Misfit reduction by a spinel layer formed during the epitaxial growth of ZnO on sapphire using a MgO buffer layer

The quality of the ZnO films, epitaxially grown on sapphire, can be substantially improved when a MgO buffer layer is used. The role and the structural characteristics of this buffer layer were studied by transmission electron microscopy. It was found that during the MgO deposition and the subsequent ZnO deposition Mg and Al interdiffusion occurs between the MgO buffer layer and the Al2O3 substrate forming an intermediate spinel MgAl2O4 layer at the deposition temperature of 700 °C. This layer has the following epitaxial relation with the substrate, [11¯00]sapphire//[11¯0]spinel, and [112¯0]sapphire//[112¯]spinel, also [0 0 0 1]sapphire//[1 1 1]spinel. The misfit between the two lattices is about 2.6% considering that every second (303¯0) sapphire plane corresponds to a (2 2 0) spinel plane. Thus, the original misfit between ZnO and sapphire is progressively decreased by the spinel layer. The formation of the spinel intermediate layer during the early stage of growth was also studied during the deposition of the MgO buffer layer. The stability of the spinel layer was tested by annealing at 1000 °C.