Density of states, hybridization, and band-gap evolution in Al_xGa_1-xN alloys

The electronic structure of the wurtzite Al_xGa_1-xN alloy system has been studied for numerous values of Al concentration x ranging from 0 (pure GaN) to 1 (pure AlN). The occupied and unoccupied partial density of states was measured for each alloy using synchrotron radiation excited soft x-ray absorption and emission spectroscopies. High-resolution x-ray emission spectroscopy allowed the motion of the elementally resolved bulk valence-band maximum to be measured as a function of Al concentration. Using this technique we estimate that the value of the band-gap bowing parameter for Al_xGa_1-xN is zero. Furthermore, the x-ray emission spectra revealed resonantlike emission at approximately 19 eV below the GaN valence-band maximum. By measuring the intensity of this feature as a function of Ga content we prove conclusively that this emission arises from hybridization of N 2p and Ga 3d states. Finally, we find that the N K- and Al K-absorption spectra depend strongly on the photon angle of incidence with respect to the surface normal. We explain this in terms of orbital anisotropy in Al_xGa_1-xN.