Role of the surface in the electrical and optical properties of GaN

We have studied the effect of ambient environment and temperature on the electrical and optical properties of n-type GaN in a setup combining a Kelvin probe with an optical cryostat. Specifically, the value of band bending at the surface and its change due to illumination (photovoltage) were studied. Complementary information about the depletion region was also obtained from photoluminescence (PL) data. We measured the surface photovoltage (SPV) signal at photon energies between 1.2 and 4.0 eV and found a maximum SPV value of ∼0.6 eV for band-to-band photon energies, indicating that the initial (dark) upward band bending decreased by at least this amount under ultraviolet illumination with an intensity of about 0.03 W/cm2. Illumination was performed with intensities ranging from 10-9 to 3x10-2 W/cm2, where the SPV signal increased as a logarithm of light intensity. In air ambient the SPV signal for high-intensity, band-to-band illumination increased quickly to a maximum and then gradually decreased during illumination. This decrease is explained by the photo-induced adsorption of negatively charged oxygen species in air.