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Atomic layer deposition was employed to deposit relatively thick (∼30 nm) aluminum oxide (Al2O3) using trimethylaluminum and triply-distilled H2O precursors onto epitaxial graphene grown on the Si-face of silicon carbide. Ex situ surface conditioning by a simple wet chemistry treatment was used to render the otherwise chemically inert graphene surface more amenable to dielectric deposition. The obtained films show excellent morphology and uniformity over large (∼64 mm2) areas (i.e., the entire sample area), as determined by atomic force microscopy and scanning electron microscopy. X-ray photoelectron spectroscopy revealed a nearly stoichiometric film with reduced impurity content. Moreover, from capacitance-voltage measurements a dielectric constant of ∼7.6 was extracted and a positive Dirac voltage shift of ∼1.0 V was observed. The graphene mobility, as determined by van der Pauw Hall measurements, was not affected by the sequence of surface pretreatment and dielectric deposition. [ABSTRACT FROM AUTHOR]

Two different photothresholds of the carbon vacancy-carbon antisite pair defect (VCCSi-) in 4H-SiC are observed using photoinduced electron paramagnetic resonance. The defect appeared after illumination with photon energy greater than 0.75 eV in two samples grown by halide chemical vapor deposition (HCVD) and one by physical vapor transport (PVT), all with activation energies (Ea) below 0.80 eV. In HCVD and PVT samples with higher activation energies, the defect was not detected with photon excitation less than 2.1 eV. The increase in VCCSi- parallels changes in the negatively charged carbon vacancy VC- in the samples with low Ea. For SiC samples with high Ea, the concentration of the anitisite-vacancy pair also increases with VC- and, in addition, is accompanied by a simultaneous decrease in VC+. The results are interpreted in terms of two defect levels, one for the double negative-to-negative transition of VCCSi at 0.75 eV below the conduction band edge and another for the negative-to-neutral transition of the defect at 2.1 eV above the valence band edge. The model yielding these levels also suggests that the neutral-to-negative and negative-to-double negative transitions of the isolated carbon vacancy are within a few tenths of an eV of each other and are located close to the conduction band edge. [ABSTRACT FROM AUTHOR]

The efficacy of diffusing cobalt into window-grade polycrystalline ZnSe during high-temperature anneals has been studied. Absorption, photoluminescence (PL), time-resolved PL, and electron paramagnetic resonance (EPR) were used to characterize samples with cobalt concentrations ranging from 1017 to 1019 cm-3. Absorption and PL were used to monitor the optical properties of Co2+ impurities, as well as detect Ni2+ and Fe2+ ions due to unintentional contamination. The temperature dependence of the 3 μm emission due to Co2+ ions was measured and lifetimes were determined. EPR was used to monitor the paramagnetic charge states (Co2+, Ni3+, and Fe3+) of the transition-metal ions in our samples. The effects of nickel and iron impurities on the Co2+ infrared emission intensity and lifetime are discussed. [ABSTRACT FROM AUTHOR]

An intense infrared absorption band has been observed in a hydrothermally grown ZnO crystal. At 12 K, the band peaks near 3577.3 cm-1 and has a half width of 0.40 cm-1, and at 300 K, the band peaks at 3547 cm-1 and has a half width of 41.3 cm-1. This absorption band is highly polarized, with its maximum intensity occurring when the electric field of the measuring light is parallel to the c axis of the crystal. Photoinduced electron-paramagnetic-resonance experiments show that the crystal contains lithium acceptors (i.e., lithium ions occupying zinc sites). Lithium and OH- ions are present in the crystal because lithium carbonate, sodium hydroxide, and potassium hydroxide are used as solvents during the hydrothermal growth. In the as-grown crystal, some of the lithium acceptors will have an OH- ion located at an adjacent axial oxygen site (to serve as a passivator), and we assign the 3577.3-cm-1 band observed at 12 K to these neutral complexes. Our results illustrate the role of hydrogen as a charge compensator for singly ionized acceptors in ZnO. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) has been used to investigate chromium ions in single crystals of CdGeAs[sub 2] grown by the horizontal gradient freeze technique. Signals from Cr[sup 2+] and Cr[sup 4+] ions were observed near 12 K. The Cr[sup 2+] ions have the 3d[sup 4] configuration with S=2 and substitute at the cadmium sites, while the Cr[sup 4+] ions have the 3d[sup 2] configuration with S=1 and substitute at the germanium sites. As a result, each impurity represents a neutral center. Both EPR spectra display the tetragonal symmetry of the lattice, which suggests that the centers most likely are not perturbed by nearby defects. The ratio of Cr[sup 2+] ions to Cr[sup 4+] ions in a particular CdGeAs[sub 2] crystal depends on the growth conditions and stoichiometry. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Optical absorption and electron paramagnetic resonance (EPR) techniques have been used to characterize the production and thermal decay of point defects in undoped single crystals of KD[sub 2]PO[sub 4] grown at Lawrence Livermore National Laboratory. A crystal was irradiated at 77 K with x rays, and then warmed to room temperature. Immediately after irradiation, broad optical absorption bands were observed to peak near 230, 390, and 550 nm. During warming, these absorption bands thermally decayed in the 80–140 K range. Another absorption band peaking near 450 nm appeared as the three bands disappeared. This last band decayed between 140 and 240 K. Correlations with EPR data suggest that the band at 230 nm is associated with interstitial deuterium atoms, the two bands at 390 and 550 nm are associated with self-trapped holes, and the band at 450 nm is associated with holes trapped adjacent to deuterium vacancies. Additional EPR spectra from several oxygen-vacancy centers and a silicon-associated hole center were observed as the crystal was warmed. All the electron and hole traps reported in this study are expected to participate in the room-temperature response of KD[sub 2]PO[sub 4] crystals to pulsed high-power ultraviolet laser beams. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thermally stimulated luminescence (TSL), optical absorption, and electron paramagnetic resonance (EPR) have been used to characterize the emission of ultraviolet light from undoped LiTaO[sub 3]. The crystals in this study were grown from a congruent melt and then subjected to a vapor-transport-equilibration (VTE) treatment. Two overlapping TSL peaks occur at 94 and 98 K, with each showing a 350 nm maximum in its spectral emission. These peaks are observed after a 77 K exposure of the crystals to x rays or lasers (266, 325, or 355 nm). Congruent crystals from the same boule (but not VTE treated) produced no measurable emission in similar experiments. During excitation of the VTE-treated crystals at 77 K, holes are localized on oxygen ions, either self-trapped or with a nearby stabilizer, and electrons are trapped at regular Ta[sup 5+] ions and at Fe[sup 3+] impurities (present at trace levels), where they form Ta[sup 4+] and Fe[sup 2+] ions. The hole centers and the Fe[sup 3+] ions have characteristic EPR spectra, and a broad optical absorption band peaking near 1600 nm is attributed to the Ta[sup 4+] electron center. Upon warming, the trapped hole centers formed during the initial excitation become unstable below 100 K and move to Ta[sup 4+] ions, thus causing radiative recombination in the TaO[sub 6] units. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) has been used to investigate molecular nitrogen and isolated nitrogen acceptors in single crystals of ZnO. These samples were grown by the seeded chemical vapor transport method with N[sub 2] added to the gas stream. A five-line EPR spectrum is observed at low temperature in the as-grown bulk crystals and is assigned to N[sup -, sub 2] molecules substituting for oxygen. This structure arises from nearly equal hyperfine interactions with two nitrogen nuclei ([sup 14]N, 99.63% abundant, I = 1). The spin Hamiltonian parameters for the N[sup -, sub 2] center are g[sub ] = 2.0036, g[sub ⊥] = 1.9935, A[sub ⊥] = 9.8 MHz, and A[sub ⊥] = 20.1 MHz, with the unique directions parallel to the c axis. Laser excitation at 9 K, with 325 or 442 nm light, eliminates the N[sup -, sub 2] spectrum (when the N[sup -, sub 2] convert to N[sup 0, sub 2]) and independently introduces the EPR spectrum due to isolated nitrogen acceptors (when N[sup -] acceptors convert to N[sup 0]). Removing the laser light and warming to approximately 100 K restores the crystal to its preilluminated state. In separate experiments, heating between 600 and 800 °C increases the number of N[sup -, sub 2] and N[sup 0] acceptors that can be observed. We suggest that the activation of these nitrogen acceptors occurs when complexes of hydrogen and nitrogen thermally dissociate. Further heating above 800 °C drives the two nitrogen acceptors to inactive forms. [ABSTRACT FROM AUTHOR]

An optical absorption investigation from 10 to 296 K has been performed on bulk crystals of ZnGeP[sub 2] grown by the horizontal-gradient-freeze method. We identify three broad absorption bands in the spectral range from 1 to 4 μm that are due to native defects. At low temperature, a band peaking near 1.2 μm and another band peaking near 2.2 μm have intensities that correlate. The strength of these two absorption bands can be increased or decreased by illuminating the crystal with selected laser wavelengths. The 2.2 μm band is strongly polarized, with its greatest intensity occurring when E is parallel to the c axis of the crystal. A third absorption band, peaking near 2.3 μm and extending from 1.5 μm to beyond 4 μm, was observed at low temperature, during and immediately after illumination. Comparison of photoinduced changes in absorption with photoinduced changes in electron paramagnetic resonance spectra allowed specific defects to be associated with each of the three absorption bands. Both the 1.2 and the 2.2 μm bands involve the singly ionized zinc vacancy. The third band at 2.3 μm involves neutral phosphorus vacancies. Specific models for the optical transitions are discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thermoluminescence (TL), optical absorption, and electron paramagnetic resonance (EPR) have been used to characterize point defects in Mg-doped stoichiometric LiNbO[sub 3]. A broad TL emission, peaking at 440 nm, is observed near 94 K when these crystals are irradiated at 77 K and then rapidly warmed. X rays and below-band-gap lasers (325 and 355 nm) are equally effective in producing the TL peak. During excitation, holes are trapped on oxygen ions adjacent to lithium vacancies and electrons are trapped on niobium ions at regular lattice sites. These defects both have characteristic EPR spectra, and the trapped electron center has an optical absorption band peaking at 1200 nm. Upon warming, the electrons become thermally unstable near 94 K and migrate to the trapped-hole sites where radiative recombination occurs. We suggest that the near-edge charge-transfer absorption band from Fe[sup 3+] impurities, nominally present, provides the mechanism by which below-band-gap light initially produces the separately trapped holes and electrons. [ABSTRACT FROM AUTHOR]

Bulk ZnGeP[sub 2] crystals were diffusion doped with copper and investigated using photoluminescence (PL) spectroscopy, photoluminescence excitation (PLE), and Fourier-transform-infrared spectroscopy. A PL band at 1.3 eV at 5 K was observed. The temperature dependence of the 1.3-eV PL band gives a thermal activation energy of 0.11 ± 0.01 eV. PLE data show a resonance in intensity occurred at 0.1 eV below the conduction band when monitoring the 1.3-eV band. The 1.3-eV emission band is found to be unpolarized in contrast to PL bands observed in nominally undoped ZnGeP[sub 2] crystals. The copper-diffused samples have an absorption band at 10 K peaking at about 0.62 eV. We suggest that the 1.3-eV luminescence band is donor-acceptor-pair recombination with the donor energy level about 0.1 eV below the conduction band and a Cu acceptor energy level about 0.62 eV above the valence band. This transition model is also supported by optical quenching experiments. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) have been used to characterize the dominant hole trap in undoped Czochralski-grown yttrium–orthovanadate (YVO[sub 4]) crystals. A silicon impurity, present inadvertently, replaces a vanadium ion and allows a hole to be trapped on one of the four adjacent oxygen ions. The unpaired spin resides in an oxygen p orbital oriented perpendicular to the plane defined by the silicon ion, the electron-deficient oxygen, and the two yttrium ions nearest the oxygen. Principal values of the g matrix (2.0033, 2.0090, and 2.0771) were obtained from EPR data taken at 15 K. Direct verification of the participation of silicon was obtained from ENDOR data taken at 12 K. We have found that this trapped-hole center appeared in large concentrations in all of our Czochralski-grown YVO[sub 4] crystals that were exposed to ionizing radiation (i.e., x rays or an ultraviolet laser beam) while the crystal was at 77 K. Interestingly, a small concentration of this trapped-hole center was present in some as-grown YVO[sub 4] crystals before exposure to ionizing radiation. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) has been used to characterize a hole trap and several electron traps in single crystals of potassium dihydrogen phosphate (KH[sub 2]PO[sub 4] or KDP). The paramagnetic charge states of these centers are produced by ionizing radiation (e.g., x rays or a 266 nm beam from a pulsed Nd:YAG laser) and are stable for days and even weeks at room temperature. One center consists of a hole trapped on an oxygen ion adjacent to a silicon impurity located on a phosphorus site. This defect has a small, but easily observed, hyperfine interaction with the adjacent substitutional proton. The other centers are formed when an electron is trapped at an oxygen vacancy. These latter defects are best described as (PO[sub 3])[sup 2-] molecular ions, where the primary phosphorus nucleus is responsible for a large hyperfine splitting (500-800 G in magnitude). Five EPR spectra representing variations of these oxygen vacancy centers are observed, with the differences being attributed to the relative position of a nearby cation vacancy, either a missing proton or potassium. An angular study of the EPR spectra, conducted at room temperature, provided principal values and principal directions for the g matrices and hyperfine matrices for the hole center and two of the electron centers. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Describes a study which investigated three platinum centers induced in potassium titanyl phosphate by ionizing radiation. Experimental details; Results of the electron paramagnetic resonance investigation.