Control of Phase in Tin Sulfide Thin Films Produced via RF-Sputtering of SnS₂ Target with Post-deposition Annealing

Abstract

Tin (II) Monosulfide (SnS) has become an interesting new material for thin film photovoltaics. SnS-based devices have achieved limited success in improved solar cell efficiency. While annealing is a typical post-deposition treatment used to improve thin film quality, sulfur volatility is an issue, despite strong Sn-S bonds in tin sulfide compounds. Annealing of sulfur-rich sputtered tin sulfide thin films in a vacuum environment has not been previously reported. In the present work, we investigated the optoelectronic properties, crystallographic phase, and morphology of annealed, sputtered tin sulfide thin films. Specifically, we studied the phase change and improvement in material quality as a result of post-deposition heat treatments. Tin sulfide thin films were sputtered with and without substrate heating. These samples were then annealed between 300°C and 500°C under moderate vacuum (<1 × 10⁻⁴ Pa) in the deposition chamber to find the optimal annealing process for producing α-SnS. Significantly improved crystallinity and morphology were seen in sulfur-rich thin films annealed at 400–500°C for 60 min. Annealed films had resistivity in the range of 30–300 Ω-cm. Experimental observations were confirmed by calculated phase diagrams, which show that annealing around 400°C at low pressure is optimal to obtain a phase-pure α-SnS film from an amorphous SnS₂ film.