Observation of thermally activated conduction at a GaN–sapphire interface

Temperature-dependent differential Hall measurements have been performed on an undoped GaN epitaxial layer grown by metalorganic chemical vapor deposition on a sapphire substrate. The resultant depth profile shows that, for this sample, the epitaxial material is mostly insulating, and that the observed thermally activated conduction arises from an interface region ⩽0.65 μm thick. Comparison with cross-sectional transmission electron microscope micrographs suggests that this conducting region is correlated to the highly defective three-dimensional growth region, while the two-dimensional growth mode beyond this thickness corresponds to the insulating portion with a lower dislocation density. Such behavior is consistent with the presence of an impurity band in a heavily doped interface region formed by oxygen outdiffusion from the substrate.