Papers by Keyword: Electron Paramagnetic Resonance (EPR)

Abstract: The spin Hamiltonian parameters (SHPs, g factors and hyperfine structure constants) defect structure for LiMgPO₄ doped with 0.1% Co²⁺ at 4.2 K are theoretically investigated from the perturbation formulas of the SHPs for a rhombically distorted octahedral 3d⁷ cluster. The impurity Co²⁺ on host Mg²⁺ site is found to suffer the larger axial relative compression ratio ρ (≈ 0.76%) and the planar angular variation Δφ (≈ 6.64^°) related to the host oxygen octahedron due to size mismatch. The calculated SHPs based on the above defect structure show good agreement with the measured results. Present study can be helpful to the preparation and characterization for the local structures for transition-metal impurities in lithium-magnesium phosphate, which may is helpful to search for the phosphor materials with better dosimetric characteristics.

Abstract: The carbonized silica (SiO₂:C) nanopowders were prepared by chemical modification of fumed silica (aerosil) by phenyltrimethoxysilane followed by thermal annealing at temperature in range of 500-800 °C in nitrogen flow. Their magnetic properties were investigated by electron paramagnetic resonance (EPR) in the temperature range from 4.2 K to 292 K. The initial and annealed SiO₂:C samples revealed carbon (C) related defects. The carbon related radicals (CRR) in annealed SiO₂:C nanopowders with g-factors 2.0042, 2.0039 were attributed to the oxygen (O)-centered CRR and C-centered CRR with a nearby O heteroatom, respectively. The EPR data were compared with infrared (IR) and photoluminescence (PL) data. It was found that the position of the PL band depends on the type of CRR formed after sample annealing. The PL with maximum intensity at 440 nm was found for the sample annealed at 500°C in which O-centered CRR was observed while in the sample annealed at 600°C in which C-centered CRR with a nearby O heteroatom was observed and graphite-like amorphous C clusters were appeared the peak of the PL band was shifted to the 510-520 nm.

Abstract: RF magnetron sputtering of two separate silicon and oxide (SiO₂ or Al₂O₃) targets in pure argon plasma was used for deposition of Si_x(SiO₂)_1-x and Si_x(Al₂O₃)_1-x films with x=0.15-0.7 on long fused quarts substrate. The effect of post-fabrication treatments on structural and light emitting properties of the films with different x values was investigated by means of Raman scattering, electron paramagnetic resonance and X-ray diffraction as well as by photoluminescence (PL) methods. The formation of amorphous Si clusters upon deposition process was found for the both types of films. The annealing treatment at 1150°C during 30 min results in formation of Si nanocrystallites (Si-ncs). The latter were found to be larger in Si_x(Al₂O₃)_1-x films than that in Si_x(SiO₂)_1-x counterparts with the same x values and are under tensile stresses. The investigation of photoluminescence properties of annealed films of both types revealed the appearance of visible-near infrared light emission. The Si_x(SiO₂)_1-x films demonstrated one broad PL band which peak position shifts gradually to from 1.4 eV to 1.8 eV with the x decrease. Contrary to this, for the Si_x(Al₂O₃)_1-x films two overlapped PL bands were observed in the 1.4-2.4 eV spectral range with peak positions at ~2.1 eV and ~1.7 eV accompanied by near-infrared tail. Comparative analysis of PL spectra of both types samples showed that the main contribution to PL spectra of Si_x(SiO₂)_1-x films is given by exciton recombination in the Si-ncs whereas PL emission of Si_x(Al₂O₃)_1-x films is caused mainly by carrier recombination either via defects in matrix or via electron states at the Si-ncs/matrix interface.

Abstract: The isotropic g factor and hyperfine structure constant for Ir⁴⁺ in CdO are theoretically studied from the perturbation formulas of these parameters for an octahedral 5d⁵ cluster based on the cluster approach. The calculated results show good agreement with the experimental data. The ligand orbital contributions should be taken into account due to significant covalency of the system with high impurity valence state even in the oxide.

Abstract: The high-Al-content Al_xGa_1-xN alloys, x>0.70, and AlN is the fundamental wide-band-gap material system associated with the technology development of solid-state LEDs operating at the short wavelengths in the deep-UV (λ < 280 nm). Yet, their properties are insufficiently understood. The present study is intended to bring elucidation on the long-time debated and much speculated Si transition from shallow donor in GaN to a localized deep DX defect in Al_xGa_1-xN alloys with increasing Al content. For that purpose electron paramagnetic resonance is performed on a particular selection of high-Al-content epitaxial layers of Al_0.77Ga_0.23N, alternatively Al_0.72Ga_0.28N, alloy composition.

Abstract: For powder XRD measurement at a higher temperature, there is a great difference (ΔT) between the temperature (T) detected by a thermocouple in a specimen stage and the actual temperature (T_a) of the ceramic powder surface irradiated by X-ray. The BaTiO₃ ceramic powder was employed to make an temperature correction on heating in light of the change of the ~ 45º characteristic peak in the vicinity of its tetragonal-cubic phase transition point (T_C = 130 °C). The thermal relaxation of BaTiO₃ is considered. When the BaTiO₃ ceramic powder was measured at T_C, the phase transition occurred at T = 170 °C and ΔT was determined as 40 °C, which is 10 °C higher than that of the ceramic bulk. The error of temperature correction is less than ± 3 °C at T_C. The approximation of a linear dependency T_a - T was given as a temperature correction line between 25 and 130 °C (T_a here).

Abstract: The formation mechanisms of paramagnetic centers originating from Zr³⁺ and Cr³⁺ ions and the influence of the nanoparticle composition on thermal generation processes of these paramagnetic centers in ZrO₂ structure were studied by electron paramagnetic resonance. A set of nanosized zirconium oxide powders (nominally pure ZrO₂, ZrO₂ doped with correspondingly Y₂O₃ and Sc₂O₃, Cr₂O₃ and Y₂O₃, as well as Cr₂O₃) was investigated: The influence of annealing on the EPR lines of Zr³⁺ and Cr⁵⁺ ions was found to be different. Annealing curves of EPR signals caused by Cr⁵+ ions have a maximum in temperature range 500–600 °C. Mechanisms of Zr³⁺ and Cr⁵⁺ ions formation were discussed.

Abstract: Defects in unintentionally Nb-doped 6H-SiC grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). An EPR spectrum with a hyperfine (hf) structure consisting of ten equal-intensity lines was observed. The hf structure is identified to be due to the hf interaction between an electron spin S=1/2 and a nuclear spin of 93Nb. The hf interaction due to the interaction three nearest Si neighbors was also observed, suggesting the involvement of the C vacancy (VC) in the defect. Only one EPR spectrum was observed in 6H-SiC polytype. The obtained spin-Hamiltonian parameters are similar to that of the Nb-related EPR defect in 4H-SiC, suggesting that the EPR center in 6H-SiC is the NbSiVC complex in the neutral charge state, NbSiVC0. Photoexcitation EPR experiments suggest that the single negative charge state of the NbSiVC complex is located at ~1.3 eV below the conduction band minimum.

Abstract: The g factors for the cubic Fe⁺ centers in LiF and NaF are theoretically investigated from the perturbation formula of the g factor for an octahedral 3d⁷ cluster including the contributions from the ligand orbital and spin-orbit coupling interactions. The increasing order of the g factor (i.e., LiF < NaF) can be ascribed to the decrease in covalency and the strength of cubic crystal-field of the systems. The validity of the results is discussed.

Abstract: The spin Hamiltonian parameters (g factors and the hyperfine structure constants) for the octahedral interstitial Fe⁰ and Mn in silicon are theoretically investigated using the perturbation formulas of these parameters for a 3d⁸ ion under octahedral environments based on the cluster approach. The theoretical results show good agreement with the experimental data, and the ligand contributions should be considered due to the strong covalency of the systems. The interstitial occupation of the above novel 3d⁸ impurities of rare valence states in silicon is discussed.