Electronic level diagram and structure of metastable centers in CdF2:Ga and CdF2:In semiconducting crystals

A study is reported of the optical properties of wide-gap, predominantly ionic cadmium fluoride crystals in photo-and thermally stimulated transformations of metastable indium and gallium centers. An analysis of these properties leads one to a conclusion of gallium having two metastable states (two types of deep centers). The deep-center binding energies and the barriers separating the shallow (hydrogenic) and deep centers have been determined for both impurities. Configuration-coordinate diagrams are developed, and microscopic models for the deep centers are proposed. It is concluded that these centers are identical with the metastable DX centers in typical semiconducting crystals. Thus cadmium fluoride is the most ionic among the crystals where DX centers have thus far been found. The potential of using such crystals for optical information recording is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 2130–2136 (December 1997)     

Donor impurities and DX centers in the ionic semiconductor CdF2

Group-III impurities in the wide-gap ionic crystal CdF₂ are examined. After being heated in a reducing atmosphere, crystals with these impurities acquire semiconductor properties, which are determined by electrons bound in hydrogen-like orbitals near an impurity. Besides these donor states, nontransition impurities form “deep” states accompanied by strong lattice relaxation, i.e. they are strongly shifted along the configuration coordinate. These states are a complete analog of DX centers in covalent and ionic-covalent semiconductors. The difference of the behavior of nontransition impurities from that of transition and rare-earth impurities is analyzed. This difference is attributed to the character of the filling of their valence shells by electrons. A deep, multilevel analogy is drawn between the properties of deep centers in typical semiconductors with an appreciable fraction of a covalent bond component and in predominantly ionic crystal CdF₂ with semiconductor properties. Fiz. Tverd. Tela (St. Petersburg) 39, 1050–1055 (June 1997)     

Paramagnetic susceptibility of additively colored CdF2:In photochromic crystals

The paramagnetic susceptibility K _para of CdF₂:In crystals with metastable indium centers has been measured in darkness after photobleaching the crystals in visible light in the temperature interval 4–300 K. For crystals cooled in darkness to liquid-helium temperature K _para is wholly determined by the accompanying impurity Mn²⁺ with magnetic moment J=5/2. Illumination of the crystals leads to the appearance of an induced signal δ K _para due to the formation of centers with J=1/2. The results of the experiments indicate the absence of paramagnetism in the deep state of the indium center and its existence in the shallow (donor) state, i.e., they confirm the two-electron (negative-U) nature of the deep indium level in CdF₂. Fiz. Tverd. Tela (St. Petersburg) 39, 1205–1209 (July 1997)     

Thermostimulated luminescence of CdF2:Eu crystals

Using the method of fractional thermostimulated luminescence (FTSL), the temperature dependence of the mean activation energy of recombination processes in CdF₂:Eu³⁺ crystal was obtained. After thermal annealing of the crystal, thermostimulated luminescence peaks were identified. Anomalously low frequency factor (s=10⁷ s ^–1) of the recombination processes can be explained by the dependence of the resonance energy transfer probability on intercentre distance.The authors are very grateful to Dr. E. Kotomin for valuable comments and Dr. C. Paracchini for supply of CdF₂:Eu crystals.     

Properties of CdF2:Ga as a medium for real-time holography

The metastable nature of the excited state of bistable Ga centers in semiconducting CdF₂ crystals allows for the use of CdF₂:Ga and CdF₂:Ga,Y crystals as materials for real-time holography over a wide range of response times (1–1000 ms). The characteristics of these materials and optimal conditions for their use are discussed in the framework of a model that describes the decay of photo-induced gratings written in them. Received: 20 November 2000 / Published online: 20 April 2001     

Energy barrier between states of a bistable center in photochromic crystals CdF2:Ga and CdF2:In

The establishment of thermal equilibrium between photoinduced (shallow) and ground (deep) states of bistable DX centers in photochromic crystals CdF₂:In and CdF₂:Ga, which are used as real-time holographic media, is studied based on the notions of chemical kinetics. Two mechanisms of mutual transformation of shallow and deep centers—the tunnel mechanism and the mechanism with the participation of free charge carriers—are considered. Equations describing the decay of a photoinduced shallow state are obtained. These equations take into account the distribution of electrons between the photoinduced and ground states and the conduction band. Analysis of the experimental kinetic curves of the decay of photoexcited shallow centers makes it possible to determine the activation energies and barrier height for thermally activated processes of mutual transformation of shallow and deep centers. In CdF₂:In and CdF₂:Ga, this barrier, which determines the decay kinetics of holograms, lies above the bottom of the conduction band by ～10 and ～500 meV, respectively.     

Dynamic reflection holograms in CdF2 crystals with bistable centers

The diffraction efficiency and the recording and relaxation times of dynamic reflection holograms, recorded in CdF₂ crystals with bistable centers are studied experimentally in the temperature range 20–100°C. In the model experiments which measured the quality of the wave reflected from the hologram, the dynamic wavefront distortions are demonstrated to be efficiently compensated using a holographic corrector based on these crystals. CdF₂ crystals with bistable centers are likely to be useful in solving problems of correction of laser light wavefront and image correction in observation telescopes with nonideal primary mirrors.     

Phase transitions in the (Cs1−x Rbx)2ZnI4 system

(Cs_1−x Rb_x)₂ZnI₄ crystals were grown by two different methods with Rb concentrations varying from x=0 to 2.5%. ¹²⁷I NQR and calorimetric measurements showed that crystals grown by the Bridgman technique contain residual impurities (∼0.5%) for all x. While x=0 crystals grown from solution do not contain detectable impurities, they allow incorporation only of a low Rb concentration, not above 0.5%. A transition-temperature-concentration (x) phase diagram has been constructed for Bridgman-grown crystals from NQR data. Rb doping shifts the normal-incommensurate and incommensurate-ferroelastic phase-transition points toward higher temperatures with different rates. The P2₁/m↔P1 first-order transition shifts toward lower temperatures. The region of low Rb concentrations lies closest to the critical point. Fiz. Tverd. Tela (St. Petersburg) 41, 143–147 (January 1999)     

Specific features in the kinetics of thermally stimulated luminescence of α-Al2O3 crystals containing defects

A study of specific features in the kinetics of thermoluminescence (TL) of defective α-Al₂O₃ crystals is reported. The TL properties are experimentally shown to be related to the presence of oxygen vacancies of thermochemical or radiation origin. It is established that the differences in the TL kinetics in α-Al₂O₃ samples grown under different conditions can be accounted for by the hypothesis that the energy spectra of the trapping levels have different widths. As is demonstrated with a titanium impurity, this broadening of the energy spectrum can originate from the presence of traces of native impurities in the α-Al₂O₃ lattice. Fiz. Tverd. Tela (St. Petersburg) 39, 1538–1543 (September 1997)     

New long-lived memory effects in LiNbO3 crystals

This paper describes a new way of recording information by changing the potential contour at the surface of z-cut crystals of LiNbO₃. The effect of various kinds of external perturbations on the rate of decay of optical images written using this method is investigated, along with the possibility of writing information on crystals with transition-metal impurities. Fiz. Tverd. Tela (St. Petersburg) 40, 337–339 (February 1998)     

Ionic,electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

Abstract

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF₂ BaF₂ and LiBaF₃ crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF₃ indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by V_k center detrapping and hole-diffusion controlled tunnel recombination within pairs like {D_n e^?…V_k }.     

Fluctuation-induced broadening of Mn2+ EPR lines in the incommensurate phase of Rb2ZnCl4 crystals

The EPR spectra of Mn²⁺ ions in Rb₂ZnCl₄ crystals is investigated in the vicinity of the transition from the paraelectric phase to an incommensurate modulated phase. When these crystals are cooled below the transition temperature T _i =304 K, a splitting of the resonance lines is observed in the singular spectrum. A one-harmonic model is used to discuss the contributions that fluctuations in the amplitude and phase of the incommensurate displacement wave make to the local width of the singular spectra. It is shown that anomalies in the local width of the low-temperature singular peaks observed in the vicinity of T _i are caused by amplitude fluctuations. Fiz. Tverd. Tela (St. Petersburg) 41, 1668–1674 (September 1999)     

Optical properties and radiation effects in layered crystals of (CnH2n+nNH3)2MCl4: n = 1, 2, 3; M = Cd,Cu, Mn

Abstract

A new absorption band has been found at 5.10 eV in (C _n H_{2n + 1}NH₃)₂CdCl₄: n = 1, 2, 3 in addition to the absorption bands of CdCl₂ whose electronic structure resembles the former crystals. The energy of the additional peak shifts with temperature by as much as 0.38 eV from 5.10eV at room temperature (RT) to 5.48 eV at liquid nitrogen temperature. This large peak shift is attributed to a structural phase transition between these two temperatures. A new type of electron center has been found in these crystals (M = Cd, Mn; n = 1, 2, 3) irradiated with X-rays at 15 K in addition to the Cl₂ ^?. This shows optical absorption bands (IR bands) in the infrared region of 10 ～ 20 kcm ^?1. The IR bands are assigned to an electron center where an electron is trapped at an ammonium site in the neighborhood of a Cl^? vacancy.     

Dielectric relaxation in the weak ferroelectric Li2Ge7O15 near the ferroelectric phase transition

The dielectric permittivity of Ni-doped Li₂Ge₇O₁₅ crystals was studied in the vicinity of the ferroelectric phase transition. Introduction of Ni has been shown to suppress the dielectric anomaly and to reduce substantially the transition temperature. A temperature hysteresis in ɛ (T) has been observed in nominally pure and Ni-doped Li₂Ge₇O₁₅ crystals near the transition point. Measurements performed under cooling from the paraphase reveal dispersion of dielectric permittivity at Debye relaxation frequencies of the order of 10⁴–10⁵ Hz at T _c . It is proposed that the hysteresis phenomena and the low-frequency dispersion are caused by residual defects (of the type of random local fields), which become polarized in the ferroelectric phase and become disordered above T _c . Fiz. Tverd. Tela (St. Petersburg) 40, 2198–2201 (December 1998)     

The effect of perturbedk-selection and gap shrinkage on the optical transitions in III–V semiconductors

Optical transitions in direct semiconductors are governed according to simple one-electron treatment by ak-selection rule, which in doped or mixed crystals is but lifted to some extent (k) caused by 1) shallow impurities, 2) isoelectronic impurities or 3) alloy scattering. Values of k for these mechanisms are given and the implications for line shapes of optical spectra are discussed.Furthermore, as to the position of lines in the optical spectra of highly doped or highly excited crystals, gap shrinkage effects have to be considered, three main mechanisms of which can be distinguished: 1) Quadratic response of the band edges with respect to the fluctuating potential of the (screened) impurities, 2) polaron effects and 3) carrier exchange and correlation effects. Results of theoretical calculation are compared with experimental findings for GaAs and GaP. The practical importance of gap shrinkage effects for gain measurements and for the operation of (Ga, Al) As laser diodes is pointed out.From the Coulomb interaction of the carriers an additional mechanism for the lifting of thek-selection rule (especially valid for pure, but excited semiconductors) can be derived (plasmon coupling): Estimations show that the simplek-selection rule is almost never fulfilled within the energy range of emission spectra and that these spectra can be well explained simply assuming no-k-selection even for pure direct material.     

Muon-spin-relaxation investigation of EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

The magnetic properties of the EuMn₂O₅ multiferroic (samples consisting of single crystals and ceramic samples) have been investigated by the muon-spin-relaxation (μSR) method in the temperature range of 10–300 K. Below the magnetic ordering temperature T _N = 40 K, the loss of the polarization of muons and the effect of the external magnetic field have been observed. Both phenomena can be explained by an additional channel of the depolarization of muons owing to the appearance of muons in a medium with a low electron density due to the charge separation process (the redistribution of the electron density in the phase transition process). The “memory” phenomenon has been revealed in a sample in the external magnetic field; the memory relaxation time depends on the size of the structure units of the samples (single crystals or ceramic grains).     

Physical properties of n-CdGeAs2 single crystals prepared by low-temperature crystallization

The first results obtained in studies of the temperature dependences of electrical conductivity and Hall constant of n-CdGeAs₂ single crystals prepared by low-temperature crystallization are reported. It has been established that the method developed permits growing single crystals with a free-electron concentration ⋍(1−2)×10¹⁸ cm⁻³ and a Hall mobility ⋍10000 cm²/(Vs) at T=300 K. It is shown that the temperature dependence of Hall mobility exhibits a behavior characteristic of electron scattering by lattice vibrations, whereas below 150 K a deviation from this law is observed to occur evidencing an increasing contribution of static lattice defects to scattering. The Hall mobility in the crystals prepared was found to reach ⋍36000 cm²/(Vs) at 77 K. Photosensitive heterojunctions based on n-CdGeAs₂ single crystals were prepared. The spectral response of the photosensitivity of these structures is analyzed. It is concluded that this method is promising for preparation of perfect CdGeAs₂ crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1190–1193 (July 1999)     

Experimental investigation of superconductivity in 2H-NbSe2 single crystal

The basic quantities characterizing the superconducting behaviour of pure 2H-NbSe₂ single crystals have been determined from specific heat measurements, performed between 0.3 and 10 K in magnetic fields up to 55 KG. When Ti impurities are added, changes are observed both in the superconducting parameters and in the onset of charge density waves, strengthening the idea that a connection exists between the two phenomena.     

Accurate depolarization ratio measurements for all diatomic hydrogen isotopologues

The Raman depolarization ratios for individual Q₁(J”) branch lines of all diatomic hydrogen isotopologues – H₂, HD, D₂, HT, DT, and T₂ – were measured, for all rotational levels with population larger than 1/100 relative to the Boltzmann maximum at room temperature. For these measurements, the experimental setup normally used for the monitoring of the tritiated hydrogen molecules at KArlsruhe TRItium Neutrino experiment was adapted to optimally control the excitation laser power and polarization, and to precisely define the Raman light collection geometry. The measured Raman depolarization values were compared to theoretical values, which are linked to polarizability tensor quantities. For this, the ‘raw data’ were corrected taking into account distinct aspects affecting Raman depolarization data, including (1) excitation polarization impurities; (2) extended Raman excitation volumes; and (3) Raman light collection over finite solid angles. Our corrected depolarization ratios of the hydrogen isotopologues agree with the theoretical values (based on ab initio quantum calculations by R.J. LeRoy, University of Waterloo, Canada) to better than 5% for nearly all of the measured Q₁(J”) lines, with 1σ confidence level. The results demonstrate that reliable, accurate Raman depolarization ratios can be extracted from experimental measurements, which may be substantially distorted by excitation polarization impurities and by geometrical effects. Copyright © 2013 John Wiley & Sons, Ltd.     

Optically stimulated depolarization currents in Ca(NbO3)2 crystals

In Ca(NbO₃)₂ crystals a depolarization current is obtained in short-circuit measurements by light irradiation at 90 K after application of an external electrical field at cooling down. The spectral response of this current shows a broad band over the whole visible wavelength range. The position and height of the current maximum depend on the amount and direction of the spectral scanning speed. Since the short-circuit current is assumed to be associated with the rate of heterocharge neutralization by the drift of photo-excited carriers these phenomena are discussed in the model of persistent internal polarization rather than in that of moving zero-field plane. The results of calculation show good agreement with the experimental data.