ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy.

J Ji,A M Colosimo,T T Trinh,R Krause-Rehberg,T E Cowan,F A Selim,L A Boatner,P S Stepanov,W Anwand,M O Liedke,A Wagner

doi:10.1038/srep31238

Abstract

The luminescence and scintillation properties of ZnO single crystals were studied by photoluminescence and X-ray-induced luminescence (XRIL) techniques. XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. It also provided bulk luminescence measurements that were not affected by surface states. The origin of a green emission, the dominant trap emission in ZnO, was then investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials. The measurements showed a single positron decay curve with a 175 ps lifetime component that was attributed to Zn vacancies passivated by hydrogen. Both oxygen vacancies and hydrogen-decorated Zn vacancies were suggested to contribute to the green emission. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE. This study reports the first application of GIPS to semiconductors, and it reveals the great benefits of the XRIL technique for the study of emission and scintillation properties of materials.

Highlights

Recombine to give near-band emission or transfer their energy to luminescence centers thereby inducing defect luminescence
X-ray-induced luminescence (XRIL) spectra are dominated by the 520 nm green emission, and the results showed that the relative emission between the near-band emission (NBE) and defect luminescence strongly depends on the annealing atmosphere
The first peak is observed at a lower energy than the near-band emission (NBE), which corresponds to band-to-band transitions and involves free excitons, donor acceptor pairs, and excitons bound to acceptors and donors and their electron satellites[29]

Summary

Introduction

Recombine to give near-band emission or transfer their energy to luminescence centers thereby inducing defect luminescence. PALS measurements in ZnO have been a subject of discrepancies and associated intense debate[19,20] Because of these factors, in his work we have performed PALS using gamma-induced positron spectroscopy (GIPS)[21,22], a unique method that provides a positron decay curve that is free from background or source contributions[23]. In GIPS, high-energy γ-rays directly produce positrons inside the sample24 - completely eliminating unwanted contributions from positron annihilation in either the source or cladding materials and, thereby, leading to more accurate measurements of positron lifetimes in general This method probes the entire volume of the crystal. By combining XRIL with scintillation measurements on ZnO crystals, we have demonstrated a strong dependence of the fast scintillation signal on the ratio between NBE and the defect luminescence

Methods

Results

Conclusion