Structural and optical properties of non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy

This work investigates the structural and optical properties of non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells (MQWs), which have been prepared on -plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). The MQWs are () oriented (-plane) as identified by the X-ray diffraction pattern. Structural properties are anisotropic and surfaces of MQWs show stripes running along the ZnO -axis direction. Sharp interfaces between the well layers and barrier layers can be clearly resolved by the secondary ion mass spectroscopy (SIMS) analysis. The room-temperature photoluminescence (PL) resulting from the well regions exhibits a significant blueshift with respect to ZnO single layer. Exciton emission in the ZnO QW is resolved into two components in the temperature dependence of the PL spectra. Two types of excitons are responsible for this feature. The excitons trapped by the potential minima dominate at low temperature, and the excitons localized in the "free exciton states" dominate at relatively high temperature. An activation energy of 7.3 meV for quenching of the exciton emission is in good agreement with the transition of the two types of excitons.