Physica B 405 (2010) 3101 3105 Contents lists available at ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb Structural, morphology and optical properties of chemically deposited Sb 2 S 3 thin films H. Maghraoui-Meherzi a,n, T. Ben Nasr b, N. Kamoun b, M. Dachraoui a a Laboratoire de chimie Analytique et Electrochimie, Faculté des Sciences de Tunis, Campus universitaire 2092 Tunis El Manar, Tunisie b Laboratoire de physique de la Matiere Condensée, Faculté des Sciences de Tunis, Campus universitaire 2092 Tunis El Manar, Tunisie article info Article history: Received 18 March 2010 Received in revised form 10 April 2010 Accepted 13 April 2010 Keywords: Antimony sulphide Thin films Chemical bath deposition Structural and optical properties abstract Metal chalcogenide thin films prepared by chemical methods are currently attracting considerable attention, as they are relatively inexpensive, simple and convenient for large area deposition. Antimony sulphide (Sb 2 S 3 ) films were deposited on glass substrate by chemical bath deposition from solution containing SbCl 3 and Na 2 S 2 O 3. Characterization of the films was carried out with X-ray diffraction (XRD), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and UV Vis spectrophotometry. Using these techniques, we have specified the effect of temperature and time deposition on the crystallinity structure of antimony sulphide films. Homogeneous films were found to be crystallized on orthorhombic structure, and indicate a direct band gap of 2.24 ev. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Recently, considerable attention has been turned towards the preparation and characterization of thin metal chalcogenide films using several techniques [1 2]. Among the various metal sulphides, antimony trisulphide (Sb 2 S 3 ) is a V VI semiconductor compound with some special applications in target material for television cameras [3], microwave devices [4], switching devices [5] and various opto-electronic devices [6 7]. Chemically deposited Sb 2 S 3 thin films are potential absorber materials in devices for photovoltaic conversion of solar energy. The use of Sb 2 S 3 thin films in schottky barrier of n-sb 2 S 3 /p-ge [8] structures with conversion efficiencies of 7.3% and heterojunction solar cells SnO 2 :F-CdS-Sb 2 S 3 -Ag (paint) with open circuit voltage (V oc ) 470 mv and short circuit current density (J sc ) 0.02 ma/cm 2, have been reported [9]. Many workers have obtained Sb 2 S 3 thin films by various techniques. Badachhape and Goswami [10] prepared Sb 2 S 3 films by vacuum evaporation. Yesugade et al. [11] electrodeposited Sb 2 S 3 polycrystalline thin films from aqueous medium. Bhosale et al. [12] prepared amorphous Sb 2 S 3 thin films by spray pyrolysis method using oxalic acid as a complexing reagent. Savadogo and Mandal [13] have obtained amorphous Sb 2 S 3 thin films from aqueous chemical bath using ethylene diamine tetra acetic acid (EDTA) as a complexing reagent. Desai and Lokhande [14] deposited amorphous Sb 2 S 3 thin films by using a thioacetamide bath and n Corresponding author. Tel.: +216 98 538 091; fax: +216 71 885 008. E-mail address: hajer.maghraoui@laposte.net (H. Maghraoui-Meherzi). tartaric acid as complexing reagents. Among all of the above deposition techniques, chemical bath depositions (CBD) are very attractive, since they are relatively simple, low cost and convenient for larger area deposition of thin films. The present work describes the chemical bath deposition of Sb 2 S 3 thin films. In order to avoid hydrolytic precipitation of antimony (III) basic salts in water media, one has to work in very acidic solutions which are inconvenient for film deposition. Another way of avoiding basic salt precipitation is to complex the Sb 3+ ion using a suitable complexing agent such as EDTA and tartaric acid, and then another chalcogenide source agent such as thiourea, thioacetamide or selenosulfate. In this work the two required agents were replaced by sodium thiosulfate which is both a complexing agent for Sb 3+ and at the same time a source of sulphide ion upon hydrolysis. The Sb 2 S 3 thin films were deposited at different deposition time and temperature, respectively, in the range 10 70 min and 40 70 1C. The structure, morphology and optical properties of these films were analyzed using X-ray diffraction, atomic force microscopy, Auger spectroscopy and UV Vis spectrophotometry. 2. Experiments Orange-yellow antimony sulphide thin films were prepared on SnO 2 /glass substrates by chemical bath deposition. The bath for Sb 2 S 3 was prepared in a 100 ml beaker as follows: 650 mg of SbCl 3 dissolved in 10 ml of acetone. This was followed by the addition of 25 ml of 1 M Na 2 S 2 O 3 and 65 ml of bi-distilled water. The glass substrates (35 25 1 mm) were dipped into hydrochloride and 0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.04.020
3102 H. Maghraoui-Meherzi et al. / Physica B 405 (2010) 3101 3105 nitric acid solution (in the proportion: 1/3 HCl and 2/3 HNO 3 ), then they are rinsed thoroughly with distilled water and chemically treated with fluorohydric acid solution (HF 5% V/V) during 5 min. Finally, they are rinsed and dried in air during 15 min at 80 1C. These substrates were placed vertically into a hermetical closed deposition cell. The chemical bath was heated, the colorless solution turned into orange-yellow at 35 1C indicating initiation of chemical reaction. The studies were carried out for various bath temperatures varying from 40 to 70 1C and at different time deposition ranging from 10 to 50 min. The thickness of the films is determined by double weight method, using the relation: e ¼ m rs where m is the thin film mass, S the area of the deposited films and r the density of Sb 2 S 3 material. Calculations for film thickness were found between 0.166 and 0.386 mm. The obtained layers of Sb 2 S 3 were well homogeneous, uniform and compact. The XRD spectra were obtained by means of X Pert PRO Alpha-1 using Co Ka monochromatic radiations. The wavelength, accelerating voltage and current were, respectively, 1.789 Å, 40 KV and 20 ma with 2y ranging from 01 to 701. AFM was carried out with a Veecoo Dimension 3100 Atomic Force Microscopy, in the so-called mode tapping mode of imaging. Auger measurements were carried out with a Riber system equipped with a cylindrical mirror analyzer (CMA) and working on the first derivative mode. Transmittance and reflectance measurements at near normal incidence were performed, at room (a) (a) (020) (311) (132) (020) (311) 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 (b) (b) (020) (311) (421) (132) (020) (311) (421) (132) 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 (c) (c) (020) (311) (421) 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 Fig. 1. (A) X-ray diffraction patterns of Sb 2 S 3 thin films deposited at 40 min with different deposition temperature. (a) 40, (b) 60 and (c) 70 1C. (B) X-ray diffraction patterns of Sb 2 S 3 thin films deposited at 60 1C with different times deposition. (a) 30, (b) 40 and (c) 50 min.
H. Maghraoui-Meherzi et al. / Physica B 405 (2010) 3101 3105 3103 temperature, over a large spectral range (0.2 2.5 mm) on Sb 2 S 3 films deposited on glass substrates by a Varian cary 5000 UV/Vis/ NIR spectrophotometer. 3. Results and discussion The formation of antimony sulphide in the thiosulphate bath is based on hydrolytic decomposition of antimony (III) thiosulphate compounds formed in aqueous media. It has been suggested that thiosulphate forms a complex with metals ion following reaction [15]: 2Sb 3+ +3 S 2 O 3 2 -Sb 2 (S 2 O 3 ) 3 In acidic media, thiosulphate ions are able to gradually release sulphide ions upon hydrolytic decomposition according the reactions: S 2 O 3 2 +H + -S+HSO 3 S+2e -S 2 These sulphide ions then combine with the antimony (III) ions released from the thiosulphate complexes, upon hydrolysis reacting on the glass substrates to form yellow-orange Sb 2 S 3 : (2) observed at the diffraction angle of 28.431 is assigned to the orthorhombic phase matching with (1 0 1) plane of stibnite structure (PDF 74-1046). The other characteristic peaks of (2 0 0), (2 2 0), (4 1 0), (3 4 0) and (5 2 1) of stibnite structure were present in all X-ray spectra with low intensity. Also, the (1 0 1) peak intensity increases when bath temperature increases reaching 60 1C and decreases at 70 1C, which reveal a better crystallinity structure of antimony sulphide films. This result can be explained by the fact that films grown at 60 1C were thicker than the other deposited layers. Fig. 1B shows the X-ray diffraction spectrum obtained for 60 1C at different deposition times. We note the presence of the same diffraction peaks already obtained, with (1 0 1) high intensity peak. The best deposition parameters for elaboration of Sb 2 S 3 thin films were found at temperature and time deposition, respectively, equal to 60 1C and 40 min. In order to study the grain size of Sb 2 S 3 thin film particles, films having a (1 0 1) peak are studied by taking the slow scan XRD pattern around the (1 0 1). The grain size is calculated using Scherrer s relation d ¼ l Dcosy where D is the full width at half maximum of the peak and l¼1.789 Å the wavelength with Co Ka target. It is found that the value of grain size listed in Table 1 is relatively higher for the film 2Sb 3+ +3 S 2 -Sb 2 S 3 The formation of Sb 2 S 3 thin film starts to deposit when ionic product (IP) [S 2 ] 3 [Sb 3+ ] 2 is greater than solubility product (Ks¼10 92.77 )ofsb 2 S 3 [14]. X-ray diffraction measurements were performed to follow the change of the layer crystallinity induced by the variation of temperature and time deposition. Fig. 1A shows the diffraction patterns of polycrystalline Sb 2 S 3 films deposited at different bath temperatures ranging from 40 to 70 1C. The one main peak, Table 1 The calculated values of film thickness and grain size for as-deposited Sb 2 S 3 using different deposition parameters Intensity (a. u) C Sb Deposition parameters Film thickness (mm) Grain size (Å) 40 1C 40 min 0.166 258 60 1C 40 min 0.386 310 70 1C 40 min 0.243 270 60 1C 30 min 0.251 260 60 1C 50 min 0.326 301 S 100 200 300 400 500 600 Kinetic Energy (ev) Fig. 3. AES spectra of Sb 2 S 3 grown at 60 1C for 40 min. 5.00 400.0 nm 600.0 nm 2.50 200.0 nm 0 0 2.50 5.00 μm 0.0 nm 1 2 3 4 μm Fig. 2. Two (a) and three (b) dimensional AFM images of CBD antimony sulphide thin films.
3104 H. Maghraoui-Meherzi et al. / Physica B 405 (2010) 3101 3105 prepared at 60 1C and 40 min and decreases for any change in deposition parameters. The relatively higher XRD peak intensity for this film confirms these results. In fact many researchers have reported that the grain size of film increase with film thickness [16]. The grain size values were in good agreement with the values reported in the literature [17]. Fig. 2 shows two (a) and three (b) dimensional AFM images of antimony sulphide thin films grown at 60 1C for 40 min having thickness 0.386 mm. Two-dimensional grain size of Sb 2 S 3 thin films were measured by using nanoscale reading. As it can be seen from this figure, the film is composed of small particles of average grain size between 200 and 300 Å. Grain size is calculated from XRD studies using Scherrer s formula. The result (310 Å) is in close agreement with the directly observed from AFM studies. Auger studies are useful in order to get information on the surface layer composition and a possible contribution of CBD parameters to this composition. In this work, Auger studies are only used to obtain qualitative information on the role of these parameters and to deduce the main features of the surface layer. The first step of this work concerned the surface composition homogeneity. In order to check this homogeneity, we selected several points on the surface and we plotted Auger spectra for each of them. These spectra being similar, we concluded that the surface homogeneity was good at the scale of the Auger measurements accuracy. Fig. 3 shows the AES survey for Sb 2 S 3 grown at 60 1C during 40 min. From this figure, we note in addition to the constituent elements Sb and S the presence of C as an impurity. The Sb/S ratio is found to be equal to 0.85, value close to the stoichiometric composition. The source for carbon contamination may be due to the exposure of the samples to atmospheric air, and the laboratory environment in which samples were manipulated. On the other hand the optical properties were not affected by the variation of deposition time and temperature, so we have presented in Fig. 4 and 5 just the spectrum of Sb 2 S 3 prepared at 60 1C for 40 min. In Fig. 4 we reported the optical transmittance and reflectance spectra of Sb 2 S 3 film. The presence of the maxima and minima of the reflectance spectrum at the same wavelength positions indicates the optical homogeneity of the deposited films and no absorption occurs at long wavelengths. Two regions are indicated in Fig. 4; (i) a strong absorption region is observed in the optical range 425 585 nm. The values of the Fig. 5. The optical transmittance of Sb 2 S 3 corrected for reflection losses (a) and the plot of the square of the optical absorption coefficient (a 2 ) versus photon energy (hn) (b). Fig. 4. Optical transmittance (T) and reflectance (R) spectra of Sb 2 S 3 elaborated at 60 1C for 40 min. film absorption coefficient, a, in this region were derived from the transmittance and reflectance spectra and (ii) a weak absorption region in the optical range 585 2500 nm was also apparent, in which the measured transmittance was treated. The observed values corrected due to the loss of reflectance at the air film interface give T corr (Fig. 5a) T% T corr ¼ 100 100 R% The optical absorption coefficient (a) of the material is, a ¼ 1 100 ln d,whered is the Sb 2S Tcorr 3 film thickness. The values of a 2 are plotted against photon energy hn in Fig. 5b. The extrapolation of thedatapointtothephotonenergyaxiswherea 2 ¼0 illustrates the presence of direct gap located near 2.24 ev for this material. Such a value agrees well with the values of 2.25 ev for crystalline Sb 2 S 3 thin films [18] and of 2.26 ev reported in [17], but it is somewhat higher than the value of 1.73 ev for samples annealed in nitrogen atmosphere [19 20]. The enhancement in the value of E g for the
H. Maghraoui-Meherzi et al. / Physica B 405 (2010) 3101 3105 3105 films annealed under a N 2 atmosphere can be attributed to quantum confinement effect due to increase in the particle size when the nanocrystalline grains transformed into polycrystalline form [16,18,21]. 4. Conclusion Chemical bath deposition of antimony sulphide has been achieved using thiosulphate as precursor in aqueous solution. The films were well crystallized with orthorhombic structure. Spherical crystallites with average size of about 200 Å were uniformly distributed showing a dense structure. The Sb/S ratio was found to be equal to 0.85, value close to stoichiometric composition. A direct band gap of 2.24 ev was obtained from the transmittance and reflectance spectra. This result is encouraging for use of these films as new base materials for photovoltaic applications. Further work is needed to improve the quality of the films through better understanding and control of the deposition parameters. Acknowledgments The author are grateful to institut d electronique du sud UMR 5214 (Université Montpellier II, France) for providing some experimental facilities. References [1] K.C. Mandal, O. Savadogo, J. Mater. Chem. 1 (1991) 301. [2] C.D. Lokhande, Mater. Chem. Phys. 27 (1991) 1. [3] D. Cope, US Patent 2 (875) (1959) 359. [4] J.m Grigas, J. Meshkauskas, A. Orliukas, Phys. Status Solidi (A) 37 (1976) k39. [5] M.S. Ablova, A.A. Andreev, T.T. Dedegkaev, B.T. Melekh, A.B. Peutsov, N.S. Shendel, L.N. Shumilova, Sov. Phys. Semicond. 10 (1976) 629. [6] M.J. Chockalingam, K. Nagarajo Rao, N. Ranjarajan, C.V. Suryanarayana, J. Phys. D 3 (1970) 1641. [7] J. George, M.K. Radhakrishana, Solid State Commun. 33 (1980) 987. [8] S.M. Sze, Physics of Semi-conductor Devices, Wiley, New York, 1981 849 and 751. [9] S. Messina, M.T.S. Nair, P.K. Nair, Thin Solid Films 515 (2007) 5777. [10] S.B. Badachhape, A. Goswami, Ind. J. Pure Appl. Phys. 5 (1967) 477. [11] N.S. Yesugade, C.D. Lokhande, C.H. Bhosale, Thin Solid Films 263 (1995) 145. [12] C.H Bhosale, M.D. Uplane, P.S. Pastil, C.D. Lokhande, Thin Solid Films 248 (1994) 137. [13] O. Savadogo, K.C. Mandal, Sol. Energy Mater. Sol. Cells 26 (1992) 117. [14] J.D. Desai, C.D. Lokhande, Thin Solid Films 237 (1994) 29. [15] I. Grozdanov, Semicond. Sci. Technol. 9 (1994) 1234. [16] R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 82 (2003) 347. [17] K.Y. Rajpure, C.H. Bhosale, J. Phys. Chem. Solids 61 (2000) 561. [18] A.M. Salem, M.S. Selim, J. Phys. D: Appl. Phys. 34 (2001) 12. [19] B. Krishan, A. Arato, E. Cardenas, T.K. Das Roy, G.A. Castillo, Appl. Surf. Sci. 254 (2008) 3200. [20] I. Grozdanov, M. Ristov, G.j Sinadinovski, M. Mitreski, J. Non-Cryst. Solids 175 (1994) 77. [21] M.T.S. Nair, Y. Peña, J. Campos, V.M. Garcia, P.K. Nair, J. Electrochem. Soc. 145 (1998) 2113.