D. Schoemaker

Nitrogen and hydrogen in thick diamond films grown by microwave plasma enhanced chemical vapor deposition at variable H2 flow rates

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H2 gas flow rates, keeping a constant [CH4]:[H2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic at...

Electron and hole trapping in PbCl2 and PbCl2:Tl crystals

Abstract Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl 2 single crystals. Besides Pb 2 3+ self-trapped electron (STEL) centers the PbCl 2 :Tl crystals exhibit trapped-electron (PbTl) + -type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl 2+ centers the holes are self trapped at pairs of neighbouring Cl − anions resulting in V k type centers with various orientation and length of the Cl–Cl axis.

Electron spin resonance of rhodium-vacancy complexes in solution-grown NaCl crystals

Three different paramagnetic [RhCl6]4− complexes were detected in x-ray irradiated solution-grown NaCl single crystals: RhCl64−⋅nVac, n=2, 1, 0. These complexes all have a 4d7 ground state, with the unpaired electron spin mainly in a 4dz2 orbital and differ only by the presence of two, one, or none next-nearest cation vacancy (Vac). The RhCl64−⋅2Vac is formed at 77 K and partially converts into RhCl64−⋅1Vac at about 190 K. At room temperature the RhCl64−⋅0Vac is dominant, but traces of the RhCl64−⋅1Vac and RhCl64−⋅2Vac centers remain. A thermally induced reorientation motion of the 4dz2 molecular orbital is used to explain the temperature dependence of the RhCl64−⋅1Vac electron spin resonance spectrum.

Electron nuclear double resonance of stable Rh centers in solution-grown NaCl single crystals

Unlike the photographically important silver halides, large NaCl single crystals can be grown from solution. In such NaCl crystals, with doping comparable to practical AgCl and AgBr microcrystals, three stable Rh centers were detected and studied by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). The primary center was identified as a nonlocally compensated [RhCl6]4− complex, at variance with earlier reported EPR work. By lack of sufficient EPR and/or ENDOR evidence, the identity of the two other Rh related centers with orthorhombic and isotropic symmetry respectively, remains rather speculative.

Electron spin resonance study of aquated hexachloro-rhodate complexes in AgCl microcrystals

The electron and hole trapping and detrapping properties of three types of rhodium centers were determined by electron spin resonance in Rh3+-doped AgCl microcrystals. Three paramagnetic Rh2+ centers were identified after ultraviolet irradiation: RhCl64−, and the singly or doubly aquated RhCl5(H2O)3− and RhCl4(H2O)22−. All three defects have a 4d7 orbital ground state in a strong (distorted) octahedral field. The RhCl64−, RhCl5(H2O)3−, and RhCl4(H2O)22− centers act as deep electron traps (lifetime at 300 K greater than 50 min) and as recombination centers. These properties of the rhodium centers are consistent with photographic experiments.

Electron nuclear double resonance of a nearly axial Rh2+ center in solution-grown NaCl crystals

Rh3+-doped NaCl single crystals grown from aqueous solution were studied with electron paramagnetic resonance and electron nuclear double resonance after irradiation at liquid nitrogen temperature. [RhCl6]4− complexes with two next-nearest neighbor charge compensating vacancies have been detected. The two vacancy-Rh2+ axes make an angle of 900 to each other and are perpendicular to the center symmetry axis. After warming the sample, this center with C2v symmetry decays consecutively into two other [RhCl6]4− centers reported earlier in Rh3+-doped NaCl and AgCl Bridgman crystals on the one hand, and/or in solution-grown NaCl:Rh3+ crystals irradiated at room temperature on the other hand.

Off‐Center Displacement of Fe+ Ions in Irradiated SrCl2: Fe Crystals Grown in Chlorine

Chlorinated SrCl 2 :Fe 2+ crystals exhibit, after X-ray irradiation, two trapped-electron Fe + (I) and Fe + (II) centers both with axial symmetry, but different EPR spectrum parameters and production properties. The analysis of the experimental data strongly suggests a substitutional eight-fold coordination of the Fe + ion in the Fe + (I) center and a strong off-center displacement to a fourfold coordinated site in the Fe + (II) center resulting in S = 3/2 and S = 1/2 ground states, respectively.

Point defects in cubic boron nitride crystals

Abstract The results of a low-temperature study by high frequency (94 GHz) EPR on brown-to-dark colored single crystals selected from cBN crystalline powders prepared by the HPHT technique with boron excess, are presented. Previous investigations by low frequency (9.4 GHz) EPR spectroscopy on such dark polycrystalline c-BN powders resulted in the identification of two paramagnetic species D1 and D2 associated with the brown-to-dark coloration, the spectrum of the latter one being observed only above 100 K. The present research identifies the D1 species, studied by EPR at low temperatures, as consisting mainly from anisotropic paramagnetic centers with electron spin S =1/2, local symmetry axis along one of the crystal 〈111〉 axes and principal g values g ∥ =2.0032±0.0009 and g ⊥ =2.0094±0.0005 at T =10 K.

Optically Detected Microwave Resonance at 95 GHz of Exciton States in InAs/GaAs Quantum Dots

We report about the first observation of optically detected microwave resonance spectra of MBE-grown InAs/GaAs quantum dots (QD). The signal is detected via modulation of the total intensity of the QD emission induced by W-band (95 GHz) microwave excitation. Optically detected magnetic resonance (ODMR) lines are observed at higher fields when the static field is either parallel or perpendicular to the growth direction, with hole g-values |g h, ∥ | = 3.9 and |g h, ⊥ | = 3.0, respectively. These broad ODMR lines are tentatively attributed to hole spin transitions. A strongly anisotropic low-field transition is ascribed to cyclotron resonance (CR), corresponding to an effective mass of 0.059 m 0 .

Hyperfine behavior of the laser-active Tl0(1) center in alkali-halides

Abstract The hyperfine parameters, obtained from an electron-spin-resonance study, of the laser-active Tl0(1) center in four alkali-halides have been analysed and compared to the Tl0(2) center results. It is shown that the anisotropic contribution to the hyperfine components is virtually the same for all the Tl0 centers but that the isotropic contribution varies widely in size and sign. This is attributed to the varying amount of s-mixing caused by the varying odd crystal-field component throughout the alkali-halides for the Tl0(1) center. For Tl0(2) such s-mixing does not occur because this center possesses inversion symmetry. The results obtained show that an analysis of the hyperfine components provides a powerful tool to determine whether or not a given np1 atom or ion center is positioned in a crystal field possessing inversion symmetry. As an illustration the hyperfine parameters of some Pb+(6p1) centers in KCl and RbCl are analyzed.