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.     

Thermo-and photostimulated luminescence of CaI2: Tl and CaI2: Pb crystals

Thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb scintillation crystals under optical and X-ray excitation is studied. It is shown on the basis of the results obtained with account for the data of studies of photo-and X-ray-luminescent properties of these scintillators that Tl⁺ and Pb²⁺ ions form complex capture centers with host defects. These centers are responsible for the thermostimulated luminescence in the temperature range of 150–295 K, and the centers of charge carrier trapping are spatially separated from the centers of recombination emission. An assumption is made that thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb crystals under optical excitation is observed mainly due to the delocalization of charge carriers from hydrogen-containing centers responsible for the excitation band at 236 nm and the photoluminescence of CaI₂ with a maximum at 395 nm. The luminescence of CaI₂: Tl crystals in the 510-nm band and CaI₂: Pb crystals in the 530-nm band is determined by the radiative decay of near-activator excitons.     

Luminescence of La and Sc impurity centers in YAlO3 single-crystalline films

The luminescence of La_Y³⁺ and Sc_Y³⁺ and Sc_Al³⁺ centers created by lanthanum and scandium ions at Y³⁺ and Al³⁺ cation sites of YAlO₃ perovskite lattice was investigated. The features of emission of excitons localized at the mentioned centers in YAlO₃:La and YAlO₃:Sc single-crystalline films were analyzed by means of time-resolved emission spectroscopy and luminescence decay kinetics measurements under excitation by synchrotron radiation at 9 and 300 K.     

X‐ray‐excited optical luminescence and X‐ray absorption fine‐structures studies of CdWO4 scintillator

X‐ray‐excited optical luminescence (XEOL) emission and excitation spectra as well as the EXAFS signal of CdWO₄ were measured in the energy region of the Cd and W absorption edges. From EXAFS refinement, structural parameters such as number of atoms, distance from the absorbing atom and width of coordination shells in the W neighborhood were determined. The role of W–O interactions on the intrinsic luminescence of CdWO₄ is discussed. The efficiencies of conversion, transfer and emission processes involved in the scintillation mechanism showed to be high when self‐trapped excitons are formed locally by direct excitation of W ions. Annihilation of these excitons provides the characteristic scintillation of CdWO₄, a broad band emission with maximum at 500 nm. The presence of two energetically different O positions in the lattice gives rise to the composite structure of the luminescence band, and no influence of extrinsic defects was noticed. A mismatch between the X‐ray absorption coefficient and the zero‐order luminescence curves corroborates that the direct excitation of Cd ions induces secondary electronic excitations not very effective in transferring energy to the luminescent group, WO₆.     

Influence of defects on the luminescence in KI

Quenching of exciton luminescence caused by coloration with u.v.-irradiation in KI and KI:Tl has been studied. Reduction in the luminescence intensity by coloration is found to depend on the coloration temperature as well as on the concentration of Tl⁺ ions. Evidences are shown that the defects with which excitons interact are vacancies and V-centers.     

Defect assisted intrinsic luminescence in Al2O3 crystals

Abstract

The origin of the luminescence bands at 7.5 eV anv 3.8 eV appearing additionaly to the luminescence of F- and F⁺- centres in pure Al₂O₃ are investigated. The time - resolved luminescence spectra, absorption and luminescence excitation spectra as well as trap spectroscopy data depending on deviation from the stochiometry of crystals are discussed in terms of self - trapping of excitons in two configurations. The role of defects due to annihilation of excitons is considered.     

Spectroscopy of resonant excitation of exciton luminescence of GaSe–GaTe solid solutions

The luminescence excitation spectra of localized excitons in GaSe_0.85Te_0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ε > 10 mV exhibit an additional maximum M _E located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum M _E in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence (M _L ). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe_0.85Te_0.15 have been considered. It has been concluded that the maximum M _E in the excitation spectrum and the maximum M _L in the luminescence spectrum are attributed to electronic–vibrational transitions with the creation and annihilation of localized excitons, respectively.     

Luminescence of excitons and antisite defects in Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

The luminescence of excitons and antisite defects (ADs) was investigated, as well as the specific features of the excitation energy transfer from excitons and ADs to the activator (Ce³⁺ ion) in phosphors based on Lu₃Al₅O₁₂:Ce (LuAG:Ce) single crystals and single-crystalline films, which are characterized by significantly different concentrations of ADs of the Lu _Al ³⁺ type and vacancy-type defects. The luminescence band with λ_max = 249 nm in LuAG:Ce single-crystal films is due to the luminescence of self-trapped excitons (STEs) at regular sites of the garnet lattice. The excited state of STEs is characterized by the presence of two radiative levels with significantly different transition probabilities, which is responsible for the presence of two excitation bands with λ_max = 160 and 167 nm and two components (fast and slow) in the decay kinetics of the STE luminescence. In LuAG:Ce single crystals, in contrast to single-crystal films, the radiative relaxation of STEs in the band with λ_max = 253.5 nm occurs predominantly near Lu _Al ³⁺ ADs. The intrinsic luminescence of LuAG:Ce single crystals at 300 K in the band with λ_max = 325 nm (τ = 540 ns), which is excited in the band with λ_max = 175 nm, is due to the radiative recombination of electrons with holes localized near Lu _Al ³⁺ ADs. In LuAG:Ce single crystals, the excitation of the luminescence of Ce³⁺ ions occurs to a large extent with the participation of ADs. As a result, slow components are present in the luminescence decay of Ce³⁺ ions in LuAG:Ce single crystals due to both the reabsorption of the UV AD luminescence in the 4f-5d absorption band of Ce³⁺ ions with λ_max = 340 nm and the intermediate localization of charge carriers at ADs and vacancy-type defects. In contrast to single crystals, in phosphors based on LuAG:Ce single-crystal films, the contribution of slow components to the luminescence of Ce³⁺ ions is significantly smaller due to a low concentration of these types of defects.     

Luminescence study of the LuBO3 and LuPO4 doped with RE3+

Lutetium borates and phosphates doped with RE³⁺ ions are perspective scintillators. In the paper, the results of the luminescence spectroscopy of LuPO₄ and LuBO₃ doped with Pr³⁺, Ce³⁺, Tb³⁺ and Eu³⁺ under synchrotron radiation excitation are presented. The processes of the energy transfer from the host lattice to the luminescence centers are considered. The creation of excitons at the edge of fundamental absorption region is shown. The bandgap values for lutetium borate and phosphate were estimated.     

Fluctuation states and optical spectra of solid solutions with a strong isoelectronic perturbation

We propose an approach to describing the density of fluctuation states in a disordered solid solution with a strong perturbation introduced by isoelectronic substitution in the range of attraction-center concentrations below the threshold of percolation along the sites of a disordered sublattice. To estimate the number of localized states we use the results of lattice percolation theory. We describe a method for distinguishing, within the continuum percolation theory, among the various “radiating” states of the fluctuation-induced tail, states that form the luminescence band at weak excitation. We also establish the position of the band of radiating states in relation to the absorption band of the excitonic ground state and the mobility edge of the system. The approach is used to describe the optical spectra of the solid solution ZnSe_1−c Te_c, which at low Te concentrations can be interpreted as a system with strong scattering. We take into account the exciton-phonon interaction and show that the calculated and observed luminescence spectra of localized excitons are in good agreement with each other. Zh. éksp. Teor. Fiz. 115, 1039–1062 (March 1999)     

Luminescence spectroscopy of the Bi single and dimer centers in Y3Al5O12:Bi single crystalline films

Luminescence of the Bi³⁺ single and dimer centers in UV and visible ranges is studied in YAG:Bi (0.13 and 0.27 at% of Bi, respectively) single crystalline films (SCFs), grown by liquid phase epitaxy from a Bi₂O₃ flux. The cathodoluminescence spectra, photoluminescence decays, and time-resolved spectra are measured under the excitation by accelerated electrons and synchrotron radiation with energies of 3.7 and 12 eV, respectively. The energy level structure of the Bi³⁺ single and dimer centers was determined. The UV luminescence of YAG:Bi SCF in the bands that peaked at 4.045 and 3.995 eV at 300 K is caused by radiative transitions of Bi³⁺ single and dimer centers, respectively. The excitation spectra of UV luminescence of Bi³⁺ single and dimer centers consist of two dominant bands, peaked at 4.7/4.315 and 5.7/6.15 eV, related to the ¹S₀→³P₁ (A band) and ¹S₀→ ¹P₁ (C-band) transitions of Bi³⁺ ions, respectively. The excitation bands that peaked at 7.0 and 7.09 eV are ascribed to excitons bound with the Bi³⁺ single and dimer centers, respectively. The visible luminescence of YAG:Bi SCF presents superposition of several wide emission bands peaking within the 3.125-2.57 eV range and is ascribed to different types of excitons localized around the Bi³⁺ single and dimer centers. Apart from the above mentioned A and C bands the excitation spectra of visible luminescence contain wide bands at 5.25, 5.93, and 6.85 eV ascribed to the O²⁻→Bi³⁺ and Bi³⁺→Bi⁴⁺ + e charge transfer transition (CTT) in Bi³⁺ single and dimer centers. The observed significant differences in the decay kinetics of visible luminescence under excitation in A and C bands of Bi³⁺ ions, CTT bands, and in the exciton and interband transitions confirm the radiative decay of different types of excitons localized around Bi³⁺ ions in the single and dimer centers.     

Luminescence of Cr ions associated with surpassing the green-emissive defect centers in β-Ga2O3

Photoluminescence of undoped and Cr³⁺-doped β-Ga₂O₃ was investigated. The transparent, undoped β-Ga₂O₃ film was successfully prepared by thermal conversion from GaOOH. The film exhibited predominant green luminescence in response to ultraviolet light excitation at 250 nm. This luminescence behavior, which was proposed to result from the oxygen defect centers, was used in examining excitation and emission mechanisms for Cr³⁺ ions doped in β-Ga₂O₃. It was found that red luminescence of Cr³⁺ surpasses green luminescence of the host lattice, as evidenced by the dependence of the spectral structure on the Cr³⁺ concentration. The excitation of Cr³⁺ was then suggested to be caused by the energy transfer from Ga³⁺O₆ octahedra present in the monoclinic β-Ga₂O₃ lattice.     

Luminescence of fluctuation tails of disordered solid solutions

The form of the stationary luminescence spectra of excitons, localized by composition fluctuations, in disordered solid solutions under weak excitation is calculated. The tail states for which there are no nonradiative transition channels are distinguished by means of continuum percolation theory. Such states are responsible for the “zero-phonon” luminescence band. The shape of the short-wavelength luminescence band edge is determined mainly by the number of isolated localizing clusters and their smallest complexes, which decreases rapidly near the mobility threshold. The real luminescence spectrum is due to the simultaneous emission of phonons. The phonon emission determines the form of the long-wavelength wing of the emission band. The computed shape of the emission spectrum is compared with the experimental luminescence spectra of the solid solution CdS_(1−c)Se_c. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 274–279 (10 February 1997)     

Luminescence and defects creation under photoexcitation of CsI : Tl crystals in Tl-related absorption bands

Characteristics of the defects created at 4.2 K by the UV-irradiation of CsI : Tl crystals in the Tl⁺-related absorption bands (by photons of 5.8-4.8 eV energy) have been studied. The dependences of the intensities of the thermally stimulated luminescence peaks appearing near 60, 90 and 125 K and of the recombination luminescence photostimulation bands peaking at 2.35, 1.92, 1.33 and 0.89 eV on the irradiation energy and duration, uniaxial stress and thallium concentration have been examined. The mechanisms of the processes, responsible for the appearance of the intense visible (2.55 and 2.25 eV) luminescence of excitons localized near Tl⁺ ions and creation of defects pairs of the type of Tl⁰-V_K and Tl⁺-V_K with various distances between the components, have been discussed.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Disorder in laser materials

Abstract

The effects of disorder on optical lineshapes and luminescence decay in potential tunable laser materials are discussed. The particular examples considered include transition metal (TM) ions in insulators, colour centres in NaCl and recombining excitons in II-VI superlattices.     

Photoluminescence of Er<Superscript>3+</Superscript> ions in layers of quasi-ordered silicon nanocrystals in a silicon dioxide matrix

The spectra and kinetics of photoluminescence from multilayered structures of quasi-ordered silicon nanocrystals in a silica matrix were studied for undoped samples and samples doped with erbium. It was shown that the optical excitation energy of silicon nanocrystals could be effectively transferred to Er³⁺ ions, which was followed by luminescence at a wavelength of 1.5 µm. The effectiveness of energy transfer increased as the size of silicon nanocrystals decreased and the energy of exciting light quanta increased. The excitation of erbium luminescence in the structures was explained based on dipole-dipole interaction (the Förster mechanism) between excitons in silicon nanocrystals and Er³⁺ ions in silica surrounding them.     

Recombination luminescence of KI crystals with a sodium impurity

It is shown in this paper that both electron-hole and hole -electron recombination luminescence are excited effectively in KI Na crystals at low temperatures. In addition to the radiation of self-localized excitons and of excitons localized at the sodium ions, the luminescence of excitons localized at dual sodium centers is detected.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 63–66, February, 1975.     

Electronic excitation energy transfer and nonstationary processes in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>:Tl crystals

We report the results of our experimental study and numerical simulation of the electronic excitation energy transfer to impurity centers under conditions where nonstationary processes take place in the hydrogen sublattice of potassium dihydrogen phosphate (KH₂PO₄) single crystals doped with mercury-like Tl⁺ ions (KDP:Tl). We present the experimental results of our investigation of the decay kinetics of the transient optical absorption (100 ns–50 s) of intrinsic defects in the hydrogen sublattice of KDP:Tl obtained by pulsed absorption spectroscopy and the results of our study of the dynamics of the change in steady-state luminescence intensity with irradiation time (1–5000 s). To explain the transfer of the energy being released during electron recombination involving intrinsic KDP:Tl lattice defects, we formulate a mathematical model for the transfer of this energy to impurity Tl⁺ luminescence centers. Within the model being developed, we present the systems of differential balance equations describing the nonstationary processes in the electron subsystem and the hydrogen sublattice; provide a technique for calculating the pair correlation functions Y(r, t) of dissimilar defects based on the solution of the Smoluchowski equation for the system of mobile hydrogen sublattice defects; calculate the time-dependent reaction rate constants K(t) for various experimental conditions; and outline the peculiarities and results of the model parametrization based on our experimental data. Based on our investigation, the dramatic and significant effect of a gradual inertial increase by a factor of 50–100 in steady-state luminescence intensity in the 4.5-eV band in KDP:Tl crystals due to the luminescence of mercury-like Tl⁺ ions has been explained qualitatively and quantitatively.     

Luminescence properties of calcium-iodide crystals

The effect of the conditions of preparation, temperature, and the action of x rays on the luminescence properties of calcium-iodide scintillation crystals is investigated. On the basis of the results of a study of the spectral characteristics of CaI₂ and CaI₂:H₂ crystals for optical and x-ray excitation in the temperature range 90–400 K, also taking into account the results of a study of the luminescence properties of CaI₂ crystals activated by Cl⁻, Br⁻, OH⁻, and Ca²⁺ impurities, it is suggested that the 236-nm band observed in the excitation spectra of crystals of calcium iodide may be caused by an uncontrollable hydrogen impurity. The luminescence of these crystals with maximum at 395 nm is ascribed to radiative recombination of excitons trapped at H⁻ ions. Zh. Tekh. Fiz. 69, 135–136 (January 1999)