Low-temperature surface photovoltage in p- type GaN

The surface photovoltage (SPV) behavior for Mg-doped, p-type GaN was studied using a Kelvin probe at temperatures from 80 to 300 K. Under band-to-band UV illumination at room temperature, the measured SPV signal for p-type GaN becomes negative as electrons are swept to the surface. However at low temperatures, the SPV signal becomes positive under UV illumination, contrary to the SPV behavior of p-type GaN at room temperature. This positive SPV resembles the behavior of an n-type semiconductor. It is assumed that at low temperatures and under UV illumination, the concentration of photogenerated electrons exceeds the concentration of free holes, leading to n-type conductivity. The positive SPV signal is caused by the alignment of a quasi-Fermi level for electrons with the Fermi level of the probe, since the band bending and its change under illumination are negligible at 80 K. Interestingly, the characteristic temperature at which this transition from p- to n-type SPV behavior occurs is dependent on illumination intensity. This effect is explained with a simple phenomenological model.