Effect of UV exposure on the surface charge behavior for GaN

Abstract

The surface charge behavior on n-type GaN was investigated as a function of UV exposure in ambient. It was first noticed that the surface photovoltage (SPV), or change in surface contact potential under UV illumination, could slowly decrease during illumination. The SPV for as-grown samples is ~0.6 eV and can drop by up to 0.3 eV over 1 h of UV exposure in air ambient. We believe that this slow decrease is due to the photo-induced chemisorption of oxygen species. Interestingly, samples exhibit a smaller drop in SPV during continuous UV illumination after many hours of accumulated UV exposure. This can be explained by the UV-induced growth of a thicker surface oxide layer which inhibits electron transfer from the bulk to the surface. The original SPV behavior can be restored by chemical etching to remove the additional surface oxide. We have also investigated the surface behavior by locally charging the surface before and after UV-induced oxide growth. Measurements of the surface contact potential using scanning Kelvin probe microscopy (SKPM) demonstrate that negatively charged regions initially spread laterally on the UV-exposed surface, but not on the clean surface. The UV-induced oxide therefore appears to be relatively conductive and enhance the lateral motion of injected electrons. The discharge of both the negatively and positively charged regions under dark conditions is similar for both the initial and UV-exposed surfaces and shows a logarithmic time dependence.