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

We use a nearest-neighbour sp
³
s* 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
_x
As
_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
_x
As
_1-x
, in agreement with recent experimental studies of GaBi
_x
As
_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
_x
N
_y
As
_1-x-y
alloys, of relevance for future high efficiency photonic devices.