Mechanism of <Emphasis Type="Italic">F</Emphasis>-center luminescence in anion-defective aluminum oxide single crystals

New experimental data illustrating the effect of deep traps on the luminescence properties of anion-defective α-Al₂O₃ single crystals are presented. It was established that deep traps have electronic nature and their filling occurs through photoionization of F centers and is accompanied by F → F⁺-center conversion. Model concepts were developed that describe the luminescence mechanism in anion-defective aluminum oxide single crystals with inclusion of thermal ionization of the excited F-center states. The validity of the model was supported by experimental data obtained in a study of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electrical conductivity.     

The role of deep traps in luminescence of anion-defective α-Al2O3: C crystals

This paper presents the results of a study of deep traps responsible for thermally stimulated luminescence in single crystals of the anion-defective corundum α-Al₂O₃: C observed at temperatures of about 703, 778, 830, and 903 K. The effect of the filling of these traps by carriers of both signs on the sensitivity to radiation of the main thermoluminescence peak at 450 K, optically stimulated luminescence, and phototransferred thermoluminescence has been investigated. The results obtained are treated in terms of a model involving interactive competition of traps of different depth.     

Characterization of deep energy level defects in α-Al2O3:C using thermally assisted OSL

Commercially available α-Al₂O₃:C powder was studied for deep energy level defects by a newly suggested method using thermally assisted optically stimulated luminescence (TA-OSL) phenomenon. The method involves simultaneous application of continuous wave optically stimulated luminescence (CW-OSL) as well as thermal stimulation up to 400 °C, using a linear heating rate of 4 K/s. By using this method, two well-defined peaks at 121 °C and 232 °C were observed. These TA-OSL peaks have been correlated to two different types of deeper defects which can be bleached at 650 °C and 900 °C respectively on thermal treatment. These deeper defects, having larger thermal trap depth and relatively lower photoionization cross-section at room temperature for stimulation with blue LED (470 nm), are stable up to 500 °C, so they can store absorbed dose information even if the sample is inadvertently exposed to light or temperature. As only a fraction of signal is bleached during TA-OSL readout, multiple readouts could be performed on an exposed sample using this technique. The dose vs TA-OSL response from deep traps of α-Al₂O₃:C was found to be linear up to 10 kGy, thus extending its application for high dose dosimetry. The value of thermally assisted energy (E_A) associated with these traps in α-Al₂O₃:C has been determined to be 0.268 eV and 0.485 eV respectively and the corresponding values of photoionization cross-section at room temperature (25 °C), for optical stimulation with blue light (470 nm), are 5.82 × 10^?20 and 3.70 × 10^?22 cm², respectively. The process of thermally assisted OSL has been formulated analytically as well as theoretically for describing the temperature dependence of optical cross-section and evaluation of thermally assisted energy associated with deep traps.     

On luminescence stimulated from deep traps using thermally-assisted time-resolved optical stimulation in α-Al2O3:C

We report a study of charge transfer mechanisms of electrons stimulated optically from very deep traps, also known as donor traps, in α-Al₂O₃:C. The investigations were carried out using thermally-assisted time-resolved optical stimulation, thermoluminescence and by way of residual thermoluminescence from the main electron trap. When the charges are optically stimulated from the deep traps, they are redistributed via the conduction band to the main electron trap and the shallow trap from where they are optically or thermally released for recombination at luminescence centres. The luminescence is strongly quenched at high measurement temperature as evident by very short luminescence lifetimes at these temperatures. The main peak due to residual thermoluminescence is located at a higher temperature than the conventional main peak.     

Optically stimulated luminescence of dosimetric anion-defective corundum crystals

The relationship between the characteristics of optically stimulated luminescence (OSL) and thermally stimulated luminescence (TSL) of dosimetric crystals based on nominally pure single-crystal, ion-defective corundum is considered. It is established that the TSL and OSL characteristics of these crystals are closely interrelated and are caused by the same active traps. It is shown that the OSL of deep traps has a form difficult to interpret, which can be due to the effect of both more shallow and deeper traps.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 140–142, January–February, 2005.     

Influence of the light illumination on thermoluminescence properties of PbWO4

The thermally stimulated properties of PbWO₄ in the temperature range 90–400 K have been investigated on undoped and terbium-doped crystals after X-ray irradiation at 90 K. Doping with terbium changes the concentration of shallow traps, which are responsible for retrapping free electrons and holes. Light illumination can change the distribution of the traps. The optically stimulated luminescence is observed. The influence of light illumination on the TSL curves and emission properties is studied. The possible mechanism of TSL phototransformation is discussed.     

Unusual temperature dependences of the photoconductivity and recombination luminescence of amorphous molecular semiconductors doped with ionic dyes

A nonexponential increase in photoconductivity with increasing temperature is discovered for poly(N-epoxypropylcarbazole) (PEPK) films doped with polymethine dyes. It is postulated that traps for nonequilibrium charge carriers form in these films during irradiation and are destroyed as the temperature is raised. Such traps are manifested by broadening of the high-temperature shoulder on the thermally stimulated luminescence (TSL) curves following the preliminary irradiation of PEPK films doped with polymethine and xanthene ionic dyes in the visible or UV range at 250–320 K and by the appearance of a new narrow TSL maximum near the preliminary irradiation temperature. These TSL features disappear after prolonged storage of the films in the dark or heating to higher temperatures. Fiz. Tverd. Tela (St. Petersburg) 41, 203–209 (February 1999)     

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.     

Deep trapping states in cerium doped (Lu,Y,Gd)3(Ga,Al)5O12 single crystal scintillators

We study deep trapping states in Ce³⁺-doped garnet crystals with the composition (Lu,Y,Gd)₃(Ga,Al)₅O₁₂, recently shown as having remarkably high light yield. We use thermally stimulated luminescence (TSL) technique above room temperature and determine the composition Gd₃Ga₃Al₂O₁₂ as the host showing the lowest concentration of traps. This host consistently manifests very low afterglow comparable to that of the standard BGO crystal. We also perform TSL glow peak analysis based on the initial rise technique to evaluate trap depth and other characteristics associated with TSL peaks.     

High-temperature VUV spectroscopy of KYF4 crystals doped with Nd3+, Er3+ and Tm3+ ions

Thermal quenching of 5d-4f luminescence from Nd³⁺, Er³⁺ and Tm³⁺ ions doped into KYF₄ crystals has been investigated in the temperature range up to ∼750 K where this luminescence is completely quenched. The obtained temperatures of thermal quenching (T_q) are ∼270, 495, 450 K for Nd³⁺, Er³⁺, Tm³⁺, respectively. At high temperatures, thermal quenching of 5d-4f luminescence from Nd³⁺ and Er³⁺ is accompanied by the appearance of 4f-4f luminescence from the lower-energy 4f levels. It has been shown that the dominating mechanism of thermal quenching for Nd³⁺ and Er³⁺ ions is thermally stimulated non-radiative transitions (intersystem crossing) from the 5d states to lower-energy 4f levels, namely ²G(2)_9/2 and ²F(2)_7/2, respectively, whereas for the Tm³⁺ ion, thermally stimulated ionization of 5d electrons to the conduction band states is responsible for thermal quenching of 5d-4f luminescence. The energy gap between the lowest Tm³⁺ 5d level and the bottom of the KYF₄ conduction band has been estimated to be 0.66 eV.     

Dielectric relaxations in undoped,Ce-doped and Ce,Zr-codoped Lu3Al5O12 single crystals

Dielectric spectroscopy was performed on single crystals of pure, Ce-doped or Ce,Zr-codoped Lu₃Al₅O₁₂, before and after UV- or X-irradaiation, at various frequencies within the range 100 Hz–1 MHz as the temperature was scanned from 110 to 353 K. All samples previously subjected to ionising radiations gave spectra showing loss peaks with Arrhenius characteristics of permanent dipoles relaxation. We attribute the dipoles to defect-stabilised pairs of anion–anion vacancies (oxygen ions and oxygen vacancies) that have captured holes and photo-electrons separately, thus forming O^?- and F⁺-like centers. The dielectric relaxation peaks disappeared in undoped or doped samples annealed at 573 K, suggesting that charge carrier traps are relatively deep. UV–visible absorption spectra have also been measured, which tend to support our proposed interpretation. Further evidence for deep traps has come from thermally stimulated luminescence experiments.     

Energy disorder in polysilanes

The absorption spectra (T=295 K), photoluminescence spectra (T=5–295 K), and thermally stimulated luminescence curves (T=5–295 K) of poly(di-n-hexylsilane) (PDHS), poly(methyl-n-hexylsilane) (PMHS), poly(methylcyclohexylsilane) (PMCHS), and poly(methylphenylsilane) (PMPS) films are measured. The results obtained are analyzed within the model of random hoppings of excitons and charge carriers over sites with a Gaussian distribution of the density of states. It is established that the variance parameters of the density-of-state functions of excitons and charge carriers characterize the energy disorder and depend on the chemical nature of side groups of the polymer, the conformation of segments of the main chain of the polymer macromolecule, and the temperature. At room temperature, the energy disorder in crystalline regions of the PDHS film is explained in terms of fluctuations in the number of monomer units in chain segments. In polymers with nonsymmetric side groups (such as PMHS, PMCHS, and PMPS), the disorder is more pronounced due to the formation of conformers in which silicon atoms occupy different positions in the chain. In the PMPS polymer, the disorder occurs through one more mechanism associated with fluctuations of the angle between the plane of the phenyl ring and the axis of the polymer segment due to mixing of σ-and π-electron states.     

Optically Induced Effects in the Thermoluminescence of Dosimetric Anion-Defective Corundum Crystals

This paper considers the influence of light fluxes in the 200–600-nm spectral range on the thermally stimulated luminescence (TL) of TLD-500 radiation detectors based on anion-defective corundum crystals. It has been shown that the luminescent-storing action of light in the above spectral range is due to the optical ionization of F-centers and electronic trapping centers of carriers responsible for the dosimetric peak at 450 K and TL at higher temperatures. The features of the dosimetric information distortion depending on the initial state of population of deep traps and the spectral composition and power of optical radiation are discussed.     

A new interactive thermoluminescence mixed-order glow curve deconvolution function

An interactive mixed-order thermoluminescence (TL) glow curve deconvolution function is presented for the first time in which the retrapping of thermally stimulated charge carriers in deep traps during the heating stage is taken into account. Considering this transition in the set of differential equations by describing the TL process and by solving them, an analytical function for TL intensity was obtained. This equation reduces to the known deconvolution function for the mixed-order model in the limiting case of saturation of deep trapping (DT) states. In intermediate cases, where the DT states are partially occupied, the proposed function acts as a real interactive model which allows the thermally stimulated electrons to be retrapped to deep electron traps. Applicability of the proposed model in a real TL system is also presented and discussed.     

Interpretation of thermoluminescence and thermally stimulated conductivity experiments: Part II: Application to CdGa2S4

Results of thermally stimulated luminescence (TSL) and conductivity (TSC) experiments on CdGa₂S₄ are reported. In the lower temperature region of the TSl curve a decay of luminescence is observed that is probably due to donor-acceptor pair recombination since no accompanying conduction is measured. The TSL and TSC curves of the undoped material further consist essentially of two peaks. If trap depths are calculated with various methods from the literature it turns out that for one peak different values are obtained. These differences cannot be explained with the conventional model consisting of one trapping level and one recombination level. If the results are compared with more complex models it follows that thermal quenching, the presence of a trap distribution, thermally disconnected traps and recombination via excited states, are important. This is most clearly demonstrated in the cases with excess Ga and with Ag or In as dopant. Although this approach probably yields valuable information about CdGa₂S₄, no definite conclusions may be drawn since some essential questions remain unsolved.     

Energy Levels in the Forbidden Band of the Bi4Ge3O12 Ceramics

Thermally stimulated luminescence in the Bi₄Ge₃O₁₂ ceramics and also in the ceramics of the parent components Bi₂O₃ and GeO₂ is investigated. The similarity of the curves of the thermally stimulated luminescence in bismuth germanate with the structure of eulytine Bi₄Ge₃O₁₂ and sillenite Bi₁₂GeO₂₀ is explained. The relation of the thermally stimulated luminescence band in Bi₄Ge₃O₁₂ (with a maximum at 143 K) to the disruptions in the germanium sublattice and of the thermally stimulated band (with a maximum at 187 K) to the recombination processes in the bismuth sublattice is shown. It has been established that the light sum in the Bi₄Ge₃O₁₂ ceramics is stored most effectively upon excitation by light in an energy region of 4.4 eV.     

Effect of point defects on luminescence characteristics of ZnO ceramics

Photo- and thermally stimulated luminescence of ZnO ceramics are produced by uniaxial hot pressing. The luminescence spectra of ceramics contain a wide band with a maximum at 500 nm, for which oxygen vacancies V_O are responsible, and a narrow band with a maximum at 385 nm, which is of exciton nature. It follows from luminescence excitation spectra that the exciton energy is transferred to luminescence centers in ZnO. An analysis of the thermally stimulated luminescence curves allowed detection of a set of discrete levels of point defects with activation energies of 25, 45, 510, 590 meV, and defects with continuous energy distributions in the range of 50–100 meV. The parameters of some of the detected defects are characteristic of a lithium impurity and hydrogen centers. The photoluminescence kinetics are studied in a wide temperature range.     

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.     

Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE₁) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE²) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE₁ → STE₂ is revealed.     

Intrinsic defect related luminescence in ZrO2

The studies of ZrO₂ and yttrium stabilized ZrO₂ nanocrystals luminescence as well as yttrium stabilized single crystal luminescence and induced absorption showed that the intrinsic defects are responsible for luminescence at room temperature. These defects form a quasi-continuum of states in ZrO₂ band gap and are the origin of the luminescence spectrum dependence on the excitation energy. Luminescence centers are oxygen vacancies related but not the vacancies themselves. At room temperature, in ZrO₂, deep traps for electrons and holes exist. The oxygen vacancies are proposed to be the traps for electrons.