TSL) And Conductivity Research Articles

Analysis of thermally stimulated luminescence and conductivity without quasi-equilibrium approximation

Thermally stimulated luminescence (TSL) and conductivity (TSC) are considered using the classical insulator model that assumes one kind of active trap, one kind of inactive deep trap and one kind of recombination centre. Kinetic equations describing the model are solved numerically without and with the use of quasi-equilibrium (QE) approximation. The QE state is characterized by the parameter qI = (dnc/dt)/Ie, where dnc/dt is the rate of change of free electron density, and Ie is the TSL intensity. The QE state parameter qI, the relative recombination probability γ = Ie/(Ie + It) (It is the trapping intensity) and a new parameter called a quasi-stationary (QS) state parameter q* = qIγ = (dnc/dt)/(Ie + It) are used for the analysis of the TSL and TSC. The QE and QS states are determined by conditions |qI| ≪ 1 and, respectively, |q*| ≪ 1. The TSL and TSC curves and the temperature dependences of qI, q*, γ the recombination lifetime and the occupancies of active traps and recombination centres are numerically calculated for five sets of kinetic parameters and different heating rates. These calculation results show that (1) the upper limit of the heating rate for the presence of the QS state appears at a higher heating rate than that for the QE state when the retrapping process is present, and (2) the TSL (TSC) curves in the QS state have properties similar to those for the TSL (TSC) curves in the QE state. Approximate formulae for calculation of the parameters qI and q* in the initial range of the TSL and TSC curves are derived and used in the heating-rate methods, proposed in this work, for determination of those parameters from the calculated TSL curves.

Interactive kinetics in thermally stimulated luminescence and conductivity and its effect on the trap depth determination

Thermally stimulated luminescence (TSL) and conductivity (TSC) complex glow curves having overlapping peaks are calculated for a system with two kinds of interactive electron traps, one kind of inactive trap, and one kind of recombination center. The densities of electrons localized on the interactive traps and the rates of change of these densities during the TSL and TSC runs also are calculated. The calculation results show that electrons thermally released from the traps of one kind during the sample heating are partly trapped by the traps of another kind from which they are next released. These processes of transfer of electrons influence the properties (the peak-positions, shape, and intensity) of the complex TSL and TSC curves. Consequently, the accuracy of trap depths evaluated from the calculated TSL and TSC curves using eight methods known from literature is, in general, lowered. This is found, particularly, for the initial-rise and the peak-shape methods.

Thermally stimulated luminescence of PbWO 4 crystals

A study of the thermally stimulated luminescence (TSL) and conductivity (TSC) properties of PbWO 4 crystals was performed on a selected set of samples. Under X-ray irradiations at 90 K TSL peaks are observed in three temperature regions, (100–130 K, 160–200 K, 220–300 K) with corresponding TSC structures. Room temperature irradiations put in evidence a weak TSL peak around 360 K. The emission spectra of the TSL signals are also presented. On the basis of these measurements, the possible nature of the defects involved in the TSL processes is discussed.

Radiation induced thermally stimulated luminescence and conductivity in SIMOX oxides

Thermally Stimulated Luminescence (TSL) and Conductivity (TSC) induced by X-ray irradiation in SIMOX buried oxides have been studied from room temperature up to 400/spl deg/C. The characteristics of an X-ray induced TSL glow peak detected around 62/spl deg/C are presented: specifically, results on the emission wavelength and trap depth are shown. The X-ray induced TSC, observed at approximately 70/spl deg/C, is due to the same trapped species responsible of the TSL emission. The stability after irradiation and dose dependence of both TSL and TSC signals have also been investigated. The results have been compared with similar studies on high temperature annealed thermal SiO/sub 2/ films and bulk materials.

Thermally Stimulated Luminescence (TSL) and Conductivity (TSC) of synthetic crystalline quartz

A comparative and simultaneous study of TSL and TSC above room temperature (20–400°C) has been performed on “as-grown” and “hydrogen-swept” synthetic quartz crystals. Following X- irradiations, TSL spectra (heating rate = 1°C/s) feature a number of peaks: at 75°C an intense structure is observed (the well-known “100°C” peak of quartz); the analysis of this peak obtained by numerical methods has shown that it follows monomolecular kinetics, giving a value of 0.83 eV for the trap depth. Additional peaks are observed at 110°C and 160°C, followed by weaker and less resolved emissions above 200°C. TSC peaks at 80°C, 120°C and 160°C, particularly evident in as-grown samples when measured with the electric field applied along the x-axis, can be associated to the corresponding TSL peaks. However, spectra performed with the electric field applied along the z-axis evidence different features. In as-grown samples a strong and broad peak at approximately 132°C is observed, while hydrogen-swept samples are characterized by two peaks at 180°C and 275°C. Such an anisotropic character, and the fact that no TSL structures are observed in the same temperature range, support the hyporthesis of an ionic nature for the latter peaks. TSC “pre-dose” measurements of the 75°C peak show that no current enhancement is observed upon irradiational and heating treatment: this result is in accordance with previous radioluminescence and thermally stimulated exoelectron emission experiments and supports the proposed model of the dynamics of this effect.

Simultaneous thermally stimulated luminescence and conductivity in low-density polyethylene

Thermally stimulated luminescence (TSL) and conductivity (TSC) in low-density polyethylene samples were measured simultaneously, while the sample was being heated to room temperature after X-ray irradiation at -190 degrees C. The TSC curves of samples from which absorbed air had been removed showed five peaks, labelled C1-C5 in order of increasing temperature, while the TSL glow curves showed only three peaks (L1-L3). The effects on these peaks of (i) immersing the samples in fuming nitric acid, (ii) annealing them in vacuum, and (iii) exposing them to an oxygen/ozone mixture, were investigated. It is concluded that the C1 and L1 peaks are associated with the same traps, these traps being formed by the polymer chain themselves in the chain-fold regions of the sample, and being broken up by the onset of side-chain motion. The same is true for the C2 and L2 peaks, except that a different set of traps and 'crankshaft' motion are involved. There is no obvious correlation between the L3 peak and any of the TSC peaks; the L3 traps are also formed in the chain-fold regions, and are broken up by the glass transition. The C3, C4 and C5 traps are probably structural defects of various kinds in the crystalline regions of the samples, from which electrons escape by thermal excitation. When the samples contain absorbed air, a hitherto unreported peak is observed in the TSC curve, corresponding to the TSL 'gas' peak.

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.

Interpretation of thermoluminescence and thermally stimulated conductivity experiments: Part I: Extensions of the conventional model

The conventional model that one often uses for the interpretation of thermally stimulated luminescence (TSL) and conductivity (TSC) experiments does usually not describe adequately the actual situation in phosphors since in most of the practical cases there is a significant difference between the shapes of measured and calculated TSL and TSC curves. We calculated the influence of several model extensions such as recombination via excited states, the presence of a trap distribution, donor-acceptor pair recombination, the presence of additional centres, temperature dependent rate parameters, thermal quenching of luminescence and scattering due to ionized defect centres on TSL and TSC curves. It is concluded that such extensions may strongly alter these curves. We also found that trap depths determined after application of some methods from the literature are unreliable if the model that can be used for the interpretation of experimental results is not accurately known. Only the method of Hoogenstraaten appears to be rather insensitive for most of the extensions considered in this paper.

Interpretation of thermoluminescence and thermally stimulated conductivity experiments: Part II: Application to CdGa 2S 4

Results of thermally stimulated luminescence (TSL) and conductivity (TSC) experiments on CdGa 2S 4 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 CdGa 2S 4, no definite conclusions may be drawn since some essential questions remain unsolved.

Evaluation of the model for thermally stimulated luminescence and conductivity; reliability of trap depth determinations

A review of the basic theory on thermally stimulated luminescence (TSL) and conductivity (TSC) based on a certain simple model is given. Approximate analytical expressions for the shapes of the TSL and TSC curves are derived. Methods from the literature for trap depth determination, some of which can easily be derived from these expressions, are applied to numerically calculated TSL and TSC curves. It turns out that the methods of Bube, Haering and Adams, Hoogenstraaten and Unger yield a correct value for the trap depth independent of the values of the retrapping ratio and the frequency factor. The method of Garlick and Gibson is reliable if applied below 15% of the maximum TSL intensity. Some methods, as most of those derived by Chen, yield a correct trap depth for a specific value of the retrapping ratio in the case of TSL only.