From pseudo-direct hexagonal germanium to direct silicon-germanium alloys

TL;DR

This study uses ab initio calculations to explore the electronic and optical properties of hexagonal SiGe alloys, revealing their potential as efficient optical emitters for integrated optoelectronics.

Contribution

It demonstrates that alloying hexagonal Ge with Si can convert it from a pseudo-direct to a direct band gap material, enhancing optical transition strength.

Findings

Hexagonal Si has an indirect band gap.

Hexagonal Ge has a pseudo-direct gap with weak optical transitions.

Alloying Ge with Si makes the band gap direct below 45% Si content.

Abstract

We present ab initio calculations of the electronic and optical properties of hexagonal SiGe alloys in the lonsdaleite structure. Lattice constants and electronic band structures in excellent agreement with experiment are obtained using density-functional theory. Hexagonal Si has an indirect band gap, while hexagonal Ge has a pseudo-direct gap, i.e. the optical transitions at the minimum direct band gap are very weak. The pseudo-direct character of pure hexagonal Ge is efficiently lifted by alloying. Already for a small admixture of Si, symmetry reduction enhances the oscillator strength of the lowest direct optical transitions. The band gap is direct for a Si content below 45 %. We validate lonsdaleite group-IV alloys to be efficient optical emitters, suitable for integrated optoelectronic applications.