Compared optical properties of ZnO heteroepitaxial, homoepitaxial 2D layers and nanowires

The spectroscopic properties of ZnO epilayers grown by molecular beam epitaxy are investigated. Three samples are compared: a homoepitaxial layer grown on a Crystec substrate, a sample directly grown on c-sapphire and a layer grown on c-sappire using an MgO buffer. In the latter case, in spite of the high dislocation density, temperature-dependent photoluminescence measurements show only a small decrease of the luminescence intensity between 4 and 300 K, with an activation energy of 108 meV. Cathodoluminescence presents an inhomogeneous emission on the micrometric scale: a stronger emission is measured in small nanometric areas. A tentative explanation of this behavior is proposed. The optical properties of these three samples are also compared to those of two-dimensional homoepitaxial layers and ZnO nanowires grown on sapphire and p-type GaN by metal organic vapor-phase epitaxy.