Epitaxial growth of non-polar ZnO films on MgO substrate

The pulsed laser deposition of pure and doped (Nd or Fe) ZnO films on (001) oriented MgO substrates was investigated. Polar (alternate Zn and O planes along the main axis growth), or non-polar ZnO films were obtained according to the growth conditions, and epitaxial relationships were observed in both cases. For the nonpolar films, different in-plane orientations were observed, and the domain matching epitaxy was used in order to describe the in-plane relationships of the film with respect to the MgO substrate. These relationships correspond to different angles between the in-plane [001](ZnO) and [100](MgO) directions and are different from those previously proposed for the epitaxy of non-polar ZnO on MgO. In addition to the lattice mismatch between film and substrate, an important parameter governing the in-plane orientation is the minimum size of the epitaxial domain which mainly determines the in-plane alignment. Indeed, for doped-ZnO films, the coherence length, i.e. the spatial extension of the crystallites is limited by the presence of the dopants. This leads to different epitaxial relationships with respect to undoped films, and which are related to the epitaxial domain size.