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. 1. 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 Cu2SnS3, Cu2Sn3S7, Cu5Sn2S7, Cu4SnS4, and Cu10Sn2S13, 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]. Cu4SnS4 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 Cu4SnS4 films using physical methods. Therefore, thermal coevaporation technique has been used in this study for the growth of Cu4SnS4 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 Cu4SnS4 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