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The interface properties between nonpolar ZnO and sapphire have been studied using high resolution transmission electron microscopy. Two nonpolar orientations are investigated: a- and m-orientations corresponding to [1120] and [1010] crystallographic directions. After the definition of the epitaxial relationships and the resulting initial lattice mismatch, we show that nonpolar ZnO can be grown on sapphire with perfectly flat interfaces. Geometrical misfit dislocations are observed at the interface ZnO/sapphire and their density gives the residual strain in the layer. A strong anisotropy in the strain relaxation is found along the two perpendicular in-plane directions. This anisotropy may be explained in terms of initial anisotropic mismatch yielding different relaxation processes. A domain matching epitaxy is observed in m- and a-oriented layers for mismatches larger than 9% while a lattice matching epitaxy, in which the relaxation is driven by nucleation and glide of dislocations, is observed in a-oriented ZnO along the [0001] in-plane direction. In order to explain the observed relaxation the activated slip systems are calculated for both nonpolar orientations as a function of the in-plane stress due to the anisotropic mismatch. There is a major difference from the polar orientations. Low energy prismatic slip systems can be effective for plastic relaxation in the nonpolar orientations because they are no longer parallel to the growth direction, which is the case of c-oriented layers, nor to the applied stress. Our results can be directly extended to other nonpolar wurtzite structures such as III-nitrides. [ABSTRACT FROM AUTHOR]

CdO(110) layers with a self-organized surface structure have been grown on (1010) sapphire (m Plane) substrates by metal-organic vapor phase epitaxy. The epitaxial relationships between layer and substrate have been determined and a crystallographic model that accounts for the CdO in-plane orientation, which results in a reduced lattice mismatch when the CdO[001] direction is perpendicular to the sapphire c axis, has been proposed. Although the measured lattice parameters indicate that the layers are almost fully relaxed, an anisotropic mosaicity is detected with symmetrical rocking curves attaining minimum values when measured along the CdO[110] direction. The layer morphology consists of a regular ridge-and-valley structure which defines, again, a preferential in-plane direction. The grooves run parallel to the CdO[001] axis and exhibit lateral surfaces sloped at 28° with respect to the (110) surface. The influence of growth temperature and VI/II molar ratio on the anisotropic mosaicity and morphology has been analyzed. [ABSTRACT FROM AUTHOR]

We investigate the dependence on Mg content of the lattice parameters and the surface morphology of nonpolar [formula] Zn1-xMgxO (x≤0.55) grown by molecular beam epitaxy. The anisotropy of the lattice parameters gives rise to an unusual in-plane strain state in the ZnO QWs: tensile strain along [1100] and compressive strain along [0001]. For a Zn0.6Mg0.4O barrier, the strain in a ZnO QW reaches -1.3% along [0001] and +0.3% along [1100]. This induces a strong blueshift of the excitonic transitions, in addition to the confinement effects, which we observe in photoluminescence excitation experiments. [ABSTRACT FROM AUTHOR]