Kind Of Recombination Center Research Articles

Light-induced lifetime degradation effects at elevated temperature in Czochralski-grown silicon beyond boron-oxygen-related degradation

The effect of ‘Light and elevated Temperature Induced Degradation’ (LeTID) of the carrier lifetime is well known from multicrystalline silicon (mc-Si) wafers and solar cells. In this contribution, we perform a series of carrier lifetime measurements to examine, whether the same effect may also be observable in boron-doped Czochralski-grown silicon (Cz-Si). The Cz-Si samples of our study are illuminated (i) at room temperature, (ii) under standard regeneration conditions eliminating the boron-oxygen (BO) related defect (i.e. at 185 °C) and (iii) at a temperature of 80 °C, typical for the examination of the LeTID effect in mc-Si. We observe the typical decay of the carrier lifetime due to the activation of the BO-related defect. Beyond the BO degradation, applying standard solar cell processes, there is no indication for the activation of a second defect. On samples, whose surfaces are passivated by fired hydrogen-rich silicon nitride layers, an additional bulk lifetime degradation effect on a long timescale is observed in the Cz-Si material. However, defect generation rate and injection dependence of the lifetime suggest another defect type than the mc-Si-specific LeTID defect. We conclude that by applying processing steps that trigger LeTID in mc-Si, the same defect does not occur in the Cz-Si samples examined in this study. On a long timescale, however, a hitherto unknown type of defect is activated, which is different from the mc-Si-specific LeTID defect. A careful differentiation between the various kinds of recombination centres which may form during illumination at elevated temperatures is hence of utmost importance.

The basic principles of quartz radiofluorescence dynamics in the UV – analytical, numerical and experimental results

An analytical solution for the behaviour of quartz radiofluorescence (RF) in the UV-band is described based on a kinetic model involving one (deep) electron trap and two kinds of recombination centres. This model has been previously used to provide a qualitative description of quartz UV-RF. The derived numerical solution of differential equations describing charge transport in quartz can successfully reproduce experimental data. Here, this set of differential equations is solved analytically by assuming a dynamic balance during the RF stimulation. The analytical results are compared with numerical solutions and experimentally derived data. With the analytical solutions a better understanding of common natural quartz UV-RF behaviour is provided, and several experimentally observed phenomena can now be explained. Furthermore the comparison of two different kinetic models shows that the characteristic decay of the UV-RF signal in preheated quartz is attributed to an increasing competition of radiative and non-radiative centres during RF.

Dose dependence effect of thermoluminescence process in TlInS2:Nd single crystals

In this paper, thermoluminescence (TL) properties of TlInS2:Nd single crystals were modeled using an interactive model (one trap and one kind of recombination center). The simulated work presented here was based on the experiment conducted by Delice et al. The thermoluminescence glow curve of TlInS2:Nd single crystals has been characterized by one main peak at 26K, which confirm the presence of one active electron trap level in the forbidden band of this material. The model used in this work is similar to other models cited previously in the literature. The calculated glow curve was in good agreement with the experimental one. Our numerical results show also that the thermoluminescence intensity increases with the increase of the dose rate (D). In the selected dose level range; the thermoluminescence response is linear and no saturation can be observed.

Evaluated thermoluminescence trapping parameters–What do they really mean?

The main two trapping parameters in thermoluminescence (TL), the activation energy and the frequency factor, are often calculated and used for the evaluation of the stability of the TL signal at a given temperature. In several cases, “anomalous” values of these parameters, either very high or very low have been reported in the literature. In practically all of these cases, the values reported have been recognized to be effective values which resulted from some special circumstances related to the specific materials in hand. Obviously, these effective values are not associated directly with the real rate of thermal release of carriers from traps at the ambient temperature, prior to heating, and therefore, they do not indicate the real decay time of the TL signal or, in other words, the stability of the signal which may be used in TL dosimetry or dating of archaeological or geological samples. In the present paper, we discuss briefly some of these cases and add, in more detail, a rather elementary situation of very low effective activation energy and frequency factor. A model with two trapping states and one kind of recombination center is used and the simulation includes the numerical solution of the relevant sets of coupled differential equations in the three stages of the measurement, namely, excitation, relaxation and heating for a given set of the trapping parameters. The parameters are chosen in such a way that two overlapping TL peaks occur, which look together like a single first-order peak, but with anomalously low evaluated effective activation energy and frequency factor. Implications regarding the possible results in glow curve deconvolution are discussed.

Modeling of the concentration quenching of thermoluminescence in dolomite using the 5TOR model

In the present paper, the concentration quenching (CQ) of thermoluminescence (TL) in dolomite using the model of five trapping centers and one kind of recombination center (5TOR) has been studied. In 2011, Reuven Chen proposed a model for the concentration quenching (CQ) of thermoluminescence, including a model of 3TOR in calcite. The simulated results obtained from our model are in good agreement as compared with the experimental results presented previously in the literature.

Intrinsic superlinear dose dependence of thermoluminescence and optically stimulated luminescence at high excitation dose rates

Superlinear dose dependence of thermoluminescence (TL) and optically stimulated luminescence (OSL) has been reported for many materials. The theoretical explanation has been ascribed to competition of either traps or recombination centers, during the excitation stage or during the read-out phase. There has been an account in the literature on superlinearity of OSL associated with merely one trapping state and one kind of recombination center. This had to do with the process taking place during the read-out stage, namely the optical stimulation. In the present work, we report on a model of one trapping state and one kind of recombination center which results in a superlinear filling of the center. Thus, one can expect a superlinear dose dependence of the area under the resulting TL glow peak as well as the OSL signal. We follow this situation by writing the simultaneous nonlinear rate equations for the one-trap-one-recombination-center (OTOR) model and study the expected results by numerical simulation consisting of solving the equations with sets of the trapping parameters. We also present analytical results based on simplifying assumptions, and compare the analytical and numerical results. The effect is significant at relatively high dose rates. The main implication is that when one tries to evaluate by TL dosimetry a dose applied at a high rate, calibration of the TL dosimeter using much smaller dose rates may result in inaccurate results.

Modeling TL-like thermally assisted optically stimulated luminescence (TA-OSL)

In this work we present a model for thermally assisted optically stimulated luminescence (TA-OSL), an effect previously observed in experimental work. The model consists of one trapping state with an excited state beneath the conduction band and one kind of recombination center. In one version, we assume that an electron is thermally elevated to the excited state. From there it can either be raised optically into the conduction band or it can be de-excited back into the ground state of the trap. Once in the conduction band, the electron may retrap in the excited state or recombine with a hole in a center, yielding a luminescence photon. The other version is similar except that once the electron is in the excited state, it may be elevated to the conduction band either optically or thermally, with activation energy E2 and frequency factor s2. Using approximations along with analytical considerations as well as numerical simulations consisting of the solution of the simultaneous differential equations with linear heating function, the resulting TL-like curves are studied. First-order like and second-order like cases as well as intermediate cases are identified. The conditions for reaching these situations are considered; it is shown that they are not simply the same as in regular TL, and in addition to the trapping parameters, the intensity of the stimulating light also plays a role.

Radioluminescence in Al2O3 : C – analytical and numerical simulation results

The phenomenon of radioluminescence (RL) has been reported in a number of materials including Al2O3 : C, which is one of the main dosimetric materials. In this work, we study RL using a kinetic model involving two trapping states and two kinds of recombination centres. The model has been previously used to provide a quantitative description of the thermoluminescence and optically stimulated luminescence processes in Al2O3 : C. Using appropriate sets of trapping parameters for the kinetic model, the RL signal along with the occupancies of the relevant traps and centres are simulated numerically. The set of differential equations is also solved analytically by assuming dynamic balance during sample irradiation. Analytical expressions are obtained for the concentrations of traps and centres in the material during irradiation with short irradiation pulses, by assuming that quasi-steady conditions hold during irradiation. Several experimentally observed characteristics of the RL signals are explained by using the model. Good quantitative agreement is found between the analytical expressions and the numerical solutions of the model for short irradiation pulses.

A new look at the linear-modulated optically stimulated luminescence (LM-OSL) as a tool for dating and dosimetry

Optically stimulated luminescence (OSL) has been in use for dosimetry and dating in the last two decades. Since the OSL dependence on time is a featureless decaying function, a linear-modulation of the stimulating-light intensity has been suggested [Bulur, E., 1996. An alternative technique for optically stimulated luminescence. Radiat. Meas. 26, 701–709.], which resulted in a peak-shaped curve. The properties of this curve have been studied, assuming first-, second- and general-order kinetics. In a recent paper we have shown [Chen, R., Pagonis, V., 2008. A unified presentation of thermoluminescence (TL), phosphorescence and linear-modulated OSL (LM-OSL). J. Phys. D: Appl. Phys. 41, 035102 (1–6).] that for general-order curves, the peak maximum cannot be expected to depend linearly on the dose of excitation. A new presentation of the LM-OSL has been suggested, in which the peak maximum is linear with the filling of trapping states, which, in turn, may be expected to be linear with the dose under appropriate conditions. In the present work, we report on results of numerical simulation of the LM-OSL using the one trap-one recombination center (OTOR) model, dealing with the traffic of carriers between one trapping state, one kind of recombination center and the conduction and valence bands during excitation and read-out, and without making any simplifying assumptions. The process during optical read-out has been followed in the simulation that consisted of the numerical solution of the relevant sets of coupled differential equations, and also by analytical treatment. Sets of parameters leading to approximately first- and second-order kinetics, and to intermediate cases, have been used and the results presented in the original and the new ways are shown. The consequences concerning dating and dosimetry are discussed.

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.

Non-monotonic dose dependence of thermoluminescence

The thermoluminescence (TL) intensity in different materials is usually a monotonic increasing function of the dose, which quite often reaches a saturation value. In several materials, however, non-monotonic dose dependence has been observed. The TL intensity reached a maximum at a certain dose and decreased at higher ones. Some authors refer to this effect as 'radiation damage'. In the present work, we show that the non-monotonic dependence can easily be demonstrated to result from competition between transitions model with two trapping states and two kinds of recombination centres. Two kinds of competition are considered. One in which competition during excitation dominates, the filling of the active luminescence centre is non-monotonic, and the resulting TL is non-monotonic. In the other, the filling of traps and centres is monotonically increasing, but the competition during heating causes TL intensity to reach a maximum and decline at higher doses.

The influence of trapping centres on the photoelectron decay in silver halide

Photoelectron is the foundation of latent image formation, the decay process of photoelectrons is influenced by all kinds of trapping centres in silver halide. By analysing the mechanism of latent image formation it is found that electron trap, hole trap and one kind of recombination centre where free electron and trapped hole recombine are the main trapping centres in silver halide. Different trapping centres have different influences on the photoelectron behaviour. The effects of all kinds of typical trapping centres on the decay of photoelectrons are systematically investigated by solving the photoelectron decay kinetic equations. The results are in agreement with those obtained in the microwave absorption dielectric spectrum experiment.

A model for non-monotonic dose dependence of thermoluminescence (TL)

In the applications of thermoluminescence (TL) in dosimetry and archaeological andgeological dating, a desirable dose dependence of TL intensity is a monotonically increasingfunction, preferably linear. It is well known that in many dosimetric materials, nonlineardependence is observed. This may include a superlinear dependence at low doses and/orsublinear dose dependence at higher doses, where the TL intensity approaches saturation.In quite a number of materials, non-monotonic dose dependence has been observed, namely,the TL intensity reached a maximum value at a certain dose and decreased at higher doses.This effect is sometimes ascribed to ‘radiation damage’ in the literature. In the presentwork we show, both quasi-analytically and by using numerical simulation, that such dosedependence may result from a simple energy level scheme of at least one kind oftrapping state and two kinds of recombination centres. One does not necessarilyhave to assume a destruction of trapping states or recombination centres at highdoses. Instead, the main concept involved is that of competition which takes placeboth at the excitation stage and the read-out stage during the heating of thesample. This may explain the fact that the phenomenon in question, although veryoften ignored, is rather common. Cases are identified in which competition duringexcitation dominates, and others in which competition during read-out dominates.

The role of retrapping in dose dependence of pulsed optically stimulated luminescence.

Superlinear behaviour has been observed in dose dependence measurements of optically stimulated luminescence (OSL). Previous theoretical work has shown that superlinearity of the integral under the OSL decay curve may occur, resulting from competition with either radiationless luminescence centres or disconnected trapping states. Also, it has recently been shown that if OSL is measured during a relatively short pulse of stimulating light, quadratic dose dependence may take place even when only one trapping state and one kind of recombination centre take part. Here, the conditions for superlinear dose dependence, quadratic and more, to occur are considered. Also, the dose dependence of 'pulsed OSL' is discussed. In this procedure, the luminescence is only detected after the end of the stimulating light pulse, it is shown that when retrapping is relatively strong and initially the traps and centres are empty, superlinear dose dependence is expected.

Dose dependence and dose-rate dependence of the optically stimulated luminescence signal

The expected behavior of the dose dependence of the optically stimulated luminescence (OSL) signal has been studied using numerical simulation. A simple model of one trapping state and one kind of recombination center is presented, and the sequence of sets of simultaneous differential equations governing the processes during excitation, relaxation, and light exposure are numerically solved. With the choice of reasonable trapping and recombination parameters, it has been shown that a quadratic dose dependence of this effect results from the model when the irradiation stage starts with empty trapping states. This may explain reports in the literature of an initial supralinear dose dependence of OSL. It is also shown that within the same model, one can get an initial linear dose dependence of OSL if one starts with partly filled traps. Also has been studied the influence of dose rate on the measured OSL signal for a constant total dose, and some effect has been seen for a certain dose-rate range. The similarities and dissimilarities of OSL as compared to thermoluminescence with respect to these phenomena are discussed.

Determination of the recombination coefficient and the density ofdeep traps from the simultaneously measured thermally stimulatedconductivity and luminescence

New methods are derived for the determination of the recombinationcoefficient and the relative density of inactive deep traps from thesimultaneously measured thermally stimulated conductivity (TSC) andluminescence (TSL) multi-peak curves. It is assumed that theTSC and TSL curves are generated by releasing electrons from several kindsof active traps and following recombination in one kind of recombinationcentre. The derivation of the methods is performed for the case of thetemperature-dependent free electron recombination lifetime and withoutapplication of the quasi-equilibrium approximation. Three methods are basedon the relations between the TSC and TSL intensities and the areas underthe TSC and TSL curves. The fourth method uses the derivative of the TSCintensity at the temperature of the TSL peak-maximum. The methods areexamined by applying them to the simultaneously calculated single- andmulti-peak TSC and TSL curves.

A model for dose-rate dependence of thermoluminescence intensity

In the applications of thermoluminescence (TL) in dosimetry and archaeological dating, it is usually assumed that the measured TL depends on the total dose applied and it is independent of the dose rate. Thus, calibration of a TL specimen is usually performed at a significantly higher dose rate than that of the dose to be determined. A few experimental accounts in the literature report on dose-rate dependences of TL intensity for a given total dose. One theoretical work published gave a numerical solution of the simultaneous differential equations governing the filling of traps and centres at different dose rates. In the present work, the numerical solution is extended so that it includes the other important stage of TL, namely the heating phase. It is shown that with a rather simple model of one trapping state and two kinds of recombination centres, dose-rate effects may occur. An appropriate choice of the relevant parameters indeed yields one emission, which increases with increasing dose rate, whereas another emission decreases with the dose rate with a constant total dose, in agreement with an experimental result in quartz quoted in the literature.

Determination of the kinetic parameters by the simultaneous measurements of thermally stimulated conductivity and luminescence

Methods for determination of the recombination coefficient, the relative density of the inactive deep traps, the trap depth, and the frequency factor from the simultaneously measured peaks of thermally stimulated conductivity and luminescence are derived in the case of the temperature-dependent electron recombination lifetime. The derivation of these methods is based on the classical model of an insulator with one kind of active trap, one kind of inactive deep trap, and one kind of recombination center, and the assumption of the quasiequilibrium. Examples of application of the methods are also presented.

Interactive kinetics in thermally stimulated luminescence in insulators and semiconductors

The interactive kinetics in thermally stimulated luminescence (TSL) in an insulator is considered using the recombination model with two kinds of active traps having different depths, one kind of inactive (deep) trap, and one kind of recombination centre. Complex TSL curves with two peaks and other TSL characteristics are numerically calculated by solving the kinetic equations describing the model. The characteristics suggest that during the generation of the high-temperature TSL peak electrons thermally released from the active deeper traps partly recombine and partly are captured by the shallower traps from which they are released again and recombine. Consequently, the high-temperature TSL peak is composed of two partial peaks that are due to the recombination of electrons released from two different kinds of traps.

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.