Tight-binding analysis of the electronic structure of dilute bismide alloys of GaP and GaAs

TL;DR

This study develops a tight-binding model to analyze how dilute bismuth incorporation affects the electronic structure of GaP and GaAs alloys, explaining experimental observations and the impact of Bi clustering.

Contribution

We introduce a detailed atomistic tight-binding model that captures Bi-related defect states and their interaction with host bands in dilute bismide alloys, explaining band gap and spin-orbit splitting variations.

Findings

Bi incorporation introduces defect states in the band gap.

Band anti-crossing explains the variation in band gap and spin-orbit splitting.

Model accurately reproduces experimental crossover at 10.5% Bi.

Abstract

We develop an atomistic, nearest-neighbor sp3s* tight-binding Hamiltonian to investigate the electronic structure of dilute bismide alloys of GaP and GaAs. Using this model we calculate that the incorporation of dilute concentrations of Bi in GaP introduces Bi-related defect states in the band gap, which interact with the host matrix valence band edge via a Bi composition dependent band anti-crossing (BAC) interaction. By extending this analysis to GaBiAs we demonstrate that the observed strong variation of the band gap Eg and spin-orbit-splitting (SO) energy with Bi composition can be well explained in terms of a BAC interaction between the extended states of the GaAs valence band edge and highly localized Bi-related defect states lying in the valence band, with the change in Eg also having a significant contribution from a conventional alloy reduction in the conduction band edge energy. Our calculated values of Eg and SO are in good agreement with experiment throughout the investigated composition range x less than 13%. In particular, our calculations reproduce the experimentally observed crossover to an Eg < SO regime at approximately 10.5% Bi composition in bulk GaBiAs. Recent x-ray spectroscopy measurements have indicated the presence of Bi pairs and clusters even for Bi compositions as low as 2%. We include a systematic study of different Bi nearest-neighbor environments in the alloy to achieve a quantitative understanding of the effect of Bi pairing and clustering on the GaBiAs electronic structure.