Giant and Composition-Dependent Optical Bowing Coefficient in GaAsN Alloys

Using first-principles supercell calculations we find a giant (7--16 eV) and composition-dependent optical bowing coefficient in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ alloys. We show that both effects are due to the formation in the alloy of spatially separated and sharply localized band edge states. Our analysis suggests that in semiconductor alloys band gap variation as a function of $x$ can be divided into two regions: (i) a bandlike region where the bowing coefficient is relatively small and nearly constant, and (ii) an impuritylike region where the bowing coefficient is relatively larger and composition dependent. For ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ the impuritylike behavior persists even for concentrated alloys.