Strong Electron–Phonon Coupling in β-Ga2O3: A Huge Broadening of Self-Trapped Exciton Emission and a Significant Red Shift of the Direct Bandgap

Abstract

The temperature dependence of self-trapped exciton (STE) emission and the optical absorption edge of monoclinic gallium oxide (β-Ga2O3) has been carefully studied. According to this research, it is found that as temperature increases (from 10 to 300 K), the STE and the direct bandgap of β-Ga2O3 exhibit a huge broadening (∼120 meV) and a significant red shift (∼250 meV), respectively. Combined with theoretical analysis, these temperature-dependent change trends are found related to the strong electron-phonon coupling (EPC) effect in crystal, which is caused by the high localization (i.e., self-trapping) of carriers in β-Ga2O3. This finding further indicates that in the transition process of carriers' absorbing and releasing photons, the influence of lattice vibration needs to be considered and described by the configuration coordinate model. The strength of EPC can be measured by the Huang-Rhys factor, which is about S ≈ 9 for β-Ga2O3 with the polar longitudinal optical phonon mode of lower energy (∼31 meV) being involved.