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Electron paramagnetic resonance (EPR) is used to monitor photoinduced changes in the charge states of sulfur vacancies and Cu ions in tin hypothiodiphosphate. A Sn2P2S6 crystal containing Cu+ (3d10) ions at Sn2+ 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 Cu2+ (3d9) ions, as electrons move from Cu+ ions to sulfur vacancies. A g matrix and a 63,65Cu hyperfine matrix are obtained from the angular dependence of the Cu2+ 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 Cu2+ ions and restore Cu+ ions. Analysis of isothermal decay curves acquired by monitoring the intensity of the Cu2+ 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 Cu2+ ions. The photoinduced EPR spectrum from a small concentration of unintentionally present Ni+ ions at Sn2+ sites is observed near 40 K in the Sn2P2S6 crystal.

Optically stimulated luminescence (CW-OSL) is observed from Ag-doped lithium tetraborate (Li 2 B 4 O 7 ) crystals. Photoluminescence, optical absorption, and electron paramagnetic resonance (EPR) are used to identify the defects participating in the OSL process. As-grown crystals have Ag + ions substituting for Li + ions. They also have Ag + ions occupying interstitial sites. During a room-temperature exposure to ionizing radiation, holes are trapped at the Ag + ions that replace Li + ions and electrons are trapped at the interstitial Ag + ions, i.e., the radiation forms Ag 2+ (4d 9 ) ions and Ag 0 (4d 10 5s 1 ) atoms. These Ag 2+ and Ag 0 centers have characteristic EPR spectra. The Ag 0 centers also have a broad optical absorption band peaking near 370 nm. An OSL response is observed when the stimulation wavelength overlaps this absorption band. Specifically, stimulation with 400 nm light produces an intense OSL response when emission is monitored near 270 nm. Electrons optically released from the Ag 0 centers recombine with holes trapped at Ag 2+ ions to produce the ultraviolet emission. The OSL response is progressively smaller as the stimulation light is moved to longer wavelengths (i.e., away from the 370 nm peak of the absorption band of the Ag 0 electron traps). Oxygen vacancies are also present in the Ag-doped Li 2 B 4 O 7 crystals, and their role in the OSL process as a secondary relatively short-lived electron trap is described.

Cadmium silicon diphosphide (CdSiP2) is a nonlinear material often used in optical parametric oscillators (OPOs) to produce tunable laser output in the mid-infrared. Absorption bands associated with donors and acceptors may overlap the pump wavelength and adversely affect the performance of these OPOs. In the present investigation, electron paramagnetic resonance (EPR) is used to identify two unintentionally present acceptors in large CdSiP2 crystals. These are an intrinsic silicon-on-phosphorus antisite and a copper impurity substituting for cadmium. When exposed to 633 nm laser light at temperatures near or below 80 K, they convert to their neutral paramagnetic charge states (SiP0 and CuCd0) and can be monitored with EPR. The corresponding donor serving as the electron trap is the silicon-on-cadmium antisite (SiCd2+ before illumination and SiCd+ after illumination). Removing the 633 nm light and warming the crystal above 90 K quickly destroys the EPR signals from both acceptors and the associated donor. Broad optical absorption bands peaking near 0.8 and 1.4 μm are also produced at low temperature by the 633 nm light. These absorption bands are associated with the SiP0 and CuCd0 acceptors.Cadmium silicon diphosphide (CdSiP2) is a nonlinear material often used in optical parametric oscillators (OPOs) to produce tunable laser output in the mid-infrared. Absorption bands associated with donors and acceptors may overlap the pump wavelength and adversely affect the performance of these OPOs. In the present investigation, electron paramagnetic resonance (EPR) is used to identify two unintentionally present acceptors in large CdSiP2 crystals. These are an intrinsic silicon-on-phosphorus antisite and a copper impurity substituting for cadmium. When exposed to 633 nm laser light at temperatures near or below 80 K, they convert to their neutral paramagnetic charge states (SiP0 and CuCd0) and can be monitored with EPR. The corresponding donor serving as the electron trap is the silicon-on-cadmium antisite (SiCd2+ before illumination and SiCd+ after illumination). Removing the 633 nm light and warming the crystal above 90 K quickly destroys the EPR signals from both acceptors and the associated donor....

Electron paramagnetic resonance (EPR) is used to identify a new and unique photoactive silicon-related point defect in single crystals of rutile $\mathrm{Ti}{\mathrm{O}}_{2}$. The importance of this defect lies in its assignment to interstitial silicon ions and the unexpected establishment of silicon impurities as a major hole trap in $\mathrm{Ti}{\mathrm{O}}_{2}$. Principal $g$ values of this new $S=1/2$ center are 1.9159, 1.9377, and 1.9668 with principal axes along the $[\overline{1}10],[001]$, and $[110]$ directions, respectively. Hyperfine structure in the EPR spectrum shows the unpaired spin interacting equally with two Ti nuclei and unequally with two Si nuclei. These silicon ions are present in the $\mathrm{Ti}{\mathrm{O}}_{2}$ crystals as unintentional impurities. Principal values for the larger of the two Si hyperfine interactions are 91.4, 95.4, and 316.4 MHz with principal axes also along the $[\overline{1}10],[001]$, and $[110]$ directions. The model for the defect consists of two adjacent Si ions, one at a tetrahedral interstitial site and the other occupying a Ti site. Together, they form a neutral nonparamagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}\ensuremath{-}\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{0}$ complex. When a crystal is illuminated below 40 K with 442-nm laser light, holes are trapped by these silicon complexes and form paramagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}\ensuremath{-}\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defects, while electrons are trapped at oxygen vacancies. Thermal anneal results show that the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}\ensuremath{-}\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ EPR signal disappears in two steps, coinciding with the release of electrons from neutral oxygen vacancies and singly ionized oxygen vacancies. These released electrons recombine with the holes trapped at the silicon complexes.

When used as optical parametric oscillators, CdSiP2 crystals generate tunable output in the mid-infrared. Their performance, however, is often limited by unwanted optical absorption bands that overlap the pump wavelengths. A broad defect-related optical absorption band peaking near 800 nm, with a shoulder near 1 µm, can be photoinduced at room temperature in many CdSiP2 crystals. This absorption band is efficiently produced with 633 nm laser light and decays with a lifetime of ∼0.5 s after removal of the excitation light. The 800 nm band is accompanied by a less intense absorption band peaking near 1.90 µm. Data from eight CdSiP2 crystals grown at different times show that the singly ionized silicon vacancy (VSi−) is responsible for the photoinduced absorption bands. Electron paramagnetic resonance (EPR) is used to identify and directly monitor these silicon vacancies.

Doping photorefractive single crystals of Sn2P2S6 with antimony introduces both electron and hole traps. In as-grown crystals, Sb3+ (5s2) ions replace Sn2+ ions. These Sb3+ ions are either isolated (with no nearby perturbing defects) or they have a charge-compensating Sn2+ vacancy at a nearest-neighbor Sn site. When illuminated with 633 nm laser light, isolated Sb3+ ions trap electrons and become Sb2+ (5s25p1) ions. In contrast, Sb3+ ions with an adjacent Sn vacancy trap holes during illumination. The hole is primarily localized on the (P2S6)4− anionic unit next to the Sb3+ ion and Sn2+ vacancy. These trapped electrons and holes are thermally stable below ∼200 K, and they are observed with electron paramagnetic resonance (EPR) at temperatures below 150 K. Resolved hyperfine interactions with 31P, 121Sb, and 123Sb nuclei are used to establish the defect models.

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...

An electron paramagnetic resonance(EPR) spectrum in neutron-irradiated ZnO crystals is assigned to the zinc-oxygen divacancy. These divacancies are observed in the bulk of both hydrothermally grown and seeded-chemical-vapor-transport-grown crystals after irradiations with fast neutrons. Neutral nonparamagnetic complexes consisting of adjacent zinc and oxygen vacancies are formed during the irradiation. Subsequent illumination below ∼150 K with 442 nm laser light converts these ( V Z n 2 − − V O 2 + )0 defects to their EPR-active state ( V Z n − − V O 2 + )+ as electrons are transferred to donors. The resulting photoinduced S = 1/2 spectrum of the divacancy is holelike and has a well-resolved angular dependence from which a complete g matrix is obtained. Principal values of the g matrix are 2.00796, 2.00480, and 2.00244. The unpaired spin resides primarily on one of the three remaining oxygen ions immediately adjacent to the zincvacancy, thus making the electronic structure of the ( V Z n − − V O 2 + )+ ground state similar to the isolated singly ionized axial zincvacancy. The neutral ( V Z n 2 − − V O 2 + )0 divacancies dissociate when the ZnO crystals are heated above 250 °C. After heating above this temperature, the divacancy EPR signal cannot be regenerated at low temperature with light.