Kelvin Probe Measurements of p-type GaN

We have studied the surface photovoltage (SPV) for band-to-band illumination on a variety of p-type (Mg-doped) GaN samples. In particular, differences in the steady-state and transient SPV have been investigated in air and vacuum for samples grown by hydride vapor phase epitaxy (HVPE) or metal-organic chemical vapor deposition (MOCVD). The SPV spectra for both samples behave in a similar manner, but larger SPVs are generated for HVPE vs. MOCVD samples under identical illumination conditions. Interestingly, we have found that p-type GaN can be sensitive to the illumination geometry, where illumination of the electrical contacts results in an anomalous "offset" of the SPV signal Regardless of illumination geometry, such offsets always appear in the case of MOCVD samples, whereas they do not appear for HVPE samples when the contacts are not illuminated. Since we have never observed such behavior for n-type samples, it appears that the stability of p-type samples under illumination may be an issue.