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 less than or equal to0.65 mum 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. (C) 2001 American Institute of Physics.