Thermally Stimulated Conductivity Research Articles

Investigation of thermally stimulated conductivity of cobalt silicide

Mono- and disilicide of cobalt were obtained in the near-surface region of diffusion-doped silicon by cobalt atoms. The thermally stimulated conductivity (TSC) of cobalt disilicide was investigated. The energy of the adhesion levels of 0.32 eV was determined for the structures of cobalt disilicide – doped silicon - cobalt disilicide. The influence of the illumination intensity on the TSC curves is investigated and it is shown that the illumination intensity does not affect the shape of the curve and weakly affects the magnitude of the maximum of the current peak.

Surface processes on KBr single crystals examined by thermostimulated exo-electron emission and desorption

Potassium bromide (KBr) is a material with many potential uses in various branches of physics and technology (in scintillators, for a Fourier spectrometer, electro-optical elements such as IR laser windows, sensors and optical fibers). We conducted a series of combined measurements of thermally stimulated exo-electron emission (TSEE) and thermally stimulated desorption (TSD). At low doses of UV irradiation, the most prominent process is the combined emission of negative charges with desorption of the components of O, H2O and mainly K, KOH and Br, as described below. At higher irradiation doses, the KBr F-center plays the most important role (Tm = 160 °C) and at temperatures above +200 °C also the decomposition of chemically bonded atoms and other components occurs. This process is accompanied by the corresponding emission of electrons and at the same time some of the already chemically bonded structures such as HBr3 are released. TSD maxima are defined and the same temperature maxima for TSD and TSEE are shown, mainly at the already mentioned higher temperatures. These maxima are linked to the defects and interactions in the surface layer. The obtained results fully confirm the applicability of our apparatus for combined measurements of TSD, TSEE, thermally stimulated luminescence (TSL) and possibly also thermally stimulated conductivity (TSC).

Deep levels in metamorphic InAs/InGaAs quantum dot structures with different composition of the embedding layers

Deep levels in metamorphic InAs/InxGa1−xAs quantum dot (QD) structures are studied with deep level thermally stimulated conductivity (TSC), photoconductivity (PC) and photoluminescence (PL) spectroscopy and compared with data from pseudomorphic InGaAs/GaAs QDs investigated previously by the same techniques. We have found that for a low content of indium (x = 0.15) the trap density in the plane of self-assembled QDs is comparable or less than the one for InGaAs/GaAs QDs. However, the trap density increases with x, resulting in a rise of the defect photoresponse in PC and TSC spectra as well as a reduction of the QD PL intensity. The activation energies of the deep levels and some traps correspond to known defect complexes EL2, EL6, EL7, EL9, and EL10 inherent in GaAs, and three traps are attributed to the extended defects, located in InGaAs embedding layers. The rest of them have been found as concentrated mainly close to QDs, as their density in the deeper InGaAs buffers is much lower. This an important result for the development of light-emitting and light-sensitive devices based on metamorphic InAs QDs, as it is a strong indication that the defect density is not higher than in pseudomorphic InAs QDs.

Density of States in Thin Boron-Doped Microcrystalline Silicon Films Estimated from the Thermally Stimulated Conductivity Method

In this work, a series of boron-doped microcrystalline silicon samples [μc-Si:H(B)] were deposited by plasma-enhanced chemical vapor deposition, using silane (SiH4) diluted in hydrogen, and diborane (B2H6) as a dopant gas. The concentration of B2H6 in SiH4 was varied in the range of 0 100 ppm. The density of states was obtained from the thermally stimulated conductivity technique and compared with results obtained by the modulated photoconductivity methods. To explain the poor agreement between the density of states obtained from the thermally stimulated conductivity and the other methods, it is shown by means of numerical simulations that the density of states is very sensitive to experimental errors introduced in the calculation of the μnτn product (mobility of electron × lifetime of the electron). The thermally stimulated conductivity method is applied here for the rst time to calculate the density of defect states in the forbidden band of μc-Si:H samples.

On the expected order of kinetics in a series of thermoluminescence (TL) and thermally stimulated conductivity (TSC) peaks

As reported in the literature, both in experimental results and in simulated glow curves, in a series of TL peaks associated with a series of trapping states and a single recombination center, the peaks tend to be of first order. In the present work we show theoretically and demonstrate by examples of numerical simulations that the last peak in a series obtained by a model of a single recombination center and multiple traps may be of second order whereas the lower-temperature peaks are usually of first order. This is the case even when retrapping is significantly faster than recombination. In some cases, the last peak has a long tail, longer than that of second-order peaks, which has to do with a different mechanism that has been discussed in a recent paper. Similar simulations of a more complex and more realistic situation of a model with multiple trapping states and multiple recombination centers have been performed. The prevalence of first-order appearance of both the curves of free electrons, associated with thermally stimulated conductivity (TSC) and of TL, evaluated for randomly chosen sets of trapping parameters, is shown by histograms. The occurrence of a small number of very high values of the symmetry factor is also discussed.

Photoelectric and electric properties of four-component copper chalcogenides

The results of investigation of the electrophysical and photoelectric properties of complex copper chalcogenides are presented, namely, the properties of CuSnAsSe3, which exhibits ferroelectric properties, and CuInAsS3, which exhibits ionic conductivity. The spectral and temperature regions of photosensitivity of these crystals are determined. The depth of the level of carrier trapping centers, which manifest themselves under thermal activation, are evaluated from the analysis of thermally stimulated conductivity (TSC) curves in CuInAsS3.

Effect of carrier capture by deep levels on lateral photoconductivity of InGaAs/GaAs quantum dot structures

Having used thermally stimulated conductivity (TSC) technique, we identified deep electron traps that produce strong effects on charge carrier transport and photoconductivity in InGaAs/GaAs quantum dot (QD) structures. The values of deep levels below the conduction band of GaAs at 0.16, 0.22, and 0.35 eV are obtained from the analysis of the shapes of TSC curves after the excitation with the quanta energy hv = 0.9, 1.2, and 1.6 eV. The level 0.16 eV in depth is an effective electron trap that provides crossing of lateral conductivity with a high-resistance mode and, therefore, causes a high photocurrent sensitivity of about 3 A/W at 77 K with excitation by interband transitions in QDs. We determined the charge density of electrons captured by the (Ec – 0.16 eV) level to be 2 × 10−6 C/cm2 at 77 K that induces electric field ∼ 105 V/cm in a vicinity of QDs. The state at Ec – 0.22 eV is shown to be related to the recombination center that can hold non-equilibrium holes over a long time under the condition that the non-equilibrium holes are localized by the quantum states of QDs. In the course of long-term electron storage in a vicinity of QDs, an electron trapped at the (Ec – 0.16) eV level can be recaptured by a deeper spatially remote (Ec – 0.22 eV) level that allows the TSC peak observation at 106 K.

Simulations of time-resolved photoluminescence experiments in α-Al2O3:C

This paper presents simulations of time-resolved photoluminescence (TR-PL) experiments in α-Al2O3:C, which is one of the main dosimetric materials. During TR-PL experiments, short pulses of UV-light are followed by relaxation periods of the charge carriers. The model used in these simulations was previously used to explain radioluminescence (RL), thermoluminescence (TL) and photoluminescence (PL) experiments for this material, and is based on optical and thermal ionizations of excited F-centers. There are no published simulations of TR-PL experiments in this important dosimetric material in the literature. In this paper, we present new simulations using two slightly different versions of the model. In the first original version of the model, thermal quenching is explained via thermal ionization of the recombination centers. In the new proposed modified version of the model, thermal quenching is described by a Mott–Seitz type of mechanism, based on competitions between radiative and radiationless electronic transitions occurring within the recombination center. We simulate a typical TR-PL experiment in Al2O3:C at different stimulation temperatures, and compare the simulation results with available experimental data. It is found that both versions of the model provide a reasonable quantitative description of the luminescence lifetime and luminescence intensity as a function of the stimulation temperature. However, very significant differences between the two models are found for the behavior of the simulated integrated thermoluminescence (TL) and thermally stimulated conductivity (TSC) as a function of the heating rate used during such experiments. Only the results from the modified version of the model, (which is based on Mott–Seitz mechanism), are in good agreement with previously reported experimental results. The two models also predict very different behaviors for the dependence of the optically integrated stimulated luminescence (OSL) and TSC signals, as a function of the stimulation temperature. The results from the two models suggest that it may be possible to decide between the two mechanisms of thermal quenching in this material, by carrying out accurate measurements of the TL, TSC and OSL signals under different experimental conditions. The effect of shallow traps on the luminescence lifetimes is also studied and compared with available experimental data.

Density-of-states in microcrystalline silicon from thermally-stimulated conductivity

The technique of thermally-stimulated currents has been applied to extract the density-of-states profile in microcrystalline silicon. Exploiting the experimental parameter space a consistent density-of-states profile emerges with an exponential conduction band tail and a broader deeper distribution. Calibrating the absolute density-of-states profile from other techniques like modulated photoconductivity, steady-state photocarrier grating technique and intensity-dependent photoconductivity allows a determination of the capture coefficient of the probed localized states.

On the verification of the simple trap model by simultaneous TL/TSC measurements

Simultaneous detection of thermoluminescence (TL) and thermally stimulated conductivity (TSC) allows acquiring a lot of information concerning basic trapping processes in dielectrics. The values of activation energies and other trap parameters determined this way noticeably depend on the kinetic model applied. This paper shows that assuming some general conditions the TL/TSC ratio should yield a monotone decreasing function. This simple property holds for the standard model consisting of an arbitrary number of trap levels and recombination centres including also continuous distribution of traps. Counter examples considering simultaneous release of electrons and holes and temperature-dependent recombination probability are given. The later case, in some situations, could be ruled out by plotting a special test function. Thus, the TL/TSC ratio provides a test for a broad class of one-carrier trap models that are used for theoretical description of trapping and recombination phenomena.

Defect characterization of spray pyrolised ? -In2S3 thin film using Thermally Stimulated Current measurements

Thermally Stimulated Conductivity (TSC) has been used to analyse defects in the novel material β -In2S3. These films were deposited using spray pyrolysis technique, varying In:S concentration ratio in the spraying solution. TSC measurements allowed the study of electrical property and non-radiative transitions, due to the defects present in the material. TSC spectra revealed four defects with their prominence varying with In:S concentration ratio. Samples with lower In concentration showed the presence and prominence of indium vacancy. A chemical impurity level due to the presence of chlorine was also detected. Even though sulphur vacancy existed in all the samples irrespective of the variation of In:S concentration ratio its effect decreased with the increase of sulphur concentration. Another defect level was also detected from TSC measurements at high temperature that was attributed to the replacement of sulphur by oxygen which was maximum in films prepared from a spray solution of In:S = 2:3 and minimum for 2:8. This high temperature defect level acts as a neutral center while all the other three levels were seen to be coulomb repulsive. Results from XPS analysis are found to be in good agreement with the TSC results.

Determination of the density of defect states by thermally stimulated conductivity studied from numerical simulations

Starting from the multiple trapping rate equations that define the non-equilibrium concentrations of electrons and holes in extended states, the thermally stimulated conductivity (TSC) experiment is examined. A system of non-linear coupled differential equations is solved to get the temporal evolution of the occupation functions and the carrier concentrations during the initial isothermal waiting time and the subsequent heating at a constant rate. The simulated TSC spectra reproduce the reported dependence of the measured spectra on the heating rate and the starting temperature. An approximate expression to obtain the DOS distribution in the upper half of the band gap from TSC spectra is deduced. The application of this expression to simulated TSC curves provides an accurate reconstruction of the introduced DOS. The TSC method compares favourably to the modulated photoconductivity experiments, both from the quality of the DOS reconstruction and the experimental simplicity of the method.

Recombination in complex systems – an analytical approach

AbstractA new analytical model is proposed to describe the kinetics of trapping and recombination of charge carriers in complex systems with an arbitrary spatial distribution of traps and recombination centres. The structural properties of a material are described by two functions Γm and Γn irrespective of the thermal history of the sample. A simple method is proposed to determine the function Γm from simultaneous thermoluminescence (TL) and thermally stimulated conductivity (TSC) measurements.

Light-induced defects in KTaO3

Photoconductivity (PC), thermally stimulated conductivity (TSC), photoluminescence (PL), thermoluminescence (TL), and electron spin resonance (ESR) measurements have been made on single crystals of potassium tantalate over the temperature range 4.2–290 K. We revealed two sorts of O− shallow hole centers which are responsible for the two temperature regions of PL and PC enhancement: T<70 K and 100–150 K. Both O− centers were identified by their ESR spectra. While at low temperatures PL and PC have a rather intrinsic origin, i.e., they do not depend essentially on the sort or quality of crystals, at 100–150 K both quantities strongly depend on the defect content and vanish in well-oxidized crystals. We show that O− centers serve as radiative electron–hole recombination centers. Their energy levels are situated at 0.08 and 0.16 eV above the top of the valence band. Measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18–30 K and 65–70 K. There is a good correlation between TSC and TL intensity in different samples as well as after annealing in O2 and H2 atmospheres. Because electrons are mobile species in KTaO3, we attribute both TSC and TL to the thermal ionization of the same shallow donor centers related with isolated oxygen vacancies. The experimental data were treated in a one-trap/one-recombination center model, which takes into account the presence of “thermally disconnected” deep electron traps.

Heating-rate method for determination of kinetic parameters from thermally stimulated conductivity and luminescence

A method for determining the ratio of the recombination and trapping coefficients and the relative density of inactive deep traps on the basis of simultaneously measured thermally stimulated conductivity (TSC) and luminescence (TSL) is derived in the case of a temperature-dependent free electron lifetime. The method is based on the relation between the preexponential factor of the temperature dependence of the thermally stimulated free electron density (TSC) and the occupancy of active traps. Values of the preexponential factor and the trap depth corresponding to different values of the relative TSC intensity can be found from the TSC peak measured at different heating rates of the sample. Similar analysis of the TSL peak produces accurate values of the trap depth as well. The method for determination of the trap depth also yields accurate results from multipeak TSC and TSL curves. These results are more accurate than those found using the heating-rate method of Haering and Adams [Phys. Rev. 117, 451 (1960)], the initial-rise method of Garlick and Gibson [Proc. Phys. Soc. (London) 60, 574 (1948)], and the fractional TSL method of Gobrecht and Hofmann [J. Phys. Chem. Solids 27, 509 (1960)].

Determination of ionisation energies and attempt-to-escape factors using thermally stimulated conductivity.

A procedure for determining an arbitrary distribution of activation energies (E) and attempt-to-escape frequencies (s) from overlapping contributions to thermoluminescence (TL) or thermally stimulated conductivity (TSC) is described. For the case of no retrapping, i.e. first order kinetics, the glow curve can be described by a two-dimensional Fredholm equation representing a superposition of Randall-Wilkins first-order peak shapes. The solution to this equation gives the distribution of trapping energies and attempt-to-escape-frequency factors necessary to obtain the TL or TSC peak shape. Analysis of simulated TL/TSC data for trap distributions distributed in both E and s demonstrates that the arbitrary E and s values can be determined from the solution of the Fredholm equation. The procedure is demonstrated for experimental TSC data from gamma-irradiated Al2O3:C.

Characterisation of the thermally stimulated conductivity and thermoluminescence of natural topaz.

Thermally stimulated conductivity (TSC) and thermoluminescence (TL) measurements were conducted to investigate the mechanisms of charge transfer and luminescence emission in natural samples of Brazilian topaz irradiated with beta particles from a 90Sr/90Y source or with a 1.75 MeV Van de Graaff electron beam. The luminescence and conductivity were simultaneously monitored during the heating of the samples, allowing direct comparison of the TL and TSC peaks. The results show that the three main TL peaks are accompanied by corresponding TSC peaks, usually shifted to higher temperatures. Comparison of the relative TL/TSC intensities of peaks 2 and 3 indicates that the process of thermal quenching of the luminescence is probably active, which is also supported by TL/TSC measurements at different heating rates. Results on the dose response of TL/TSC peaks also reveal an interesting feature: the TL intensity shows a monotonic increase with dose in the range of study (50 Gy-3 kGy) comprising a linear-supralinear-saturation characteristic, while the TSC peaks exhibit an increase from 50 Gy to 1 kGy, followed by a small decrease for doses greater than 1 kGy. This result is interpreted in terms of a model involving multiple traps and one recombination centre.

Numerical analysis of simultaneous TL/TSC measurements.

Thermoluminescence (TL) and thermally stimulated conductivity (TSC) are closely related phenomena. Simultaneous detection of luminescence and conductivity allows one to acquire more information concerning the basic trapping processes occurring during heating of a sample. In this paper a new formula for simultaneous TL/TSC measurement is derived under quasiequilibrium (QE) conditions. Numerical analysis of the obtained experimental data requires fitting of only two unknown parameters. The new method of analysis was verified for computer generated TL/TSC curves. The practical usefulness of this method is demonstrated in application to the measurements performed for the a-alumina single crystal. The analysis, in general, confirms earlier results for the peak B'. The results obtained for the peak D' are not so clear. Most likely they reveal some complex factors that are not taken into account.

Shallow traps in pure KTaO 3 crystals

We made Thermally Stimulated Conductivity (TSC), Thermoluminescence (TL) and Electron Spin Resonance (ESR) measurements on single crystals of potassium tantalate in the temperature range 4.2-290 v K. We revealed two sorts of O m shallow hole centers which are responsible for Photoconductivity (PC) and Photoluminescence (PL) enhancement. Both O m centers were identified by their ESR spectra. We show that these centers serve as radiative electron-hole recombination centers. The measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18-30 v K and 65-70 v K. Both TSC and TL are attributed to the thermal ionization of the same shallow donor centers related with oxygen vacancies. Experimental data were treated in a simple one-trap/one-recombination center model, which takes into account the presence of "thermally disconnected" deep electron traps.

Comparison of some models for nonlinear fitting analysis of TSC measurements

Basic differential equations for thermally stimulated conductivity (TSC) measurement have no analytical solutions even for the simplest cases. Therefore, analysis of the obtained results is very difficult. The same equations are used also for thermoluminescence (TL). Nevertheless, TL analysis is much easier due to a number of approximate models, such as general order kinetics and mixed order kinetics. In this paper we discuss usefulness of the approximations in description of TSC with respect to determination of trap parameters using nonlinear curve fitting analysis. For this purpose appropriate equations were derived. The results are compared to the recently derived method based on the quasi equilibrium approximation.