Epitaxial growth of (1 1 (2)over-bar 0) ZnO on (0 1 (1)over-bar 2) Al2O3 by metalorganic chemical vapor deposition

There has been increased interest in high quality ZnO films for use in a diverse range of applications such as in high frequency surface acoustic wave filters, buffer layers for GaN growth, transparent and conductive electrodes, and solid state lasers. In the present paper, ZnO films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range 350-450 degrees C. X-ray diffraction and electron microscopy results indicate that the ZnO films are epitaxially grown on (0 1 (1) over bar 2) Al2O3 surface with the (1 1 (2) over bar 0) plane parallel to the surface. Cross-sectional resolution-transmission electron microscopy imaging of the as-grown film shows that the interface is semi-coherent and atomically sharp, with misfit dislocations relieving the misfit strain between ZnO and sapphire. In order to check the thermal stability of the as-grown ZnO films, annealing in an O-2 + N-2 ambience at 850 degrees C for 30 min was performed. The annealed films showed improved crystallinity. At the same time, limited reaction between ZnO and sapphire occurred, resulting in the formation of a 15-20 nm thick spinel layer at the interface.