GaAs:N vs GaAs:B alloys: Symmetry-induced effects

Electronic structure of GaAs with a 3% addition of B and N is studied by ab initio molecular dynamics. We find that the influence of B on the band gap and on its pressure dependence is weaker than that of N. Hydrostatic pressure strongly reduces the oscillator strength of the fundamental transition in GaAs:B, but not in GaAs:N. Group-theoretical analysis shows that the differences are symmetry induced, and stem from the different substitution sites of B (cation sublattice) and N (anion sublattice). For both alloys we find an additional optical transition at about 0.5 eV above the band gap involving a state derived from L1c; its features agree with these of the E₊ transition in GaAs:N.