Li-doping effects on the native defects and luminescence of Zn2GeO4 microstructures: Negative thermal quenching

Abstract

Novel transparent semiconducting oxides have emerged as key materials for a variety of optoelectronic applications due to their ultra-wide band gap. Owing to the critical role of native defects and impurities in the optoelectronical properties, investigation in detail of the electronic structure of these oxides has become a challenge, being luminescence technique a powerful way to gain insight into the electronic levels. Here, the photoluminescence temperature dependence of Li doped Zn2GeO4 microrods is analysed and discussed in the framework of the electronic levels structure related to native defects and Li impurities. Unlike the undoped material, the blue emission (2.8 eV) of Li-doped germanate exhibits a negative thermal quenching as its photoluminescence intensity increases with increasing temperature. The mechanisms underlying this anomalous behaviour are discussed with the aid of X-ray photoemission spectroscopy and first principles calculations. This study has helped us to understand the role of Li in the electronic structure and optical response of Zn2GeO4. Our results highlight the versatility of Zn2GeO4 luminescence through Li doping, which could be exploited in optoelectronic devices with bespoke properties.