Luminescence and surface properties of MgxZn1−xO thin films grown by pulsed laser deposition

Abstract

We investigated the surface roughness and the luminescence properties of MgxZn1−xO thin films (0⩽x⩽0.19). The thin films were grown on a-plane sapphire substrates by pulsed laser deposition. The root-mean-square surface roughness depends on the oxygen partial pressure p(O2) applied during deposition and takes a minimal value of 0.6nm for p(O2)=1×10−3mbar. Deposition of the thin films on a ZnO buffer layer further diminishes the surface roughness. The photoluminescence maximum peak from the MgxZn1−xO thin films is due to free excitons, which are localized in potential minima at low temperatures, and shows a linear high-energy shift with increasing x. The full width at half maximum (FWHM) of the photoluminescence depends on p(O2). Samples with the smallest FWHM values were grown at p(O2)=1.6×10−2mbar. We analyze the contributions of alloy broadening and of the lateral Mg-concentration inhomogeneity to the photoluminescence FWHM. In optimized samples FWHM is limited by random alloy broadening. Scanning cathodoluminescence shows that the lateral inhomogeneity of the Mg distribution decreases with p(O2) if the samples are grown directly on the sapphire substrates. Deposition of the MgxZn1−xO thin films on a ZnO buffer layer leads to a laterally homogeneous Mg distribution in the samples.