The theory of thermally stimulated conductivity (TSC) for a single trap depth in the presence of deeper traps, and a single type of recombination center, has been developed in detail without making the customary restrictive approximations and assumptions based on the relative importance of recombination or retrapping, or on the constancy of the lifetime. General conclusions about the shape of the TSC curve are presented, depending on the ratio of recombination to trapping probabilities, and on the ratio of the density of the traps of interest to that of deeper traps. The results of the theory are applied to the particular cases of first-order kinetics, strong retrapping with constant lifetime, and strong retrapping with varying lifetime. A critical discussion is given of the analysis of TSC data according to methods involving the quasi-Fermilevel, the half-widths of the TSC curve, varying heating rates, and the initial activation energy at the beginning of the TSC curve. It is concluded that the method of decayed TSC provides the most reliable determination of trap depth. The results of the theory can be readily adapted for application to systems involving more than one type of recombination center.
.it' field strengths which can lie niarkedly belo\\. the critical values for impact ionization and Zener extraction, field induced changes in the capture cross section. barrier height, and frequency factor of Coulomb-at.tractive centers due to a deformation of the potential around the center and induced by the external field can cause a marked redistribution of carriers over these centers (field-enhanced ionization). These changes in the kinetic parameters of t,he centers are estimated, using a classical model. and are given as a function of the external field. For a typical semiconductor a t T 1 200 "K, a possible decrease of trap-occupat.ion 111) t'o a factor of two, already, at fields of the order of lo2 V/cm is calculated.Fur Feldstarken. die hetrachtlich iinterhalb der kritischen Werte ftir StoBionisation und fiir Feldemission liegen, konnen feldbedingte Anderungen des Einfangquerschnittes. der Rarrierenhohe und des Frequenzfaktors von Coulomb-at.traktiven Zentren. verursacht ciurch eine Potentialdeformation in der Niihe des Zentrums durch das aul3ere elektrischr Feld, eine merkliche Umverteilung..von T,adungstrlgern iiber diese Zentren hervorrufen (.,field-enhanced ionization"). Die Anderungen der kinetischen Parameter dieser Zentren werden in einem klassischen Model1 und als Funktion des auBeren Feldes angegeben. Es wird ausgerechnet, daB fur einen typischen Halbleiter bei T = 200 O K die Hafttermbesetzung bis um einen Faktor zwei schon bei Feldstarken in der Xiihe von 100 V/cm absinken kann.
A detailed investigation of different methods for determining electron trap parameters has been made on crystals of CdS-CdSe. The principal techniques involved are decay of photoconductivity and thermally stimulated conductivity (TSC). Direct evidence of a quasicontinuous trap distribution with total density of 5×1015 cm−3, trap depth range of 0.1–0.7 eV, and capture cross sections of the order of 10−16 cm2 is obtained, for which correct values of the parameters can be calculated from Fermi-level analysis of either decay or TSC data. In the same crystals a discrete trap level with density of 2×1014 cm−3, depth of 0.73 eV, and apparent cross section of 10−14 cm2 is also found. In spite of the large value of cross section derived from the freeing of trapped electrons, these traps exactly obey monomolecular kinetics. A temperature threshold at 180°K is found, below which it is not possible to fill these traps. Examination of a number of possibilities favors the proposal that these traps are characterized by a Coulomb-repulsive barrier.
The effects of moderate electric fields (≤3×103 V/cm) on the trapping processes in photosensitive CdS-CdSe single crystals have been investigated using photoelectronic techniques. Possible mechanisms such as injection of electrons, extraction of holes, dielectric polarization due to inhomogeneities, Joule heating, electrochemical effects, impact ionization, field-assisted tunneling, and field-associated changes in the capture cross sections and/or thermal emission probabilities of traps are considered. Evidence is presented for the reality of field-associated changes in trapping parameters in the absence of all the other possible effects. Results are consistent with a field emptying of Coulomb-attractive traps by a decrease in the trap depth and a decrease in the capture cross section of traps. The conclusions may be relevant to the interpretation of space-charge-limited current data and to mechanisms capable of leading to improved photoconductor speed for low intensity excitation.
It is shown that the observed steep decrease of the electron density in photoconducting CdS(A1, Ag) with field in the range between 20 and 70 kV/cm is caused by a redistribution of holes from slow to fast recombination centres (field quenching). This redistribution is produced by field-enhanced ionization of holes from Coulomb-attractive slow recombination centers. The abrupt onset of the field quenching occurs because of the slow recombination traffic masking the fast center traffic until i t becomes predominant. Competing infrared quenching reduces the masking effect and uncovers the earlier phases of field quenching :&heady near 1 kV/cm (at 200 OK). Impact ionization and Zener extract,ion of holes from slow centers cannot explain the observed behavior. However, quantitative agreement between experiment and field quenching via field-enhanced ionization can be reached.Es wird gezeigt, dal3 der steile Abfall der Elektronenkonzentration in photoleitendem CdS(A1, Ag) mit dem elektrischen Feld zwischen 20 und 70 kV/cm durch eine Umverteilung \-on Liichern von langsamen zu schnellen Rekombinationszentren verursacht wird (Feldtilgnng). Diese Umverteilung wird durch ,,field-enhanced ionization" von Lochern aus Coulomb-attraktiven langsamen Zentren verursacht. Der abrupte Einsatz der Feldtilgung tritt auf, weil die lsngsame Rekombination eine Rekombination iiber schnelle Zentren vertleckt, bis diese iiberwiegt. Eine konkurrierende Infrarottilgung vermindert den Verdeckungseffekt und macht eine friihere Phase der Feldtilgung bereits in der Nahe von 1 kV/cm sichtbar. Eine Stoljionisation oder Feldemission von Lochern aus langsamen Zentren kann das experimentell beobachtete Verhaken nicht erklaren; jedoch kann eine q uantitat'ive nbereinstimmung zwischen Experiment und Feldtilgung durch ,,field-enhanced ionization" erreicht werden. I n the case of' photoconducting CdS, the mobility [8] and the conductivity [9] ns it function of the electric field have been determined and indicate a strong field quenching of majority carriers 72 as the mechanism for NDC. The low velocity of propagation of moving domains [lo] indicates that the redistribution of space charge is trap controlled. Introduction
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