Stabilization of the cubic π-phase of SnS by calcium substitution

Abstract

The cubic phase of tin monosulphide, π-SnS, is of significant interest due to its attractive properties, such as a wider band gap suitable for solar photovoltaic application and being easier to epitaxially deposit onto technologically relevant semiconductors compared to the thermodynamically stable orthorhombic phase of α-SnS. Recently, we reported cation-assisted phase control for obtaining π-SnS rather than α-SnS using Pb²⁺ cations with a concentration of ∼20 cation percent (cat%). However, replacing Pb²⁺ with alternative non-toxic, environmentally friendly cations for cubic phase stabilization would be clearly advantageous. We have computationally investigated the energetics and electronic properties of calcium ion impurities in both SnS polymorphs. We found that addition of Ca²⁺ cations enables phase control of SnS grown from solution from α-SnS to π-SnS. Experimentally, we observed compact films of π-SnS after incorporating Ca²⁺ cations. Computational results indicated that ∼11 cat% of Ca²⁺ ions are required for preferred growth of π-SnS over α-SnS. Furthermore, the presence of an intermediate layer of CaS is computationally predicted to significantly contribute to the stabilization of the π-SnS phase, thereby reducing the Ca concentration required, which aligns well with experimental observations. Subsequently, we find that CaS is a promising substrate for epitaxial growth of π-SnS in the (111) orientation. Moreover, the bandgap of π-SnS decreased slightly with increasing concentration of Ca cations in the material. These results can facilitate the bulk scale synthesis of π-SnS material, bringing it closer to practical utility for a range of applications.