Giant optical oscillator strengths in perturbed hexagonal germanium

TL;DR

This study demonstrates that perturbations in hexagonal germanium significantly enhance its optical oscillator strengths, making it a promising material for infrared optoelectronic devices.

Contribution

The paper reveals that chemical and structural modifications, including atomic substitutions and strain, dramatically increase optical oscillator strengths in hexagonal germanium.

Findings

Replacing Ge with Si boosts oscillator strength by orders of magnitude.

Structural modifications and tensile strain can invert conduction band order, enhancing optical transitions.

Perturbations make hexagonal Ge suitable for light-emitting applications.

Abstract

We present ab initio calculations of electronic and optical properties of perturbed hexagonal germanium and demonstrate that it is a superior material for active optoelectronic devices in the infrared spectral region. It is known that perfect lonsdaleite Ge is a pseudodirect semiconductor, i.e., with direct fundamental band gap but almost vanishing oscillator strength for the lowest-energy optical transitions. Perturbing the system by replacing a Ge atom in the unit cell with a Si atom boosts of the oscillator strength at the minimum direct gap by orders of magnitude, with a concurrent blue shift of the interband distances. This effect is mainly due to the increased s character of the lowest conduction band because of the perturbation-induced wave function mixing. A purely structural modification of the lonsdaleite unit cell of hexagonal Ge yields as well increased optical oscillator strengths, but their magnitude significantly depends on the actual details of the atomic geometry. In particular, moderate tensile uniaxial strain can induce an inversion of the order of the two lowest conduction bands, immediately leading to an extremely efficient enhancement of optical transitions. In general, chemical and/or structural perturbations of the lonsdaleite lattice are shown to be the key to make hexagonal germanium suitable for light emitting devices.