Exciton relaxation in chalcogenide semiconductor investigated by time-resolved photoluminescence measurement

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Amorphous chalcogenide semiconductors which contain the elements of group VI show various kinds of photo-induced structural changes such as photo-darkening, photo-bleaching, photo-crystallization, etc.. These changes are caused by lattice deformations which result from the relaxation of the photo-excited electrons through the strong electron-phonon interaction. It is also important that the amorphous states are not in the thermal equilibrium. The chalcogen atoms which are in two-fold coordinated are considered to play an important role, but the details of the mechanism is still not clear. For example, it is still unknown how the excited electron distorts the lattice.To know the relaxation process of the excited electron in the amorphous and crystalline GeSe_2 which is a typical chalcogenide semiconductor, the time resolved photo-luminescence measurement was carried out. Both phases have the band-gap in visible region (2.2-2.7eV). The excitation by band-gap light causes a broad Gaussian-shaped photo-luminescence in near-infra-red region (-1eV). The large Stokes-shift is due to the strong electron-phonon interaction. The continuous wave laser light is modulated by an acousto-optic modulator and then used for the excitation source of the time-resolved measurement. The measured time region was 100ns-3ms after the cut-off of the excitation light. There are many similarities in the spectral shape between the amorphous and crystalline GeSe_2. This result shows that the initial states of the luminescence are similar in both materials. As for the time-profile, the crystalline GeSe_2 shows the single exponential decay, while the amorphous GeSe_2 shows the extended exponential decay. The later is caused by the energy distribution of the luminescent centers by structural fluctuation in the glass.