Structure and optical properties of ZnO/MgO multi-quantum wells deposited on oxidated sapphire substrate

ZnO/MgO multi-quantum wells with modulation structure are grown on oxidated Al₂O₃ (0001) substrates using radio-frequency reactive magnetron sputtering method. X-ray reflectivity and X-ray diffraction measurement, electronic probe, atom force microscopy, transmission spectrum and PL spectrum are used to characterize the samples. The XRD scan and phi-scan results show the films are highly (001) textured and have epitaxial relationship with the substrates. The width of quantum well is determined to be between 8.38 nm and 21.78 nm by XRR and EPMA. The AFM results show that the RMS roughness of the MQWs increases from 6.4 nm to 21.2 nm with the decrease of period of modulation. Low temperature PL spectrum shows the peak can be assigned to the radiative recombination of bound excitons, and the activation energy is estimated to be about 30 meV. The emission due to spatially separated carriers caused by quantum-confined Stark effect is also found in the spectrum of sample with smaller well width.