Optical properties of a-plane non-polar Zn1−x Mg x O/ZnO multiple quantum wells with different barrier compositions

Abstract

The optical properties of a-plane non-polar Zn1−x Mg x O/ZnO multiple quantum wells (MQWs) with different barrier compositions have been investigated. It is found that the dominant photoluminescence (PL) emission at 16 K as well as at room temperature exhibits monotonous blue shift as Mg content in the barriers increases due to quantum confinement effect. Moreover, increasing the Mg content in the barriers improves carrier confinement efficiency as well as degrades the crystal quality of MQWs. In a-plane Zn0.85Mg0.15O/ZnO MQWs, exciton emission in the ZnO wells at 16 K is dominated by localized excitons (LEs) at 3.413 eV and free excitons (FEs) at 3.459 eV. Temperature-dependent PL spectra indicate that the intensity of the LE peak gradually decreases and the PL spectra become dominated by FE emissions at the temperature above 120 K. The behavior of LEs and FEs as the temperature increases is interpreted by the formation of potential minima in ZnO wells due to exciton localization. The deduced activation energy of 11 meV is in good agreement with quenching temperature of LEs.