Thickness-Dependent Physical Properties of Coevaporated Cu4SnS4 Films

Cu4SnS4 films of different thicknesses were prepared by thermal coevaporation technique on glass substrates at a constant substrate temperature of 400°C. The layer thickness was varied in the range 0.25–1 μm. The composition analysis revealed that all the evaporated films were nearly stoichiometric. The XRD patterns indicated the presence of a strong (311) peak as the preferred orientation, following the orthorhombic crystal structure corresponding to Cu4SnS4 films. Raman analysis showed a sharp peak at 317 cm−1, also related to Cu4SnS4 phase. The optical transmittance spectra suggested that the energy band gap decreased from 1.47 eV to 1.21 eV with increase of film thickness. The hot-probe test revealed that the layers had p-type electrical conductivity. A decrease of electrical resistivity was observed with the rise of film thickness.


Introduction
In recent years, extensive efforts have been made in finding novel and new semiconductors for application in energy conversion using the photovoltaic route. One such new material system is the Cu-Sn-S that contains inexpensive, nontoxic, and earth-abundant elements. This ternary system exhibited different stable phases such as Cu 2 SnS 3 , Cu 2 Sn 3 S 7 , Cu 5 Sn 2 S 7 , Cu 4 SnS 4 , and Cu 10 Sn 2 S 13 , that was reported by Wu et al. [1]. All these compounds are represented by the general formula I-IV-VI. Their optimum optical, thermal, and mechanical properties with appropriate energy band gap of these materials have attracted much attention for their applications in solar cells, sensor and other optoelectronic devices [2]. Cu 4 SnS 4 is one such material that had an optimum energy band gap for solar energy conversion along with suitable properties [3,4]. Hence, many researchers have attempted to grow thin films of this material using a variety of wet chemical methods such as chemical bath deposition [5] and electrodeposition [6]. However, to our knowledge, there were no attempts made to form Cu 4 SnS 4 films using physical methods. Therefore, thermal coevaporation technique has been used in this study for the growth of Cu 4 SnS 4 films. It is one of the best techniques because of the possibility of large-scale production, high quality of the grown layers, and minimum wastage of components. In previous work, we have shown that thin films of Cu 4 SnS 4 could be obtained by coevaporating SnS and CuS powders [7].
In general, the physical properties of thin films strongly depend on the deposition technique and growth parameters as well as the film thickness [8]. Film thickness plays an important role in controlling the film properties unlike its bulk counterpart, and all the physical properties of the films may oscillate with thickness [9][10][11][12]. The influence of crystallite size on the physical properties has been of much interest in semiconductor devices. The structure and surface morphology of the film changes with the increase of film thickness, which will in turn affect the optical transmittance [13,14] and electronic properties [15,16]. Hence, in the present work, the influence of film thickness on physical properties of Cu 4 SnS 4 films has been investigated and reported.

Experimental Analysis
Cu 4 SnS 4 thin films were prepared by thermal coevaporation technique using Box-type Hind Hi Vac system (model BC-300). 5N pure CuS and SnS were used as the source materials, and the depositions were carried out under a pressure better than 10 −5 Torr. In this technique, the distance between the source and substrate is maintained as 20 cm. The material is kept in a tantalum boat covered with quartz wool, which is heated by resistive heating process. Ultrasonically cleaned Corning 7059 glass slides were used as substrates for depositing Cu 4 SnS 4 films. The films were prepared for different thicknesses that vary in the range 0.25-1 m, keeping the substrate temperature at 400 ∘ C. The rate of deposition and thickness of the experimental films were determined using the quartz crystal thickness monitor (model QTM-101) placed just below the substrate holder. The as-grown layers were characterized by studying the composition, structural, optical, and electrical properties. The elemental composition and surface morphology of Cu 4 SnS 4 films were examined using scanning electron microscope (SEM) (model: Zeiss EVO MA 15) attached to the energy dispersive X-ray analyser (EDAX) system (model: INCA Penta FET X3). The structural properties of Cu 4 SnS 4 films were measured using an (model: Seifert 3003TT) X-ray diffractometer (XRD) with Cu-K radiation source ( = 1.5402Å). Raman analysis was carried out at room temperature with HORIBA JOBIN YVON (model: Lab Ram HR800) Raman spectrometer. The optical transmittance of the films was recorded as a function of wavelength using Perkin Elmer Lambda-950 UV-Vis-NIR spectrophotometer in order to determine the optical band gap, absorption coefficient, and refractive index. The electrical properties of the films were recorded by using hot-probe method, four-probe method, and Hall effect studies.

Results and Discussion
The as-grown Cu 4 SnS 4 layers were pinhole free, uniform, and well adherent to the substrate surface. With the increase of film thickness, the color of the films was changed from palebrownish yellow to dark-yellowish brown.

Compositional Analysis.
The composition of the different elements present in Cu 4 SnS 4 films, grown at a constant substrate temperature, was approximately constant although there is an increase of film thickness ( ). Figure 1 shows the EDAX spectrum of a typical Cu 4 SnS 4 film grown with a thickness of 1 m. It can be seen from the spectrum that there were peaks corresponding to Cu, Sn, and S, and no other impurities were present. The elemental composition of the layer evaluated using this spectrum was Cu = 43.35 at. %, Sn = 15.56 at. %, and S = 41.09 at. %. The films showed (311) plane as the dominant orientation, and its intensity increased with the increase of film thickness, which indicates an improvement in the crystallinity of the layers. The crystal structure was evaluated to be orthorhombic. Nair et al. [17] also observed a similar behavior with the (311) plane as preferred orientation in Cu 4 SnS 4 films formed by heating SnS-CuS layers grown by chemical bath deposition method.
The lattice parameters corresponding to orthorhombic crystal structure were calculated using the relation where is the interplanar distance and (ℎ ) are the Miller indices. The evaluated lattice parameters were = 13.52Å, = 7.67Å, and = 6.43Å. The change in lattice parameters ISRN Condensed Matter Physics 3 , , and with increase of film thickness was found to be marginal. The evaluated lattice parameters are in close agreement with the standard JCPDS data file (Card no. 29-0584). The crystallite size, , of the as-deposited layers was evaluated using the Debye-Scherrer formula [18] given by where is the full width at half maximum of the (311) peak, is the diffraction angle, and is the X-ray wavelength. The crystallite size of the films increased from 39 nm to 76 nm with the increase of film thickness. The increase in crystallite size might be interpreted in terms of columnar grain growth in the film structure. The observed less grain size at lower thicknesses might be due to the strong interaction between the substrate and vapour atoms, which restricts the mobility of ad-atoms. The improvement in the crystallinity of Cu 4 SnS 4 layers can provide activation energy for the adsorbed atoms to occupy the minimum energy positions, enhancing the recrystallization due to the coalescence of the islands by increasing the volume and surface diffusions.

Raman Analysis.
The Raman spectroscopy measurements were conducted on the as-grown Cu 4 SnS 4 films in the wavenumber range 100-600 cm −1 as shown in Figure 3. From the Raman spectra, only one characteristic Raman line could be observed at 317 cm −1 for all thicknesses. This vibrational mode is not related to any other binary or ternary phases of Cu-Sn-S such as SnS, SnS 2 , Sn 2 S 3 , CuS, Cu 2 S, Cu 2 SnS 3 , and Cu 3 SnS 4 and is therefore ascribed due to the contribution of Cu 4 SnS 4 phase as it is also confirmed from the XRD analysis. It could be seen from the Raman spectra that the intensity of the peak increased with the increase of film thickness, which could be due to the improvement in the crystallinity of the layers. Figure 4 shows the SEM pictures of Cu 4 SnS 4 films grown in the thickness range 0.25-1 m. All the films showed worm-like grains that were uniformly distributed across the surface of the substrate. The size of the grains increased with the increase of the film thickness, and the layers became more densely packed with rough surface morphology. Cu 4 SnS 4 films deposited at lower thicknesses <0.5 m exhibited uniformly distributed small grains with regular shapes. Although the size of the grains is small, the grains were grown uniformly. The surface structure of the films grown at higher thicknesses >0.75 m contained grains with complex and multiple structures. This is the evidence for the presence of Cu 4 SnS 4 phases with different orientations in accordance with the XRD data. Here, the size of the grain is higher than in the films grown at other thicknesses. Figure 5 shows in the films. With the increase of film thickness, the absorption edge was shifted towards longer wavelength side, indicating a decrease in the energy band gap of the films. In order to evaluate the energy band gap, , of the films, ( ℎ]) was plotted against the photon energy (ℎ]), where is an integer whose value reveals the type of optical transition in the films. In the present study, the nature of the transition was found to be direct, and the energy band gap was calculated using the relation [19] ( ℎ ) = (ℎ − )

Optical Properties.
where is a constant, ℎ] is the incident photon energy, and is the absorption coefficient, calculated using the relation where is the transmittance and is the thickness of the film. The evaluated absorption coefficient of the layers was >10 4 cm −1 . Figure 6 shows the plots of ( ℎ]) 2 versus ℎ] where the extrapolation of the plot onto ℎ] axis gives the band gap. The energy band gap of the films decreased from 1.47 eV to 1.21 eV with the increase of film thickness from 0.25 m to 1 m. The energy band gap can be affected by the increase of crystallite size, decreasing of stacking faults and crystal imperfections, which results in the orientation of the individual crystallites [20], quantum size effect [21], and disorder at the grain boundaries [22]. In the present study, the change in energy band gap with film thickness can be understood by the decrease of structural disorder as well as increase in crystallite size in the films as observed from the microstructural analysis. Patel et al. also observed a similar behavior in CZTS films prepared by spray pyrolysis technique [23]. In our previous work [7], we observed that the energy band gap varied between 1.7 eV and 1.9 eV where the film thickness was approximately 200 nm, and the layers were grown using different substrate temperatures varied in the range 200-350 ∘ C. These films showed that the crystallite size varied in the range 18-35 nm. Hence, the reported higher energy band gap in these layers is attributed to the quantum confinement due to small crystallite size. However, in this study, the layers thickness varied over a wide range of 0.25-1.0 m, where the evaluated grain size varied from 39 nm to 76 nm. Therefore, the observed decrease in the band gap in the present work compared to our previous work is attributed to the improvement in the crystallite size.  3.6. Electrical Properties. The hot-probe test revealed that all the as-grown films exhibited p-type electrical conductivity. The electrical properties of the Cu 4 SnS 4 layers were examined at room temperature by four-probe method and Hall effect studies. Figure 7 shows the variation of electrical resistivity with film thickness. The resistivity of the films decreased from 5.8 × 10 2 Ω cm to 2.5 × 10 2 Ω cm with the increase of film thickness from 0.25 m to 1 m. This decrease of film resistivity with thickness might be due to the decrease ISRN Condensed Matter Physics of residual stress and defect density with the increase of crystallite size in the films [24]. The increase of crystallite size could mainly decrease the grain boundary scattering thereby decreasing the film resistivity. The carrier concentration of the film increased from 9 × 10 14 cm −3 to 3.6 × 10 15 cm −3 , and the mobility of the film decreased from 12 cm 2 V −1 s −1 to 7 cm 2 V −1 s −1 . Avellaneda et al. [3] also reported similar observation for the thin films of Cu 4 SnS 4 prepared by chemical bath deposition technique by heating SnS-CuS for 1 h each at different temperatures: 315, 350, and 400 ∘ C under 300 mTorr of nitrogen and the chamber was evacuated to about 20 mTorr.

Conclusions
Cu 4 SnS 4 films have been successfully grown by thermal coevaporation of CuS and SnS on glass substrates at a substrate temperature of 400 ∘ C and varying the film thickness in the range 0.25-1 m. The XRD patterns suggested that all the as-deposited films were polycrystalline in nature with the (311) plane as the preferred orientation and exhibited orthorhombic structure. The Raman studies revealed only a single peak corresponding to Cu 4 SnS 4 phase. The films had high optical absorption, and the evaluated energy band gap decreased with the increase of film thickness and varied in the range 1.47-1.21 eV. The electrical resistivity of the films also showed a similar trend with the increase of film thickness.