Growth evolution of surface nanowires and large anisotropy of conductivity on MgZnO/ZnO quantum wells based on M-nonpolar (10−10) ZnO

Self-organized surface nanowires were formed on M-nonpolar (10−10) ZnO homoepitaxial layer surfaces. High-resolution transmittance electron microscopy showed that the surface nanowires possessed a flat plateau at the top and a side facet with constant step spacing at the atomic scale. In and ex situ observations of layer growth revealed that anisotropic islands appeared after growing two-dimensional surfaces and that these gradually changed to nanowire structures with increasing layer thickness. Their growth origin, identified from theoretical parameter fittings based on a step-edge barrier model, was close to that of O-polar ZnO growth. A large difference in electron transport was clearly observed in Mg0.12Zn0.88O/ZnO quantum wells (QWs). Transport anisotropy correlated strongly with the surface morphology and with the structural quality of QWs, as confirmed by polarized photoluminescent spectroscopy. Finally, as a possible mechanism of anisotropic electron transport, we suggest that the electron motion perpendicular to the nanowire arrays was strongly restricted due to a potential barrier caused by an interface roughness of the surface nanowires.