Relaxation of localized excitons by phonon emission at oxygen vacancies in Cu₂O

We have studied the relaxation mechanism of localized excitons at optically active defect centers in cuprous oxide by the photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. We find that the temperature dependence of the PL and PLE spectra of the oxygen vacancy emission is well explained by the adiabatic potential model. The Huang-Rhys factors for the absorption and emission processes are obtained as ∼8, indicating the medium coupling of the exciton-phonon interaction. The effective frequencies of the vibrational levels are obtained as 16.5 meV and 16.1 meV, respectively, for the excited and ground states. These results indicate the Stokes shift of 0.269 ±0.008 eV which is released from the localized excitons as the lattice relaxation energy by phonon emission. Our energy level diagram resolves the previous discrepancy between the small activation energy and the large separation of the luminescence photon energies for free and localized excitons.