Photoinduced trapping of charge at sulfur vacancies and copper ions in photorefractive Sn_2P_2S_6 crystals

摘要

Electron paramagnetic resonance (EPR) is used to monitor photoinduced changes in the charge states of sulfur vacancies and Cu ions in tin hypothiodiphosphate. A Sn_2P_2S_6 crystal containing Cu~+ (3d~(10)) ions at Sn~(2+) sites was grown by the chemical vapor transport method. Doubly ionized sulfur vacancies (V_S~(2+)) are also present in the as-grown crystal (where they serve as charge compensators for the Cu~+ ions). For temperatures below 70 K, exposure to 532 or 633 nm laser light produces stable Cu~(2+) (3d~9) ions, as electrons move from Cu~+ ions to sulfur vacancies. A g matrix and a ~(63,65)Cu hyperfine matrix are obtained from the angular dependence of the Cu~(2+) EPR spectrum. Paramagnetic singly ionized (V_s~+) and nonparamagnetic neutral (V_S~0) charge states of the sulfur vacancies, with one and two trapped electrons, respectively, are formed during the illumination. Above 70 K, the neutral vacancies (V_S~0) are thermally unstable and convert to V_S~+ vacancies by releasing an electron to the conduction band. These released electrons move back to Cu~(2+) ions and restore Cu~+ ions. Analysis of isothermal decay curves acquired by monitoring the intensity of the Cu~(2+) EPR spectrum between 74 and 82 K, after removing the light, gives an activation energy of 194 meV for the release of an electron from a V_S~0 vacancy. Warming above 120 K destroys the V_S~+ vacancies and the remaining Cu~(2+) ions. The photoinduced EPR spectrum from a small concentration of unintentionally present Ni~+ ions at Sn~(2+) sites is observed near 40 K in the Sn_2P_2S_6 crystal.