Surface modification of sapphire (0001) by Ga can eliminate multiple rotation domains in ZnO films. The existence of Ga at ZnO/sapphire interface was confirmed by x-ray energy dispersive spectroscopy in a transmission electron microscope. Atomic detail of mismatch dislocations at interface was imaged by high resolution transmission electron microscopy. Inside the ZnO film, there is a high density of stacking fault. Both pure gliding of ZnO (0001) plane and condensation of vacancies or interstatials are possible mechanisms to generate the stacking fault.