The crossover of preferred orientation in heteroepitaxial ZnO/MgO(0 0 1) films

Orientational crossover in heteroepitaxial ZnO/MgO(0 0 1) thin films deposited by radio frequency sputtering was examined by X-ray diffraction and scanning electron microscopy. At the initial stage of growth, highly strained c-domains with the (0 0 0 2) planes aligned to the surface normal were observed. As the film thickness was increased, the growth behavior then changed gradually to strain free a-domains with (1 0 1̄ 0) planes. Both the a- and c-domains showed a ±15° rotated in-plane domain configuration with respect to the MgO substrate. Nano-scale ZnO islands were formed on the sufficiently thick ZnO thin film at the later stages of growth. The orientational crossover might have been caused by interplay between the strain and surface energy.