Effects of AlOx-cap layer on the luminescence and photoconductivity of ZnO thin films

Abstract

The effects of AlOx-cap layer on the optical and photoelectrical properties of ZnO films have been studied by cathodoluminescence (CL), photoluminescence (PL), and photoconductivity (PC). Both the PL and CL show that the cap layer improves the emission characteristics of ZnO by enhancing the band-edge emission while at the same time reducing the deep-level emissions. To study the origin of improvement, depth-resolved CL has been carried out to map out the emissions at different depths. It shows that the improvement occurs primarily at the film surface, which indicates the cap layer acts as a passivation layer that suppresses the detrimental surface states. The PC measurement on the capped ZnO at room temperature shows a distinctive excitonic feature at 3.29 eV and an overall increment of photoresponse above the band gap. Therefore, our results suggest a higher sensitivity of UV detection can be achieved in ZnO simply be employing a thin AlOx-cap layer.