The defect characteristics of ZnO film grown on (0 0 0 1) sapphire substrate using an ultrathin Ga wetting layer are investigated by transmission electron microscopy and X-ray diffractometry compared to that of the ZnO film without Ga. It was found that the defects of the ZnO film with a Ga layer were prominently reduced and a high-quality film was formed. Within this ZnO film, most defects near the interface are mixed-type dislocations that interact strongly, leading to a remarkable reduction of dislocations in the upper part of the epitaxial film with a total dislocation density of as low as 8 x 10(8) cm(-2). Almost no pure screw dislocations were observed. Furthermore, the film exhibits a single domain structure, and no inversed domains were found. The role of the ultrathin gallium layer in the defect reduction and inversion domain suppression is discussed. (C) 2004 Elsevier B.V. All rights reserved.