Theory of electronic structure of BGaAs and related alloys

Previous experiments on B_xGa_1-xAs containing a few percent boron show a dramatic increase in electron effective mass, m*_e , similar to that observed in many GaN_xAs _1-x samples. By contrast, there is a near-linear blue-shift of the energy gap, which can be conventionally described using the virtual crystal approximation. We use a tight-binding model to show that isolated B atoms have little effect either on the band gap or lowest conduction band dispersion in B_xGa_1-xAs. By contrast, B pairs and clusters introduce defect levels close to the conduction band edge (CBE) which, through a weak band-anticrossing (BAC) interaction, significantly reduce the band dispersion in and around the Γ-point, thus accounting for the strong increase in m*_e and reduction in mobility observed in these alloys. Calculations show that replacing gallium by aluminium shifts the CBE upwards, leading to a large density of B-related states in the energy gap. By contrast, indium shifts the band edge downwards, leading eventually to a band edge m*_e close to that predicted by the virtual crystal approximation.