Electronic structure of BAs and boride III-V alloys

Boron arsenide, the typically ignored member of the Group-m-V arsenide series BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma conduction-band minimum is p-like (Gamma (15)), not a-like (Gamma (1c)), it has an X-1c-like indirect band gap, and its bond charge is distributed almost equally an the two atoms in the unit cell, exhibiting nearly perfect covalency. The reasons for these are trucked down to the anomalously low atomic p orbital energy in the boron and to the unusually strong s-s repulsion in BAs relative to most other Group-III-V compounds. We find unexpected valence-band offsets of BAs with respect to GaAs and AlAs. The valence-band maximum (VBM) of BAs is significantly higher than that of AlAs, despite the much smaller bond length of BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects result from the unusually strong mixing of the cation and anion states at the VBM. For the BAs-GaAs alloys, we find (i) a relatively small (similar to3.5 eV) and composition-independent band-gap bowing. This means that while addition of small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of boron to GaAs raises the gap; (ii) boron "semilocalized" states in the conduction band (similar to those in GaN-GaAs alloys); and (iii) bulk mixing enthalpies that are smaller than in GaN-GaAs alloys. The unique features of boride Group-III-V alloys offer new opportunities in band-gap engineering.