Quality assessment of GaN epitaxial films: Acidification scenarios based on XPS-and-DFT combined study

Abstract X-ray photoelectron spectroscopy (XPS) of electronic structure and surface states of GaN epitaxial films grown on Al2O3 and SiC substrates established the presence of only traces of surface oxidation after three years storage in standard air media conditions. No additional treatment was applied during this storage process. The slight shift (~1 eV) of Valence Band (VB) spectra was found which occurs without any visible contribution of gallium oxide electronic states to the valence band area. Additionally, GaN-typical N 2s band is present in VB-spectra of studied epitaxial films what is confirming their high quality grade. First-principle modelling demonstrates favorability of GaN surface acidification because of humidity and much slower oxidation process in the dry-air conditions. Theoretical models of acidification scenarios also demonstrate that oxidation of defect-free and defective surfaces (N-vacancies) are a good leveler: electronic structures of all considered surfaces became almost identical that is in a perfect agreement with results of XPS measurements and analytics.