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Applied Surface Science 257 (2011) 3670–3676

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

tructural and optoelectronic properties of vacuum evaporated SnS thin filmsnnealed in argon ambient

iswajit Ghosh, Rupanjali Bhattacharjee, Pushan Banerjee, Subrata Das ∗

dvanced Materials & Solar Photovoltaic Division, School of Energy Studies, Jadavpur University, Kolkata 700032, India

r t i c l e i n f o

rticle history:eceived 23 April 2010eceived in revised form6 November 2010ccepted 16 November 2010vailable online 24 November 2010

a b s t r a c t

In this work, 650 nm polycrystalline SnS thin films were grown by thermal evaporation of high puritytin sulfide powder at 250 ◦C substrate temperature, followed by post deposition annealing at 200 ◦C and300 ◦C for 2, 4 and 6 h, and at 400 ◦C for 2 and 4 h in argon ambient. The XRD pattern of the as-deposited andannealed SnS films led to the conclusion that the as-deposited films were polycrystalline in nature withpreferentially oriented along (1 1 1) direction. The direct bandgap of all the films was found to be observed

◦

eywords:nSvaporation-ray difraction

between 1.33 and 1.53 eV. Except for annealing at 400 C all the films were nearly stoichiometric innature, suggesting lower rate of desulfurization at that ambient. However, higher annealing temperaturehas resulted in the segregation of tin phase. All the films showed good absorption in the visible range.The as-deposited and annealed films showed p-type conductivity. Hall measurement revealed the carrierconcentration and mobility ranging from 1015 to 1016 cm−3 and 0.8 to 31.6 cm2 V−1 s−1 respectively. The

remeve film

ESEMhotoresponse

photoconductivity measuresistance of the respecti

. Introduction

Recent investigations on new photovoltaic materials havettracted considerable interest in SnS thin films due to theirast potential for use in thin film solar photovoltaics [1–3] andther optoelectronic devices like holographic recording system4], solar control device [5], near-infrared detector [6], etc. SnSs an orange-to-grey colored p-type semiconductor with layeredrthorhombic crystal structure belonging to the space group sym-etry D16

2h (pnma). Their unit cells spans two layers which stacklong the c-axis of the crystal where layers of S and Sn atomsre tightly bound together by van der Waals force [7] and itsirect and indirect bandgap energies were reported to be 1.2–1.5nd 1.0–1.2 eV, respectively [8,9]. SnS has some unique charac-eristics: (i) bandgap lies in between to that of Si (1.12 eV) andaAs (1.43 eV), (ii) High absorption coefficient (>104 cm−1) near

he fundamental absorption edge similar to that of CdTe, (iii) non-oxic nature and (iv) both the constituent materials Sn and S arebundant and cheap. In addition, doping by Cu, Ag, Al, Cl hasncreased its p-type conductivity by several orders of magnitude

10–12]. These factors have made SnS as the potential candidateor nontoxic, cheap absorber layer for thin film solar cell withdS [1–2] and ZnO [3] as the wide bandgap window material.in sulfide is available in several binary sulfide forms such as

∗ Corresponding author. Tel.: +91 33 2414 6823; fax: +91 33 2414 6853.E-mail address: neillohit@yahoo.co.in (S. Das).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.11.103

nts of all the SnS films were carried out by recording the lowering ofs with time under illumination.

© 2010 Elsevier B.V. All rights reserved.

SnS, SnS2, Sn2S3, Sn3S4 [13] and all of them are semiconductors.Ristov et al. [10] reported that SnS exhibits both p and n typecarrier conductivity and the interconversion takes place at highertemperature where presumably the doubly ionized tin vacancyconverts to doubly ionized sulfur vacancy. Annealing processesare normally accompanied with different structural and compo-sitional changes and the reduction of intrinsic stress associatedin the structure. Some published works demonstrated the fabri-cation and subsequent characterization of vacuum evaporated SnSthin films [8,14]. The most characteristic feature of chalcogenidesemiconductor is the variation of stoichiomerty owing to desul-phurization. Furthermore, annealing is expected to generate newlocalized defect levels, as the sulfur vacancy is mainly created atthe surface compared to that of the bulk. These sets of discreteenergy levels may give rise to change in photoconductivity. John-son et al. [8] studied the photoconductivity to dark conductivityratio of SnS thin films with variable thickness after annealing in airand argon ambient for small duration and also using different metalcontacts.

In the present work, we mainly studied the structural, com-positional and optical properties of argon annealed SnS films as afunction of temperature and duration of annealing in argon ambi-ent. The response to illumination of as-grown and annealed SnS

films was carried out to at least ascertain the rough estimation ofthe variation of photosensitivity with annealing parameters. Thereason for annealing carried out for prolonged duration in thepresent studies is to investigate the possibility of phase change andtype conversion of SnS.

ce Science 257 (2011) 3670–3676 3671

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Table 1Sample nomenclature.

Sample specification Nomenclature

As-deposited TS AAnnealed at 200 ◦C 2 h TS 202Annealed at 200 ◦C 4 h TS 204Annealed at 200 ◦C 6 h TS 206Annealed at 300 ◦C 2 h TS 302Annealed at 300 ◦C 4 h TS 304

B. Ghosh et al. / Applied Surfa

. Experimental details

Polycrystalline tin sulfide thin films of about 650 nm thicknessere grown on previously cleaned sodalime glass substrate by

vaporating high purity (99.5%) SnS powder (Alfa-Aesar – 8 mesh)rom a quartz crucible as the effusion source. The deposition wasarried out in a high vacuum system at a base pressure of 10−5 mbar,ith a source-to-substrate distance of 17 cm and at a substrate tem-erature of 250 ◦C as evident from the temperature sensor attachedo the substrate. The rate of evaporation was kept at about 10 A s−1

nd the thickness of the as-grown films was monitored with theelp of digital quartz crystal thickness monitor. It may be men-ioned here that three glass slides having dimension of 7.5 × 2.5 cm2

ere used as substrate in a single run. The as-grown films weremooth, pinhole free and strongly adherent on the substrate andrownish black in appearance. Each of the slides were cut intohree equal parts and they were subsequently annealed at 200 ◦Cnd 300 ◦C for 2, 4 and 6 h, and at 400 ◦C for 2 and 4 h in argonmbient. Prior to annealing, the chamber was evacuated using aotary-diffusion pump combination to a pressure of 10−5 mbar, fol-owed by introduction of argon gas (99.99% purity) to maintain aressure of 0.1–0.2 mbar.

The SnS films thus prepared were analyzed structurally using X-ay diffraction (Brukers D8), FESEM (Jeol 6700F) and EDX (Carl ZeissVO 40) as well as optically through UV-visible spectrophotometryPerkin-Elmer Lambda-35).

The photoconductivity of the all SnS films were studied after

abricating indium contacts of 1 mm2 area, 1 cm apart and using aungsten-halogen lamp of 100 mW/cm2 intensity as the source ofight and subsequently the change in resistance across the indiumontacts was noted as a function of the time of illumination. Van derauw method of Hall measurement was also applied to determine

Fig. 1. XRD spectra of the SnS films as-deposite

Annealed at 300 ◦C 6 h TS 306Annealed at 400 ◦C 2 h TS 402Annealed at 400 ◦C 4 h TS 404

the bulk resistivity, carrier concentration, Hall mobility and type ofconductivity of the SnS films.

3. Results and discussion

The as-deposited and all of the annealed films were found tobe p-type in nature as revealed from thermoelectric measurement,where the direction of change in thermoelectric voltage was mea-sured using a hot and a cold probe, placed on the SnS films. Thenomenclature of the nine samples has been shown in Table 1.

3.1. Structural characterization using XRD

The observed XRD patterns of SnS films (Fig. 1) confirmed

orthorhombic, polycrystalline structure of the films. All thepredominant peaks (1 0 1), (1 1 1), (1 3 1) in the pattern closelymatched to those of orthorhombic SnS (herzenbergite-syn) cor-responding to JCPDS Card No. 39-0354 having lattice constantsa = 0.4329 nm, b = 1.1192 nm and c = 0.3984 nm. The as-deposited

d and annealed at different temperatures.

3672 B. Ghosh et al. / Applied Surface Science 257 (2011) 3670–3676

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Figs. 3 and 4 show the optical transmission and the reflectionspectra of the SnS films. All the films showed low transmissionin the visible range. The presence of interference fringes of the

Table 2Composition from EDX.

Sample Sn atomic percentage S atomic percentage Sn/S ratio

TS A 49.49 50.51 0.97TS 202 49.51 50.49 0.98TS 204 49.60 50.40 0.98TS 206 49.99 50.01 0.99

ig. 2. FESEM images of SnS films (a) as-deposited at 30k magnification, (b) as-depot 200 ◦C for 6 h at 50k magnification (e) annealed at 300 ◦C for 6 h at 30k magnific0k magnification (h) annealed at 400 ◦C for 4 h at 50k magnification.

lms were preferentially oriented along (1 1 1) plane. Many othereaks with less intense reflection were also observed, amonghich those corresponding to (1 2 0) and (2 1 1) of SnS may have

een possibly merged to that of (1 1 1) and (4 2 1) peaks respec-ively of Sn2S3 (JCPDS Card No. 30-1379) because of the closeroximity of their d-spacing. It may be noted that in case of sampleS 402 separation of Sn phase from SnS phase took place (asvident from the appearance of Sn peak corresponding to JCPDSard No. 18-1380) and it was even more pronounced when thelm was annealed for longer duration, i.e. TS 404. XRD pattern

urther indicated that with the progress of annealing there was aubstantial growth of (1 0 1) plane and it was more significant andredominant peak for TS 404. At that ambient the suppression of1 1 1) plane and the growth of (0 4 2) were also observed.

.2. Surface morphology and compositional analysis

The surface morphologies of the films (recorded using FESEMfter coating a thin layer of platinum over the film surfaces) haveeen shown in Fig. 2a–d. FESEM images revealed that the worm-likerains were arranged in an irregular order and each grain was found

o be composed of several crystallites (30–50 nm). There were voidsn between the worm-like grains. It has been observed that slighthange in the orientation and shape of the grains had been takenlace on annealing at 300 ◦C and agglomeration between them-elves was evident at higher annealing temperature (400 ◦C).

t 50k magnification, (c) annealed at 200 ◦C for 6 h at 30k magnification (d) annealed(f) annealed at 300 ◦C for 6 h at 50k magnification (g) annealed at 400 ◦C for 4 h at

The variation of Sn to S atomic ratio of the SnS films with respectto the annealing temperature has been determined from the EDXstudies and presented in Table 2. It may be mentioned here thatdesulfurization is most common for SnS thin films via annealingowing to the re-evaporation of sulfur from SnS layer. Therefore theannealed films would be increasingly sulfur deficient [14]. It maybe mentioned here that the long Sn–S bond in the crystal structureis quite responsible for the release of sulfur owing to annealing.

3.3. Optical characterization

TS 302 50.16 49.84 1.00TS 304 50.50 49.50 1.02TS 306 50.98 49.02 1.04TS 402 61.73 38.27 1.61TS 404 74.51 25.49 2.92

B. Ghosh et al. / Applied Surface Science 257 (2011) 3670–3676 3673

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voids in between the worm like grains.With the progress of annealing the gradual reduction of the

resistivity of the films was noticed presumably owing to theannealing-induced grain size enlargement and the increase in the

Table 3Bandgaps of SnS thin films.

Sample Bandgap (eV)

TS A 1.53TS 202 1.48TS 204 1.47TS 206 1.46TS 302 1.45TS 304 1.45

Fig. 3. Transmission spectra of SnS films.

amples indicated the thickness uniformity of the samples [15].owever when the films annealed at higher annealing temperaturend also for longer duration (400 ◦C, 2 h and 4 h) the interferenceringes were destroyed and the transmission curves were compar-tively smooth indicating the non-uniform thickness of the films.t was presumably due to re-evaporation of SnS from the film itself.he comparatively low transmission of TS 404 might be due to theresence of excess tin component, as discussed earlier.

Except TS 404, the total reflectance of all the films was around0% and it is quite evident as the higher annealing temperatureaused surface roughness.

The relation between the absorption coefficient (�) and the inci-ent photon energy h� is given by:

˛h�)n = A(h� − Eg) (1)

here A is a constant and Eg is the bandgap energy. The inter-ept of the tangent drawn to (˛h�)2 vs. h� plot, as shown in Fig. 5,ave a good approximation of the direct bandgap energy of the

aterial, measured to lie between 1.33 and 1.53 eV, as shown in

able 3. Interestingly, it has been observed that with the progressf annealing, there has been a subsequent reduction in the bandgap.

Fig. 4. Reflection spectra of SnS films.

3.4. Annealing effect and the electrical characterization

The bulk resistivity of the SnS samples were determined usingvan der Pauw Hall measurement method and the parameters aregiven in Table 4. In the present studies the comparatively high resis-itivity of the films observed was due to the discontinuity of therespective grains present in the films. Low magnification micro-graphs clearly show that the film is not continuous and there are

TS 306 1.44TS 402 1.42TS 404 1.33

3674 B. Ghosh et al. / Applied Surface Science 257 (2011) 3670–3676

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in component due to desulfurisation. The resisitivity was drasti-ally reduced for TS 404 where the separation of the tin phase wasvident, as discussed earlier.

Hall measurements confirmed that all the films showed p-ype conductivity and with annealing there has been a substantialncrease in the carrier concentration from the order of 1015 to 1016

wing to the increasing metallic component (tin). Although anneal-ng induced a significant increase in the carrier concentration, theall mobility reduced drastically in TS 402 and TS 404 comparedo other samples. It is presumably due to scattering caused by thencreased concentration of defects present in the crystal lattice.

.5. Photoresponse measurement

The SnS films were kept under a tungsten halogen lamp at00 mW/cm2 intensity and the changes in the resistance measuredcross two indium electrodes by applying 9 V bias voltage and wereecorded up to the illumination time of 180 s. Eventually, the darkesistances were taken as that at time “0”. The normalized values of

able 4arameters from Hall measurement.

Sample Bulk resistivity (� cm) Carrier concentrati

TS A 127.0 2.58 × 1015TS 202 126.5 4.21 × 1015TS 204 124.6 1.75 × 1015TS 206 120.9 2.12 × 1015TS 302 125.0 1.88 × 1015TS 304 123.7 1.79 × 1015TS 306 114.4 1.72 × 1015TS 402 85.6 1.66 × 1016TS 404 83.2 9.00 × 1016

for direct band gap.

the resistances have been plotted by taking the dark resistance asunity and have been shown in Fig. 6 against the time of illumination.

The photoresponse for all the films showed a characteristicof exponential fall in resistance at first and then a tendency ofsaturation. This was due to the fact that the rate of photogen-eration of carriers decreased with time. And simultaneously therecombination process gave rise to the possibility of reducing thephotocurrent. Consequently a steady state was obtained where therate of generation of charge carriers would be equal to the recom-bination rate under constant illumination. This phenomenon wasquite responsible for obtaining a nearly flat profile of the normal-ized resistance at the end (Fig. 6a–d).

It has been observed that the photoresponse of SnS thin filmsonly slightly be reduced with increase in annealing temperature as

well as duration of annealing at a particular temperature (Fig. 6a–d).However, the effect was more pronounced with the change inannealing temperature rather than the duration of annealing.

These characteristics can possibly be explained by referringto the change in the orientation of the grain structure which

on (cm−3) Hall mobility (cm2 V−1 s−1) Type

19.0 p11.7 p28.6 p24.3 p26.6 p28.2 p31.6 p

4.4 p0.8 p

B. Ghosh et al. / Applied Surface Science 257 (2011) 3670–3676 3675

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ubsequently increases the density of trap states (Fig. 2a–h). So,lthough the carrier density might be slightly increased owing toittle change in stoichiometry on annealing, the subsequent largeumber of trap states resulted in increasing the surface recombi-ation rate.

. Conclusion

Annealing up to 300 ◦C in argon ambient (2, 4 and 6 h) did notring any noticeable structural change. However the annealing atigher temperature for prolonged duration resulted in segregationf tin phase. It may be mentioned here that although the variationf Sn/S ratio was reported earlier subject to change in annealingemperature, however, no separate tin phase was reported [14]. Theresence of large excess of tin in the films is presumably owing toigher rate of desulfurization. This fact suggests the stability of SnS

hin films well within the range of 300 ◦C annealing temperature inrgon ambient.

Furthermore, FESEM analysis showed randomly oriented grainsonsisting of several crystallites and the presence of significantmount of porosity in between the grains which gave rise to com-

r of SnS films with time.

paratively higher resistivity. XRD studies confirmed the presenceof small amount of Sn2S3 phase as contamination to SnS.

All the films showed p-type conductivity and the appreciableincrement of the carrier concentration was taken place subject toannealing. However the drastic reduction in the Hall mobility ofthe samples annealed at 400 ◦C is the indicative of the deterioratingquality of the crystal lattice structure.

Acknowledgement

The authors would like to acknowledge Dr. Arghya NarayanBanerjee, assistant professor at Yeungnam University, South Koreafor helpful discussions.

References

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