S. H. Choh

Electron‐paramagnetic‐resonance study of the Mn2+ luminescence center in ZnS:Mn powder and thin films

The number of isolated Mn2+ ions and Mn2+ clusters in ZnS:Mn powder and thin films has been studied using Mn2+ spectra measured at room temperature with an X‐band electron‐paramagnetic‐resonance spectrometer. While the concentration of the isolated Mn2+ ions decreases with increasing Mn concentration, the concentration of the clusters increases. At low Mn concentration, the Mn2+ ion substitutes for the Zn ion in ZnS:Mn in the cubic phase. At high Mn concentrations, where the ZnS powder has a dominant hexagonal phase, the Mn ion still prefers to substitute for Zn in ZnS:Mn at the cubic site rather than at the hexagonal site.

Electron paramagnetic resonance study of the Eu2+ ion in a PbWO4 single crystal

The electron paramagnetic resonance of the Eu2+ ion in a PbWO4 single crystal, grown by the Czochralski method, has been investigated using an X-band spectrometer. The rotation patterns in the crystallographic planes together with spin-Hamiltonian parameters of Eu2+ show that the local site symmetry of the Eu2+ ion is tetragonal. The spectroscopic splitting tensor g, zero-field splitting parameters Bkq, and hyperfine tensor A are determined with the effective spin Hamiltonian. Energy levels of the ground state for Eu2+ embedded in the PbWO4 crystal are calculated. It turns out that the Eu2+ ion substitutes for the Pb2+ ion without any charge compensation.

Electron paramagnetic resonance studies of Mn2+ ions in β-Ga2O3 single crystal

A Mn2+ ion-doped β-Ga2O3 single crystal was grown by using a floating zone method. By employing an X-band electron paramagnetic resonance (EPR) spectrometer, Mn2+ EPR spectra were recorded at room temperature. The rotation patterns in the crystallographic planes together with spin-Hamiltonian parameters of Mn2+ show unequivocally that the actual local site symmetry of the Mn2+ ion is monoclinic. A Mn2+ ion lies on a site with a two-fold rotation symmetry, parallel to the monoclinic axis of the crystal. The spectroscopic splitting tensor g, zero-field splitting parameters Bkqs, and the hyperfine tensor A are determined with the effective spin Hamiltonian. It turns out that the Mn2+ ion lies at only one site, the substitutional site for the Ga3+ ion in the oxygen octahedron rather than in tetrahedron. Energy levels of the ground state for a Mn2+ ion embedded in the β-Ga2O3 crystal are calculated.

Electron paramagnetic resonance characterization of Cr3+ impurities in a β-Ga2O3 single crystal

β-Ga2O3 single crystals doped with the Cr3+ ion were grown in an O2 atmosphere using the floating zone method. Electron paramagnetic resonance (EPR) spectra of the Cr3+ ion were recorded with an X band EPR spectrometer at 20 °C. The rotation patterns of Cr3+ spectra in the crystallographic planes together with spin-Hamiltonian parameters showed unequivocally that the actual local site symmetry of the Cr3+ ion in β-Ga2O3 is not orthorhombic, as previously reported, but monoclinic. The spectroscopic splitting tensor g and the zero-field splitting parameters Bkq s were determined with an effective spin Hamiltonian. The Cr3+ ions replace Ga3+ ions in oxygen octahedra rather than in oxygen tetrahedra. The energy levels of the ground state of a Cr3+ ion embedded in the β-Ga2O3 crystal were calculated.

Isothermal capacitance transient spectroscopy (ICTS) study for midgap levels in Hb-GaAs by rapid thermal annealing

We studied the midgap levels by using isothermal capacitance transient spectroscopy (ICTS) in Hb-GaAs which had been processed by rapid thermal annealing (RTA). As the annealing time at 850 °C increased, the EL2 trap (Ec−0.81 eV) was transformed to the EX2 trap (Ec−0.73 eV) and eventually to the EX1 trap (Ec−0.87 eV). The diffusivity of the EL2 trap obtained from the experimental result of the heat treatment was about 1.02·10−8cm2/s at 850 °C. This result indicate that the EL2 trap contains an interstitial arsenic atom. The result of the transformation to the EX1 and EX2 traps suggests that, when the EL2 trap is VAsASiVGaAsGa, the EX2 trap may be VAsVGaAsGa, which Asi is diffused out during a thermal annealing.

Luminescence and electron paramagnetic resonance studies of white‐light emitting SrS: Pr, F thin film electroluminescent devices

The fluorescence emission and excitation spectra of white‐light emitting SrS: Pr, F thin film electroluminescent devices have been investigated. It was determined from the results obtained that the dominant electroluminescence mechanism was that the ionization of Pr3+ centers occurs first, then subsequently recombination with electrons occurs, and finally Pr3+ center transitions give rise to luminescence. The emission mechanism of SrS: Pr, F seems to be the same as that of a SrS: Pr, K electroluminescent device, except for the appearance of strong peaks around 610–670 nm. The impurity excitation peak in the lower excitation energy, longer‐wavelength region in the FL spectrum may be an important factor for the selection of an effective white‐light emitting EL material. The electron paramagnetic resonance experiment of SrS: Pr, F was performed on powder and thin film specimens. The hyperfine structure of an isolated Mn2+ ion was observed in this SrS: Pr, F thin film. This Mn center which was substituted for...

Cr3+(I) and Cr3+(II) centers in alexandrite single crystal

Nuclear magnetic resonance spectra of the 9Be and 27Al nuclei as well as electron paramagnetic resonance spectra of Cr3+ ion in an alexandrite single crystal (BeAl2O4:Cr) have been investigated by employing a pulse nuclear magnetic resonance spectrometer and an X-band electron paramagnetic resonance spectrometer, respectively. Be has two magnetically inequivalent sites. While Al(I) at inversion symmetry has four magnetically inequivalent Al sites, Al(II) at mirror symmetry has two. The Cr3+(I) center has four magnetically inequivalent sites, whereas the Cr3+(II) center has two. All resonance sets of nuclear magnetic resonance for 9Be and 27Al nuclei and electron paramagnetic resonance for Cr3+ ion are analyzed in terms of crystal symmetry, ionic size, and charge state. It is concluded that the Cr3+(I) and Cr3+(II) ions substitute for Al3+(I) at inversion symmetry and Al3+(II) at mirror symmetry, respectively, intact of Be2+ sites.

Nuclear magnetic relaxation studies of 69Ga, 71Ga, and 75As nuclei in GaAs single crystals doped with paramagnetic impurities

Abstract The GaAs, GaAs:In,Cr, and GaAs:Mn single crystals were grown by the vertical gradient freeze (VGF) technique. The spin-lattice relaxation processes of 69Ga, 71Ga and 75As have been studied for the undoped GaAs single crystals and crystals doped with paramagnetic impurities Cr and Mn. The relaxation processes in all samples are well described by a single exponential function. The T2 dependence of the relaxation rate was in accordance with that predicted for the Raman mechanism of nuclear spin-lattice relaxation for all crystals investigated. The manganese doping showed a tendency to shorten slightly the spin-lattice relaxation times.

Electron paramagnetic resonance characterization of impurity Gd3+ ions in a PbWO4 single crystal

Lead tungstate single crystals doped with Gd2O3 were grown by the Czochralski method in Ar atmosphere. The electron paramagnetic resonance of the Gd3+ ion in a PbWO4 single crystal has been investigated at 9.4 GHz. The spectroscopic splitting tensor g and zero-field splitting parameters Bkq were determined using the effective spin Hamiltonian. The rotation patterns in the crystallographic planes together with spin Hamiltonian parameters of Gd3+ ion show that the local site symmetry of the Gd3+ ion in PbWO4 crystal is tetragonal and that the Gd3+ ion substitutes for the Pb2+ ion without local charge compensation at the nearby surroundings.

Compensation center of Cr3+ in GaAs codoped with Cr and In for obtaining a semi-insulating property

Abstract The dependences of Cr 2+ and Cr 3+ electron paramagnetic resonance (EPR) signals as a function of Cr concentration have been investigated in conjunction with clarifying the atomic configuration for obtaining semi-insulating properties in a vertical gradient freeze (VGF)-GaAs single crystal codoped with Cr and In (VGF-GaAs:Cr,In). Cr 2+ EPR signals are observed in both samples of semi-insulating and semiconducting GaAs:Cr,In single crystals, whereas Cr 3+ EPR signals can be observed only in semi-insulating samples. The semi-insulating properties become obvious when increasing the intensity of Cr 3+ and Cr 2+ EPR signals in VGF-GaAs:Cr,In crystals. Isolated Cr Ga 3+ ions play an important role in GaAs as a charge compensation center for obtaining semi-insulating property.