Comparison of surface photovoltage behavior for n-type versus p-type GaN

Abstract

Using a Kelvin probe, the authors have studied changes in surface contact potential during illumination, i.e., surface photovoltage (SPV), for n- and p-type GaN films grown by hydride vapor phase epitaxy. Short ultraviolet (UV) exposures (3 s) generate a positive SPV of about 0.5 eV for n-type and a negative SPV of about −0.6 eV for p-type GaN, which is consistent with the expected surface band bending for these two surface types. The fast component of the SPV is attributed to the accumulation of photogenerated holes (n-type) or electrons (p-type) at the surface, which results in a decrease in band bending. During long UV exposures (1 h), however, slower photoinduced processes can cause the SPV signal to change as a function of the ambient conditions. For both n- and p-type GaN, UV illumination causes the adsorption of negatively charged oxygen species on the surface in air and their subsequent desorption in vacuum. It appears that this adsorbate effect is most prevalent for n-type GaN in vacuum and p-type GaN in air, with significant changes in the SPV of up to 0.3 eV (n-type) and 0.6 eV (p-type). One important difference between the GaN films is the sensitivity of p-type GaN to the illumination geometry. When the sample contacts are exposed to near-bandgap light, unexpected offsets of the SPV signal can result. If such offsets are appropriately taken into account, however, then the SPV behaviors for both n- and p-type GaN can be reasonably modeled by thermionic expressions.