Band bending and photoemission-induced surface photovoltages on clean n- and p-GaN „0001... surfaces

Photoelectron spectroscopy ~PES! has been used to measure the dependence of core-level shifts on temperature T and source intensity for the clean ~0001! surfaces of pand n-GaN grown by metalorganic chemical vapor deposition. In the dark, the Fermi level at the surface occurs 2.55 eV above the valence band maximum for both carrier types. The surface photovoltage ~SPV! induced by laboratory PES sources exceeds 1 eV on p-GaN at room temperature ~RT!. Hence PES at RT may prove impractical for determining Schottky barrier heights for optically thin metal films on p-GaN. The source-induced SPV falls rapidly with increasing T, persisting only to ;150 °C with a standard ultraviolet PES source. This strong T dependence cannot be explained quantitatively by standard SPV models. Band bending is relatively immune to pyroelectric and piezoelectric polarization but is sensitive to chemisorbed oxygen; thermal conversion of the chemisorbed layer to an ‘‘oxide’’ reduces the effect of the contaminant.