Preparation of stoichiometric GaN(0001)−1×1: an XPS study

Abstract

We report on the investigation of GaN(0001)−1×1 using synchrotron radiation x-ray excited photoelectron spectra from the core levels Ga 3p and N 1s, as well as from the contaminants O 1s and C 1s. Measurements were done after outgassing (a) and during three surface treatment methods performed in sequence; (b) ammonia (NH3) flux anneals, (c) Ga deposition with sample held at room temperature followed by vacuum anneal, and (d) Ga deposition on a heated sample followed by Ga desorption during NH3 flux anneal. We have found that the initial NH3 flux anneals increased the amount of present N on the surface and enabled the formation of a well-ordered surface structure, according to the low energy electron diffraction (LEED) pattern. After treatment (b) and (d) the core level spectra of Ga 3p are much improved showing clearly distinct features indicative of increased Ga-N bonding. The Ga to N concentration ratio decreases during the surface treatments from 4.0 to 1.1, hence towards stoichiometry. The amounts of C (and O) present on the surface after outgassing corresponded to 1.1 (0.9) monolayers (ML) but reduced to 0.1 (0.1) ML after the final treatment (d). The Fermi level position in the band gap shifts down by 0.55 eV during the surface treatments, indicating a change of states present in the band gap. We have also found strong support that this Ga-polar sample is initially Ga-terminated.