Point Defects in Lithium Gallate and Gallium Oxide

Electron paramagnetic resonance (EPR), Fourier-Transform Infrared spectroscopy (FTIR), photoluminescence (PL), thermoluminescence (TL), and wavelength-dependent TL are used to identify and characterize point defects in lithium gallate and β-gallium oxide doped with Mg and Fe acceptor impurities single crystals. EPR investigations of LiGaO2 identify fundamental intrinsic cation defects lithium (V−Li) and gallium (V2−Ga) vacancies. The defects’ principle g values are found through angular dependence studies and atomic-scale models for these new defects are proposed. Thermoluminescence measurements estimate the activation energy of lithium vacancies at Ea = 1.05 eV and gallium vacancies at Ea \u3e 2 eV below the conduction band minimum. Mg and Fe doped β-Ga2O3 crystals are investigated with EPR and FTIR and concentrations of Ir4+ ions greater than 1 × 1018 cm3 are observed. The source of the unintentional deep iridium donors is the crucible used to grow the crystal. In the Mg-doped crystals, the Ir4+ ions provide compensation for the singly ionized Mg acceptors contributing to the difficulties in producing p-type behavior in bulk single crystals. A large spin-orbit coupling causes Ir4+ ions to have a low-spin (5d5, S = 1/2) ground state. The Ir4+ ions have an infrared absorption band representing a d − d transition within the t2g orbitals. Using these same techniques the Fe2+/3+ level in Fe-doped β-Ga2O3 crystals is determined. With these noncontact spectroscopy methods, a value of 0.83 ± 0.04 eV below the conduction band is obtained for this level. These results clearly establish that the E2 deep level observed in DLTS experiments is due to the thermal release of electrons from Fe2+ ions

Oxygen Vacancies in LiB\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e Crystals and Their Role in Nonlinear Absorption

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

Lithium and Gallium Vacancies in LiGaO\u3csub\u3e2\u3c/sub\u3e Crystals

Lithium gallate (LiGaO2) is a wide-band-gap semiconductor with an optical gap greater than 5.3 eV. When alloyed with ZnO, this material offers broad functionality for optical devices that generate, detect, and process light across much of the ultraviolet spectral region. In the present paper, electron paramagnetic resonance (EPR) is used to identify and characterize neutral lithium vacancies (V0Li) and doubly ionized gallium vacancies (V2−Ga) in LiGaO2 crystals. These S = 1/2 native defects are examples of acceptor-bound small polarons, where the unpaired spin (i.e., the hole) is localized on one oxygen ion adjacent to the vacancy. Singly ionized lithium vacancies (V−Li) are present in as-grown crystals and are converted to their paramagnetic state by above-band-gap photons (x rays are used in this study). Because there are very few gallium vacancies in as-grown crystals, a post-growth irradiation with high-energy electrons is used to produce the doubly ionized gallium vacancies (V2−Ga). The EPR spectra allow us to establish detailed models for the two paramagnetic vacancies. Anisotropy in their g matrices is used to identify which of the oxygen ions adjacent to the vacancy has trapped the hole. Both spectra also have resolved structure due to hyperfine interactions with 69Ga and 71Ga nuclei. The V0Li acceptor has nearly equal interactions with Ga nuclei at two Ga sites adjacent to the trapped hole, whereas the V2−Ga acceptor has an interaction with Ga nuclei at only one adjacent Ga site

Cu 2+ and Cu 3+ Acceptors in β-Ga 2 O 3 Crystals: A Magnetic Resonance and Optical Absorption Study

Electron paramagnetic resonance (EPR) and optical absorption are used to characterize Cu2+ (3d9) and Cu3+ (3d8) ions in Cu-doped β-Ga2O3. These Cu ions are singly ionized acceptors and neutral acceptors, respectively (in semiconductor notation, they are Cu− and Cu0 acceptors). Two distinct Cu2+ EPR spectra are observed in the as-grown crystals. We refer to them as Cu2+(A) and Cu2+(B). Spin-Hamiltonian parameters (a g matrix and a 63,65Cu hyperfine matrix) are obtained from the angular dependence of each spectrum. Additional electron-nuclear double resonance (ENDOR) experiments on Cu2+(A) ions give refined 63Cu and 65Cu hyperfine matrices and provide information about the nuclear electric quadrupole interactions. Our EPR results show that the Cu2+(A) ions occupy octahedral Ga sites with no nearby defect. The Cu2+(B) ions, also at octahedral Ga sites, have an adjacent defect, possibly an OH− ion, an oxygen vacancy, or an H− ion trapped within an oxygen vacancy. Exposing the crystals at room temperature to 275 nm light produces Cu3+ ions and reduces the number of Cu2+(A) and Cu2+(B) ions. The Cu3+ ions have an S = 1 EPR spectrum and are responsible for broad optical absorption bands peaking near 365, 422, 486, 599, and 696 nm. An analysis of loops observed in the Cu3+ EPR angular dependence gives 2.086 for the g value and 22.18, 3.31, and −25.49 GHz for the principal values of D (the fine-structure matrix). Thermal anneal studies above room temperature show that the Cu3+ ions decay and the Cu2+ ions recover between 75 and 375 °C

Optically Active Selenium Vacancies in BaGa\u3csub\u3e4\u3c/sub\u3eSe\u3csub\u3e7\u3c/sub\u3e Crystals

Barium gallium selenide (BaGa4Se7) is a recently developed nonlinear optical material with a transmission window extending from 470 nm to 17 μm. A primary application of these crystals is the production of tunable mid-infrared laser beams via optical parametric oscillation. Unintentional point defects, such as selenium vacancies, cation vacancies (barium and/or gallium), and trace amounts of transition-metal ions, are present in BaGa4Se7 crystals and may adversely affect device performance. Electron paramagnetic resonance (EPR) and optical absorption are used to identify and characterize these defects. Five distinct EPR spectra, each representing an electron trapped at a selenium vacancy, are observed at low temperature (there are seven crystallographically inequivalent selenium sites in the crystal). One spectrum is stable at room temperature and is present before illumination. The other four are produced at lower temperatures with 532 nm laser light and are thermally unstable at room temperature. Each S = 1/2 singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with 69Ga and 71Ga nuclei at one neighboring Ga site. A significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion and gives principal values of the hyperfine matrices in the 3350–6400 MHz range. Broad photoinduced optical absorption bands in the visible and near-infrared are assigned to the selenium vacancies

Ir \u3csup\u3e4+\u3c/sup\u3e Ions in β-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e Crystals: An Unintentional Deep Donor

Electron paramagnetic resonance (EPR) and infrared absorption are used to detect Ir4+ ions in β-Ga2O3 crystals. Mg and Fe doped crystals are investigated, and concentrations of Ir4+ ions greater than 1 × 1018 cm−3 are observed. The source of the unintentional deep iridium donors is the crucible used to grow the crystal. In the Mg-doped crystals, the Ir4+ ions provide compensation for the singly ionized Mg acceptors and thus contribute to the difficulties in producing p-type behavior. The Ir4+ ions replace Ga3+ ions at the Ga(2) sites, with the six oxygen neighbors forming a distorted octahedron. A large spin-orbit coupling causes these Ir4+ ions to have a low-spin (5d5, S = 1/2) ground state. The EPR spectrum consists of one broad line with a significant angular dependence. Principal values of the g matrix are 2.662, 1.815, and 0.541 (with principal axes near the crystal a, b, and c directions, respectively). Ionizing radiation at 77 K decreases the Ir4+ EPR signal in Mg-doped crystals and increases the signal in Fe-doped crystals. In addition to the EPR spectrum, the Ir4+ ions have an infrared absorption band representing a d-d transition within the t2g orbitals. At room temperature, this band peaks near 5153 cm−1 (1.94 μm) and has a width of 17 cm−1. The band is highly polarized: its intensity is maximum when the electric field E is parallel to the b direction in the crystal and is nearly zero when E is along the c direction

Deep Donors and Acceptors in β-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e Crystals: Determination of the Fe\u3csup\u3e2+/3+\u3c/sup\u3e Level by a Noncontact Method

lectron paramagnetic resonance (EPR), infrared absorption, and thermoluminescence (TL) are used to determine the Fe2+/3+ level in Fe-doped β-Ga2O3 crystals. With these noncontact spectroscopy methods, a value of 0.84 ± 0.05 eV below the conduction band is obtained for this level. Our results clearly establish that the E2 level observed in deep level transient spectroscopy (DLTS) experiments is due to the thermal release of electrons from Fe2+ ions. The crystals used in this investigation were grown by the Czochralski method and contained large concentrations of Fe acceptors and Ir donors, and trace amounts of Cr donors. Exposing a crystal at room temperature to 325, 375, or 405 nm laser light converts neutral Fe3+ acceptors to their singly ionized Fe2+ charge state and, at the same time, converts a similar number of neutral Ir3+ donors to the Ir4+ charge state. The Fe3+ EPR spectrum slowly recovers after the light is removed, as electrons are thermally released from Fe2+ ions to the conduction band. Most of these released electrons recombine nonradiatively with holes at the deep Ir4+ donors. Using a general-order kinetics model, the analysis of isothermal recovery curves for the Fe3+ EPR signal taken between 296 and 310 K gives the activation energy for the decay of the photoinduced Fe2+ ions. A TL peak, with emitted light having wavelengths longer than 500 nm, occurs near 349 K when a few of the electrons released from Fe2+ ions recombine radiatively with holes at Ir4+ and Cr4+ donors. Photoluminescence and EPR verify the presence of Cr3+ ions. Abstract ©2019 Author(s)

Experimental Determination of the (0/−) Level for Mg Acceptors in β-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e Crystals

Electron paramagnetic resonance (EPR) is used to experimentally determine the (0/−) level of the Mg acceptor in an Mg-doped β-Ga2O3 crystal. Our results place this level 0.65 eV (±0.05 eV) above the valence band, a position closer to the valence band than the predictions of several recent computational studies. The crystal used in this investigation was grown by the Czochralski method and contains large concentrations of Mg acceptors and Ir donors, as well as a small concentration of Fe ions and an even smaller concentration of Cr ions. Below room temperature, illumination with 325 nm laser light produces the characteristic EPR spectrum from neutral Mg acceptors (Mg0Ga). A portion of the singly ionized Ir4+ donors are converted to their neutral Ir3+ state at the same time. For temperatures near 250 K, the photoinduced EPR spectrum from the neutral Mg0Ga acceptors begins to decay immediately after the laser light is removed, as electrons are thermally excited from the valence band to the Mg acceptor. Holes left in the valence band recombine with electrons at the deeper Ir3+ ions and restore the Ir4+ ions. An activation energy for the thermal decay of the Mg0Ga acceptors, and thus a value for the (0/−) level, is obtained by using a general-order kinetics model to analyze a set of five isothermal decay curves taken at temperatures between 240 and 260 K

Deep Donor Behavior of Iron in β-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e Crystals: Establishing the Fe\u3csup\u3e4+/3+\u3c/sup\u3e Level

The Fe4+/3+ donor level is experimentally determined to be 0.70 eV (±0.05 eV) above the valence band maximum in β-Ga2O3. Electron paramagnetic resonance (EPR) is used to monitor Fe3+ ions that are unintentionally present in an Mg-doped β-Ga2O3 crystal (with a low Fermi level). For temperatures near 255 K, exposure to 325 nm laser light converts a portion of the Fe3+ ions to Fe4+ and Fe2+ ions and, at the same time, forms neutral magnesium acceptors (Mg0Ga) and neutral Ir donors (Ir3+). After removing the light, the intensity of the Fe3+ EPR spectrum has a significant additional decrease as holes thermally released to the valence band from rapidly decaying neutral Mg acceptors are trapped at Fe3+ ions and form even more Fe4+ ions. This demonstrates that the Mg0/- acceptor level, near 0.65 eV, is closer to the valence band than the Fe4+/3+ level. Following the fast initial post-light decrease, the Fe3+ spectrum then slowly recovers as Fe4+ ions are destroyed by electrons thermally excited from the valence band. An activation energy for the thermal decay of the Fe4+ donors, and thus a value for the Fe4+/3+ level, is obtained from the analysis of five Fe3+ isothermal recovery curves taken from the Mg-doped crystal between 250 and 270 K. A first-order kinetics model is used, as minimal retrapping is observed. In separate experiments, EPR shows that Fe4+ ions are also produced in an Fe-doped β-Ga2O3 crystal (without Mg acceptors) during exposures to laser light at temperatures near 255 K. Abstract © 2020 Author(s)

Self-trapped Holes (Small Polarons) in Ferroelectric KH\u3csub\u3e2\u3c/sub\u3ePO\u3csub\u3e4\u3c/sub\u3e Crystals

Density functional theory is used to establish the ground-state structure of the self-trapped hole (STH) in KH2PO4 crystals. The STHs in this nonlinear optical material are free small polarons, a fundamental intrinsic point defect. They are produced with ionizing radiation in the low-temperature orthorhombic structure of KH2PO4 and are only stable (i.e. long-lived) below approximately 70 K. A large 129-atom cluster, K19H40P14O56, is constructed to model the STH. The ωB97XD functional with the 6−31+G* basis set is used and geometry optimization is performed. Our results show that two of the oxygen ions in a PO4 unit relax toward each other and equally share the hole. These two oxygen ions do not initially have close hydrogen neighbors. This equal sharing of the hole is related to the presence of isolated, slightly distorted, PO4 units and is significantly different from the small-polaron behavior often observed in other oxide crystals where the hole is localized on only one oxygen ion. The computational results provide a detailed description of the lattice relaxation occurring during formation of the STH. Characteristic spectral features of this defect are a larger hyperfine interaction with one 31P nucleus and equal, but smaller, hyperfine interactions with two 1H nuclei. The computed values for these isotropic and anisotropic hyperfine coupling constants are in excellent agreement with results obtained from electron paramagnetic resonance experiments.Abstract © 2019 IOP