  The electronic structure and the anisotropy of the Al - N {\pi} and {\sigma}
chemical bonding of wurtzite AlN has been investigated by bulk-sensitive total
fluorescence yield absorption and soft x-ray emission spectroscopies. The
measured N K, Al L1, and Al L2,3 x-ray emission and N 1s x-ray absorption
spectra are compared with calculated spectra using first principles
density-functional theory including dipole transition matrix elements. The main
N 2p - Al 3p hybridization regions are identified at -1.0 to -1.8 eV and -5.0
to -5.5 eV below the top of the valence band. In addition, N 2s - Al 3p and N
2s - Al 3s hybridization regions are found at the bottom of the valence band
around -13.5 eV and -15 eV, respectively. A strongly modified spectral shape of
Al 3s states in the Al L2,3 emission from AlN in comparison to Al metal is
found, which is also reflected in the N 2p - Al 3p hybridization observed in
the Al L1 emission. The differences between the electronic structure and
chemical bonding of AlN and Al metal are discussed in relation to the position
of the hybridization regions and the valence band edge influencing the
magnitude of the large band gap.
