Light-induced charge transfer in nominally pure and iron doped Sn_2P_2S_6: a study by combined EPR-optical absorption spectroscopy

摘要

The light-induced charge transfer in ferroelectric tin hypothiodiphosphate Sn_2P_2S_6 is investigated by means of optical absorption and electron paramagnetic resonance spectroscopy and their combination. By creating electrons and holes via optical bandgap excitation defect states in the gap are recharged. This situation is highly metastable, even at room temperature. EPR reveals that hole capture probably transforms Sn~(2+) at one of the two inequivalent Sn sites into Sn~(3+). By 1.4 eV illumination such holes are transferred to the other Sn site. Optical absorption measurements at 298 K give evidence that the incorporation of iron leads to an absorption at about 700 nm. Dielectric loss measurements show relaxation phenomena in the frequency range of 20 Hz to 1 MHz and the influence of band gap illumination on the behavior of the relaxing carriers. The recombination of the metastable electron and hole arrangement is detected by thermoluminescence. It occurs only if the pyroelectric field of the crystal has the right polarity. The phenomenon appears to be connected to a Poole-Frenkel effect.