range of 50e700 nm.It is clear from Fig. 3 that SnSb2S4 layer prepared with thicknessof 50 nm is amorphous in nature while those prepared withthickness above or equal to 150 nm are polycrystalline in naturewith a preferred orientation along the SnSb2S4 (621) plane withorthorhombic structure, as found by several authors[20]. Theoverall feature of this compound SnSb2S4 is that all the depositedfilms are crystallized, in spite without heating the substrates.Furthermore, with increasing film thickness, the crystallinequality of SnSb2S4 layers is enhanced. Also, the (621) peak in-tensity increases gradually with increasing film thickness. Thisshows an improvement in crystallinity with increasing film

Fig. 2 XRD patterns of standard and as-synthesized SnSb2S4 powder.

Fig. 3 XRD patterns of SnSb2S4 films grown with different thickness.

Table 1 FWHM, grain size, strain, dislocation density, roughness andoptical band gap values for SnSb2S4 films grown with differentthickness

Thickness(nm)

FWHM(deg.)

Grain size(nm)

d(�1014 m�2)

ε

(�10�4)RMS(nm)

700 0.26 30.87 10.49 41.15 6.6650 0.27 30.11 11.02 42.19 6.1500 0.28 28.84 12.01 44.10 5.1400 0.29 27.73 12.99 45.85 3.3250 0.32 25.30 15.61 50.28 2.7150 0.32 25.30 15.61 50.25 2.650 e e e e 2.0

1008 N. Khedmi et al.: J. Mater. Sci. Technol., 2014, 30(10), 1006e1011

thickness. Similar results are also reported by others[21,22]. Inorder to study the microstructural parameters such as grain size,strain and dislocation density of SnSb2S4 thin film particles,films having a (621) peak are studied by taking the slow scanXRD pattern around the (621). The crystallite size (D) of theSnSb2S4 films is calculated by using Scherrer’s formula[23]:

D ¼ Kl

b cos q(1)

where D is the crystallite size, l is the wavelength of the X-rayused, b is the full width at half maximum (FWHM) of diffractionpeak in radians, q is the corresponding Bragg angle and K is aconstant of approximate 0.9. The lattice strain (ε) wascalculated by using the Scherrer’s relation[23]:

ε ¼ b

4 tan q(2)

The dislocation density (d), defined as the length of disloca-tion lines per unit volume of the crystal, was evaluated from theformula[24]:

d ¼ 1

D2(3)

The crystallite size and the FWHM of the layers were evalu-ated for the samples, using the (621) diffraction line of theSnSb2S4 compound. The grain size D and the FWHM values arepresented in Table 1. It can be shown that as film thickness in-creases from 50 to 700 nm, the FWHM decreases from 0.32� to0.26� and the crystallite sizes increases from 25 to 30 nm.Indeed, the crystallite size increases with increasing film thick-ness. This implies that the reduced film thickness will lead todegradation in the crystallinity of the films. Furthermore, theincrease in the grain size indicates a decrease in the lattice de-fects, which in turn reduces internal strain and dislocation den-sity. Also, the lattice strain and the dislocation density ofSnSb2S4 films decrease from 15.61 � 1014 to 10.49 � 1014 m�2

and 50.25 � 10�4 to 41.15 � 10�4, respectively, with increasingthickness (Table 1). The increase in peak intensity and thedecrease of FWHM are due to the improvement in the crystal-linity and a reduction in the micro strain. Similar thickness

dependence of the microstructural parameters on the depositionof other materials has been reported[25,26]. The microstructuralparameters of the investigated films are summarized in Table 1.

3.2. Surface morphology

Fig. 4 displays the 3D AFM micrographs (1 mm � 1 mm) ofSnSb2S4 samples with different thickness ranging from 50 to700 nm. The surface morphologies are constituted of sphericalgrains with a relatively uniform distribution. The surface ofSnSb2S4 layers are influenced by the film thickness. In fact, thesample obtained at low thickness (50 nm) is constituted ofsmaller crystallites agglomerated in an irregular distributed is-land. This can be related to the amorphous state of the films atthe initial stage of growth. As the film thickness increases from150 to 700 nm, the smaller crystallites agglomerate together toform relatively large aggregates. The improvement of the gran-ularity of the layers with thickness can be related to the intensityincrease of the peak (621) in the XRD spectra (Fig. 3). Inaddition, the root-mean-square (RMS) average roughness asdetermined from AFM measurements increases from 2 to 6.6 nmwhen the films thickness increases from 50 to 700 nm and filmsurface becomes rough (Table 1). With increasing film thickness,the crystallinity is improved and the grain size becomes larger.This result agrees with the XRD observation.

3.3. Optical study

3.3.1. Optical transmittance and reflectance spectra. Figs. 5and 6 show the typical transmittance (T) and reflectance (R)spectra of SnSb2S4 thin films deposited at normal incidence inthe wavelength range of 300e1800 nm. When the value of thefilm thickness is higher than 50 nm, a curve step appears in thetransmittance and the presence of ripples is noted in trans-mittance and reflectance spectra. The ripples are attributed tooptical interference effects. This change is due to the decrease incrystallinity and can be attributed to the films thickness. Thisbehavior indicates an increase in the disorder with decreasingthickness, which confirms the X-ray results (Fig. 3). It is clearfrom these figures that the number of the interference fringes isincreased with increasing film thickness and the absorption edgeshifts from lower wavelength region to higher wavelength. Theaverage transmittance of all the films in the visible range (400e800 nm) is about 60%. The reduction of transmittance at higherthickness may be attributed to the increased scattering of photonsby the increase in the roughness of the surface morphology. Theincrease of surface roughness with increasing thickness isevident from the AFM results[27]. The reduction in transmittance

Fig. 4 3D AFM micrographs (1 mm � 1 mm) for SnSb2S4 films grown with different thickness.

N. Khedmi et al.: J. Mater. Sci. Technol., 2014, 30(10), 1006e1011 1009

can also be understood from the fact that, the thicker filmsconsist of more atoms and therefore more states will be availablefor the photons to be absorbed[28].

3.3.2. Absorption coefficient. The optical absorption coefficientwas evaluated from the transmittance (T) and reflectance (R) databy using the following formula[29]:

a ¼ 1

dln

"ð1� RÞ2

T

#(4)

where a is the absorption coefficient, T is the transmittance, R isthe reflectance and d is the film thickness. Fig. 7 shows theabsorption coefficients a (hn) vs the photon energy (hn) ofSnSb2S4 films with different thickness. It can be seen that theabsorption coefficient increases from 7 � 104 cm�1 to2 � 105 cm�1 with increasing films thickness from 50 to700 nm, in the visible spectral range. This result is veryimportant because the spectral dependence of the absorptioncoefficient affects the solar cell conversion efficiency forphotovoltaic applications[30].

3.3.3. Energy gaps. The dependence of the absorption coeffi-cient on photon energy was analyzed in the high absorption re-gions to obtain detailed information about the energy band gaps.The absorption coefficient (a) and photon energy can be related

Fig. 5 Optical transmittance spectra for SnSb2S4 films grown withdifferent thickness.

by applying the Tauc model, and the Davis and Mott model inthe high absorbance region[31,32]:

ahn ¼ A�hn� Eg

�n(5)

where A is a constant depending on the transition probability, hnis the photon energy, Eg is the optical band gap and n equals 1/2for direct gap and 2 for an indirect gap. For SnSb2S4 thin filmsunder investigation, the variation of absorption coefficient withphoton energy is found to follow the above relation withn ¼ 1/2 indicating that the transition corresponds to a directlyallowed electronic transition. The energy band gap isdetermined by extrapolating the linear portion of (ahn)2 vs hncurve to (ahn)2 ¼ 0. Fig. 8 depicts the variation of (ahn)2 withthe photon energy (hn) for different SnSb2S4 film thickness. Itis clearly seen that the Eg of the films decreases from 2.1 to1.65 eV as the thickness increases from 50 to 700 nm(Table 1). The decrease in the optical band gap with filmthickness can be attributed to the improvement in thecrystallinity due to the increase in crystallite size and surfaceroughness and the decrease in structural disorder and latticestrain[33]. Also, at higher thicknesses the films are morehomogenous with reduced defects and disorder, whichdecreases the density of localized states in the band structureand consequently decreases the optical energy gap[34]. Similarresults were also observed by other researchers[35,36].

3.3.4. Refractive index and extinction coefficient. Due to theexistence of the interference patterns, which exhibits severalmaxima and minima as the wavelength increases, the method ofSwanepoel[37] was used to calculate the refractive index n. Themethod is based on the approach of Manifacier et al.[38]. Thisapproach suggested creating an upper and lower envelope of thetransmission spectrum beyond the absorption edge. The refrac-tive index can then be calculated from the relation[37]:

n ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN2 � S2

pq(6)

N ¼ 2S

�Tmax � Tmin

TmaxTmin

�þ S2 þ 1

2(7)

Fig. 6 Optical reflectance spectra for SnSb2S4 films grown withdifferent thickness.

Fig. 7 Absorption coefficient spectra for SnSb2S4 films grown withdifferent thickness.

Fig. 9 Variation of the refractive index n(l) with wavelength forSnSb2S4 films grown with different thickness.

Fig. 10 Variation of the extinction coefficient k(l) with wavelength forSnSb2S4 films grown with different thickness.

1010 N. Khedmi et al.: J. Mater. Sci. Technol., 2014, 30(10), 1006e1011

where S is the refractive index of the glass substrate. Tmax

and Tmin are the values of the envelope at the wavelengthswhere the upper and lower envelops and the experimentaltransmittance spectrum are tangential, respectively. Theextinction coefficient k can be calculated from the relation[39]:

K ¼ al

4p(8)

where a is the absorption coefficient and l is the correspondingwavelength. The variations of refractive index n and extinctioncoefficient k vs wavelength for SnSb2S4 films grown atdifferent thickness are shown in Figs. 9 and 10. It is obvious

Fig. 8 Plot of (ahn)2 vs (hn) for SnSb2S4 films grown with differentthickness.

that a remarkable variation is observed in both n and k valueswith film thickness. The refractive index is found to decreasewith increasing wavelength of the incident photon. At higherwavelength of the incident photon, the refractive index tendsto be constants[40]. The values of n and k are influenced by thefilm thickness. As can be seen in Fig. 9, n increases graduallyas the film thickness increases. There is a good correlationbetween the refractive indices and the band gaps ofsemiconductors, where typically, n increases as Eg decreases.Additionally, k also increases with increasing film thickness(Fig. 10). The extinction coefficient in the visible range of550 nm is found to be of the order of 10�1 and such lowvalue of k in visible and near infrared region is a qualitativeindication of excellent surface smoothness of the SnSb2S4 thinfilms.

4. Conclusion

SnSb2S4 powder has been successfully synthesized by thehorizontal Bridgman method and SnSb2S4 thin films have beendeposited with different thickness (50e700 nm) by vacuumthermal evaporation method on non-heated glass substrates.XRD results reveal that SnSb2S4 films prepared with 50 nm thickis amorphous in nature while those prepared with thicknessabove or equal to 150 nm are polycrystalline in nature with apreferred orientation along the SnSb2S4 (621) plane withorthorhombic structure. The overall feature of this compoundSnSb2S4 is that the deposited films are crystallized, in spitewithout heating the substrates. The analysis of the optical

N. Khedmi et al.: J. Mater. Sci. Technol., 2014, 30(10), 1006e1011 1011

absorption spectra reveals high absorption coefficient of105 cm�1 and one directly allowed transition in the range of1.65e2.10 eV, which is attributed to the fundamental edge. Themicrostructural parameters of SnSb2S4 films such as FWHM,crystallite size, strain, dislocation density and roughness arecalculated and the refractive index and extinction coefficient aredetermined. We have observed that film thickness can be used tomodify structural and optical properties of thin films, because themicrostructural and optical constants of thin films depend on thedeposition parameter such as films thickness. This propertyconfers to the material interest perspectives for its application inmany physical domains such as optical, photovoltaic conversionand others.

REFERENCES

[1] Y. Moelo, E. Makovicky, N.N. Mozgova, J.L. Jambor, N. Cook,A. Pring, W. Paar, E.H. Nickel, S. Graeser, S. Karup-Moller,T. Balic-Zunic, W.G. Mumme, F. Vurro, D. Topa, L. Bindi,K. Bente, M. Shimizu, Eur. J. Mineral. 20 (2008) 7e46.

[2] G.N. Kryukova, M. Heuer, G. Wagner, T. Doering, K. Bente, J. Sol.Chem. 178 (2005) 376e381.

[3] P.P.K. Smith, J. Solid State Chem. 58 (1985) 78e86.[4] K. Hoang, S.D. Mahanti, J. Androulakis, M.G. Kanatzidis, Mater.

Res. Soc. Symp. Proc. 886 (2006), 0886eF05-06.1.[5] M.Y. Chen, K.A. Rubin, U.S. Patent, No. 5242784, 1992.[6] M. Hoheisel, H. Dittrich, World Patent, No. 98/28800, 1998.[7] H. Dittrich, German Patent, No. 196 13 683, 1996.[8] H. Dittrich, D.J. Vaughan, R.D.A. Pattrick, S. Graser, E. Makoviky,

M. Lux-Steiner, R. Kunst, D. Lincot, in: Proceedings to the 10thInternational Conference on Ternary and Multinary Compounds,Stuttgart, Crystal Research and Technology, Berlin, 1996.

[9] H. Dittrich, K. Herz, In: Proceeding to the 11th InternationalConference on Ternary and Multinary Compounds, ICTMC-11,Salford, September 8e12, 1997.

[10] H. Dittrich, A. Stadler, D. Topa, H.J. Schimper, A. Basch, Phys.Status Solidi A-Appl. Mat. 206 (2009) 1034e1041.

[11] S.A. Manolache, A. Duta, L. Isac, M. Nanu, A. Goossens,J. Schoonman, Thin Solid Films 515 (2007) 5957e5960.

[12] T. Wagner, M. Krbal, P. Nemec, M. Frumar, Th Wagner, M. Vlcek,V. Perina, A. Mackova, V. Hnatovitz, S.O. Kasap, Appl. Phys. 79(2004) 1563e1565.

[13] J. Gutwirth, T. Wagner, P. Nemec, S.O. Kasap, M. Frumar, J. Non-Cryst. Solids 354 (2008) 497e502.

[14] B. Pejova, I. Grozdanov, D. Nesheva, A. Petrova, Chem. Mater. 20(2008) 2551e2565.

[15] I. Gaied, A. Gassoumi, M. Kanzari, N. Yacoubi, In: Proceeding tothe 10th International Conference on Application of ContemporaryNon-Destructive Testing in Engineering, Ljubljana, Slovenia,September 1e3, 2009.

[16] A. Gassoumi, M. Kanzari, J. Optoelectron. Adv. Mater. 14 (2012)272e276.

[17] N. Ali, S.T. Hussainb, M.A. Iqbala, K. Hutchingc, D. Lane, Optik124 (2013) 4746e4749.

[18] P.P.K. Smith, J.B. Parise, Acta Crystallogr. B 41 (1985) 84e87.[19] O.S. Heavens, Optical Properties of Thin Solid Films, Butterworths,

London, 1950.[20] A. Gassoumi, M. Kanzari, Med. J. Elec. Commun. 5 (2009) 66e72.[21] X. Hao, J. Ma, D. Zhang, T. Yang, H. Ma, Y. Yang, Ch. Cheng,

J. Huang, Appl. Surf. Sci. 183 (2001) 137e142.[22] M.M. El-Nahass, E.M. El-Menyawy, Mater. Sci. Eng. B 177 (2012)

145e150.[23] B.D. Cullity, Elements of X-ray Diffraction, Addison-Welsley,

London, 1978.[24] G.B. Williamson, R.C. Smallman, Philos. Mag. 1 (1956) 34e45.[25] R. Swanepoel, J. Phys. E-Sci. Instrum. 16 (1983) 1214e1222.[26] A.R. Balu, V.S. Nagarethinama, M.G. Syed Basheer Ahamed,

A. Thayumanavan, K.R. Murali, C. Sanjeeviraja, V. Swaminathan,M. Jayachandra, Mater. Sci. Eng. B 171 (2010) 93e98.

[27] X. Xiu, Z.Y. Pang, M. Lv, Y. Dai, L. Ye, S. Han, Appl. Surf. Sci.253 (2007) 3345e3348.

[28] N. Tigau, V. Ciupina, G. Prpdan, J. Optoelectron. Adv. Mater. 8(2006) 37e42.

[29] T.S. Moss, G.J. Burrell, B. Ellis, Semiconductor Opto-Electronics,Wiley, New York, 1973.

[30] V.V. Kindyak, V.F. Gremenonok, I.V. Bodnar, V. Rud Yu,G.A. Madvedkin, Thin Solid Films 250 (1994) 33e36.

[31] J. Tauc, Amorphous and Liquid Semiconductors, Plenum Press,London, New York, 1974.

[32] E.A. David, N.F. Mott, Philos. Mag. 22 (1970) 903e922.[33] S. Venkatachalam, D. Mangalaraj, S. Narayandass, J. Phys.

D-Appl. Phys. 39 (2006) 4777e4782.[34] A. Ubale, Mater. Chem. Phys. 121 (2010) 555e560.[35] S. Sonmezoglu, A. Arslan, T. Serin, N. Serin, Phys. Scr. 84 (2011)

065602.[36] M.G. Varnamkhasti, H.R. Fallah, M. Mostajaboddavati,

A. Hassanzadeh, Vacuum 86 (2012) 1318e1322.[37] R. Swanepoel, J. Phys. E 16 (1983) 1214e1222.[38] J.C.Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E 9 (1976) 1002e1004.[39] M. Caglar, S. Ilican, Y. Caglar, F. Yakuphanoglu, J. Mater. Sci.-

Mater. Electron. 19 (2008) 704e708.[40] K. Prabakar, S. Venkatachalam,Y.L. Jeyachandran, S.K. Narayandass,

D. Mangalaraj, Sol. Energy Mater. Sol. Cells 81 (2004) 1e12.