Sn vacancies in photorefractive Sn2P2S6 crystals: An electron paramagnetic resonance study of an optically active hole trap

Abstract

Electron paramagnetic resonance (EPR) is used to identify the singly ionized charge state of the Sn vacancy (VSn−) in single crystals of Sn2P2S6 (often referred to as SPS). These vacancies, acting as a hole trap, are expected to be important participants in the photorefractive effect observed in undoped SPS crystals. In as-grown crystals, the Sn vacancies are doubly ionized (VSn2−) with no unpaired spins. They are then converted to a stable EPR-active state when an electron is removed (i.e., a hole is trapped) during an illumination below 100 K with 633 nm laser light. The resulting EPR spectrum has g-matrix principal values of 2.0079, 2.0231, and 1.9717. There are resolved hyperfine interactions with two P neighbors and one Sn neighbor. The isotropic portions of these hyperfine matrices are 167 and 79 MHz for the two 31P neighbors and 8504 MHz for the one Sn neighbor (this latter value is the average for 117Sn and 119Sn). These VSn− vacancies are shallow acceptors with the hole occupying a diffuse wave function that overlaps the neighboring Sn2+ ion and (P2S6)4− anionic unit. Using a general-order kinetics approach, an analysis of isothermal decay curves of the VSn− EPR spectrum in the 107–115 K region gives an activation energy of 283 meV.

Dual role of Sb ions as electron traps and hole traps in photorefractive Sn_2P_2S_6 crystals

“…The triplet sets of lines strongly suggest that the wave function for the unpaired spin has significant overlap with one (P 2 S 6 ) 4− anionic unit (and thus with its two 31 P nuclei), as well as the interaction with one Sb ion at a Sn site. The recent study of the EPR spectrum from singly ionized Sn vacancies in SPS shows similar hyperfine interactions with two 31 P nuclei [14]. Table 1 provides a comparison of the 31 P hyperfine interactions for the two trapped hole centers when the magnetic field is along the three crystallographic directions.…”

“…A sample for our EPR experiments (2.0 × 2.5 × 2.5 mm 3 ) was then cut from this larger boule. Reference [14] describes details of the growth process. Starting materials (Sn 1.93 P 2.03 S 6.04 ) for this particular growth run were chosen to produce a Sn deficient crystal.…”

“…While being held at 42 K, the sample was initially exposed to 633 nm laser light. The light was then removed and the sample was warmed to 125 K and held at this temperature for 2 min to reduce interfering signals from isolated Sn and S vacancies [14,15]. These ramps up and down in temperature each took approximately 5 min.…”

“…The resulting Sb 2+ (5s 2 5p 1 ) ions are stable for sample temperatures below approximately 200 K. Their thermal decay at higher temperatures is described by second-order kinetics with an activation energy of 420 meV [13]. Also, EPR has shown that isolated Sn 2+ vacancies trap a hole during illumination at low temperatures [14]. Other defects in SPS identified by EPR include sulfur vacancies [15] and Sn 3+ holelike small polarons [16].…”

Hyperbolic decay of photo-created Sb2+ ions in Sn2P2S6:Sb crystals detected with electron paramagnetic resonance

“…There is only a small number of the published reports where photo-EPR has been utilized for isothermal trap decay measurements. 7 In those prior studies, the EPR line intensity was monitored directly in the course of a thermal decay of the traps at a temperature range, where the EPR line broadening caused by spin-lattice relaxation was still of minor significance, and the decay of the EPR detected traps occurred at a reasonably high rate. These are necessary conditions for the straightforward EPR technique; it cannot be applied to systems that have an appreciably fast decay occurring only at such high temperatures that the EPR lines become broadened beyond recognition.…”