We suggested recently [V. Virkkala et al., Phys. Rev. B 88, 035204 (2013)] that the band-gap narrowing in dilute GaAs 1−x N x alloys can be explained to result from the broadening of the localized N states due to the N-N interaction along the zigzag chains in the 110 directions. In that study our tight-binding modeling based on first-principles density-functional calculations took into account the random distribution of N atoms in a natural way. In this work we extend our modeling to GaAs 1−x Bi x alloys. Our results indicate that Bi states mix with host material states. However, the states near the valence-band edge agglomerate along the zigzag chains originating from individual Bi atoms. This leads to Bi-Bi interactions in a random alloy broadening these states in energy and causing the band-gap narrowing.
The energetics of native point defects in GaSb is studied using the density-functional theory within the hybrid functional scheme (HSE06). Our results indicate that the Ga Sb antisite has the lowest formation energy and could thus be the acceptor defect responsible for the p-type conductivity of undoped GaSb. We find also that the Sb Ga antisite has a remarkably low formation energy in Sb-rich growth conditions and it should act as a donor for all Fermi level positions in the band gap. However, we suggest that the structural metastability of the Sb Ga antisite or extrinsic point defects, namely carbon and in particular oxygen, may neutralize its compensating character.
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