Transient optical absorption of hole polarons in ADP (NH4H2PO4) and KDP (KH2PO4) crystals

A study of transient optical absorption of the ADP (NH₄H₂PO₄) and KDP (KH₂PO₄) nonlinear crystals in the visible and UV spectral regions is reported. Measurements made by absorption optical spectroscopy with nanosecond-time resolution established that the transient optical absorption (TOA) of these crystals originates from optical transitions in the hole A and B radicals and the optical-density relaxation kinetics is rate-controlled by interdefect tunneling recombination, which involves these hole centers and the electronic H⁰ centers representing neutral hydrogen atoms. At 290 K, hole polarons and the H⁰ centers undergo thermally stimulated migration, which is not accompanied by carrier ejection into the conduction or valence band. The slow components of the TOA kinetics with characteristic times from a few tens of milliseconds to a few seconds can be assigned to diffusion-controlled annihilation of hydrogen vacancies associated with impurity or structural defects.     

Luminescence of ZnO films implanted with Au+ ions and annealed in oxygen radicals

Visible and ultraviolet luminescence spectra of ZnO:Au⁺ samples with various dose of implanted gold were studied at a temperature of 77 K. Luminescence peaks of bound and free excitons, D-A pairs, and electron recombination to the gold acceptor level were detected. The optical depth (0.117 eV) of the Au_Zn impurity acceptor level was determined. Diffusive spreading of the implanted impurity profile upon annealing of the zinc oxide film was observed by SIMS.     

Recombination kinetics in nonlinear defective LiB3O5 crystals

A study of recombination kinetics in LiB₃O₅ (LBO) crystals by time-resolved luminescence and absorption spectroscopy is reported. An investigation of the kinetics of transient optical absorption (TOA) and luminescence under ns-scale electron-beam excitation performed within a broad temperature range of 77–500 K and a 1.2–5-eV spectral interval has established that the specific features in the recombination kinetics observed in LBO involve electronic, B²⁺, and hole, O⁻, trapping centers. The TOA and luminescence kinetics, as well as their temperature dependence, are interpreted by a model of competing hole centers. Relations connecting the kinetics parameters and the temperature dependence to the parameters of the main LBO point defects are presented. Fiz. Tverd. Tela (St. Petersburg) 40, 2008–2014 (November 1998)     

Impurity Cu+ centers in LiF single crystals

LiF single crystals with copper impurity (0.0004–0.002%) have been grown by the Czochralski method and investigated. The luminescence, excitation, and optical absorption spectra have been recorded. The luminescence spectrum contains a band at 450 nm upon 250-nm excitation. This band is attributed to Cu⁺ centers in the samples grown. The mechanisms of capture and recombination during thermoluminescence are considered.     

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.     

Transient optical absorption and luminescence of lithium triborate LiB<Subscript>3</Subscript>O<Subscript>5</Subscript>

The transient optical absorption and luminescence of LiB₃O₅ (LBO) nonlinear crystals in the visible and UV spectral ranges were studied. Measurements made using absorption optical spectroscopy with nsscale time resolution revealed that the transient optical absorption (TOA) in LBO originates from optical transitions in hole centers and that the kinetics of optical density relaxation are rate-limited by interdefect nonradiative tunneling recombination involving these hole centers and the Li⁰ electronic centers, which represent neutral lithium atoms. At 290 K, the Li⁰ centers can migrate in a thermally stimulated, one-dimensional manner, a process which is not accompanied by carrier delocalization into the conduction or valence band. It is shown that the pulsed LBO cathodoluminescence kinetics is rate-limited by a recombination process involving two competing valence-band-mediated hole centers and shallow B²⁺ electronic centers. The radiative recombination accounts for the characteristic σ-polarized LBO luminescence in the 4.0-eV region.     

Radiative relaxation of low-energy electron excitations and point defects in beryllium oxide

The mechanism by which elementary point defects arise and evolve in beryllium oxide crystals and the latest advances in the dynamics of electron excitations of this compound are considered. Experimental and theoretical results which touch on the generation and evolution of elementary point defects are obtained. The properties of intrinsic and impurity point defects, and the crystal-chemical fundamentals of defect generation, optical and luminescent transitions in them, and color centers are discussed. The electron structure of B²⁺, Al²⁺, Zn⁺, and [Li]⁰ defects, nonisothermal relaxation of paramagnetic centers, thermostimulated recombination processes, and the concept of the coexistence of two types of self-localized excitons in beryllium oxide, radiative annihilation of which gives rise to luminescence in the UV and far UV bands are considered. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 76–93, November, 1996.     

Edge emissions of ion-implanted CdS

Edge emissions of CdS crystals implanted with Li⁺, N⁺, Ne⁺ and P⁺ are studied. Li and P are found to form acceptors and give rise to I₁ lines in the blue and donor-acceptor pair recombination emissions in the green. N gives rise to two kinds of I₂ lines (excitons bound to neutral donors) and donor-acceptor pair recombination emissions. An acceptor due to a defect is found in N⁺ and Ne⁺ implanted CdS. The temperature range of thermal treatment where impurity centers are formed is also investigated. The relationship among impurity centers produced by N⁺ implantation is, moreover, discussed based upon the results obtained here and previous works.     

Far-infrared magneto-absorption of donor-bound excitons in germanium: Pseudo-acceptor model

The magneto-oscillatory absorption spectrum of the arsenic-bound excitons in germanium observed at 118.6 μm reveals a series of absorption lines similar to the Zeeman spectrum of the acceptor impurity. This fact indicates that the bound excitons have the excited states associated with the light-hole Landau ladders and these excited states can be described by the model of a hole bound to the D^- state, i.e. the pseudo-acceptor model. The hole binding energy of the ground state of the bound excitons has been obtained to be 4.7 meV, which is smaller compared with the binding energy of the acceptor impurity.     

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.     

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)     

1.8—4.2K GaAs1-xPx注N+,注Zn+光荧光谱

本文研究了1.8—4.2K下离子注N,注ZnGaAs_1-xP_x样品的光致发光行为。实验结果表明由N-Zn跃迁完全转变到N束缚激子复合的x值依赖N,Zn的浓度。利用Campbell局域化模型计算了N-Zn跃迁与N束缚激子复合的几率比。这一几率比是组分x和N-Zn浓度的函数。在1.8K,在同一样品上我们清晰地观测到对应N-Zn跃迁与N束缚激子复合的光谱。 关键词：     

Polar-center phase nuclei in He+-irradiated CuO single crystals

Irradiation of various single-crystal CuO faces [ac,bc,(110)] with 4.6-MeV He⁺ ions has been found to result in reduction of CuO to Cu₂O and Cu on the irradiated and unirradiated sides, lifting of forbiddenness from optical transitions in the [CuO₄]⁷⁻ electron center in the 0.7–0.95-eV energy range, a change in dichroism near the bands corresponding to transitions in the hole centers, [CuO₄]⁵⁻, and electron centers, [CuO₄]⁷⁻, as well as in a resonant increase of absorption at 0.95–1.30 eV with an unusual polarization dependence. The results of He⁺ irradiation of CuO single crystals are discussed in terms of a model of the nucleation of the phase of polar (electron and hole) centers in copper-oxygen systems. Fiz. Tverd. Tela (St. Petersburg) 40, 419–424 (March 1998)     

Dislocation spectroscopy of crystals

A new method of studying the energy characteristics of dislocations is proposed, which is based on the investigation of the interaction of moving dislocations with purposefully introduced electronic and hole centers. A study has been made of KCl, NaCl, KBr, LiF, and KI alkali halide crystals containing electronic F and hole V _K and Me ⁺⁺ (Cu⁺⁺, Ag⁺⁺, Tl⁺⁺, In⁺⁺) centers. Investigation of the temperature dependence of the dislocation interaction with the F centers permitted determination of the position of the dislocation-induced electronic band (DEB) in the band diagram of the crystal. In KCl, the DEB is separated by ≈2.2 eV from the conduction-band minimum. It is shown that dislocations transport holes from the centers lying below the dislocation-induced hole band (DHB) (X ⁺, In⁺⁺, Tl⁺⁺, V _K) to those above the DHB (the Cu⁺ and Ag⁺ centers). Such a process is temperature independent. The DHB position in the crystal band diagram has been determined; in KCl it is separated by ≈1.6 eV from the valence-band top. The effective radii of the dislocation interaction with the electronic F and hole X ⁺, V _K, and Tl⁺⁺ centers have been found. Fiz. Tverd. Tela (St. Petersburg) 41, 2139–2146 (December 1999)     

Electronic and optical properties of Cd1-xZnxS nanocrystals

We report a numerical simulation of the conduction and valence band edges of Cd_1-xZn_xS nanocrystallites using a one — dimensional potential model. Electron — hole pairs are assumed to be confined in nanospheres of finite barrier heights. Optical absorption measurements are used to fit the bandgap of the Cd_1-xZn_xS nanocrystal material. A theoretical analysis is also made to calculate the energy location of bound excitons and the oscillator strength of interband transitions as a function of zinc composition. The aim of the latter study is to investigate the optical behavior of Cd_1-xZn_xS nanocrystals. An attempt to explain all the results is presented.     

Photodynamic processes in CaF<Subscript>2</Subscript> crystals activated by Ce<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> ions

The photochemical properties of CaF₂ crystals activated by Ce³⁺ and Yb³⁺ ions are studied. A model of the photodynamic processes induced by pumping UV or VUV radiation in active media is suggested and experimentally verified. This model explains both the presence of color centers of electronic and hole nature in crystals activated by cerium and the mechanism of suppressing of solarization processes after additional activation of the samples by Yb³⁺ ions. The cross sections of the processes of free-carrier capture by various ytterbium impurity centers are estimated. These impurity centers are established to be effective centers of recombination of free carriers of both signs.     

Creation and optical properties of stable and metastable hole-trapped centres in beryllium oxide

Abstract

The paper is devoted to study of formation mechanisms and optical absorption of the hole-trapped centers in neutron, electron-impulse and X-irradiated BeO crystals. V⁰ and V^? centers are found out to be formed as a result of neutron irradiation creating cation Frenkel pairs. Within the transient absorption decay kinetics, we registered a component whose thermal-time properties coincide with those of the luminescence of triplet self-trapped excitons. A number of absorption bands from the V_B center and exciton hole nucleus are interpreted as transitions between the O^? ion p-levels splitted by the crystal field, as well as polaron transitions and transitions into the valence band.     

Recombination processes in europium-doped cadmium iodide crystals

Results of comprehensive research into optical and luminescent-kinetic characteristics of europium-doped cadmium iodide crystals excited by nitrogen laser radiation, α-particles, and x-rays are presented. Crystals under study have been grown by the Bridgman–Stockbarger method. The doping EuCl₃ admixture was introduced into the charge in quantities of about 0.05 and 1.0 mol%. Impurity absorption detected in the near-edge region of the crystals is interpreted as part of the Eu²⁺ ion long-wavelength band associated with f–d-transitions. The cation impurity and matrix defects in CdI₂:Eu²⁺ crystals create complex centers responsible for emission with a maximum in the 580–600-nm region. The short component in the luminescence decay kinetics of weakly-doped crystal excited by α-particles and x-ray photons is due to the exciton emission characteristic of CdI₂. The slow component in the scintillation pulse results from recombination of charge carriers followed by creation of exciton-like states on the defect-impurity centers. Laser or x-ray excitation induces light-sum accumulation on the trapping levels at a depth of 0.2–0.6 eV that is mainly related to matrix microdefects. Trapping centers associated with the chlorine impurity are observed in the heavily-doped crystal. Photostimulated luminescence at 85 K arising at the electron stage of the recombination process is caused by recombination of electrons released from F-type centers with holes localized near the activator. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 358–364, May–June, 2009.     

Transient hole-polaron optical absorption in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium gadolinium orthoborate Li₆Gd(BO₃)₃ (LGBO) in the visible and UV spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li⁰ centers. At 290 K, the Li⁰ centers are involved in thermostimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow components of the TOA decay kinetics, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-lived Frenkel defect pairs in the LGBO cation sublattice have been analyzed.     

Centers of charge nonuniformity and reduction of copper oxide CuO during irradiation with nitrogen ions

Long-range reduction of CuO to Cu₂O and Cu was observed by irradiating polycrystals and different planes of CuO single crystals ((110) and (020)) with 16-MeV nitrogen ions with fluence 10¹⁷ cm⁻². The infrared absorption spectra show an increase in the number of hole [CuO₄]⁵⁻ and electronic [CuO₄]⁷⁻ centers. The highest density of electronic centers and reduction occur near the surfaces of the samples. Fiz. Tverd. Tela (St. Petersburg) 41, 1564–1567 (September 1999)