Unveiling interface structure and polarity of wurtzite ZnO film epitaxially grown on a-plane sapphire substrate

Atomic-scale structure properties of the epitaxial growth of the wurtzite ZnO film prepared on an a-plane sapphire (alpha-Al2O3) substrate have been investigated by using aberration-corrected transmission electron microscopy. The crystallographic orientation relationship of (0001)[1120](ZnO)//(1120)[0001](alpha-Al2O3) has been determined between the ZnO film and the alpha-Al2O3 substrate. Two types of oxygen-terminated a-plane alpha-Al2O3 substrate surfaces have been characterized, which leads to the formation of different heterointerface structures and ZnO domains with opposite lattice polarity. The coalescence of opposite polarity domains results in the appearance of inversion domain boundaries (IDBs) on prismatic planes, and kinks occur on basal planes during the propagation of IDBs within the film. Additionally, the structure of stacking mismatch boundaries in the film with threefold coordinated Zn and O atoms has been resolved. We believe that these findings can be helpful to advance the understanding of the complex propagation of planar defects (e.g., IDBs and stacking faults) in wurtzite films and the interface structure and polarity of wurtzite films on the a-plane sapphire substrate.