Polarization dependent X-ray absorption studies of the chemical bonds anisotropy in wurtzite GaN grown at different conditions

Polarization-dependent X-ray absorption spectroscopy was used to examine the influence of crystal growth techniques and substrates type on the bond lengths and the bond structure of the single crystalline, wurtzite GaN in a form of bulk materials and epitaxial layers. The layers were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on different substrates such as SiC, sapphire and GaN. From the observed X-ray absorption near edge structure (XANES) of the Ga K-edges, it was found that MOCVD introduces a stronger disorder around Ga atoms than MBE. Comparing the Ga and N K-edges of the epilayers and the bulk crystal, we found a prevailing contribution of N-vacancies in the layers and dominance of Ga-vacancies in the bulk crystal. The bonds along the c-axis are less perfect than the bonds in the c-plane for all investigated epilayers. The performed standard extended X-ray absorption fine structure analysis (EXAFS) resulted in a direct estimate of the bond lengths in the c-plane and along the c-axis.