Giant Optical Oscillator Strengths in Perturbed Hexagonal Germanium

Abstract

By means of ab initio calculations we demonstrate that perturbed hexagonal germanium is a superior material for active optoelectronic devices in the infrared spectral region. Perfect lonsdaleite germanium is a pseudodirect semiconductor, with a direct fundamental band gap but almost vanishing optical transitions. Perturbing the system by replacing a germanium atom with a silicon atom in the primitive cell increases the oscillator strength at the onset gap by orders of magnitude, with a concurrent blue shift of the transition energies. This effect is mainly due to the increased s character of the lowest conduction band because of the perturbation‐induced wave function mixing. A structural distortion of pure lonsdaleite Ge can enhance as well optical oscillator strengths, but their magnitude significantly depends on the specific details of the resulting atomic geometry. In particular, moderate tensile uniaxial strain can induce an order inversion of the two lowest conduction bands, leading to an extremely efficient enhancement of the dipole matrix elements at the minimum gap. In general, chemical and/or structural perturbations are shown to make lonsdaleite germanium a suitable material for light emitting devices.

Theoretical insights into the amplified optical gain of hexagonal germanium by strain engineering

“… 14 Likewise, Belabbes et al evaluated the interband transition energies for uniaxially strained lonsdaleite germanium and Ge–Si alloy. 26 However, a thorough understanding of the structure–property relationship of uniaxially strained hexagonal germanium is essential to effectively comprehend the potential characteristics of light-emitting materials.…”

Stimulated Emission from Hexagonal Silicon-Germanium Nanowires

“…infrared light emission applications 14,17 . A priori, it is not obvious that hex-SiGe can serve as a gain medium for laser amplification since, according to the present status of our theoretical understanding 15,18,19 , a folded L-band remains dipole forbidden unless the translational symmetry is broken by e.g. alloying.…”