Hydrothermal epitaxial growth of ZnO films on sapphire substrates presenting epitaxial ZnAl2O4 buffer layers

This article describes our investigation of the hydrothermal epitaxial growth of c-plane ZnO films on Al2O3 substrates presenting ZnAl2O4 buffer layers. We obtained (111) ZnAl2O4 epitaxial layers on a-plane Al2O3 substrates readily through solid phase epitaxy. Although the ZnAl2O4 buffer layers grew epitaxially with a (111) out-of-plane orientation and comprised two coexisting equivalent azimuthal variants with relative 180° in-plane rotation, the ZnO epitaxial films grown upon them exhibited a c-plane orientation with unitary in-plane epitaxial orientation of <11¯00>ZnO∥<11¯0>ZnAl2O4 on the two different ZnAl2O4 variants. Taking the coincidence of the site lattices between the (0001) plane of ZnO and the (111) plane of ZnAl2O4 into account, a reduction in lattice misfit was achieved through a 30° rotation between the lattices of the ZnO and the ZnAl2O4. We used X-ray diffraction and transmission electron microscopy to obtain detailed microstructural views of the hydrothermally grown ZnO epitaxial films on the ZnAl2O4 buffer layers.