Deep impurity states and intrinsic defects in photorefractive Cd1−x FexTe crystals

AbstractStudies were made of the low-temperature optical and photoelectric properties of Cd_1−x Fe_xTe crystals (x=0.0038) which provided information on the optical quality of these crystals and the nature of their inhomogeneity, and also revealed deep Fe²⁺ impurity centers and singly charged acceptor complexes. It was established that these complexes, which include doubly charged cadmium vacancies and ionized donors, are anisotropic. It was shown that their anisotropy is determined by the nature of the donor atom and its position in the crystal lattice (at a cationic or anionic site). Since these crystals contain real deep impurity centers and acceptor complexes, a mechanism is proposed for the photorefractive effect in these crystals. It was observed for the first time that the photorefractive properties of CdTe crystals containing impurity 3d elements may exhibit anisotropy unrelated to that of the electrooptic effect. Fiz. Tverd. Tela (St. Petersburg) 40, 1216–1220 (July 1998)     

CuI晶体缺陷态电子结构及其施主-受主对发光机理的研究

运用相对论密度泛函理论和嵌入分子团簇方法，模拟计算了具有γ相CuI晶体的本征缺陷态电子结构.结果表明，四面体间隙Cu和Cu空位最有可能在禁带中引入浅施主和受主能级，从而形成施主-受主对(DAP)，产生420—430 nm的DAP复合发光. 关键词： γ-CuI晶体 密度泛函理论 电子结构 发光机理     

Low temperature thermal properties and intrinsic defects in vitreous silica

Measurements of the low temperature specific heatC _p (0.04KT2.4K) and thermal conductivityk (0.5KT20K) of high purity vitreous silica (Suprasil W) in the as-received and electron-irradiated states are reported. In the temperature range where the localized excitations inherent in glasses dominateC _p (T0.5K) no change is observed upon electron irradiation of up to 10¹⁹e^– (total dose). An anomalyC around 1.8K is observed inC _p which is reduced by 40% upon irradiation. However, ask is not affected by electron irradiation, localized excitations as origin forC can be ruled out. The density of paramagnetic centers in the most heavily irradiated sample is 510¹⁷cm^–3 as obtained from the ESR signal. It has been suggested that these centers have diamagnetic precursors in the unirradiated glass. Our measurements indicate that those defects are not associated with the localized excitations.     

Influence of structural factors on the spectral and luminescence properties of metalloporphyrins

On the basis of a study of the influence of alkyl substitution on the absorption spectra of metalloporphyrins, the nodal properties of the optical electrons, obtained in the MOLCAO method, have been confirmed. It has been established that alkyl substituents have a comparatively marked influence on the phosphorescence yield. In a study of the influence of additional complex formation on the phosphorescence, it has been shown that the effect of conjugation plays a significant part in removing the intercombination exclusion when heavy atoms are present in the molecule. It has been concluded that the D_4h symmetry of the conjugated system is preserved approximately in the pyridinates of Mg porphyrins. Phosphorescence of associates of Cu porphyrins has been detected.Presented at the Second All-Union Conference on the Use of Physical Methods in the Chemistry of Complex Compounds (Kishinev, October 1965).     

Interaction of hydrogen and deuterium with intrinsic atomic defects in high-purity electron-irradiated nickel

Abstract

High-purity nickel was irradiated with 2 MeV electrons at temperatures below 80 K to a dose of 1 × 10²³ e^?/m² in the as-prepared state and after charging with H or D. By means of magnetic after-effect measurements relaxations of anisotropic radiation-induced defects and of defect-hydrogen complexes were investigated in the temperature range between 4.2 and 500 K. The isochronal annealing behaviour of these relaxations and the isochronal recovery of the residual resistivity was measured simultaneously on the same specimens. At temperatures below the hydrogen mobility (< 160 K) in charged irradiated specimens relaxation maxima are observed at 45, 100, 115 and 140 K which show no isotope shift for H and D charging. The maxima below 160 K are explained by defect-hydrogen complexes, where radiation-induced defects reorient around immobile hydrogen atoms. Above 160 K, where hydrogen atoms get mobile, in charged irradiated specimens a broad relaxation maximum appears at 170 K which shows an inverse isotope shift for H and D charging. This 170 K maximum anneals in Stage III. A hydrogen diffusion maximum observed in charged specimens at 215 K prior to irradiation is missing after electron irradiation. The 170 K relaxation is explained by defect-hydrogen complexes, where hydrogen atoms reorient around immobile radiation-induced defects while the long-range hydrogen diffusion is suppressed by these defects. In such relaxation measurements hydrogen and deuterium atoms are used as a “probe” to investigate radiation-induced defects.     

Dimer self-organization of impurity ytterbium ions in synthetic forsterite single crystals

Paramagnetic centers formed by impurity Yb³⁺ ions in synthetic forsterite (Mg₂SiO₄) grown by the Czochralski technique are studied by X-band CW and pulsed EPR spectroscopy. These centers are single ions substituting magnesium in two different crystallographic positions denoted М1 and М2, and dimer associates formed by two Yb³⁺ ions in nearby positions М1. It is established that there is a pronounced mechanism favoring self-organization of ytterbium ions in dimer associates during the crystal growth, and the mechanism of the spin–spin coupling between ytterbium ions in the associate has predominantly a dipole–dipole character, which makes it possible to control the energy of the spin–spin interaction by changing the orientation of the external magnetic field. The structural computer simulation of cluster ytterbium centers in forsterite crystals is carried out by the method of interatomic potentials using the GULP 4.0.1 code (General Utility Lattice Program). It is established that the formation of dimer associates in the form of a chain parallel to the crystallographic axis consisting of two ytterbium ions with a magnesium vacancy between them is the most energetically favorable for ytterbium ions substituting magnesium in the position М1.     

Studies of intrinsic luminescence in KCl

Measurements have been made of the luminescence intensity, lifetime, emission spectra, and thermoluminescence associated with self-trapped exciton states in KCl, at temperatures of 7°K and above. These results indicate the presence of another exciton-like state in addition to the well-known state that decays by emission at 2.32 eV. Thermoluminescence glow peaks and spectra are correlated with F + V_k and F + H recombination processes.     

Influence of copper impurity and structure defects on the properties of compound CdSnAs2

A study was made of the mechanisms of formation and interaction of impurities and intrinsic defects in CdSnAs₂ crystals during diffusion doping of this compound with copper and during subsequent low-temperature (300°C) annealing. This was done by diffusing copper into samples with different initial defect structure under conditions of controlled arsenic vapor pressure and applying the method of quasichemical reactions to analyze the experimental data. It was found that the special features of the physical properties of n-type CdSnAs₂:Cu crystals — the low density and high mobility of electrons-were due to the high reactivity of copper resulting in binding of the original crystal defects to form neutral complexes. It was found that the anomalous annealing of CdSnAs₂: Cu samples, resulting in n-p conversion of the type of conduction, was associated with precipitation of a solid solution of an accidental donor impurity (most probably oxygen). The diffusion doping with copper was found to be one of the most effective methods for controlling the properties of CdSnAs₂, particularly in the preparation of high-resistivity crystals.Translated from Izvestiya Uchebnykh Zavedenii, Fizika, No. 5, pp, 102–108, May, 1980.     

First-principles study on the formation energies of intrinsic defects in LiNbO3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO₃): (1) the oxygen-vacancy model , (2) the niobium-vacancy model , and (3) the lithium-vacancy model . The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult.     

Terahertz impurity luminescence under the interband photoexcitation of semiconductors

The observation of intense terahertz luminescence under the interband photoexcitation of n-GaAs and p-Ge semiconductors at low temperatures is reported. The terahertz photoluminescence is caused by radiative transitions, which accompany the capture of nonequilibrium carriers by impurity centers. These centers are in turn created in the crystal due to the impurity-assisted electron-hole recombination. The external quantum yield of the luminescence reaches 0.1%.     

Quantum yields of oxazines and phenoxazines and the investigation of their spectral luminescence properties in binary mixtures

The electronic spectra of Nile Blue acetate, phenoxazines, and its fluorinated analogs have been investigated experimentally and theoretically. The spectral-luminescence properties of these molecules have been studied in detail in ethanol and in nonpolar solvents (toluyl, dimethylsulfoxide) with an alcohol admixture (propanol, butanol, and isobutyl alcohol). The role of micelle formation in non-aqueous solvent on the spectral-luminescence of these molecules is elucidated. Also considered in this paper is the dependence of the quantum fluorescence yield of several oxazines and phenoxazines in ethanol on the structure of the dye molecules.B. D. Kuznetsov Siberian Physicotechnical Scientific-Research Institute at the State University, Tomsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 88–93, May, 1992.     

Manifestation of the Hanle effect in submillimeter EPR spectroscopy of thulium impurity ions in synthetic forsterite

A signal related to the spin level crossing in a zero magnetic field—the Hanle effect—has been registered for the first time in the EPR spectrum. It has been shown that, in the general case, the shape of the signal is determined by two qualitatively different mechanisms: (i) the interference of unsteady-state contributions to the dynamics of atomic coherences (electric or magnetic quantum transition moments with certain phases) with close frequencies (“beats at a zero frequency”) and (ii) the summation of resonant signals determined by the steady-state dynamics of the same atomic coherences. The relaxation time of spin coherences has been determined for the EPR transition of Tm³⁺ ions in synthetic forsterite.