Electron statistics in CdF2 semiconductor crystals with DX centers

The degree of compensation and ionization energy of two-electron DX centers in CdF₂: In and CdF₂: Ga semiconductors are determined by studying the statistical distribution of electrons localized on impurity levels. The sharp temperature dependence of the concentration of neutral donors observed in CdF₂: Ga over the temperature range T = 250–400 K is explained by a high compensation degree, K ≥ 0.996. Thus, all Ga ions introduced into a CdF₂ crystal lattice during crystal growth form shallow donor levels. However, the concentration of Ga ions that can form bistable DX centers is rather low and is close to the concentration of electrons injected into the crystal during additive coloring. In CdF₂: In crystals, the degree of compensation is smaller than that in CdF₂: Ga but is sufficiently high and the number of bistable DX centers is not limited. It is concluded that an extremely narrow impurity band forms in the CdF₂: Ga semiconductor. For a total charged-impurity concentration of ～10²⁰ cm^?3, the width of the impurity band in CdF₂: Ga is not likely to exceed ～0.02 eV.     

Thermostimulated Luminescence of PbWO4, Bi2WO6, and Y2WO6 Ceramics

Thermostimulated luminescence (TSL) of PbWO₄, Bi₂WO₆, and Y₂WO₆ ceramics on x-ray excitation is investigated. The spectral luminosity of the thermostimulated luminescence is analyzed. The thermal activation energies conforming to the corresponding thermostimulated luminescence peaks are determined. It is established that on emptying trapping centers radiative recombination occurs at the intrinsic-luminescence centers associated with tungsten-oxygen complexes WO₄ ^2–.     

Location of the energy levels of the rare-earth ions in BaF<Subscript>2</Subscript> and CdF<Subscript>2</Subscript>

The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF₂ and CdF₂ crystals is determined. The role of RE ³⁺ and RE ²⁺ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF₂: RE and CdF₂: RE is associated with the location of the excited energy levels in the band diagram of the crystals.     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Electroluminescence in CdF2 crystals

Electroluminescence is observed in CdF₂: Eu and CdF₂: Gd single crystals at liquid nitrogen temperature. The comparison of the emission spectra with those obtained from the same materials under X-ray excitation shows that the electric field stimulates either the intrinsic luminescence amd that due to the doping substance. The dependence of the brightness on the temperature is also reported and compared with the intrinsic luminescence efficiency. The effect is used for measuring the rare earth luminescence decay times.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Slow components of the emission decay in fluoride crystals doped with Ce3+

Spatially separated defects created by photons with energies 6–8 eV in alkali-earth fluoride crystals doped with cerium are investigated with the help of thermoluminescence. Measuring the spectra of creation of V_k and H peaks of thermostimulated luminescence inBaF ₂:Ce³⁺. we demonstrate that photons with energies higher than 6eV induce H centers (self-trapped holes captured by interstitialF ions), whereas the formation of self-trapped holes begins on exposure to photons with energies greater than 7 eV. The influence of photoionization on theCe ³⁺ luminescence inBaF ₂, SrF₂, CaF₂, andCeF ₃ crystals is investigated in the range of photon energies 4–8 eV. An exponentialCe ³⁺-emisson decay was observed for excitation energy lying in the range 4–6 eV. Slow and fast decay components were observed under excitation by photons with energies higher than 6 eV. We believe that the slow and fast components are due to the tunnel recombination of trapped electrons with hole centers. A. P. Vinogradov Institute of Geochemistry of the Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 43–49, March, 2000.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

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.     

Dy-doped Lu2SiO5 single crystal: spectroscopic characteristics and luminescence dynamics

Spectroscopic and kinetics properties of Lu₂SiO₅:Dy³⁺ (LSO:Dy) single crystal with 1 and 5 at.% of activator were investigated. The polarised absorption and unpolarised emission spectra were measured at 10–300 K. Parameters characterising radiative relaxations of LSO:Dy were estimated by the Judd–Ofelt model. The crystal-field energy structure was derived from low-temperature optical spectra exhibiting the presence of two non-equivalent Dy³⁺ sites. It was found that dysprosium ions in site 1 and in site 2 do not form isolated subsystems; these subsystems are coupled by an effective spectral energy migration process. The LSO:Dy crystal exhibits a strong luminescence in the visible. Strong ion–ion interactions were observed for LSO:Dy (5 at.%); luminescence decays are non-exponential and the macro-parameter of donor–acceptor interaction C _da amounts to 5.3 (10^?52 m⁶?s^?1) and 7.8 (10^?52 m⁶?s^?1) at 10 and 300 K, respectively. Laser potential related to the ⁴F_9/2→⁶H_13/2 yellow luminescence in Dy:LSO was assessed based on evaluation of the emission cross section values. It was concluded that the crystal is a promising material for visible laser operation.     

Photo- and thermo-stimulated luminescence of CsI—Tl crystal after UV light irradiation at 80 K

Abstract

Thermo- and photo-stimulated luminescence are studied for CsI—Tl crystal after the irradiation with the UV light at 80 K. Creation spectrum of the photostimulated luminescence coincides with the D absorption band of Tl⁺ ions. Nature of the defects created by UV light at low temperatures is discussed basing on the correspondence between the thermostimulated glow curve peaks and thermal evolution of the photostimulation spectra observed after irradiation in the D absorption band. Three bands at 1400, 950 and 580 nm have been observed in the stimulation spectrum at 80 K. The 1400 and 950 nm stimulation bands are presumably explained as the optical transitions in the Tl⁰ centre forming the spatially correlated defect pair with V_k centre while the 580 nm stimulation band is connected with the unperturbed Tl⁰ centres. It is concluded that the Tl⁺ luminescence at low temperature is connected with the electron recombination with the Tl²⁺ centre.     

Temperature dependence features of the photoluminescence spectral distribution of Li2B4O7 and Li2B4O7:Cu

The temperature dependence of the photoluminescence (PhL) spectral distributions has been reported for the lithium tetraborate Li₂B₄O₇ (LTB) and Li₂B₄O₇:Cu (LTB:Cu) single crystals and for glassy LTB:Cu. It was found that the emission peaks of the LTB and LTB:Cu single crystals are non-elementary and splittable by temperature increase into several elementary peaks. By the sample heating the temperature quenching of PhL as well as the redistribution of the PhL intensity among elementary peaks was observed. Heating of the LTB:Cu single crystal samples caused no shift of the spectral maxima of the individual PhL peaks. The curve describing the temperature dependency of individual PhL peaks for the LTB:Cu single crystal is characterized by maxima resulting from combination of PhL and thermostimulated luminescence (TSL). The PhL intensity for glassy LTB:Cu is significantly lower than that for LTB:Cu single crystal with the equivalent copper dopant content. As compared to the LTB:Cu single crystal, the PhL spectral maximum for glassy LTB:Cu is wider and shifted to the lower energy range. Heating of the glassy LTB:Cu sample results in the PhL temperature quenching without any shift of the spectral maximum.     

Energy transfer and thermalization in LiYF4:Tm, Ho

Energy transfer processes are very important in solid-state laser systems because they can cause an enhancement of the luminescence emission resulting in a reduction of the laser threshold. In this work, a detailed investigation to understand the basic processes of energy transfer between Tm and Ho ions in LiYF₄, a solid-state laser crystal, was made. Data includes absorption, luminescence excitation and response to pulsed excitation. Dynamics of the energy transfer was analyzed by considering the kinetic evolution of the emissions of both ions. It was found that the energy transfer process between the ³ F ₄ spectral manifold of Tm and the ⁵ I ₇ spectral manifold of Ho results in thermal equilibration of these two manifolds. Received: 20 May 1999 / Revised version: 20 August 1999 / Published online: 27 January 2000     

Different processes responsible for blue pumped, ultraviolet and violet luminescence in high-concentrated Er:YAG and low-concentrated Er:YAP crystals

Ultraviolet and violet upconverted luminescence in high-concentrated (8.4 at%) Er³⁺:YAG and low-concentrated (1 at%) Er³⁺:YAP crystals has been investigated under blue laser excitation of the ⁴F_7/2 multiplet. The upconversion mechanisms were studied in detail based on upconversion luminescence intensity dependence and decay curves. Upconversion luminescence was attributed to energy transfer processes in Er³⁺:YAG and excited state absorption processes in Er³⁺:YAP, respectively.     

Time-resolved spectroscopy of natural and synthetic BeO crystals

The results of comparative luminescence investigation of natural and synthetic BeO crystals are presented. Time-resolved luminescence (2.5–8 eV) and luminescence excitation spectra, and the kinetics of glow decay were measured using ultraviolet-vacuum-ultraviolet (VUV) synchrotron radiation (5–22 eV) or x-radiation (50–620 eV or 3–62 keV) ranges. X-ray and thermostimulated luminescence of natural BeO crystals were compared to the glow of additively colored synthetic crystals. The characteristic luminescence of F and F ⁺ centers was found in natural crystals. In synthetic crystals similar luminescence is observed only after additive or radiation coloration by virtue of the creation of F ⁻ and F ⁺ centers on anion vacancies. The defects found in the crystal lattice of a natural BeO crystal testify to the degree of mineral metamictization of the given deposit.     

Thermo- and photostimulated luminescence in LiF : Mg,Ti single crystals irradiated by ions and VUV light

A coordinated study of the relaxation of optical absorption induced by vacuum ultraviolet radiation, x-rays, and α-particles, as well as of photo- and thermostimulated luminescence (TSL) of LiF : Mg, Ti crystals (TLD-100) in the 295–750-K interval, has revealed that TSL regions characterized by activation energies E _a = 2.2–2.4 eV and anomalously high frequency factors p ₀ = 10²¹–10²² s^?1 alternate with regions where E _a = 1.5 eV and p ₀ = 10¹²–10¹⁴ s^?1. The relative intensities of the TSL peaks produced by UV illumination (10–17 eV) differ strongly under the conditions of selective photon-induced generation of anion excitons, free electrons and holes, or near-impurity electronic excitations. The latter are responsible for the high efficiency of tunneling radiative (involving titanium centers) or nonradiative (involving hydroxyl ions) recombination. The analysis of TSL peaks of LiF: Mg, Ti and LiF took into account two-step processes, namely, thermal dissociation of three-fluorine F ₃ ^? molecules and recombination of the products of their decay (V _K and V _F centers, H interstices).     

Optical spectra and excited state relaxation dynamics of Sm<Superscript>3+</Superscript> in Gd<Subscript>2</Subscript>SiO<Subscript>5</Subscript> single crystal

Single crystals of gadolinium orthosilicate Gd₂SiO₅ containing 0.5 at% and 5 at% of Sm³⁺ were grown by the Czochralski method. Optical absorption spectra, luminescence spectra and luminescence decay curves were recorded for these systems at 10 K and at room temperature. Comparison of optical spectra recorded in polarized light revealed that the anisotropy of this optically biaxial host affects the intensity distribution within absorption and emission bands related to transitions between multiplets rather than the overall band intensity. It has been found that among four bands of luminescence related to the ⁴G_5/2→⁶H_J (J=5/2–11/2) transitions of Sm³⁺ in the visible and near infrared region the ⁴G_5/2 →⁶H_7/2 one has the highest intensity with a peak emission cross section of 3.54×10⁻²¹ cm² at 601 nm for light polarized parallel to the crystallographic axis c of the crystal. The luminescence decay curve recorded for Gd₂SiO₅:0.5 at% Sm³⁺ follows a single exponential time dependence with a lifetime 1.74 ms, in good agreement with the ⁴G_5/2 radiative lifetime τ _rad=1.78 ms calculated in the framework of Judd-Ofelt theory. Considerably faster and non-exponential luminescence decay recorded for Gd₂SiO₅:5 at% Sm³⁺ sample was fitted to that predicted by the Inokuti-Hirayama theory yielding the microparameter of Sm³⁺–Sm³⁺ energy transfer C _da=1.264×10⁻⁵² cm⁶×s⁻¹.     

Luminescence in high purity In0.53Ga0.47As

We have investigated luminescence processes in high purity In_0.53Ga_0.47As grown by liquid phase epitaxy on InP substrate. The origins of luminescence processes have been determined by studying the dependence of emission intensity and spectrum on temperature. We show that exciton-ionized donor complex dominates the luminescence at 2K. With increasing temperature, the luminescence spectrum is dominated by donor-valence band and free electron-free hole recombination. Spectrum and bandgap energy are found to be dependent on the excitation position.     

Dielectronic recombination of lithium-like gold: Towards QED tests

Dielectronic recombination (DR) and radiative recombination (RR) of lithium-like gold in the energy range of 0 to 225 eV have been studied at the Experimental Storage Ring (ESR) of the GSI in Darmstadt. Main objective of the measurements is the precise determination of the 2s_1/2−2p_1/2 energy splitting as an additional QED test. This novel method, developed at the ESR [1], is based on the extrapolation of a multitude of measured resonances Au⁷⁵⁺ (1s²2p_1/2 nl_j) up to the series limit (n = ∞). Furthermore experimental data for the Au⁷⁵⁺ (1s²2p_3/26l_j) resonance manifold are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.