Nature of Highly Conducting Interfacial Layer in GaN Films

Using several scanning probe techniques to probe electronic properties, we show that the GaN/sapphire interfacial region contains >= ten times higher electron density but with the Fermi level being 50 to 100 meV deeper in the bandgap compared to the less conducting bulk film. This anomalous behavior cannot be explained by transport in the intrinsic conduction band of GaN. Rather, it points to the existence of a partially filled donor impurity band. We relate the presence of this deep impurity band conduction to excess oxygen in the region and the defective microstructure at the GaN/sapphire interface.