Luminescence of selective area growth of epitaxial ZnO nanowires and random-growth-oriented nanobelts

Abstract

Epitaxial ZnO nanowires and random-growth-oriented nanobelts were grown on c-plane sapphire with and without a pre-coated ZnO epilayer film. On the pre-coated ZnO epilayer, ZnO nanowires are vertically aligned with good in-plane alignment as a result of homoepitaxy, whereas on the bare c-plane sapphire, besides a few nanowires vertically aligned with [0001]znO parallel to [0001](Al2O3), the nanowires were properly aligned with three-fold rotation symmetry. The ZnO nanowires are well-defined hexagonal crystals with diameters of 70-500 nm and lengths of up to several micrometres. In the junction regions between the pre-coated epilayer and the bare sapphire surface, however, ZnO nanobelts (nanoribbons) were found. Cathodoluminescence measurements revealed that the emission at 3.26 eV is correlated with free-exciton recombination and the broad green emission at 2.48 eV is attributed to surface defects. The stronger green emission implies that more surface defects exist on the side walls of nanowires and nanobelts. In Raman scattering, the E1 (LO) mode is sensitive to the orientation of nanostructure that is consistent with the cathodoluminescence results.