Observation of anomalous emissions of nonpolar a-plane MgZnO and ZnO epi-films based on XEOL and time-resolved XEOL in hybrid bunch mode

Bi-Hsuan Lin,Chien-Yu Lee,Wei-Rein Liu,Bo-Yi Chen,Wen-Feng Hsieh,Yung-Chi Wu,Yu-Hao Wu,Gung-Chian Yin,Mau-Tsu Tang

doi:10.1063/5.0015244

Abstract

X-ray excited optical luminescence (XEOL) using an x-ray nanobeam operating in the hybrid bunch mode provides not only a sufficiently high peak power density but also high-quality temporal domain measurements for studying the luminescence dynamics of photonic materials and devices. We used these features to investigate the peculiar emission behavior of the as-grown and rapid thermal annealing (RTA)-treated a-plane MgZnO epi-films, from which an anomalous emission was observed at ∼325 nm, along with a rapid increase in near-band edge (NBE) emission with increasing x-ray irradiation time. This peculiar emission behavior was also observed in the cathodoluminescence spectra obtained under electron beam excitation. Increases in the NBE emission intensity of the RTA-treated a-plane MgZnO epi-film were observed in terms of both photoluminescence and small anomalous emissions of XEOL. This can be explained by the effective transfer of Mg atoms from interstitial sites to suitable sites as a result of RTA treatment. Based on comparison with an a-plane ZnO epi-film without Mg, we conclude that the anomalous emission peak is caused by Mg-related energy states created by the high-dose x-ray or electron beam irradiation. Furthermore, the rapid increase in NBE emission and the reduction in the long decay lifetime can be attributed to charge transfer from the Mg clusters, which are thus responsible for the anomalous emissions.

Highlights

While a similar change in the near-band edge (NBE) emission has been observed using soft x-ray irradiation,[12] rapid remote plasma annealing,[13] and low-energy electron beam irradiation[14] in the materials of ZnO nanowire arrays, MgZnO/ZnO multiple quantum wells (MQWs), and O-polar and Zn-polar ZnO wafers, respectively, no new energy states have been observed in their spectra
We used these features to investigate the peculiar emission behavior of the as-grown and rapid thermal annealing (RTA)-treated a-plane MgZnO epi-films, from which an anomalous emission was observed at ∼325 nm, along with a rapid increase in near-band edge (NBE) emission with increasing x-ray irradiation time
Increases in the NBE emission intensity of the RTA-treated a-plane MgZnO epi-film were observed in terms of both photoluminescence and small anomalous emissions of X-ray excited optical luminescence (XEOL)

Summary

Introduction

While a similar change in the NBE emission has been observed using soft x-ray irradiation,[12] rapid remote plasma annealing,[13] and low-energy electron beam irradiation[14] in the materials of ZnO nanowire arrays, MgZnO/ZnO MQWs, and O-polar and Zn-polar ZnO wafers, respectively, no new energy states have been observed in their spectra. Increases in the NBE emission intensity of the RTA-treated a-plane MgZnO epi-film were observed in terms of both photoluminescence and small anomalous emissions of XEOL.

Results

Conclusion