Shigeyoshi Maeta

On the Determination of Trap Depth from Thermally Stimulated Currents

A new method for estimating energy depths of traps from a thermally stimulated current with slow retrapping is described in detail. This method uses the maximum temperature Tp and the arbitrary temperature Ti at which the TSC intensity attains 1/m of the maximum value Im in the lower temperature region. The energy depth E is given by E=A((kTpTi)/(Tp-Ti)), where the coefficient A is a parameter which is determined to be about 0.5~2.5 corresponding to the individual case of m=1.1~5. The validity of this method is examined for our experimentally obtained TSC results in a wide temperature range and for the calculated curves based on the experimental results. The coefficient A essentially can be calculated very accurately, and good accuracy for E is guaranteed by recalculating A for each TSC curve. In the special case of m=2, the value A can be determined to be 1.40 with an accuracy of more than 3 % for almost cases.

On the Determination of the Trap Depths from Thermally Stimulated Currents II

A new asymptotic estimation method (the H-method), which enables one to estimate the exact energy depth Et of a trap from the TSC peak and one of other coordinates of the higher side of the peak temperature Tp, has been proposed. From the establishment of this proposal and the L-method estimated from the lower side, more exact estimation of Et and a new application were made possible. The m characteristics that provide a basis for judging whether a TSC curve is contributed by a single trap only or by multiple traps have been reported. They show a perfect flatness with the same Et value in the range from ca. 10% to 90% of 1/m if the TSC curve is contributed by a single trap only. The m characteristics of the coefficient Ci (=Et/kTp) have been proposed. A simple and convenient equation of the H-method and an expression of the asymptotic estimation method without Tp are presented.

A New Method for Determining the Trap Depth from Thermally Stimulated Current

A new method for determining the energy depth of the trap Et from a thermally stimulated current is proposed. This method uses a slope at an arbitrary point in the Arrhenius plot of the TSC on the lower side of the temperature of the TSC maximum. In the Arrhenius plot, the current range over ca. 20% of the TSC maximum is hardly influenced by the fluctuation of zero level, but this region is not used in an initial rise method. In the present method, the range up to ca. 95 % of the TSC maximum can be used, and the more accurate value Et without the influence of background noise can be necessarily evaluated. This method is divided into five modes of graphical solution and a mode of numerical calculation. The procedures for obtaining Et are simple, and it is possible to obtain the Et value to three significant digits. The respective methods are applied to the TSC experimentally observed and numerically calculated results in an anthracene single crystal, and the results are shown.

Analyses of Thermally Stimulated Current Curves with No Peaks

A theory that enables one to accurately estimate three parameters, namely, an energetic depth of carrier trap site Et, an escape frequency factor ν and a charge carrier density nt, from thermally stimulated current (TSC) curves with no peaks is proposed. Generally, these three parameters could not be evaluated from any TSC datum without peak coordinates. Such TSC data are usually measured experimentally in order to separate signals from each other in various composite cases. Our proposed theory makes it possible to evaluate several sets of three parameters. The proposal is a method to compensate the charges lost in experiments. Three types of charge compensation processes are reported in detail. The accuracy of evaluation of each of the three parameters is very good and the Et values of three significant figures could be obtained by numerical analyses.

Trap States of Tris-8-(Hydroxyquinoline)Aluminum Degraded by Blue Laser Using Thermally Stimulated Current Method

We investigated the photodegradation phenomena of tris-8-(hydroxyquinoline)aluminum (Alq3) film after irradiation with a blue laser of 442 nm to identify the contributions of the excited states of Alq3 molecules. The photodegraded Alq3 film showed decreases in fluorescence lifetime and carrier transport ability markedly. These photodegradation phenomena are considered to be due to an increase in the energy distribution of trap states detected from the analysis by a thermally stimulated current (TSC) method, in connection with the orientation of neighboring Alq3 molecules, e.g., the disordered aggregation states of Alq3 molecules.

New exact computer analysis of the TSC curve by the asymptotic estimation method and an approach to the electrical conduction of organic crystals

Experimental results were obtained for a naphthacene-doped anthracene single crystal excited by Q-switched ruby laser pulses. Two representative TSC (thermally stimulated current) curves were obtained. The curves were analyzed by an asymptotic estimation method, involving the autoseparation of the TSC curve and the exact estimation of the energy depth of the carrier trap from the separated curve.<<ETX>>

TSC of polyaniline thin film

We report the results of thermally stimulated current (TSC) measurements of polyaniline thin films with electric conducting or insulating property and the latest application of the analytical estimation methods for TSC signals.

Computer Simulation of Thermally-Stimulated Currents in an Anthracene Single Crystal with Discrete Shallow Traps

Numerically-calculated results and discussion of the experimental results of thermally stimulated current in anthracene single crystals induced by Q-switched ruby laser are reported. It is concluded that the energies of the shallow traps in anthracene single crystals are distributed discretely.

Behavior of the discrete energy states

A study comparing composite thermally stimulated current (TSC) curves that usually are measured by the single heating method, and partial curves from repeated heating and cooling processes, is reported. For this purpose, both the auto-separation (As) and reconstruction (Re) methods are necessary. The former enables us to separate accurately composite curves obtained by one heating process into several TSC curves with a single relaxation process. The latter enables us to reconstruct TSC curves from partial curves obtained by repeated heating and cooling, even if they had no observed peak. Both methods apply a theory, using the fundamental element that was defined originally to be the smallest quantity and a coordinate determined arbitrarily on the measured TSC curves. The current type of charge compensation equation that makes accurate RC procedure possible was first proposed. From detailed analysis, it was confirmed that an electron trap and a hole trap are produced simultaneously in a naphthacene doped anthracene single crystal. Analysis suggests that repeated heating and cooling influences both trap sites and that the indirect transport mechanisms via discrete state-to-state charge transfer processes is not negligible.

Proposal of reconstructing method of partial TSC curves to some full curves for accurate evaluation

An original theory that three parameters, the energy depths of carrier traps E/sub t/, the escape frequency factor /spl nu/ and the density of carrier trap site n, can be theoretically evaluated with high accuracy from partial TSC curves is proposed. How the standard can be constructed has been a most important resultant point.