An exponential dependence of the photoconductivity on the surface photovoltage at GaN layers is predicted theoretically and confirmed experimentally. The prediction is based on the assumption that the material is mainly an ordered polycrystal, consisting of columnar grains. Accordingly, transport is expected to be limited by potential barriers at the grain boundaries, arising from the charge trapped at grain-boundary defects. The observed exponential dependence provides evidence that strongly supports the model by establishing a direct link between the bulk conductivity and the surface potential barrier. The same model is shown to successfully explain several other defect-related findings as well.

• Received 23 April 1999

DOI:https://doi.org/10.1103/PhysRevB.61.15573