Sn2Se3: A conducting crystalline mixed valent phase change memory compound

Abstract

Thin film Sn2Se3 is a promising new phase change memory material with a very high resistance contrast between insulating amorphous and conducting crystalline phases. We identify the structure of the Sn2Se3 crystalline phase using ab initio evolutionary structure search and report its properties. We find a structure based on Sn-Se ribbons with clear disproportionation of Sn into Sn(II) and Sn(IV) sites, similar to Sn2S3. The energy is only 9 meV/atom above the tie line between SnSe and SnSe2. Sn charge disproportionation is only marginally favored in this selenide, in contrast to sulfides. This leads to a semimetallic rather than semiconducting behavior. These findings, marginal stability of the crystalline phase and conducting character and close competition of disproportionated and non-disproportionated structures, are important for understanding the behavior of Sn2Se3 as a phase change memory material, specifically the fast low energy, the low temperature switching, and the high resistance contrast.