  High-resolution soft X-ray emission and absorption spectra near the N K-edge
of wurtzite GaN are presented. The experimental data are interpreted in terms
of band structure based full-potential electronic structure calculations. The
absorption spectra, compared with calculations including core hole screening,
indicate partial core hole screening in the absorption process. The resonant
emission spectra demonstrate pronounced dispersions of the spectral structures,
identifying effects of momentum conservation due to resonant inelastic X-ray
scattering (RIXS) with anisotropic electronic structure of GaN. In view of a
wide range of optoelectronic applications of GaN, our findings on the momentum
selectivity in RIXS can be utilized in development of GaN based nanoelectronics
devices by controlling direct vs indirect band gap character of GaN
nanostructures.
