Time-resolved spectroscopy of the visible emission band in strontium titanate

The luminescence spectrum of ${\mathrm{SrTiO}}_3$ from 1.6 to 3.2 eV has been studied as a function of time elapsed after excitation. A broad emission band with maximum intensity at 2.44 eV can be observed. The shape of this band, including some fine structure, can be reproduced by the Huang-Rhys model for excitation-lattice interaction. A vibron energy ħΩ=88 meV and a Huang-Rhys factor $S_0$≃6 are deduced. It is shown that those values are consistent with what is required for exciton self-trapping. The luminescence intensity has been followed for times between 100 ns and 10 ms. The decay curves indicate that two recombination processes are involved. We associate the first one with self-trapped excitons interacting with acoustic phonons and the second with the retarded formation of self-trapped excitons from localized electrons and holes. The emission appears to be quenched by a nonradiative recombination channel whose activation energy is 0.07 eV.