In this study, structural properties of epitaxial Ga-doped Mg0.1Zn0.9O layers grown on ZnO/alpha-Al2O3 templates by plasma-assisted molecular beam epitaxy have been investigated by high-resolution transmission electron microscopy (HRTEM), and high resolution X-ray diffraction (HRXRD). From analysis of the diffraction pattern, the monocrystallinity of the Mg0.1Zn0.9O layer with hexagonal structure is confirmed. The orientation relationship between Mg0.1Zn0.9O and the template is determined as (0001)(Mg0.1Zn0.9O)||(0001)(ZnO)||(0001)(Al2O3) and [2 (1) over bar(1) over bar0](Mg0.1Zn0.9O)||[2 (1) over bar(1) over bar0](ZnO)||[1 (1) over bar 00](Al2O3). The density of dislocations near the top l surface layers measured by plan-view TEM is about 3.610(10) cm(-2), one order of magnitude higher than the value obtained for ZnO layers on alpha-Al2O3 with a MgO buffer. Cross-sectional observation revealed that the majority of threading dislocations are in the [0001] line direction, i.e. they lie along the surface normal and consist of edge, screw, and mixed dislocations. Cross-sectional TEM and X-ray rocking curve experiments reveal that most of dislocations are edge dislocations. The interface of Mg0.1Zn0.9O and ZnO layers and the effect of excess Ga-doping in these layers have been also studied. (C) 2004 Elsevier B.V. All rights reserved.