L. E. Halliburton

Tin hypothiodiphosphate (Sn2P2S6) crystals, consisting of Sn2+ ions and (P2S6)4− anionic groups, are ferroelectric semiconductors below 337 K. Doping with Se forms (P2SeS5)4− anions and reduces the transition temperature, as isovalent Se ions replace S ions. In the present work, a hole is trapped on a (P2SeS5)4− unit during an exposure at 55 K to 532 nm laser light. The resulting (P2SeS5)3− defect has an unpaired spin and is best described as a small polaron. Electron paramagnetic resonance (EPR) spectra show that this polaron (i.e., trapped hole) has principal g values near 2.01 and resolved hyperfine interactions with two phosphorous nuclei. Hyperfine splittings are 7.1, 7.0, and 6.3 mT for one phosphorous nucleus and 19.9, 27.7, and 25.7 mT for the other phosphorous nucleus when the magnetic field is along the a, b, and c directions, respectively. The absence of a large hyperfine interaction with a nearby Sn nucleus indicates that the (P2SeS5)3− polaron does not have an adjacent Sn vacancy [see Golden et al., J. Appl. Phys. 120, 133101 (2016)]. Comparisons are made with similar holelike small polarons previously observed with EPR at low temperature in undoped and Te-doped Sn2P2S6 crystals. Because of the significant differences found in the distributions of their unpaired spin densities, the small polarons in these three materials provide an experimental data set of g values and hyperfine that can be used to validate results from advanced density-functional-theory modeling studies.

Cadmium silicon phosphide (CdSiP2) is a nonlinear optical material widely used in optical parametric oscillators. Intrinsic defects (vacancies and antisites) are responsible for unwanted broad optical absorption bands in these crystals that degrade the performance of the devices. In the present work, optical absorption and electron paramagnetic resonance (EPR) spectra are acquired (at room temperature and 12 K, respectively) from a neutron-irradiated CdSiP2 crystal. After the irradiation, the crystal is highly absorbing from the band edge near 600 nm to beyond 1.3 μm because of overlapping defect-related absorption bands. Heating to 550 °C removes nearly all the absorption induced by the high-energy neutrons. EPR spectra show that a primary effect of the neutrons is the production of phosphorous vacancies. A broad EPR signal attributed to perturbed intrinsic defects is seen before heating the irradiated crystal. After warming to 550 °C, an EPR spectrum representing isolated neutral phosphorous vacancies (VP0) is observed without light and an EPR spectrum from neutral phosphorous-on-silicon antisites (PSi0) can be photoinduced with 633 nm light. The formation of the PSi0 antisites (occurring when phosphorous interstitials created by the neutrons are trapped at silicon vacancies) provides direct evidence that the neutrons displaced phosphorous ions. The VP0 and PSi0 donors were not detected in EPR spectra taken from the as-grown CdSiP2 crystal (i.e., before the neutron irradiation).

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.

Zinc germanium diphosphide (ZnGeP2) is a ternary semiconductor best known for its nonlinear optical properties. A primary application is optical parametric oscillators operating in the mid-infrared region. Controlled donor doping provides a method to minimize the acceptor-related absorption bands that limit the output power of these devices. In the present study, a ZnGeP2 crystal is doped with selenium during growth. Selenium substitutes for phosphorus and serves as a deep donor. Significant concentrations of native defects (zinc vacancies, germanium-on-zinc antisites, and phosphorous vacancies) are also present in the crystal. Electron paramagnetic resonance (EPR) is used to establish the atomic-level model for the neutral charge state of the selenium donor. The S = 1/2 signal from the neutral donors is produced at 6 K by illuminating with 633 nm light (electrons excited from doubly ionized Zn vacancies convert SeP+ donors to SeP0 donors). A g matrix, with principal values of 2.088, 2.203, and 1.904, is extracted from the angular dependence of the EPR spectrum. The principal-axis direction associated with the 1.904 principal value is close to a Se–Ge bond. This indicates an asymmetric distribution of unpaired spin density around the selenium ion and thus predicts the deep donor behavior.

CdSiP₂ crystals are used in optical parametric oscillators to produce tunable output in the mid-infrared. As expected, the performance of the OPOs is adversely affected by residual optical absorption from native defects that are unintentionally present in the crystals. Electron paramagnetic resonance (EPR) identifies these native defects. Singly ionized silicon vacancies (V_Si⁻) are responsible for broad optical absorption bands peaking near 800, 1033, and 1907 nm. A fourth absorption band, peaking near 630 nm, does not involve silicon vacancies. Exposure to 1064 nm light when the temperature of the CdSiP₂ crystal is near 80 K converts V_Si⁻ acceptors to their neutral and doubly ionized charge states (V_Si⁰ and V_Si²⁻, respectively) and greatly reduces the intensities of the three absorption bands. Subsequent warming to room temperature restores the singly ionized charge state of the silicon vacancies and brings back the absorption bands. Transitions responsible for the absorption bands are identified, and a mechanism that allows 1064 nm light to remove the singly ionized charge state of the silicon vacancies is proposed.

Cadmium germanium diphosphide (CdGeP2) crystals, with versatile terahertz-generating properties, belong to the chalcopyrite family of nonlinear optical materials. Other widely investigated members of this family are ZnGeP2 and CdSiP2. The room-temperature absorption edge of CdGeP2 is near 1.72 eV (720 nm). Cadmium vacancies, phosphorous vacancies, and germanium-on-cadmium antisites are present in as-grown CdGeP2 crystals. These unintentional intrinsic point defects are best studied below room temperature with electron paramagnetic resonance (EPR) and optical absorption. Prior to exposure to light, the defects are in charge states that have no unpaired spins. Illuminating a CdGeP2 crystal with 700 or 850 nm light while being held below 120 K produces singly ionized acceptors (VCd−) and singly ionized donors (GeCd+), as electrons move from VCd2− vacancies to GeCd2+ antisites. These defects become thermally unstable and return to their doubly ionized charge states in the 150–190 K range. In contrast, neutral phosphorous vacancies (VP0) are only produced with near-band-edge light when the crystal is held near or below 18 K. The VP0 donors are unstable at these lower temperatures and return to the singly ionized VP+ charge state when the light is removed. Spin-Hamiltonian parameters for the VCd− acceptors and VP0 donors are extracted from the angular dependence of their EPR spectra. Exposure at low-temperature to near-band-edge light also introduces broad optical absorption bands peaking near 756 and 1050 nm. A consistent picture of intrinsic defects in II-IV-P2 chalcopyrites emerges when the present CdGeP2 results are combined with earlier results from ZnGeP2, ZnSiP2, and CdSiP2.

Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band. Both Ni2+ (3d8) and Ni3+ (3d7) acceptors are present in the as-grown crystal. Also present are unintentional Ir3+ (5d6) and Ir4+ (5d5) donors. The neutral Ni3+ acceptors have a low-spin S = 1/2 ground state and are easily monitored with electron paramagnetic resonance (EPR). Principal values of the g matrix for these acceptors are 2.131, 2.138, and 2.233. Although paramagnetic, the singly ionized Ni2+ acceptors are not seen with EPR at X band (9.4 GHz). The Ir4+ donors are monitored with EPR and with infrared absorption spectroscopy. Exposing the Ni-doped β-Ga2O3 crystal to 275 nm light at room temperature increases the concentration of Ni3+ ions and reduces the concentration of Ir4+ ions as electrons move from the acceptors to the donors. After illumination, heating the crystal above 375 °C restores the initial concentrations of the Ni3+ and Ir4+ ions. Broad optical absorption bands peaking near 303 and 442 nm are attributed to the Ni3+ acceptors.

LiB₃O₅ (LBO) crystals are used to generate the second, third, and fourth harmonics of near-infrared solid-state lasers. At high power levels, the material’s performance is adversely affected by nonlinear absorption. We show that as-grown crystals contain oxygen and lithium vacancies. Transient absorption bands are formed when these intrinsic defects serve as traps for “free” electrons and holes created by x rays or by three- and four-photon absorption processes. Trapped electrons introduce a band near 300 nm and trapped holes produce bands in the 500-600 nm region. Electron paramagnetic resonance (EPR) is used to identify and characterize the electrons trapped at oxygen vacancies (the unpaired electron is localized on one neighboring boron). Self-trapped holes and lithium vacancies with the hole trapped on an adjacent oxygen are also observed with EPR. At room temperature, we predict that most of the unwanted defect-related ultraviolet absorption created by a short laser pulse will decay with a half-life of 29 µs.

Electron paramagnetic resonance (EPR) is used to establish models for electron traps in Ag-doped lithium tetraborate (Li₂B₄O₇) crystals. When exposed at room temperature to ionizing radiation, electrons are trapped at interstitial Ag⁺ ions and holes are trapped at Ag⁺ ions on Li⁺ sites. The trapped electrons occupy a 5s¹ orbital on the interstitial Ag ions (some of the unpaired spin density is also on neighboring ions). Three EPR spectra are assigned to electrons trapped at interstitial Ag ions. Their g values are near 1.99 and they have resolved hyperfine structure from ¹⁰⁷Ag and ¹⁰⁹Ag nuclei. The spectrum representing the largest concentration of trapped electrons has the unpaired spin shared by the interstitial Ag ion and an adjacent boron ion at its regular lattice site. A ¹⁰B enriched crystal verifies this assignment and an analysis of spin-Hamiltonian parameters yields information about the Ag and B orbitals occupied by the unpaired spin. The second spectrum has the unpaired spin shared equally by two Ag ions, one at an interstitial site and the other at an adjacent Li site. The third spectrum has a large Ag hyperfine interaction and a weak Li interaction. Optical absorption bands associated with the trapped electrons are observed between 225 and 500 nm. Thermal release of electrons from these traps is responsible for a prominent thermoluminescence peak near 150 °C, whereas optical release of the electrons at room temperature produces intense optically stimulated luminescence. Radiative recombination occurs at Ag²⁺ ions with emission peaking near 270 nm.