Photoluminescence excitation process and optical absorption in Ge–S chalcogenide glasses

The photoluminescence (PL) excitation mechanisms in Ge–S chalcogenide glasses are investigated based on the comparison between the PL excitation and optical absorption spectra. The spectra show a remarkable dependence on the glass composition and the excitation energy. The excitation mechanism is different in the Ge-rich and S-rich glasses, and a threshold excitation energy (∼2.45–2.50 eV) exists for the PL intensity in the S-rich glasses. Interpretation of the optical absorption spectra revealed that the threshold change the excitation from the defect states in the bandgap to the excitation from the localized states in the valence band. When the excitation energy is greater than the threshold energy, the electronic transition from the top of the valence band, where the photo-excited carriers are located, tends to become trapped by the non-radiative recombination centers. On the other hand, when the excitation energy is lower than the threshold, only the electronic transition from the charged defects to the conduction band is permitted, which leads to a greater PL intensity. Interpretation of the optical absorption spectra also revealed the additional lower excitation energy states of defects in the bandgap.