Electronic structure of GaN measured using soft-x-ray emission and absorption

The electronic structure of thin-film wurtzite GaN has been studied using a combination of soft-x-ray absorption and emission spectroscopies. We have measured the elementally and orbitally resolved GaN valence and conduction bands by recording Ga $L$ and N $K$ spectra. We compare the x-ray spectra to the partial density of states from a recent ab initio calculation and find good overall agreement. The x-ray emission spectra confirm that the top of the valence band is dominated by N $2p$ states whereas they reveal that there is only weak hybridization between Ga $4s$ and N $2p$ states. Surprisingly, we found a weak feature in the N $K$ emission at approximately 19.5 eV below the valence-band maximum that arises from hybridization between N $2p$ and Ga $3d$ states. X-ray absorption spectra show that the bottom of the conduction band is a mixture of Ga $4s$ and N $2p$ states, again in very good agreement with the theory.