Dual role of Te ions as electron traps and hole traps in ferroelectric Sn2P2S6 crystals

Tin hypothiodiphosphate (Sn2P2S6) is a ferroelectric semiconductor with electro-optic and photorefractive relevance. The lattice contains Sn2+ ions and (P2S6)4− anionic groups. In the present work, an Sn2P2S6 crystal is doped with Te during growth. Our results show that the Te ions occupy Sn sites and S sites, where they serve as electron and hole traps, respectively. Electron paramagnetic resonance (EPR) spectra from five trapped holes and one trapped electron are observed at low temperatures after exposure to 633 nm light. Spin-Hamiltonian parameters are extracted from the angular dependence of each spectrum. Holes are localized on (P2TeS5)4− units and form (P2TeS5)3− anions, with the five defects corresponding to Te on different S sites within the anion. These defects have large g shifts caused by the tellurium spin–orbit coupling parameter. Four of the five also have an isotropic hyperfine interaction with one Sn ion, which suggests that they have an adjacent Sn vacancy (leaving just one close Sn neighbor). Three trapped-hole spectra are photoinduced at 20 K and two appear after removing the light, warming to 120 K, and then returning to 20 K. Electrons are trapped in pairs at Te4+ ions occupying Sn2+ sites when the crystal is exposed to 633 nm light while at 20 K. The Te2+ ions that are formed have no observable EPR spectrum. Warming to 100 K releases one electron from Te2+ ions that have an adjacent Sn vacancy, and an EPR spectrum from Te3+ ions (5s25p) is seen when the crystal is returned to lower temperatures.