Guided Growth of Horizontal GaN Nanowires on Spinel with Orientation-Controlled Morphologies

We report the guided growth of horizontal GaN nanowires (NWs) on spinel (MgAl2O4) substrates with three different orientations: (111), (100), and (110). The NWs form ordered arrays with distinct morphologies on the surface of the substrates, controlled by the interaction with the substrate. The geometry of the NWs matches the symmetry of the spinel surfaces: on MgAl2O4(111), (100), and (110) the NWs grow in six, four, and two directions, respectively. The epitaxial relations and morphologies of the NW?substrate interface were characterized by cross-sectional transmission electron microscopy. The substrate was found to be mobilized during the growth and either climb up or recede on/under one or two sides of the NW, depending on the substrate orientation. Possible reasons for the similarity and differences between the orientations of the NWs and thin GaN films grown on MgAl2O4 are proposed. These results demonstrate the generality and flexibility of the guided growth phenomenon in NWs and specifically show that MgAl2O4(111) could be a low-mismatch substrate for the growth of high-quality GaN layers and NWs.