Thermally Stimulated Current Peak Research Articles

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.

Study on Electron Traps in Polyethylene Terephthalate by Thermally Stimulated Current and Photo-Stimulated Detrapping Current Analyses

Electron traps in PET photoelectret were investigated by the analyses of the thermally stimulated current (TSC) and the photo-stimulated detrapping current (PSDC). A broad peak of TSC was observed around -100°C at which the motion of the COO group was released and a plateau was observed around 0°C. The apparent activation energy of the former peak was estimated at about 0.23 eV by the partial heating technique. On the other hand, PSDC spectra at -185°C showed an existence of deep traps at 2.3 eV, which could be thermally cleaned by heating only up to -70°C. These facts clearly show the effect of molecular motions on the carrier detrapping process during heating.

The influence of oxidation on thermally stimulated currents from trapped carriers in high density polyethylene

Thermally stimulated currents (TSC) were measured from oxidized and unoxidized high-density polyethylene with prior X-ray irradiation. Six TSC peaks due to trapped carriers were observed in the unoxidized samples. Slight oxidation introduced a new peak around 180K, which was ascribed to unstable intermediates in the oxidation process. Further oxidation reduced the TSC below 340K and enhanced the TSC peak around 360K ascribed to stable oxidation products such as carbonyl groups. The TSC without X-ray irradiation were also reduced by oxidation. These reductions are considered to be closely related to the introduction of deep traps responsible for the TSC peak around 360K.

Study of carrier mobilities by a thermally-stimulated-current technique

A new method to determine the trap-modulated mobilities of electrons and holes in insulators from thermally-stimulated-current (TSC) data is described. The formalism is based on the assumption of a box-shaped carrier distribution which maintains its rectangular form while it is spreading through the insulator at increasing temperatures. Correlating each TSC peak with the interaction of the carriers with one particular trapping level, it is possible to evaluate the effect of this trapping level on the carrier mobility and to determine the temperature dependence of the mobility. The theoretical formulation of this method, as well as some experimental results obtained on electron and hole mobilities in Teflon, will be discussed.

Thermally stimulated currents and dielectric study in amorphous selenium

Two relaxation modes have been isolated by the thermally-stimulated currents (TSC) technique in vitreous selenium. At about 0 °C, a field-independent TSC peak is observed. It may be explained by the field-independent mobility which has been measured in the same temperature range. This relaxation is thermally actived with the same activation energy as the ac conduction found at low frequency (F<1 kHz) from ac measurements in the same samples. At about −100 °C, a field-dependent TSC peak gives a conductivity varying as ω which agrees fairly well with ac measurements in the same samples above 10 kHz. The field dependency of this TSC peak is well described by the continuous distribution of thermally activated hole-drift mobility which has been measured in the same temperature range, using a time-of-flight technique.

Thermally stimulated currents in polyethylene and ethylene–vinyl-acetate copolymers

Thermally stimulated currents (TSC) from polyethylene and ethylene–vinyl-acetate copolymers with a variety of vinyl-acetate contents have been investigated using a TSC technique with x-ray irradiation. Five main TSC peaks, which we believe are due to trapped carriers, were observed in these polymers. The detrapping of carriers is closely related to molecular-chain motions, and trapping sites are assigned to physical defects such as cavities formed by local arrangements of molecular chains rather than carbonyl groups. One of the TSC peaks is associated with traps in the amorphous region, one with traps in the boundary region between the amorphous and the crystalline regions, i.e., in the folded region, and another with traps in the crystalline region. The TSC spectrum from the dipole of the vinyl-acetate unit under a bias field shows a TSC peak accompanied by a current inversion. The average effective dipole moment for the vinyl-acetate unit was determined to be about 1.8 D from a TSC analysis.

General properties of secondary relaxations in polymers as determined by the thermally stimulated current method

AbstractThe method of thermally stimulated current (TSC) has been used to study the low‐temperature dielectric β relaxations of several polymers including especially poly(vinyl chloride), poly(vinyl acetate), polyamide 6, 6,6,poly(t‐butyl acrylate), poly(methyl methacrylate), poly(ethyl methacrylate), poly(phenyl methacrylate), and poly(t‐butyl methacrylate). The distribution characteristics of the relaxation processes have been determined from the corresponding TSC peaks by a fractional polarization technique which consists of applying the electric field in several discrete steps during a slow cooling. Several common features have been found in all the polymers investigated: the β peaks are characterized by a distribution of relaxation times resulting from a distribution in activation energy and this distribution is quasisymmetrical and continuous. These facts are in agreement with the hypothesis of a relaxation involving local motions of small polar groups undergoing various interactions with the environment. Some discrepancy remains, however, between our calculated values of the mean activation energy and those obtained from the dielectric loss.

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.

Theory of thermally stimulated current in hopping systems

Using the concept of thermally activated mobility, a theory is developed to calculate the thermally stimulated current (TSC) in hopping systems as a function of temperature. Unlike the existing theories which describe only the band transport systems, the present theory is applicable to both band and hopping transport systems. Various features of this ’’mobility-induced’’ TSC are illustrated with numerical examples and compared with experiments performed on polymer films (e.g., PVK). These features form the basis of a new method (reported in an earlier paper) for determination of the activation energy, and a technique for studying very low mobilities and/or very low field transport phenomena. It is also found that an observation of shift in TSC peak temperature with applied field does not necessarily imply a field-dependent activation energy.

Dipole reorientation in polyvinylidene fluoride

The thermally stimulated current (TSC) from polarized polyvinylidene fluoride is studied below room temperature. The experimental results indicate that the maximum of the TSC observed at ∼ −50°C is due to reorientation of dipoles in a process associated with the glass transition. The TSC peak is inhomogeneously broadened by a distribution in relaxation frequencies. The results are analyzed in terms of both Arrhenius, and Williams-Landel-Ferry descriptions of the temperature dependence of the relaxation processes and the distribution function for the relaxation frequencies is calculated from the temperature dependence of the TSC. It is shown that a distribution in preexponential factors or in activation energies can account for the observed width of the TSC peak. For the latter case a distribution varying exponentially with activation energy seems most likely. From the data we obtain (3.6±0.4)×104 °K for the apparent activation energy and ∼2×10−3 sec−1 for the relaxation frequency of the dominant realignment process. The glass transition temperature is determined as Tg ∼ −51°C.

Charge Carrier Trapping in Organic Crystals

Abstract -Studies on the carrier trapping characteristics of anthracene, chrysene and tetracene crystals using thermally stimulated current (TSC) techniques and space-charge-limited-trap-limited current (SCLTLC) techniques are described. TSC studies on melt-grown, vapor-grown and radiation damaged anthracene crystals, melt-grown chrysene crystals and vapor-grown tetracene crystals have shown that a TSC peak at 255–265 °K (trap depth 0.6–0.8 e V) is obtained for all these crystals and appears to be relatively insensitive to the physical state of the crystals. SCLTLC measurements on many high purity melt-grown and thick (∼ 1–2 mm) vapor-grown anthracene crystals using a liquid hole injecting contact (AlCl3-anthracene/nitromethane), gave trap densities ranging from 1014 to 1019 traps cm−3 for both types of crystals. Hole trap densities did not correlate with the density of dislocations emerging through (001), indicating that the hole traps in these crystals are not directly related to the dislocation content.

The Effect of Various Optical Radiations and Water Vapor on the Trapping Spectrum of CdS

Trapping levels in a cadmium sulfide single crystal were studied by the thermally stimulated current method. In all, six trapping levels were detected in the crystal. A study of the shallow traps, which occurred at 0.04, 0.12, and 0.13 ev below the conduction band edge, indicated that electrons could be excited directly to the 0.13 ev level. A peak in the photoconductivity was observed at −60° when the crystal was cooled in the presence of IR and 533 mµ light. The center with level at 0.13 ev appears to behave as a recombination center for temperatures greater than −60° and as a trapping center which can be emptied optically at temperatures less than −60°C. Other experiments involving cooling during illumination indicated the presence of another trapping level which apparently could not be filled from the conduction band. Water vapor which was adsorbed on the crystal surface produced thermally stimulated current (TSC) peaks at 80°−100°C in the absence of any treatment of the crystal by light. The appearance and magnitude of these TSC peaks were affected by light as well as the partial pressure of water vapor. Pumping over the crystal with adsorbed water present also stimulated currents. Both of the latter observations are believed due to the dissociation of an adsorbed species such as on the crystal surface.