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

We use a nearest-neighbour sp³s* tight-binding Hamiltonian to investigate the electronic structure of GaAs-based dilute bismide and bismide-nitride alloys. We show that the observed strong variation of the band gap (E_g) and spin-orbit splitting energy (Δ_SO) with Bi composition in GaBi_xAs_1-x is due primarily to a band-anticrossing interaction between the extended states of the host matrix valence band maximum and highly localised Bi-related defect states lying in the valence band, with the change in E_g also having a significant contribution from a conventional alloy reduction in the conduction band edge energy. We calculate a crossover to an E_g <; Δ_SO regime at approximately 10.5% Bi composition in bulk GaBi_xAs_1-x, in agreement with recent experimental studies of GaBi_xAs_1-x epilayers grown on GaAs. Finally, we present calculations which show that the effects of N and of Bi are largely independent of each other in random GaBi_xN_yAs_1-x-y alloys, of relevance for future high efficiency photonic devices.