Electronic structure of lonsdaleite SixGe1−x alloys

Abstract

Conventional diamond-structured silicon (Si) and germanium (Ge) possess indirect fundamental band gaps, limiting their potential for applications in light-emitting devices. However, SixGe1-x alloys grown in the lonsdaleite ("hexagonal diamond") phase have recently emerged as a promising directgap, Si-compatible material system, with experimental measurements demonstrating strong room temperature photoluminescence. When grown in the lonsdaleite phase, Ge possesses a narrow (0:3 eV) "pseudo-direct"fundamental band gap. Lonsdaleite Si is indirect-gap (0:8 eV), creating the possibility to achieve direct-gap lonsdaleite SixGe1-x alloys across a Gerich composition range. We present a first principles analysis of the electronic and optical properties of lonsdaleite SixGe1-x alloys, elucidate the electronic structure evolution and direct-to indirect-gap transition, and describe the impact of alloy band mixing effects on inter-band optical transition strengths.