Unraveling Abnormal Thermal Quenching of Sub-Gap Emission in β-Ga2O3

Abstract

In this work, the optical transition of self-trapped excitons (STEs) and the emergent green emission in β-Ga₂O₃ samples with/without Sn impurities at various doping levels have been investigated via temperature- and power-dependent photoluminescence. The ultraviolet (UV) emissions ≈ 3.40 eV unanimously exhibit an excitonic nature related to STEs and typical negative thermal quenching (NTQ) characters. The NTQ activation energy decreases from 103.56 to 42.37 meV with the increased electron concentration from 2.1 × 10¹⁶ to 6.7 × 10¹⁸ cm⁻³, indicative of the reduced energy barrier that electrons should overcome to form stable STEs due to the lift-up of Fermi level. In comparison, the green emissions ≈ 2.35 eV with two quenching channels are observed only in samples with Sn impurities at cryogenic temperatures. One channel is the nsnp-ns² transition of Sn²⁺, the other is donor-acceptor pair recombination via (2V_Ga-Sn_i)²⁻ complex, which is energetically favorable as evidenced by density functional theory calculations. The semi-classical quantum theory models fitting proves the transition from green to UV emissions with elevated temperature. The enhanced STEs emission with distinguished NTQ effect strengthens evidence that the stable polarons inherently limit the transport of holes in Ga₂O₃, and also support the potential of Ga₂O₃ materials for the development of UV optoelectronics.