Effects of KOH etching on the properties of Ga-polar n-GaN surfaces

The effects of a KOH treatment on the properties of n-type GaN surfaces and associated Au/n-GaN contacts have been investigated by X-ray photoelectron spectroscopy, atomic force microscopy, reflection high-energy-electron diffraction, current–voltage and electron-beam-induced current characterization. Ga-polar surfaces grown by molecular beam epitaxy and metal–organic chemical vapour deposition were compared. A decrease in electron barrier height and an increase in non-radiative recombination properties of Au/n-GaN contacts were found with KOH treatment, correlated with an increase of surface Ga vacancies, an increase in surface N–H2 content and a decrease in surface C contamination. A 0.3-eV shift in the Ga3d peak position towards the valence band and a reduction in the dislocation contrast were observed for the case of molecular-beam-epitaxy-grown GaN only, demonstrating that surface Ga vacancies and threading dislocations play only a limited role in defining the resultant metal/GaN contact properties. Accordingly, the surface atomic content and the resulting surface states, following KOH treatment, should be taken into consideration when appraising the electrical properties of n-GaN surfaces and the performance of associated metallic contacts.