Dissymmetrization in X-irradiated RbTiOPO4 crystal

Electron paramagnetic resonance has been used to study the hole and electron paramagnetic centers formed in X-irradiated RbTiOPO₄, the crystals of the KTP family. X-irradiation of RbTiOPO₄ crystals at 77 K produced an oxygen hole center and four different trivalent titanium electron centers I₁, II, III and IV. Theg-tensors, their principal values and axes for the defects were calculated and compared with those for KTiOPO₄ centers. X-irradiation at 300 K produced another two oxygen hole centers and three electron centers I₁, I₂ and II. EPR spectra of the center II revealed dissymmetrization, i.e., irregular distribution of growth defects, between the physically equivalent sites lowering the point group symmetry of the local environment of paramagnetic centers Ti³⁺.     

Defect sites in thin films of germanium dioxide irradiated with silicon ions

Optical transmission spectra of GeO₂ films irradiated with silicon ions and subjected to postimplantation annealing in the regime of silicon nanocrystal formation are analyzed. It is shown that point defects form in the films after irradiation with doses D ～ 10²⁰ m^?2: germanium electron centers, neutral oxygen vacancies, and Ge²⁺ centers, which have been annealed at a temperature of 1000°C for an hour. At D ≥ 1 × 10²¹ m^?2, more complex defects arise in the films, which are only partially annealed under the same conditions.     

Structures of Fe3+-Vo defects in ABO3 perovskite-type crystals

The displacements of Fe³⁺ ions and hence the structures of Fe³⁺-V_o defects in ABO₃ oxide perovskites are studied by analyzing their electron paramagnetic resonance data from the high-order perturbation formula of zero-field splitting on the basis of the spin-orbit coupling mechanism. The results suggest that the Fe³⁺ ions in the Fe³⁺-V_o defect centers are displaced away from the oxygen vacancy V_o by about 0.035 nm. The reasonableness of the displacement is discussed.     

Excitation of self-trapped-exciton luminescence in the recombination of frenkel defects in BeO

Polarized luminescence and transient optical absorption (TOA) induced by pulsed electron irradiation in beryllium oxide crystals were studied. Exponential stages with decay times τ = 6.5 ms were observed to exist in luminescence bands at 4.0, 5.0, and 6.7 eV, which coincide in spectral composition and polarization characteristics with the luminescence of self-trapped excitons (STEs) of two types. The formation efficiency of centers with a 6.5-ms decay time is comparable to that of triplet STEs. The general characteristics of the kinetics and the decay times of the TOA of these centers do not depend on electron fluence and are governed by the monomolecular recombination process. The spectra of TOA centers with a decay time of 6.5 ms were found to be similar to those of V-type hole centers and STE hole components. The mechanism by which recombination of closely spaced, spatially correlated Frenkel pairs, Be⁺ and V^? centers, brings about an exponential component with a 6.5-ms decay time in the luminescence of STEs of two types in BeO is discussed.     

Correlation between defect-related electroluminescence and charge trapping in Gd-implanted SiO<Subscript>2</Subscript> layers

When amorphous silica is bombarded with energetic ions, various types of defects are created as a consequence of ion-solid interaction (oxygen deficient centers (ODC), non-bridging oxygen hole centers (NBOHC), E^′-centers, etc.). Luminescent peaks from oxygen deficiency centers at 2.7 eV, non-bridging oxygen hole centers at 1.9 eV and defect centers with emission at 2.07 eV were observed by changing the concentration of implanted Gd³⁺ ions. Charge trapping in Gd-implanted SiO₂ layers was induced using constant current electron injection to study the electroluminescence intensity with dependence on the applied voltage change. The process of electron trap generation during high field carrier injection results in an increase of the electroluminescence from non-bridging oxygen hole centers. Direct correlation between electron trapping and the quenching of the electroluminescence at 2.07 eV and 2.7 eV was observed with variation of the implanted Gd concentration. PACS 78.60.i; 72.20.Jv; 78.20.-e     

The Formation of Dimeric Centers of Copper,Iron and Gadolinium in Modified Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript>

It is discovered that the electron paramagnetic resonance (EPR) spectrum of the doubly charged copper centers occurs in single crystals of Pb₅Ge₃O₁₁ doped with gadolinium or iron after annealing in an atmosphere containing chlorine and bromine. Similar annealing of the crystals doped with copper in a chlorine and fluorine atmosphere leads to redistribution of the intensities of the EPR spectra of two types of Cu²⁺ centers. The influence of annealing on the ongoing intensity of spectra of the dimeric triclinic centers Fe³⁺–A, Gd³⁺–A (where A represent Cl^?, Br^?, O^2?, F^?) was the subject of this research. Consideration is given to the mechanisms for changing the charge state and association of copper center with defects.     

Trapped-hole centers in MgO single crystals

The properties of the majority trapped-hole centers in MgO, such as g-factors, positions of absorption and luminescence bands, and temperatures of thermal destruction, have been analyzed with the emphasis on the observed regular trends and interrelations between the properties of these centers. Particular emphasis has been placed on the positively charged [Be]⁺ and [Ca]⁺ trapped-hole centers, which have a large cross section for recombination with conduction electrons. In these centers, a hole is localized at an oxygen ion near the impurity Be²⁺ or Ca²⁺ ion located at a regular cation site. The generation and transformation of defects due to the recombination of either relaxed conduction electrons with OH⁻-containing hole centers or cold and hot electrons with [Be]⁺ and [Ca]⁺ centers have been considered. Using the interrelation of the characteristics of hole centers and taking into account that the recombination emission band revealed at ∼6.8 eV is due to the Ca²⁺-containing centers that are stable below 50 K, the prospects for the EPR detection of the [Ca]⁺ center at T < 4.2 K have been discussed.     

Electron-irradiated n+-Si as hole injection tunable anode of organic light-emitting diode

Traditionally, n-type silicon is not regarded as a good anode of organic light emitting diode (OLED) due to the extremely low hole concentration in it; however, when doped with Au element which acts as carrier generation centers, it can be, as shown in our previous work. In this study, we demonstrate a new kind of carrier generation centers in n⁺-type silicon, which are the defects produced by 5 MeV electron irradiation. The density of carrier generation centers in the irradiated n⁺-Si anode can be controlled by tuning the electron irradiation time, and thus hole injection current in the OLEDs with the irradiated n⁺-Si anode can be optimized, leading to their much higher maximum efficiencies than those of the OLEDs with non-irradiated n⁺-Si anode. For a green phosphorescent OLED with the irradiated n⁺-Si anode, the current efficiency and power efficiency reach up to 12.1 cd/A and 4.2 lm/W, respectively.     

Generation Kinetics of Nonequilibrium Charge Carriers in Crystals with Deep Impurities Involving Two-Center Transitions between Band and Impurity States

A new and efficient mechanism of nonlinear photoexcitation of a transparent crystal with deep impurity centers is proposed. It is hypothesized that the energy of a quantum of light is smaller than the energy gap between the bottom of the conduction band and the impurity level, but is larger than the gap between the impurity level and the top of the valence band. The kinetics of nonlinear cascade generation of nonequilibrium charge carriers is considered taking into account two-center processes in which the energy transfer and the photon absorption occur in one and the same elementary event. The dependences of the quasi-equilibrium concentrations of nonequilibrium charge carriers in the bands and of the occupancy of impurity states on the laser-radiation intensity are obtained. It is shown that the generation process of nonequilibrium electron–hole pairs is of a threshold nature. Depending on the concentration of impurity centers, the threshold intensities can be ~10⁵–10⁷ W/cm², while the setting time of the quasi-equilibrium occupancies of electronic states is ~10–0.1 ns.     

Laser photoelectron microspectroscopy for imaging impurity ions,color centers,and small-size irregularities in dielectric media

New photoelectron microscopy application fields are considered for imaging the arrangement of color centers, small-size defects, and impurity ions in dielectric matrices. The techniques proposed are based on the first experimental results obtained in studying the possibilities of observing stepwise laser photoelectric effect in ZrO₂:Nd³⁺, where photoelectron emission results from the stepwise absorption of light by the impurity ions Nd³⁺. In that case, the electrons being emitted originate in immediate proximity to the impurity ions, which offers possibilities to image the above structures by photoelectron microscopy techniques.     

Interaction studies of cysteine with Li+, Na+, K+, Be2+, Mg2+, and Ca2+ metal cation complexes

The coordination geometries, electronic features, metal ion affinities, entropies, and the energetics of Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, and Ca²⁺ metal cations with different possible conformations of cysteine complexes were studied. The complexes were optimized using density functional theory (B3LYP) and second order Moller–Plesset Perturbation (MP2) theory methods using 6‐311 + +G** basis set. The interactions of the metal cations at different nucleophilic sites of cysteine conformations were considered after a careful selection among several binding sites. All the metal cations coordinate with cysteine in a tridentate manner and also the most preferred position for the interaction. It is found that, the overall structural parameters of cysteine are not altered by metal ion substitution, but, the metal ion‐binding site has undergone a noticeable change. All the complexes were characterized by an electrostatic interaction between ligand and metal ions that appears slightly more pronounced for lithium and beryllium metal complexes. The metal ion affinity (MIA) and basis set superposition error (BSSE) corrected interaction energy were also computed for all the complexes. The effect of metal cations on the infrared (IR) stretching vibrational modes of amino N? H bond, side chain thiol group S? H bond, hydroxyl O? H bond, and Carbonyl C?O bond in cysteine molecules have also been studied. The nature of the metal ion‐ligand bond and the coordination properties were examined using natural bond order (NBO) at bond critical point (electron density and their Laplacian of electron density) through Atoms in Molecules (AIM) analyses. Copyright © 2010 John Wiley & Sons, Ltd.     

Photochemistry and reactions of OH− defects and F centers in Alkali Halides

Additively colored KCl:OH^? crystals show under combined UV and visible irradiation an irreversible destruction of F centers and visible absorption. This process, which is effective over the whole temperature range 4–300 K, was studied systematically in terms of the reaction efficiency and products using UV-visible and IR (local mode) spectroscopy. The UV photo-dissociation of OH^? defects and photo-ionization of F centers produce a combined net effect, in which at higher temperatures all F centers are converted into U centers, thus completely bleaching the colored crystal. By these photo-reactions, high contrast visible images can be produced which are stable under visible light at RT, and are thermally stable up to 650°C. Besides the optical information-storage aspect, these photo-reactions can be used for controlled production of U_A centers if the crystal contains alkali-ion impurities like Na⁺.     

Radiation phenomena in microinhomogeneous structures of flouroaluminate glass-like materials

The simulation of the fluoroaluminate matrix with small additives of phosphates has made it possible to create low-scattering glasses with a wide transparency window in the infrared region and to study the fundamental problem on the formation of radiation defects typical for the phosphate matrix (PO ₃ ^2? hole, PO ₃ ^2? electron, and PO ₄ ^2? hole defects). By comparing the spectra of induced optical absorption, electron paramagnetic resonance, and Raman scattering, it has become possible to investigate the generation of defects in the course of the successive formation of the phosphate matrix in fluoroaluminate glass. The feasibility of the formation of PO ₄ ^2? hole centers both in single tetrahedra and upon rupture of P-O-P bridge bonds with the simultaneous formation of PO ₃ ^2? electron centers has been shown. As a result of the revision of the nature and mechanisms of formation of radiation color centers in the studied glasses, it appears promising to introduce cerium (Ce³⁺) and europium (Eu³⁺) as protectors into these glasses.     

Interactions of gamma rays with undoped and Mn-doped sodium phosphate glasses

Ultraviolet and visible spectroscopic measurements were used to investigate prepared undoped and Mn-doped sodium phosphate glasses before and after successive gamma irradiation. The effects of both glass composition and MnO₂ content on the generation of radiation-induced defects were investigated. Undoped sodium phosphate glass shows strong UV absorption, which is attributed to the presence of trace iron impurities present in the raw materials. Mn-doped glasses reveal an additional visible broad band centered at about 500 nm due to Mn³⁺, which has recently been related to the ⁵E_g →⁵T_2g transition. The radiation-induced bands are correlated with the generation of liberated electron–hole pairs during the process of gamma irradiation and the possibility of photochemical reactions especially with trace iron impurities and manganese ions. The intensity and the position of the induced bands are observed to depend on the type and composition of glass, concentration of the dopant and also on the irradiation dose. Manganese ions when present in relatively higher content have been found to show a shielding behavior towards the effects of progressive gamma irradiation causing a retardation of the growth of the induced defects. Infrared and Raman spectra of the undoped and Mn-doped glasses were measured to investigate the structural phosphate groups present and the effect of MnO₂ on the network structure. An ESR investigation was carried out to confirm the state of manganese ions in the prepared sodium phosphate glasses.     

Radiation induced defects in KBr:Tl+ crystals

The defects produced in KBr:Tl⁺ crystals during x-irradiation at 77 K were studied using thermoluminescence (TL), thermally stimulated currents (TSC), and absorption and emission spectra. Three main glow peaks at 165, 193 and 258 K were observed both in the TL and in the TSC curves. A variety of irradiation induced absorption bands were observed in the UV, visible and infrared up to about 2 microns. The 165 K TL peak was found to emit only the 440 nm band assigned to thallium dimers, while the peaks at 193 and 258 K exhibited the UV bands at 310 and 365 nm as well as the 440 nm band.The defects produced during the irradiation were the V_k hole center, the Tl° and the Tl⁺₂ electron centers. Smaller concentrations of Tl²⁺ and (Tl⁺)⁺₂ centers were also produced.An analysis of the results including measurements on lightly and heavily doped crystals enabled to draw conclusions on the nature of the defects and on the recombination processes involved. A close correlation has been found between the temperatures at which changes in the various absorption bands take place and the temperatures of the TL peaks. The analysis enabled also a full classification of the absorption bands.     

Optical properties of UV-induced color centers in a KY3F10:Ce3+ crystal

The evolution of color centers induced in a KY₃F₁₀:Ce³⁺ crystal by UV radiation has been observed and interpreted. It has been revealed that, initially, the UV irradiation of the KY₃F₁₀:Ce³⁺ crystal induces the formation of color centers predominantly of the F-type, which, in a short time period of about ten minutes, are transformed into complex color centers of the F ₂-type, as well as into impurity color centers. Based on the data obtained, a diagram of energy states of the crystal, dopant, and color centers has been constructed, on which most probable processes that are caused by electronic transitions occurring in the KYF:Ce³⁺ crystal after its UV irradiation have been indicated.     

Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu₃Al₅O₁₂ (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of Lu_Al antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and Lu_Al antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb²⁺ ions. The structure of the luminescence centers, related to the background emission of impurity Pb²⁺ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al₂O₃, Y₂O₃) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O^? ions and electrons localized at excited levels of Lu³⁺ cations.     

Localized electronic excitations in crystalline phenacite Be2SiO4

The results of coordinated spectroscopic studies of the nature and properties of electronic excitations localized at regular and defect sites of the Be₂SiO₄ lattice are presented. The methods employed are electron-beam-excited pulsed absorption spectroscopy, pulsed cathodoluminescence, and low-temperature VUV spectroscopy with selective excitation by synchrotron radiation. The bands in luminescence spectra of Be₂SiO₄ at 2.70 and 3.15 eV are assigned to [AlO₄]^5? and [SiO₄]^4? centers formed both in direct relaxation of electronic excitations at defect levels and through the formation of exciton-defect complexes. Disruptions of beryllium-oxygen bonds (short-lived defects in the form of beryllium vacancies V _Be ^? ) are considered as initiating the formation of optically active centers with characteristic absorption bands in the range 1.5–4.0 eV. The intrinsic luminescence of the Be₂SiO₄ crystal at 3.6 and 4.1 eV is attributed to radiative decay of self-trapped excitons of two types. A mechanism of exciton self-trapping at the [SiO₄] and [BeO₄] tetrahedral groups is proposed, which involves excitation transfer from a threefold-coordinated oxygen atom to neighboring silicon or beryllium atoms.     

Optical spectra of (CdF2)0.9(InF3)0.1 semiconductor solid solutions

The differences in the optical spectra of CdF₂:In semiconductors with bistable DX centers (concentrated (CdF₂)_0.9(InF₃)_0.1 solid solutions) and “standard” samples with a lower impurity concentration used to record holograms are discussed. In contrast to the standard samples, in which complete decay of two-electron DX states and transfer of electrons to shallow donor levels may occur at low temperatures, long-term irradiation of a (CdF₂)_0.9(InF₃)_0.1 solid solution by UV or visible light leads to decay of no more than 20% deep centers. The experimental data and estimates of the statistical distribution of electrons over energy levels in this crystal give the total electron concentration, neutral donor concentration, and concentration of deep two-electron centers to be ～5 × 10¹⁸ cm^?3, ～9 × 10¹⁷ cm^?3, and more than 1 × 10²⁰ cm^?3, respectively. These estimates show that the majority of impurity ions are located in clusters and can form only deep two-electron states in CdF₂ crystals with a high indium content. In this case, In³⁺ ions in a limited concentration (In³⁺ (～9 × 10¹⁷ cm^?3) are statistically distributed in the “unperturbed” CdF₂ lattice and, as in low-concentrated samples, form DX centers, which possess both shallow hydrogen-like and deep two-electron states.     

Hole-electron mechanism of F-H pair generation in RbCl crystals with impurity electron traps

Production of F, Cl ₃ ⁻ , Ag⁰, and Tl⁰ centers in RbCl:Ag and RbCl:Tl crystals by photons having energies ranging from 5 to 10 eV has been studied at 295 and 180 K. It is shown that creation of near-impurity excitations is accompanied by formation of F centers localized in the vicinity of Ag⁺ and Tl⁺ ions. F centers are produced in direct optical generation of self-trapped excitons. In addition to the well-known mechanism of F-H pair production in nonradiative recombination of electrons with self-trapped holes, a hole-electron process has been revealed for the first time to operate in RbCl:Ag having deep electron traps. By this mechanism, F-H pairs appear in the following sequence of stages: thermally stimulated unfreezing of hopping diffusion of self-trapped holes (V _K centers), tunneling electron transfer from Ag⁰ to the approaching V _K centers, and subsequent nonradiative decay of triplet self-trapped excitons near Ag⁺ ions. Fiz. Tverd. Tela (St. Petersburg) 40, 1238–1245 (July 1998)