Correlation of self-organized surface nanostructures and anisotropic electron transport in nonpolar ZnO (10-10) homoepitaxy

Self-organized surface nanostructures were formed during laser molecular beam epitaxy of nonpolar ZnO (10-10). The growth mechanism of the nanostructures was related to a Schwoebel barrier effect generated during ZnO homoepitaxy. Small islands elongated along the [001] direction were generated on two-dimensional (2D) growing surfaces above the critical thickness. With increasing film thickness, the anisotropic islands developed homogeneous nanostripe arrays with stripe lengths above 5 mu m and connected by some branches. Highly anisotropic surface morphology markedly influenced electron transport of ZnO films and Mg0.12Zn0.88O/ZnO multiquantum wells (MQWs) with conductivity parallel to the nanostripe arrays by more than one order of magnitude larger than that observed perpendicular to the nanostripe arrays. We propose that the origin of anisotropic conductivity of ZnO films resulted mainly from anisotropically distributed scattering centers of electrons related to grain boundaries between nanostripe arrays. Furthermore, the surface nanostructures consisting of nanostripe arrays gave rise to the anisotropic surface roughness at the heterointerface between Mg0.12Zn0.88O and ZnO layers and contributed to the anisotropy of 2D electron transport of the MQWs at low temperatures.