  Anion substitution with bismuth (Bi) in III-V semiconductors is an effective
method for experimental engineering of the band gap Eg at low Bi
concentrations, in particular in gallium arsenide (GaAs). The inverse
Bi-concentration dependence of Eg has been found to be linear at low
concentrations x and dominated by a valence band-defect level anticrossing
between As and Bi occupied p levels. This dependence breaks down at high
concentrations where empirical models accounting only for the As-Bi interaction
are not applicable. Predictive models for the valence band hybridization
require a first-principle understanding which can be obtained by density
functional theory with the main challenges being the proper description of Eg
and the spin-orbit coupling. By using an efficient method to include these
effects, it is shown here that at high concentrations Eg is modified mainly by
a Bi-Bi p orbital interaction and by the large Bi atom-induced strain. This
points to the role of different atomic configurations obtained by varying the
experimental growth conditions in engineering arsenide band gaps, in particular
for telecommunication laser technology.
