Impurity Band in the Interfacial Region of GaN Films Grown by Hydride Vapor Phase

Using several deriatives of scanning force microscopy with conducting tips, we find direct evidence for impurity band conduction in GaN films near the sapphire substrate interface. Scanning current-voltage and capacitance microscopy measurements both show that the free electron density is much higher in the interfacial region. However, surface contact potential images reveal that the Fermi level in the interfacial region is 50 to 100meV deeper into the bandgap that it is in the less conducting bulk film. These results are inconsistent with a high density of electrons in the intrinsic conduction band. Rather, they point to the existene of a partially filled donor impurity band and compensating acceptors, as a result of a high concentration of oxygen and the defective microstructure at the GaN/sapphire interface.