Annealing of RF-magnetron sputtered SnS2 precursors as a new route for single phase SnS thin films

Abstract

Tin sulphide thin films have been grown on soda-lime glass substrates through the annealing of RF-magnetron sputtered SnS 2 precursors. Three different approaches to the annealing were compared and the resulting films thoroughly studied. One series of precursors was annealed in a tubular furnace directly exposed to a flux of sulphur vapour plus forming gas, N 2 + 5%H 2 , and at a constant pressure of 500 mbar. The other two series of identical precursors were annealed in the same furnace but inside a graphite box with and without elemental sulphur evaporation again in the presence of N 2 + 5%H 2 and at the same pressure as for the sulphur flux experiments. Different maximum annealing temperatures for each set of samples, in the range of 300-570°C, were tested to study their effects on the properties of the final films. The resulting phases were structurally investigated by X-Ray Diffraction (XRD) and Raman spectroscopy. Annealing of SnS 2 precursors in sulphur flux produced films where SnS 2 was dominant for temperatures up to 480°C. Increasing the temperature to 530°C and 570°C led to films where the dominant phase became Sn 2 S 3. Annealing of SnS 2 precursors in a graphite box with sulphur vapour at temperatures in the range between 300°C and 480°C the films are multi-phase, containing Sn 2 S 3 , SnS 2 and SnS. For high annealing temperatures of 530°C and 570°C the films have SnS as the dominant phase. Annealing of SnS 2 precursors in a graphite box without sulphur vapour at 300°C and 360°C the films are essentially amorphous, at 420°C SnS 2 is the dominant phase. For temperatures of 480°C and 530°C SnS is the dominant phase but also same residual SnS 2 and Sn 2 S 3 phases are observed. For annealing at 570°C, according to the XRD results the films appear to be single phase SnS. The composition was studied using energy dispersive spectroscopy being then correlated with the annealing temperature. Scanning electron microscopy studies revealed that the SnS films exhibit small grain structure and the film surface is rough. Optical measurements were performed, from which the band gap energies were estimated. These studies show that the direct absorption transitions of SnS are at 1.68 eV and 1.41 eV for annealing in graphite box with and without elemental sulphur evaporation, respectively. For the indirect transition the values varied from 1.49 eV to 1.37 eV. The results of this work show that the third approach is better suited to produce single phase SnS films. However, a finer tunning of the duration of the high temperature plateau of the annealing profile is required in order to eliminate the b-Sn top layer.

Structural and electrical properties of SnS2 thin films

“…SnS 2 films often include secondary phases of Sn 2 S 3 or SnS, which may significantly affect the structural, optical, and electrical properties of the material [29,30]. One of the most effective methods of phase analysis of Sn x S y compounds is Raman spectroscopy [31,32].…”

“…One of the most effective methods of phase analysis of Sn x S y compounds is Raman spectroscopy [31,32]. This method is mostly used together with XRD and allows one not only to study phase composition but also to identify polytypes in the samples [29,33].…”

Comparative study on the annealing types on the properties of Cu2ZnSnS4 thin films and their application to solar cells

“…In addition, the several weak peaks located at 252 cm -1 , 288 cm -1 , 350 cm -1 and 374 cm -1 were observed; these are known to result from a kesterite CZTS phase [29][30]. However, some literature surveys indicated that the Raman peaks located at 250 cm -1 and 288 cm -1 were overlapped both of CZTS and Cu and Sn-based secondary phase [31][32][33]. Based on our XRD results, they were secondary phases.…”

“…In addition, the very weak peaks located at 153 cm -1 (CF) and 165 cm -1 (RTA) were observed in the CZTS thin films, which resulted in the SnS compound [33]. Generally, it is well known that the penetration depth of 514 nm Raman excitation laser was ~ 100 nm.…”

Laser-induced SnS2-SnS phase transition and surface modification in SnS2 thin films

“…This is why films of Sn x S y compounds usually contain secondary phases [3]. The SnS 2 -SnS phase transition has been studied only for thermal annealing [20]. At the same time, laser irradiation could be considered as an effective method for controllable phase transition in Sn x S y since it provides rapid increases of temperature in materials.…”