Evolution of wurtzite ZnO films on Cubic MgO (001) substrates: a structural, optical, and electronic investigation of the misfit structures.

We investigated the interface between hexagonal ZnO films and cubic MgO (001) substrates, fabricated via molecular beam epitaxy. X-ray diffraction and (scanning) transmission electron microscopy revealed that growth follows the single [0001] direction when the temperature of the substrate is above 200 °C, while when the substrate temperature is below 150 °C, growth initially is along [0001] and then mainly changes to [0-332] variants beyond a thickness of ∼10 nm. Interestingly, a double-domain feature with a rotational angle of 30° appears during growth along [0001] regardless of the temperature, experimentally demonstrating the theoretical predictions for the occurrence of double rotational domains in such a heteroepitaxy [Grundmann et al., Phys. Rev. Lett. 105, 146102 (2010)]. We also found that the optical properties of the ZnO film are influenced greatly by the mutation of growth directions, stimulated by the bond-length modulations, as we determined from X-ray absorption spectra at Zn K edge. These results also showed the evolution of the 4p(xy) and 4p(z) states in the conduction band with the rise in the temperature for growth. We consider that our findings may well promote the applications of ZnO in advanced optoelectronics for which its integration with other materials of different phases is desirable.