Thermal Release Of Electrons Research Articles

Electron traps in Ag-doped Li2B4O7 crystals: The role of Ag interstitial ions

Electron paramagnetic resonance (EPR) is used to establish models for electron traps in Ag-doped lithium tetraborate (Li2B4O7) crystals. When exposed at room temperature to ionizing radiation, electrons are trapped at interstitial Ag+ ions and holes are trapped at Ag+ ions on Li+ sites. The trapped electrons occupy a 5s1 orbital on the interstitial Ag ions (some of the unpaired spin density is also on neighboring ions). Three EPR spectra are assigned to electrons trapped at interstitial Ag ions. Their g values are near 1.99 and they have resolved hyperfine structure from 107Ag and 109Ag nuclei. The spectrum representing the largest concentration of trapped electrons has the unpaired spin shared by the interstitial Ag ion and an adjacent boron ion at its regular lattice site. A 10B enriched crystal verifies this assignment and an analysis of spin-Hamiltonian parameters yields information about the Ag and B orbitals occupied by the unpaired spin. The second spectrum has the unpaired spin shared equally by two Ag ions, one at an interstitial site and the other at an adjacent Li site. The third spectrum has a large Ag hyperfine interaction and a weak Li interaction. Optical absorption bands associated with the trapped electrons are observed between 225 and 500 nm. Thermal release of electrons from these traps is responsible for a prominent thermoluminescence peak near 150 °C, whereas optical release of the electrons at room temperature produces intense optically stimulated luminescence. Radiative recombination occurs at Ag2+ ions with emission peaking near 270 nm.

Competition between long time excitation and fading of thermoluminescence (TL) and optically stimulated luminescence (OSL)

In the present work, we consider the filling and thermal emptying of traps and centers in a simple one-trap-one-recombination-center model in a small system such as a quartz grain in nature, when both the excitation and thermal release of electrons are very slow. Due to the nature of the very slow process, Monte Carlo simulations appear to be a very appropriate method. However, in parallel, we have applied an approximate analytical method and found practically the same results although the Monte Carlo results showed some small fluctuations due to the statistical nature of the procedure. This is in line with the experimental results which are also expected to have statistical fluctuations. The main result found is that after a long enough time, measured in hundreds or thousands years or more, the filling of the trap reaches a plateau which, depending on the parameters, may be very significantly smaller than the concentration of the trap in question. This equilibrium value is the same if we start from very low, e.g. zero concentration or very high, above the equilibrium value. This plateau level depends strongly on the relevant parameters. However, comparing simulations with activation energies of 1.2 eV and 1.3 eV shows strong dependence of the plateau level on the energy. Similarly, we can expect strong dependence on the temperature at which the sample is held. The results reached here and shown in Figs. 1–4 are based on the simplest OTOR model, but similar results of approaching a plateau level which are not due to the saturation of traps may occur in more complex systems as is demonstrated by simulations based on the Bailey model for quartz which includes several traps and centers.

On the quasi-equilibrium assumptions in the theory of thermoluminescence (TL)

The phenomenon of thermoluminescence (TL) is governed by a set of simultaneous differential equations. When one studies the properties of a single peak, resulting from the thermal release of electrons from a trap into the conduction band, followed by radiative recombination with holes in centers, the set consists of three non-linear equations. Even in this simple case, the equations cannot be solved analytically. In order to get approximate solutions, the conventional way has been to make the “quasi-equilibrium” assumptions, namely that |dnc/dt| is significantly smaller than |dn/dt| and |dm/dt|, where n and m are the occupancies of traps and centers, respectively, nc is the concentration of electrons in the conduction band, and nc≪n; nc≪m. We show, using simulations as well as analytical arguments that the former condition often does not occur; however, its consequences are valid. The reason is that the conventional quasi-equilibrium assertion must be replaced by a different condition. As for the smallness of the concentration of free electrons, we show that it may not be fulfilled at the high-temperature end of a single glow peak or in the highest-temperature peak in a series. In some cases, this condition results in a broad high-temperature tail of the TL peak, as previously observed experimentally in several materials.

Electron paramagnetic resonance study of rare-earth related centres in K 2YF 5:Tb 3+ thermoluminescence phosphors

Rare-earth related centres have been investigated in K 2YF 5:Tb 3+ crystals, exhibiting thermoluminescence (TL) below and above room temperature (RT), using electron paramagnetic resonance (EPR) spectroscopy at Q (34 GHz) and W-band (94 GHz). The spectra have been studied prior to irradiation, after exposure in the kGy range to X-rays at 77 K and subsequent pulse annealing up to 570 K. In addition to Gd 3+, previously studied in detail, we identified Er 3+ and Yb 3+ centres as accidental impurities in as-grown crystals and determined their effective g tensors. The EPR spectra of irradiated and annealed crystals provide evidence for the production of at least three distinct Tb-related trapped hole centres, two of which could definitely be identified as Tb 4+. Hence, we prove that the Tb 3+ activator ions also act as hole traps in K 2YF 5. Pulse annealing experiments indicate that the TL above RT results from thermal release of electrons, recombining at these Tb 4+ ions.

Duplicitous thermoluminescence peak associated with a thermal release of electrons and holes from trapping states

A thermoluminescence (TL) glow peak may result from a transition of electrons from traps into the conduction band, followed by a recombination with holes trapped in a luminescence center. Another possibility is that holes trapped in a hole trap are thermally released into the valence band and recombine with electrons in an electron recombination center. A series of glow peaks emitted from a given sample may include peaks of both kinds. In some cases, peaks may be identified as being of one kind or the other, say, by using thermally stimulated electron emission (TSEE), which can take place when the free carriers are electrons. In the present work, we demonstrate by the use of simulation that two peaks may result from one electron and one hole trapping states and a single hole recombination center. The first TL peak is observed when thermally stimulated electrons recombine with holes in the center. The TL peak is terminated when the holes in the center are exhausted. At higher temperatures, holes from a hole trap are released into the valence band and then captured by the hole center, thus this center is replenished. More electrons from the electron trap are thermally released now and recombine with the newly arrived holes in centers. A second TL peak may be observed which carries some information concerning the hole trap. It is thus demonstrated that some of the usual methods for distinguishing between electron and hole traps can lead to incorrect conclusions. It is possible for a hole trap, for example, to induce an increase in electron recombination in such a way that it produces a peak that looks nearly identical to TL from an electron trap. This simulation may bring about a new look at TL peaks occurring in materials used in TL dosimetry and dating. A new interpretation may also be given to “Auger” TSEE associated with the thermal release of electrons from the surface of a material, which indirectly results from the thermal release of holes from traps. The performance of some methods for evaluating the activation energies and the significance of the results in the present situation are discussed.

Photochromic Ag-TiO<sub>2</sub> Thin Films Fabricated by Dual-Target Helicon Magnetron Sputtering

Photochromic material Ag-TiO2 thin films are fabricated on quartz substrate by dual -target helicon magnetron sputtering. The phototchromic behavior is investigated for the sample loaded with 90% Ag. Spheres, ellipsoids and polyhedra shape of Ag particles with wide range size (5∼100 nm) are dispersed in the TiO2 amorphous matrix observed by transmission electron microscopy. The spectral hole burned by the irradiation of laser at the wavelength 532 nm can be explained by a particle-plasmon-assisted electron transfer from Ag nanoparticles to TiO2 and subsequent trapping by adsorbed molecular oxygen. Moreover, the mechanism of the slow recovery after photochromism is suggested as a slow thermal release of electrons from oxygen trapping centers and subsequent capture into the Ag nanoparticles.

Photoconductivity in hydrogenated amorphous silicon. I. Thermal emission and hopping of trapped charges

Abstract The photoconductivity of hydrogenated amorphous silicon is found to be the result of three different processes that have been identified using the dual-beam-modulated photoconductivity technique: the release of electrons trapped either in D− states or in the conduction-band tail, and a quenching process related to recombination. Each process follows its own generation rate dependence. A model has been established in which the above three processes add up to give the observed steady-state photoconductivity. The relative contributions of these processes have been determined over large temperature and light flux ranges. This allowed us to show firstly that the thermal release of electrons from gap states, either from D− or from the band tail, is still an important process at 90 K, secondly that the relative contributions of D− and tail states change drastically at about 165 K, that is when the electron quasi-Fermi level is located at about 0.3 eV below the mobility edge and thirdly that at low tempe...

Point Defects and the Pre-Dose Effect in Quartz

The thermal annealing properties of (H3O4)0 centres in single crystal SiO2 were studies by means of electron spin resonance (ESR). The data indicate that these centres take part in the TL process at 110 oC in this material. The mechanism suggested for this TL signal is the thermal release of electrons from (GeO4)- centres and their recombination with holes trapped at (A104)0 and (H3O4)0 centres. We have also monitored the radiation-induced infra-red absorption bands in order to examine the thermal stability of A10H centres. Correlations between the A10H annealing curves and the pre-dose thermal activation curves indicate that hydrogen impurities are mobile in the quartz lattice during the pre-dose activation sequence.

Point Defects and the Pre-Dose Effect in Quartz

Abstract The thermal annealing properties of (H3O4)0 centres in single crystal SiO2 were studies by means of electron spin resonance (ESR). The data indicate that these centres take part in the TL process at 110 oC in this material. The mechanism suggested for this TL signal is the thermal release of electrons from (GeO4)- centres and their recombination with holes trapped at (A104)0 and (H3O4)0 centres. We have also monitored the radiation-induced infra-red absorption bands in order to examine the thermal stability of A10H centres. Correlations between the A10H annealing curves and the pre-dose thermal activation curves indicate that hydrogen impurities are mobile in the quartz lattice during the pre-dose activation sequence.

Light induced ESR spectra in rutile powders

A series of rutile powders has been examined by electron spin resonance techniques before and after ultraviolet irradiaton at low temperatures. Prior to irradiation a broad line was seen at 4.2 K whose g-value (g ≈ 1.955) was similar to that reported by lyengar in hydrogen-reduced rutile. After ultraviolet irradiaton at 4.2 K a new line was observed. Computer simulation techniques showed that the best fit to the observed lineshape was obtained by taking g1 = g2 = 2.0077 and g3 = 2.015 with Lorentzian broadening. Isochronal annealing of this centre showed marked decreases in the signal intensity at both 10 and 80 K; these are attributed, respectively, to thermal release of electrons from a shallow and a deep electron trap giving recombination at the centre.

Thermally stimulated relaxation in insulating CdF2:Eu3+

The mechanism is investigated by which photogenerated Eu2+ ions in CdF2:Eu3+ relax back to the Eu3+ charge state upon thermal relaxation. Thermally stimulated luminescence (TSL) and conductivity (TSC) techniques are combined with photoluminescence and EPR measurements as a function of temperature at various Eu3+ concentrations. Experimental evidence is presented for this relaxation to occur via thermal release of electrons, trapped in the Eu2+ donor state, into the conduction band and subsequent radiationless recombination with F0 interstitials. Analysis of the thermoluminescence emission spectra suggests TSL to be due to energy transfer from the so produced F--interstitials to neighbour Eu3+ ions which are located in a tetragonal crystal environment.

Radiation damage in single crystal L-Histidine as studied by low-temperature thermoluminescence techniques

X-irradiation of single crystal L-Histidine at 10 K produces TL glow peaks at 38, 72, 84, 122, 162, 204, and 245 K. The 84 K peak is the most intense one and is characterized by a thermal activation energy of 0.073 eV and frequency factor of 1.1×10 3s -1. Moreover, it is readily photobleachable, whereas the other glow peaks are not, and is tentatively correlated with the thermal decay of a carboxyl anion radical. Computer simulation of the Randall-Wilkins first-order TL expression provided a check on the experimentally derived parameters characterizing the 38 and 84 K peaks. The initial-rise method did not produce accurate parameters for the 38 K peak; however, computer simulation yielded an activation energy of 0.022 eV and a frequency factor of 20 s -1 which were in agreement with the experimental shape of the glow curve. This TL peak is attributed to the thermal destruction of an imidazole cation radical. Emission spectra measurements of the 84 K luminescence (other peaks were of insufficient intensity) indicated that TL results from thermal release of electrons and their subsequent de-excitation to the ground state via the singlet and triplet manifolds. At sufficiently high temperatures (∼78 K) one only observes singlet state emission due to intersystem crossing.

Trapping and luminescence mechanisms in manganese-doped zinc silicate phosphorsa tunneling model

We have investigated electron trapping and luminescence (phosphorescence, thermoluminescence and photostimulated luminescence) mechanisms in Mn and Mn+As doped Zn2SiO4 phosphors. The intensities of both the phosphorescence and photostimulated luminescence are found to decay with time approximately as (time)-1. The generally accepted model of such a decay of phosphorescence, based on thermal release of electrons from a broad distribution of trap energy levels, cannot explain the observed decay of the photostimulated luminescence. In order to explain both results we have developed a new model based on radiative recombination by tunneling between holes trapped on Mn ions and electrons in shallow traps or in excited states of the deeper traps. We derive a simple expression which describes for both experiments at all times the decay of the light intensity and its dependence on experimental conditions. The agreement between theory and experiment is found to be excellent.

Thermally stimulated and stationary Hall effect measurements on CdGa2S4

Hall effect measurements have been carried out on CdGa2S4, a semiconductor with a high native defect concentration (about 1027 m-3). The mobility under stationary conditions of illumination is found to be proportional to T1.35, indicating that the scattering mechanism is mainly due to charged centres and probably also to neutral defect centres and dipoles. The mobility under transient conditions existing during the presence of thermally stimulated conductivity has also been measured. The behaviour can be explained by an increase of the number of ionized defect centres due to thermal release of electrons that were trapped at these centres during illumination at lower temperatures. In both cases n-type conductivity is found.

Thermoluminescence Mechanism in Dosimetry LiF

The high-temperature dosimetry glow peaks in LiF: Mg are thought to result from thermal release of electrons from traps associated with the Mg impurity. There is also evidence, however, that the visible-light emission in the same glow peaks is due to recombination of mobile holes. These observations were reconciled by a mechanism proposed by Mayhugh in which the thermally ionized electron frees a V3-center hole, which then emits a photon at another recombination center. We find that this model is supported by new measurements of Podgoršak, Moran, and Cameron on glow peaks which occur below room temperature.

GLOW CURVES FROM PHOTOSYNTHETIC BACTERIA*

Abstract— Cells and chromatophores of the photosynthetic bacterium Rhodopseudomonas viridis were frozen in the dark and illuminated while frozen or were illuminated during freezing. Upon being heated they emitted light. The conditions under which glow curves could be produced and the kinetics of the light emission indicate that the glows resulted from the thermal release of electrons from secondary electron acceptors and their recombination with oxidized reaction center bacteriochlorophyll b.Glow curves could not be obtained with Rhodospirillum rubrum.

Recombination Kinetics in GaP Red-Emitting Diodes Determined by Photocurrent and Decay Characteristics

The photocurrent characteristic of GaP red light-emitting diodes excited in the band centered at 5900 Å below the band edge at room temperature was studied in a temperature range of 170–500°K. The characteristic band is explained in accordance with the thermal release of electrons from optically excited excitons bound to the Zn–O nearest neighbor pairs to the conduction band. The Zn–O pair concentration in the light-emitting region is determined accurately by analysing the results, The time-decay characteristics of the red electroluminescence were also measured to obtain the transition rates of electrons trapped at the Zn–O centers. At room temperature, the thermal release rate of electrons WII is 1.8±0.2×106 sec-1 and the electron capture cross section of the Zn–O center is deduced to be 5.9±1.5×10-16 cm2 by using the observed binding energy of the center 300±10 meV. The radiative recombination rate WI and the non-radiative recombination rate WIII are estimated to be 1.6×106 and 2.4–3.4×106 sec-1, respectively. WIII seems to be determined by Auger process and depends largely on the free hole concentration in the active p region.

Thermoluminescence and Color Center Correlations in Dosimetry LiF

Dosimetry LiF was γ irradiated up to 10 000 R, and the resulting thermoluminescence was studied above room temperature in conjunction with the optical absorption between 190 and 400 nm. Changes in the glow curves and absorption spectra after various optical and thermal treatments show that the thermoluminescence process begins with thermal release of electrons from centers causing optical absorption in the 310–380-nm region. Moreover, the centers causing the important dosimetry glow peaks near 200°C appear to have two excited states, so that these centers have optical absorption bands at both 310 nm and under the F band at 250 nm. In particular two glow peaks near 200°C are correlated with two 310-nm absorption bands which are distinguished by different optical bleaching rates. For the glow peak usually used for dosimetry, the correlation is supported by an equality of thermal activation energies for the glow peak and 310-nm band. To interpret all the data in terms of these correlations requires assuming a restriction in the availability of luminescence sites. Implications for the photon production mechanisms and the trap structures are discussed. An asymmetry in the F band of LiF, as if a large K band were present, is also found.

Optical absorption and photochromism in iron-doped rutile

Photochromism has been observed in single-crystal iron-doped rutile. Colouring occurs on irradiation with light of photon energy greater than 2.5 eV. The ratio of the activation energies for optical bleaching (2.5 eV) and thermal bleaching (0.5 eV) agrees with approximate theory. The colouring process is interpreted as the optical transfer of an electron from the valence band to an Fe3+ Square Operator centre where Square Operator signifies an iron-adjacent anion vacancy: the majority of the resultant holes are trapped by Fe3+ Square Operator ions on sites without an adjacent vacancy. Bleaching is a combination of optical and thermal release of electrons from Fe2+ Square Operator centres direct and thermal release of electrons from Fe2+ Square Operator centres into the conduction band followed by recombination with the trapped holes, and direct recombination between the Fe2+ Square Operator centres and non- trapped holes.

Transient Polarization in Insulating CdS

Transient-current measurements have been carried out on single crystals of insulating CdS having non-injecting contacts. Application of step voltages to optically pre-excited crystals produced currents which lasted from 3000 sec and decayed as much as three orders of magnitude at temperatures in the neighborhood of 200\ifmmode^\circ\else\textdegree\fi{}K. The total transported charge varied approximately as the square root of the applied voltage, suggesting that the crystal polarized with the formation of a positively charged cathode depletion layer produced by the thermal release of electrons from bound states. A description of the polarization process in terms of the dynamics of the formation of the cathode depletion layer is given, and a method for determining the concentrations, activation energies, and frequency factors of the trapping states taking part in the process is presented. For the crystals investigated the traps were found to lie at a single discrete energy level; the concentration of traps was (2.5\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}${10}^{14}$/${\mathrm{cm}}^{3}$ and the activation energy was 0.63\ifmmode\pm\else\textpm\fi{}0.01 eV. The trapping cross section was found to be approximately 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ ${\mathrm{cm}}^{2}$.