Mechanism of twin formation in sputter-grown ZnO/Al2O3(0001) heteroepitaxial films

The mechanism of the twin formation and strain evolution of highly mismatched sputter-grown ZnO/Al2O3 (0001) heteroepitaxial films was investigated. Based on the real time synchrotron X-ray scattering measurements, the existence of the transition thickness at which the twin of the 30° rotated domains start to nucleate within the ZnO films was revealed. It was shown that the twin growth above the transition thickness stops as the strain almost fully relaxes, which indicates that the twin in the sputter-grown ZnO is a mechanical twin, not a growth twin. We presented a schematic modeling of the twin formation during the early growth stage.