Characterization of tin (II) sulphide thin film synthesized by successive chemical solution deposition

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1 Indian J Phys DOI /s ORIGINAL PAPER Characterization of tin (II) sulphide thin film synthesized by successive chemical solution deposition A Mukherjee and P Mitra* Department of Physics, The University of Burdwan, Golapbag, Burdwan, West Bengal, India Received: 3 December 2014 / Accepted: 17 February 2015 Abstract: In the present work, tin (II) sulphide (SnS) thin films were grown on glass substrate by successive chemical solution deposition method using ammonium sulphide as anionic precursor solutions. Characterization techniques of X-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy and energy-dispersive x-ray were utilized to study the microstructure of the films. Energy-dispersive x-ray confirmed formation of nearly stoichiometric film with slight excess of tin under optimized deposition conditions. Particle size estimated from Rietveld refinement of X-ray diffraction data using MAUD software was *41 nm which compared well with field emission scanning electron microscopy measurements. The value of the energy gap of *1.51 ev was found to be near the optimum needs for photovoltaic solar energy conversion (1.5 ev) with high absorption in the visible region. An enhancement in energy gap was observed for tin-enriched films. Keywords: PACS Nos.: Tin sulphide; Successive chemical solution deposition; Thin film; Particle size; Band gap z; cp; Dz 1. Introduction Among the many chalcogenide semiconductors, SnS, SnSe, GeS and GeSe are promising materials for solar energy conversion [1]. Tin (II) sulphide (SnS) films are highly suitable for many applications in a number of solid-state devices, such as photovoltaic, photoelectrochemical (PEC), photoconductive cells and intercalation battery systems [2]. SnS could be of interest for photovoltaic cells, since it has a direct optical energy gap of *1.3 ev and it has a high absorption coefficient ([10 4 cm -1 ) and a high conversion efficiency of about 25 % [3]. The constituent elements, Sn and S, are available in abundance. Besides, these elements are cheap and non-toxic. SnS is a brown-black or grey solid, insoluble in water having meting point of 880 C and density 5080 gm/cm 3. Among several binary tin sulphides known (SnS, SnS 2, Sn 2 S 3, Sn 3 S 4, Sn 4 S), SnS, a p-type semiconductor with a tunable band gap in the visible region, is most important [4]. SnS has a orthorhombic-layered structure having lattice parameters a = Å, b = Å and c = 3.98 Å, which can be synthesized *Corresponding author, mitrapartha1@rediffmail.com by numerous fabrication methodologies [4]. Accordingly, the synthesis and characterization of tin sulphide thin films via different techniques have attracted considerable attention. Although physical routes of film preparation produce uniform and compact films and also render better control over stoichiometry, they require expensive capital instruments and are difficult to expand to large scale. Consideration of simplicity, economy and input energy suggests that thin films of these materials be deposited by simpler chemical route. Various chemical techniques such as cathodic electrodeposition [5], electrochemical deposition [6], spray pyrolysis [7, 8], chemical bath deposition (CBD) [2, 9 13] have been used to prepare and characterize SnS films. The characteristics of materials prepared depend very much on the synthesis route. The film in this study is grown by SCSD, which creates a thin film by successive dipping of a solid substrate in separately placed cationic and anionic precursor for reaction at chosen temperatures [14]. In chemical bath deposition (CBD) method, deposition of thin films occurs due to substrate maintained in contact with dilute chemical baths containing cationic and anionic solutions. Accordingly, SCSD may be termed as modified version of chemical bath deposition (modified CBD). The technique is similar to successive ion layer adsorption and Ó 2015 IACS

2 A Mukherjee and P Mitra reaction (SILAR) technique, in which one single dipping in anionic and cationic precursors gives a thickness comparable to one monolayer. Every technique of thin film deposition has its own merits and demerits [4]. SCSD has some advantages such as easy control of growth parameters, simple, less expensive and highly feasible for large area deposition. Also SCSD offers preparation of film on any kind of substrate at low temperatures and under mild working conditions. Although the fabrication of metal sulphide thin films by SILAR method is quiet common, reports on preparation of SnS thin films by successive dipping technique in separately placed cationic and anionic precursors (SILAR/SCSD) are scarce [4, 15 17] since first reported by Ristov et al. [18] who named it as chemical deposition (CD). While Safonova et al. [17] have used sodium thiosulphate as anionic precursor, all the other workers [4, 15, 16, 18] have used sodium sulphide anionic precursor thus inevitably introducing the possibility of highly mobile sodium ion incorporation in the deposited films. In this communication, we report the preparation of SnS thin films by SCSD using ammonium sulphide as anionic precursor which removes the possibility of incorporation of undesired species (highly mobile sodium impurities) in the films. We have investigated the detailed structural and morphological properties of SCSD-deposited SnS thin films. 2. Experimental details Tin sulphide thin film was synthesized by dipping a precleaned glass substrate (commercially available microscope slides), alternately in tin chloride (SnCl 2 ) solution as cationic precursor was kept at *70 C and 2 vol. % ammonium sulphide [(NH 4 ) 2 S] solution as anionic precursor was kept at room temperature. Between every immersion in cationic and anionic precursors, it was rinsed with distilled water. The concentration of cationic precursor was varied in the range of M. The tin chloride solution bath was prepared by adding SnCl 2 2H 2 O (E-Merck) in distilled water. Similarly, ammonium sulphide (40 48 wt%, Sigma-Aldrich) solution was prepared by adding two (2) cc ammonium sulphide in 100-cc distilled water. Concentrations of the reacting baths were found to be optimized in order to have good-quality adherent films. For concentrations of cationic bath less than 0.02 M, the bath solution was found to be unstable for film deposition. For concentrations higher than 0.05 M, nucleation process was affected possibly due to the presence of higher amount of reactant (i.e. large number of ionic species) at the substrate solution interface. Accordingly, three different concentrations of cationic precursor viz M (Bath A), M (Bath B) and 0.05 M (Bath C) were selected for film synthesis. One of the problems with stannous chloride solution was that it did not dissolve completely. A drop of acetic acid was added to a clear transparent solution of tin chloride. The solution of ammonium sulphide was stable and no stirring was required. Dipping of the substrate in cationic and anionic precursors led to the formation of SnS thin film. The film deposition process consisted of the following steps: (i) dipping of the substrate in the hot SnCl 2 which carries a thin layer of tin ion adhered to the substrate surface and (ii) introduction of the substrate into ammonium sulphide bath kept at room temperature. The reaction between tin ions and sulphur ions resulted in the formation of SnS. Thus, one complete dipping cycle involved dipping the substrate into SnCl 2 bath and its withdrawal from the bath followed by dipping into the ammonium sulphide bath. The dipping time in each bath was 5 s. The number of dipping cycles was optimized to give a film thickness of *500 nm measured gravimetrically using an electronic balance (DENVER instrument, Germany, TP-303). Nearly 120 dipping cycle gave *500 nm thickness for film deposited from M cationic bath concentration. This indicated a growth rate of more than 4 nm per dipping cycle (*4.17 nm per sipping cycle), which was much higher than one monolayer thickness of SnS. For higher concentrations of cationic bath, relatively lower number of dipping cycle was required to get the thickness of *500 nm indicating enhanced growth rate for films deposited from higher concentrations of cationic precursors. Ghosh et al. [15] reported a growth rate of *1.1 nm on glass and *1 nm on ITO substrate and named the technique as SILAR possibly because the values were comparable to one monolayer thickness (almost twice) of SnS. X-ray diffraction (XRD) with CuK a radiation (k = Å) was performed to assess the overall structure and phase purity. The experimental peak positions were compared with the standard JCPDS files and the Miller indices were indexed to the peaks. SEM (Model S530, Hitachi, Japan) was used to study the surface morphology and to illustrate the formation of crystallites on the film surface. FESEM was used to measure the particle size. The same instrument was used for EDX analysis carried out to qualitatively measure the sample stoichiometry. UV VIS spectrophotometric measurements were taken using a double-beam spectrophotometer (Shimadzu, UV 1800) at room temperature. The spectra were recorded by using a similar glass as a reference and hence, the absorption due to the film only was obtained. The band gap of the films was calculated from the absorption edge of the spectrum. 3. Results and discussion A typical XRD pattern of SnS thin film in the scan range o is shown in Fig. 1 (dotted curve). The major peaks

3 Characterization of tin (II) sulphide thin film cps/ev sample S Sn Si S Sn Fig. 1 XRD pattern of SnS thin film [experimental (dotted) and fitted (continuous)] at o, o and o can be well indexed to the orthorhombic structure of SnS [JCPDS no ]. The corresponding reflecting planes are (120), (101) and (111). Other peaks with relatively small intensities match well with those of orthorhombic SnS. The lattice constants of assynthesized SnS, estimated from Rietveld refinement of XRD data using MAUD software [19 21], are a = 0.409, b = and c = nm, which are very close to that of bulk SnS (JCPDF ). An appropriate fit (continuous line) has been observed with the goodness of fit (GOF) value *1.2. The average particle size that has been determined from Rietveld fitting is *41 nm. Since the data have been fitted using Rietveld refinement technique, no background subtraction is made for the present experiment. Figure 2 shows the EDX spectra for SnS film deposited from M concentration of cationic bath (Bath A). The corresponding Sn:S ratio is observed to be 54:46 indicating good stoichiometry of the deposited films with small excess amount of tin. Elemental composition is obtained by comparing the relative concentration of the elements; the later is obtained from the normalized atomic per cent concentration. Figure 3 on the other hand shows the same for that deposited from 0.05 M concentration of cationic bath (Bath B). The corresponding Sn:S ratio is observed to be 70:30 indicating strongly rich tin component. This might be due to the fact that for higher concentration of cationic complex, the release of tin ion is not counterbalanced by the availability of sulphur ions. The peaks appeared at 0.52 kev and at 1.72 kev are due to oxygen and silicon coming from the substrate component. Oxygen inclusion in the film is also possible which is difficult to estimate since EDX measurement has been taken on glass substrate. Na, Mg and Ca elemental peaks at *1.1, *1.23 and kev Fig. 2 EDX spectrum of SnS film deposited from M cationic bath cps/ev S Sn Si S kev *3.64 kev have resulted due to the elements in glass substrates and distilled water. The microstructure of SnS thin film prepared from Bath A is shown in Fig. 4. The SEM photograph illustrates the formation of sub-micrometre crystallites distributed more or less uniformly over the surface. Agglomeration of small crystallites seems to be present in certain regions on the film surface. Figure 5 shows the FESEM image of the same film. Tin sulphide film with grains distributed mostly in nanometre regime and well covering the substrate is clearly seen. In many areas, nanosized SnS particles form globular Sn sample Fig. 3 EDX spectrum of SnS film deposited from 0.05 M cationic bath

4 A Mukherjee and P Mitra Fig. 6 FESEM image of tin-enriched SnS film Fig. 4 SEM image of SnS film deposited from M cationic bath 35 Absorbance (a. u.) (a) M (b) M (c) 0.05M 5 0 (c) (b) (a) Wavelength (nm) Fig. 5 FESEM image of SnS film deposited from M cationic bath aggregates of microdimensions. Average particle size is *46.4 nm. Figure 6 shows the FESEM image of SnS film deposited from 0.05 M cationic bath. Tin enrichment results in grain coalescence and homogeneous structure with average particle size of *50 nm. Curves (a), (b) and (c) of Fig. 7 show the plot of absorbance (A) as a function of wavelength for 0.025, and 0.05 concentrations of cationic precursors, respectively. Optical band gaps (e g ) of the films are evaluated from the dependence of absorption coefficient (a) on photon energy (hm) using Tauc s formula for direct transition Fig. 7 Absorbance versus photon wavelength for cationic bath concentrations of (a) M, (b) M and (c) 0.05 M ðahmþ n ¼ Bhm e g where B is a parameter that depends on the transition probability and is a function of index of refraction and hole/electron effective masses [12, 22] and n is the index that has value equal to 1 2 for the direct allowed transitions and 2 for indirect allowed transitions. SnS thin films have been reported to exhibit both direct [4, 23, 24] as well as indirect [25 28] transitions. The graphical representation of (ahm) 2 against hm for evaluating the direct band gap value is shown in Fig. 8. Curves (a), (b) and (c) of Fig. 8 show the plot of (ahm) 2 against hm for 0.025, and 0.05 concentrations of cationic precursors, respectively. Absorption coefficient (a) has been evaluated from the

5 Characterization of tin (II) sulphide thin film ( h ) 2 (ev-cm -1 ) (a) M (b) M (c) 0.05M relation A = at and using a value of thickness *500 nm measured gravimetrically. The direct optical energy gap was obtained when the straight-line portion of the (ahm) 2 versus hm plot is extrapolated to intersect the energy axis at a = 0. The direct band gap of SnS deposited from M cationic bath concentration calculated is *1.51 ev, which compares well with the reported value of 1.43 ev [4]. The optical band gap for indirect transition has been estimated from the intercepts of the straight-line parts of the curves with the x-axis using a graphical presentation of the dependence of ðahmþ 1=2 on photon energy hm (Fig. 9). Curves (a), (b) and (c) of Fig. 9 show the plot of ðahmþ 1=2 against hm for three different concentrations of cationic precursors. The indirect band gap of SnS deposited from M cationic bath concentration calculated is (a) h (ev) Fig. 8 Plot of (ahm) 2 versus hm of SnS for cationic bath concentrations of (a) M, (b) M and (c) 0.05 M ( h ) 2 (ev-cm -1 ) (a) h (ev) (c) (c) (b) (b) (a) M (b) M (c) 0.05M Fig. 9 Plot of ðahmþ 1=2 versus hm of SnS for cationic bath concentrations of (a) M, (b) M and (c) 0.05 M *1.28 ev, which compares well with reported value of 1.18 ev [28]. The increase in band gap for tin-enriched films may be due to density of point defects arising from nonstoichiometry [29] orpresenceofsns 2 and Sn 2 S 3 in SnS. The optical band gap for SnS thin films also varies depending on the variation of the tin-to-sulphur ratio leading to the change in energy band structure and also the density of states (DOS) of valence band and conduction band [4]. 4. Conclusions Phase-pure SnS thin film could be successfully synthesized by SCSD using non-sodium-containing anionic precursor. The technique of SCSD is simple, cost-effective and offers an easily scalable alternative to industrial level. Use of ammonium sulphide as anionic precursor removes the possibility of incorporation of highly mobile sodium ions in the films. Nearly stoichiometric film with slight excess of tin could be obtained under optimized concentration of cationic precursor. The growth rate per dipping cycle under this optimized concentration of cationic precursor is little larger than 4 nm (*4.17 nm), which is much higher than one monolayer thickness of SnS. The films are polycrystalline with particle size of *41 nm estimated from Rietveld analysis using MAUD software, which compares well with FESEM measurements (*46.4 nm). The value of the direct energy gap of *1.51 nm is near the optimum needs for photovoltaic solar energy conversion (1.5 ev). An enhancement in particle size as well as in direct and indirect band gap has been observed for tin-enriched films. Acknowledgments The authors wish to thank the University Grants Commission (UGC), India, for granting Centre for Advanced Study (CAS) under the thrust area Condensed Matter Physics including Laser applications to the Department of Physics, Burdwan University, under the assistance of which the work has been carried out. References [1] R K Ramakrishna, R N Koteswara and R W Miles Sol. Energy Mater. Sol. Cells (2006) [2] R Mariappan, M Ragavendar and V Ponnuswamy Opt. Appli. XLI 989 (2011) [3] J J Hong, C Shu-Ying, W Xin-Kun and Y Yong-Li Nat. Sci (2010) [4] B Ghosh, M Das, P Banerjee and S. Das Appl. Surf. Sci (2008) [5] A Ghazali, Z Zainal, M Z Hussein and A Kassim Sol. Energy Mater. Sol. Cells (1998) [6] M Ichimura, L Takeuchi, Y Ono and E Arai Thin Solid Films (2000) [7] N Koteeswara Reddy and K T Ramakrishna Reddy Mater. Chem. Phys (2007) [8] N Koteeswara Reddy and K T Ramakrishna Reddy Mater. Res. Bull (2006)

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