Band gap bowing and optical polarization switching in Al_1-xGa_xN alloys

We present a detailed theoretical study of the band gap bowing of wurtzite AlGaN alloys over the full composition range. Our theoretical framework is based on an atomistic tight-binding model, including local strain and built-in potential variations due to random alloy fluctuations. We extract a bowing parameter for the band gap of b=0.94 eV, which is in good agreement with experimental data. Our analysis shows that the bowing of the band gap mainly arises from bowing of the conduction band edge; for the composition dependence of the valence band edge energy we find a close to linear behavior. Finally, we investigate the wave function character of the valence band edge as a function of GaN content x. Our analysis reveals an optical polarization switching around x=0.75, which is in the range of reported experimental data.