  The interface between hexagonal ZnO films and cubic MgO (001) substrates,
fabricated through molecular beam epitaxy, are thoroughly investigated. X-ray
diffraction and (scanning) transmission electron microscopy reveal that, at the
substrate temperature above 200 degree C, the growth follows the single [0001]
direction; while at the substrate below 150 degree C, the growth is initially
along [0001] and then mainly changes to [0-332] variants beyond the thickness
of about 10 nm. Interestingly, a double-domain feature with a rotational angle
of 30 degree appears for the growth along [0001] regardless of the growth
temperature, experimentally demonstrated the theoretical predictions for
occurrence of double rotational domains in such a heteroepitaxy [Grundmann et
al, Phys. Rev. Lett. 105, 146102 (2010)]. It is also found that, the optical
transmissivity of the ZnO film is greatly influenced by the mutation of growth
directions, stimulated by the bond-length modulations, as further determined by
X-ray absorption Spectra (XAS) at Zn K edge. The XAS results also show the
evolution of 4pxy and 4pz states in the conduction band as the growth
temperature increases. The results obtained from this work can hopefully
promote the applications of ZnO in advanced optoelectronics for which its
integration with other materials of different phases is desirable.
