Excitation-energy dependence of the photoluminescence total-light decay in arsenic chalcogenides

Abstract

Previous measurements of transient photoluminescence (PL) in a-As₂S₃ have indicated that the PL spectrum is extremely sensitive to the excitation photon energy. In the present paper we show that this sensitivity is also manifest in the total-light decay (TLD), the spectrally integrated PL intensity as a function of time. The most surprising result of this study is that, whereas the TLD decays approximately as a power law of the form t ^−0·75 for times between 10⁻⁸ and 10⁻³ s for high excitation energy, low excitation energy reveals two well-defined rates of 10⁶ and 10³ s⁻¹. The dependence of the TLD on excitation energy is found to be consistent with a model in which excitations are localized at low energy and delocalized at high energy. TLD results are also presented for a-As₂Se₃, c-As₂S₃ and c-As₂Se₃. Well-defined rates are observed in all samples, and it is found that a process at long times (∼ 10⁻³ s) usually exhibits the highest relative quantum efficiency. Arguments are presented which suggest that this long-time process arises from transitions from a triplet excited state to a singlet ground state.