Nature of the highly conducting interfacial layer in GaN films

Using several scanning probe techniques to investigate local electronic properties, we show that the GaN/sapphire interfacial region contains ⩾ ten times higher electron density but with the Fermi level being 50–100 meV deeper in the band gap 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 impurity band conduction to excess oxygen in the region and the defective microstructure at the GaN/sapphire interface.