Self-trapped excitons in orthorhombic SnBr2

Luminescence properties of SnBr2 have been studied to reveal the photo-excited exciton relaxation process. Two types of luminescence with large Stokes shifts are found at low temperatures; the 2.2-eV luminescence band produced under the photo-excitation in the first exciton region, and the 2.5-eV luminescence band stimulated by photons with energies above the bandgap. The time-resolved photoluminescence measurements have revealed that the 2.2-eV luminescence comprises fast (1.2 ps) and slow (6.4 ps) exponential decay components, whereas the 2.5-eV luminescence shows the time dependence of I(t)proportional tot(-0.9). These results suggest that the former band is attributed to the radiative decay of self-trapped excitons, and the latter band would originate from tunneling recombination of holes with the STEL as in the case of lead halides.