Bistable Fermi level pinning and surface photovoltage in GaN

Band bending and Fermi level pinning at GaN(0001) surfaces have proved controversial despite their fundamental importance. By combining Kelvin probe, surface photovoltage and X-ray photoemission measurements, we clarify how the Fermi level pinning affects the band bending for n- and p-type GaN(0001). The presence of two different mid-gap pinning levels is confirmed on native oxide-covered samples, and they lie at remarkably similar energies to those previously found for atomically clean GaN(0001). The population of these bistable pinning levels and their influence on the band bending both depend on the bulk doping and level of illumination by above-gap photons.