Using several derivatives of scanning force microscopy with conducting tips, we find direct evidence for the existence of a highly compensated donor impurity band 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 of these films. However, surface contact potential images reveal that the Fermi level in the interfacial region is 50 meV to 100 meV deeper into the bandgap than 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 existence of a partially filled donor impurity band with the Fermi level in the impurity band. We show that this anomalous conduction behavior most likely originates from a high concentration of oxygen and the defective microstructure at the GaN/sapphire interface.