Quasiparticle excitations in ${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$${\mathrm{N}}_{\mathit{x}}$ and ${\mathrm{AlAs}}_{1\mathrm{-}\mathit{x}}$${\mathrm{N}}_{\mathit{x}}$ ordered alloys

We have performed a quasiparticle study of the fundamental excitations in ordered alloys of III-V semiconductors, ${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{N}}_{\mathit{x}}$ and ${\mathrm{AlAs}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{N}}_{\mathit{x}}$. The experimentally observed anomalous redshift in ${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{N}}_{\mathit{x}}$ of the direct band edge is explained here by effects arising from the 20% lattice mismatch. The fact that the bottom of the conduction band is mainly dominated by nitrogen leads to a large reduction of the fundamental gap at Γ as the volume increases. This reduction continues until the charge density begins to locate on the arsenic site. Based on this simple scenario, we predict that a similar redshift may be observed in the indirect gap ${\mathrm{AlAs}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{N}}_{\mathit{x}}$ alloys. Also an indirect to direct band-gap transition will be observed for intermediate N concentrations. The possibility of closing the band gap is discussed in terms of the quasiparticle results.