Local epitaxial growth of CuO films on MgO.

Abstract

We have investigated the structure of molecular-beam-epitaxy-grown CuO thin films on MgO substrates using reflection high-energy electron diffraction, x-ray diffraction, and high-resolution electron microscopy (HREM). The results show that CuO(111) planes grow parallel to (001)MgO planes. Three main in-plane epitaxial relations are observed: [1\ifmmode\bar\else\textasciimacron\fi{}10]CuO\ensuremath{\parallel}[110]MgO, [01\ifmmode\bar\else\textasciimacron\fi{}1]CuO\ensuremath{\parallel}[110]MgO, and [101\ifmmode\bar\else\textasciimacron\fi{}]CuO\ensuremath{\parallel}[100]MgO. Close to the interface, strains related to the matching conditions imposed by the substrate affect both symmetry and lattice constants of the monoclinic CuO structure. Cross-sectional HREM shows that the exact epitaxial orientation is only partially preserved as the film thickness increases. The loss of in-plane epitaxy is affected by a poor matching of lattice spacings and by misorientations between planes of both crystals, which join at the interface. We investigate the relative orientations between both crystals on the basis of a mapping technique developed for the epitaxy of the axial-commensurate system. The results show that all the experimentally observed epitaxial orientations are successfully predicted by this technique.