  The electronic structure and chemical bonding of wurtzite-GaN investigated by
N 1s soft x-ray absorption spectroscopy and N K, Ga M1, and Ga M2,3 emission
spectroscopy is compared to that of pure Ga. The measurements are interpreted
by calculated spectra using first-principles density-functional theory (DFT)
including dipole transition matrix elements and additional on-site Coulomb
interaction (WC-GGA+U). The Ga 4p - N 2p and Ga 4s - N 2p hybridization and
chemical bond regions are identified at the top of the valence band between
-1.0 and -2.0 and further down between -5.5 and -6.5 eV, respectively. In
addition, N 2s - N 2p - Ga 4s and N 2s - N 2p - Ga 3d hybridization regions
occur at the bottom of the valence band between -13 and -15 eV, and between
-17.0 and -18.0 eV, respectively. A band-like satellite feature is also found
around -10 eV in the Ga M1 and Ga M2,3 emission from GaN, but is absent in pure
Ga and the calculated ground state spectra. The difference between the
identified spectroscopic features of GaN and Ga are discussed in relation to
the various hybridization regions calculated within band-structure methods.
