Thermally Stimulated Current Method Research Articles

The Thermally Stimulated Current (TSC) Technique on Evaluating Degradation of ZnO Varistors

Thermally stimulated current (TSC) is a simple and effective test technique to study the thermal activated charge, electron trap and activation energy of dielectric and semiconducting materials. It is well known that the addition of glass frits can improve the degradation property of ZnO varistors. An activation energy of 0.45 eV has been found on the ZnO varistors with/without glass frits after the DC degradation by using TSC method. The tested quantity of thermal activated charges, QTSC, may represent the numbers of migrated zinc interstitials. ZnO visitors doped with glass frits have less QTSC. Experiment indicates that TSC test is a convenient method to evaluate the degradation behavior of ZnO varistors.

Temperature Dependence of the Bistable Photoconductivity of Thin DNA: PEDOT Films

Thin DNA:PEDT-PSS layers were investigated. The functionalization of DNA by PEDT-PSS rendered the material electrically active, its conductivity being about (1–5) × 10−10 Ω−1cm−1 at the room temperature. The samples remained ohmic down to 77 K. The thermal activation energy of the conductivity near the room temperature was about 0.033 eV, and it decreased under 0.014 eV below 170–180 K. The weak carrier trapping was identified by the Thermally Stimulated Current method, proving the recombination of light-generated carriers. Notably, by constant light excitation a “bistable” photoconduction below the room temperature was evidenced. The photosensitive state could be induced by the light from the spectral region from ∼ 500 nm up to ∼1000 nm, with a maximum effect in the range of 650–800 nm. A remarkable increase of the photocurrent could be observed below 145–155 K by cooling the samples. Meanwhile by heating the photosensitivity remained increased up to 235–245 K. The long characteristic relaxation times after the light excitation in this state were proportional to the relative photosensitivity of material. This indicates that such phenomenon could presumably be attributed to the light-induced changes associated with PEDT-PSS, i.e., modification of the sample material morphology and/or induced variation of carrier transport conditions.

Study of traps in P3HT:PCBM based organic solar cells using fractional thermally stimulated current (FTSC) technique

Thermally Stimulated Current (TSC) measurement, well known technique for trap level detection, was applied to derive the density of occupied states in organic solar cells. The device structure is Glass/ITO/P3HT:PCBM/Al. Analysis of the TSC spectra allows to determine the Fermi level position from the edge of the nearest band (LUMO or HOMO) (~0.42eV). At that time, TSC peaks cannot be evaluated due to multiple traps that contribute to the current signal. In order to achieve a rough picture of the trap structure, Fractional TSC (FTSC) method was applied. The procedure is based on multiple temperature cycles which evaluate activation energies for different ranges of temperatures. The results reveal several plateaus, pointing out the presence of four trap levels (0.12eV, 0.16eV, 0.20eV and 0.35eV). Additionally, a curve-fitting procedure is done in order to characterize the trap parameters and achieved with five trap levels (0.059eV, 0.12eV, 0.14eV, 0.20eV and 0.35eV) in Gaussian or exponential distribution. Four of them are in complete agreement with measured FTSC ones. Those results are very well correlated with our other characterizations by Admittance Spectroscopy (AS). The results presented here show that, in spite of the chemical differences between inorganic materials and conjugated polymers, FTCS technique can still be applied to determine the density of occupied states in these materials, as long as care is taken. Polymers are low-mobility materials with large concentrations of deep impurities.

Measurement of energy levels in a silicon detector damaged by neutrons

The level of defects in a semiconductor silicon detector diode made of high resistivity N type material and exposed to neutrons in a research nuclear reactor was examined by measuring the thermally stimulated current (TSC). A modified TSC method was employed where the released charge was measured in the reverse direction on a diode with zero bias voltage. Electrons captured in cooled traps due to the photoelectric effect are released when the material is heated. The detector was irradiated with an integral neutron flux of 7.63 × 1015 n/cm2.

Influence of Rubber Annealing on Behavior of the Three-Phase Model in PET Study by the Thermally Stimulated Current (TSC) Method

Polymers are generally formed by two phases: a crystalline phase where chains are arranged in parallel, one over the other, and an amorphous phase where there is absence of order. Several recent works have supposed the existence of a three-phase model where there coexist simultaneously a crystalline phase, a rigid amorphous phase, and a mobile amorphous phase. The objective of our work is to put in evidence the existence of three phases separately within polyethylene terephtalate (PET). This polymer is widely used in electrical insulation and especially in packaging. To this effect, samples of PET have undergone a heat treatment of isothermal annealing time from 10 to 240 min. The samples thus treated were studied by the thermally stimulated current method, which tracks the variation of the depolarization current as a function of sample temperature. Recordings of thermocurrent are made after increasing temperature with a linear heating rate of 7°C/min. Then we can calculate the areas under the thermocurrent peak and hence estimate the corresponding degrees for the various phases present in the annealed material, χc and χam. Analysis of the values obtained for these degrees introduce a third component, denoted χar, which characterizes the rigid amorphous phase. Monitoring the evolution of these degrees with duration of annealing is realized. It is clear from all results that PET annealed rubber has a three-phase behavior with simultaneous presence of three phases, a crystalline phase χc, an amorphous mobile phase χam, and a rigid amorphous phase χar.

Electrical and optical properties of thin films of DNA:PEDOT

We report investigations of functionalized DNA:PEDT–PSS films. The electrical conductivity of the sample material at the room temperature was about (1–5) × 10 −10 Ω −1 cm −1. The IV curves of the samples were linear and symmetrical in the region from the room temperature down to the liquid nitrogen temperature. The thermal activation energy of the conductivity near the room temperature was about 0.033 eV independently on the applied bias. The weak carrier trapping was identified by the Thermally Stimulated Current method, proving the fast recombination of light-generated carriers. Notably, by constant light excitation a “bistable” photoconduction below the room temperature was evidenced, i.e., upon excitation by a white light a remarkable increase of the photocurrent could be observed below 145–155 K by cooling the samples. Meanwhile by heating the photosensitivity remained increased up to 235–245 K. Such phenomenon could presumably be attributed to the light-induced changes of the sample material morphology and/or associated variation of carrier transport conditions.

Thin Films of DNA:PEDOT-PSS – Electrical and Optical Properties

Electrical, charge transport and optical properties of DNA:PEDOT-PSS thin films were investigated. Sample conductivity at room temperature was about (1–5) × 10−10 Ω−1 cm−1, IV curves being linear and symmetrical down to Liquid Nitrogen (LN) temperature. The thermal activation energy of the dark conduction near the room temperature was about 0.033 eV independently on the applied bias. The small effect of carrier trapping was evidenced by the Thermally Stimulated Current method, proving the fast recombination of light-generated carriers. Though, by constant light excitation a “bistable” photoconduction below the room temperature was identified. I.e., upon excitation by light from the spectral region ∼500–800 nm a notable increase of the photocurrent could be observed below 140–160 K by cooling the samples. Meanwhile by heating the photosensitivity remained increased up to 230–240 K. Most probably such phenomenon could be attributed to the light-induced morphology changes of the samples.

DC electrical treeing phenomena and space charge

Influence of space charge on dc tree propagation is studied using many insulating polymer materials under divergent fields. The inception voltage and the propagation of dc treeing are affected strongly by space charge. Space charge formation in polymers was observed directly by using thermally stimulated current (TSC) techniques under divergent field. These results of the space charge effect on dc treeing phenomena are consistent with those obtained by the TSC method. From these results, the origin of space charge in polymers and the suppression techniques for space charge formation were discussed.

Characterization of polymers using the thermally stimulated current technique

Thermally Stimulated Current (TSC) technique is a relaxation technique, such as DMA, but works at very low equivalent frequency (10−3–10−4Hz) and thus clearly resolves the peaks related to the molecular mobility. The effect of orientation on Polypropylene fibers, the relaxation of internal stresses on Polycarbonate disks, and the effect of aging on acrylic airplane windows have been investigated using the TSC method.

Neutron induced defects in silicon detectors characterized by DLTS and TSC methods

Neutron induced defects in silicon detectors fabricated from n-type float zone material of different resistivity (100–6000Ω cm) have been studied using the C-DLTS (Capacitance-Deep Level Transient Spectroscopy) and TSC (Thermally Stimulated Current) method. While the application of the C-DLTS technique for high resistivity material is limited to neutron fluences below about 10 11 cm −2 the TSC method remains a powerful tool for the defect characterization even at high fluences. Up to 5 defect levels were observed in some of the unirradiated samples. These partly are due to thermal treatments during the fabrication process. After neutron irradiation defect levels at E c − 0.17, −0.23 and −0.42 eV and at E v + 0.36 eV were found. A detailed analysis of the predominant peak at about −0.42 eV has shown that it is a superposition of two levels at −0.39 and −0.42 eV. For these defect levels introduction rates, annealing effects and a comparison between the DLTS and TSC technique are presented. Possible correlations of these results with macroscopic detector properties are discussed.

What is TSC?

The technical background for the thermally stimulated current (TSC) technique is presented. The effect of metal polymer contacts on the TSC method is discussed. The transitions in amorphous polymers are described. The dynamic properties of polyethylene using TSC method are given. >

Detection of deep levels and compensation mechanism in undoped, liquid-encapsulated Czochralski n-type GaAs

Undoped n-GaAs with a 300 K resistivity between 10−1 and 108 Ω cm (electron concentration between 1×107 and 5×1015 cm−3 ) grown in quartz crucibles by the liquid-encapsulated Czochralski (LEC) technique was investigated by thermally stimulated current (TSC), temperature-dependent Hall effect (TDH), and deep-level transient spectroscopy (DLTS). Using Schottky contacts the TSC method could be extended to medium-resistivity samples. The strongly varying electron concentrations are correlated to varying TDH activation energies. The correlation between the donors dominating the electrical equilibrium properties and the electron traps detected by TSC and DLTS is discussed. Medium-deep and deep levels are present in this LEC material in such high concentrations that they must be taken into account in the compensation mechanism.

Oh − reorientability in hydroxyapatites: Effect of F − and Cl −

The effects of partially substituted F − and Cl − in hydroxyapatite on the co-operative reorientation motions in the OH-dipole chains associated with the monoclinic-hexagonal phase transition have been examined with the fractional-polarization thermally stimulated current (TSC) method. The motions can be sensed with the TSC method even though various chain segments may be oppositely oriented and thus mandate the hexagonal phase. Substitution of one in four OH − ions by F − causes the OH − dipole chains to be too short, and too bound at the ends, to display this co-operative reorientation. With 16% F substitution, there are enough OH − chains of sufficient length to show the co-operative motion, identifiable by the occurrence of a compensation phenomenon. The occurrence of three compensations with essentially the same temperature (209–216°C) but different characteristic relaxation times is ascribed to the existence of different OH − chain lengths between interrupting F − ions. Chlorhydroxyapatite (Cl 0.5OH 0.5) shows no compensation below 800°C even though both end-members do, at the monoclinic-hexagonal phase transition temperature. This is ascribed to short chain lengths and to direct OH −−Cl − interactions. Fluorapatite shows no compensation below 1200°C, and no phase transition in that temperature range is known.

Influence of tacticity on Tβ, Tg, and TLL in poly(methyl methacrylate)s by the method of thermally stimulated current (TSC)

AbstractThe thermally stimulated‐current method (TSC) has been employed to determine the temperatures and intensities of Tβ, Tg, and T > Tg for pure isotactic, pure syndiotactic, and five atactic specimens with syndiotactic triad content from 49.5 to 75%; Tg was found to increase linearly with syndiotactic triad content as Tg (°C) = 48.0 + 0.856 (% syn), with R2 = 0.970 standard error 5.6°C; Tg for the syndiotactic specimen is 136.6°C measured, 133.6°C calculated. Several atactic specimens exhibit a second glass temperature 15 to 35 K above the regression line ascribed to some pure syndio content, and/or some isotactic–syndiotactic stereocomplexes. All specimens exhibited the liquid–liquid or TLL transition (relaxation) which increases linearly with 100‐% isotactic triad content. Isotactic PMMA shows a T′LL relaxation 50 K above TLL. The Tg and TLL values obtained correlate extremely well with values from differential scanning calorimetry (DSC) determined in a separate study, as well as with most literature data. Intensities of Tg and TLL by TSC are greatest for isotactic, next for syndiotactic, with a broad, low minimum for atactic materials. The intensity of a β relaxation increases slowly from isotactic to syndiotactic. The TLL found by TSC compares well with literature values for isotactic PMMA obtained by several methods, and TLL in the atactic region compares well with literature values for atactic material. The ratio TLL/Tg ranges from 1.09 to 1.20 with no dependence on tacticity. Tg follows simple Arrhenius behavior with enthalpies of activation about one‐half of the values normally calculated from dielectric and mechanical loss. The frequency dependences of TLL and T′LL follow a Vogel–WLF relationship with temperature. The origin of TLL is discussed in terms of the Frenkel hypothesis of segment–segment interaction. Evidence for TLL and TLL from a variety of methods indicates that these two temperatures are not artifacts of the TSC method.

Thermally stimulated current (TSC) study of the Tg and Tll transitions in anionic polystyrenes

AbstractThe thermally stimulated current (TSC) technique has been used to investigate three anionic polystyrenes of M̄n 17,000, 71,700, and 1.55 × 106, i.e., M < Mc, M > Mc, and M ≫ Mc, where Mc is the entanglement molecular weight. A current maximum near Tg designated TMg, has relaxation times which follow an Arrhenius equation. A second current maximum at T > Tg appears to be the Tll process and is designated TMll. Relaxation times for it follow a Vogel equation. TMg and TMll vary with molecular weight, increasing below Mc and leveling off above Mc at a temperature of about 170°C. Values of TMg and TMll are compared with values of Tg and Tll obtained from torsional braid analysis, which involves melt flow; and with differential‐scanning‐calorimetric values on fused films, where there is no transport of polymer. It is concluded from such cross‐comparisons that TSC, at least for polystyrene, is a quasistatic test which may involve microscopic viscosity. Macroscopic viscosity does not play a role. The ratio TMll/TMg is in the range 1.10–1.16, similar to Tll/Tg values by other methods. A few comments about Tll in atactic poly(methyl methacrylate) by the TSC method are given.

Determination of dielectric relaxation time as a function of pressure and temperature by the thermally stimulated current method

The thermally stimulated current (TSC) has been used over a wide range of pressure to determine the principal characteristics of the polymers such as the activation volume ΔV, the activation energy ΔE, and the preexponential term τ0 of the Arrhenius-type relaxation time: τ=τ0 exp[(PΔV+ΔE)/kT]. Experimentally, the 6-Polyamide sample is subjected to a high hydrostatic pressure before application of the classical TSC method. The variation law of the relaxation time, a function of temperature and pressure, τ (P,T), of this polymer is given by the determination of these characteristics with the initial slope analysis of the TSC experimental curves.

Water-Dependent Relaxation in Polymers. Study by the Thermally Stimulated Current Method

Water-Dependent Relaxation in Polymers. Study by the Thermally Stimulated Current Method

Thermally Stimulated Current of CdTe Doped with Zn

Three traps in p-type CdTe doped with Zn are found using the TSC (thermally stimulated current) method between 77 and 200 K. Their depths are estimated to be approximately 0.24, 0.25 and 0.40 eV. The TSC peak for the 0.24 eV trap is considerably quenched by the infrared rays. A quantitative analysis of the processes for the 0.24 eV trap is presented. It is proved that there are two different processes involved in the occupation of 0.40 eV trap at 77 K: one is a fast process and the other is slow. A model is proposed to explain qualitatively the complicated processes of the 0.40 eV trap.

Trap-modulated mobility of electrons and holes in Teflon FEP

The trap-modulated mobilities of electrons and holes in 25-μm Teflon FEP, charged by application of a high dc field at room temperature, are determined from thermally stimulated current (TSC) measurements performed under open-circuit conditions. Use of the TSC method allows one to evaluate approximately the effect of various trapping levels in the material (corresponding to different TSC peaks) on carrier mobility. For example, the first and second above-room-temperature TSC peaks for positive carriers yield mobilities of 10−11 cm2/V sec at 50 and 80 °C, respectively. Similarly, for negative carriers, the second above-room-temperature TSC peak yields a mobility of 10−11 cm2/V sec at 185 °C. The temperature dependence of the mobilities follows an Arrhenius-type behavior with activation energies of 0.7 and 1.0 eV for the positive-carrier peaks and 1.8 eV for the negative-carrier peak.

Photoelectronic Properties of CdSe Evaporated Films

Pure CdSe films deposited on 100°–400°C substrates show high photosensitivity after a treatment of subsequent baking in a temperature range of 500°–700°C in argon gas. The maximum value of light-to-dark current ratio at 1000 lx is of the order of magnitude of 106. The maximum peak of spectral sensitivity is in a range of 700–710 mµ. Spectral distribution of absorption coefficient of baked films is almost equal to that of single crystals. Effect of baking on deposited films was studied by measuring thermally stimulated current (TSC) and superlinearity. Trap denisty of deposited films estimated by TSC method is 1018–1019 cm-3. Whereas, the decrease of trap density is observed in highly photosensitive films after baking, particularly for deep traps (trap density is smaller than 1014 cm-3). Properties of sensitizing centers in baked films are observed to be similar to those of single crystals.