Browsing by Subject "Photoconductivity"
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Item A Photo-Initiated Self-Sustaining Conduction State in Semi-Insulating GaAs(Texas Tech University, 1984-05) Jiang, ChunHigh power switching using GaAs:Cr and undoped GaAs initiated by 1.06 micrometer YAG laser and 0.5 micrometer DYE laser at liquid nitrogen temperature was demonstrated. Some unusual properties of photoconductivity, such as the negative differential motility and the change of the relaxation time of the induced carriers due to the different intensity of light injection, have been investigated. For field strength above 20 kv/cm a self-sustaining conduction state was observed, following the photoconductive current pulse with a delay of 1-UOO ns. Various features of this second signal are presented. In order to understand the mechanism responsible for the self-sustaining conduction state, a heating model was investigated analytically and found to be inadequate. Possible mechanisms are discussed.Item A theory of lock-on and electrical breakdown(Texas Tech University, 2003-12) Kambour, Kenneth EIn this dissertation, a theory of electrical breakdown of insulators is developed. This theory is based on collective impact ionization, which includes both the electric field dependence and the carrier density dependence of impact ionization. This theory is applied to photoconductive semiconductor switches (PCSS's) and is used to explain the lock-on effect, an optically triggered breakdown that occurs in GaAs PCSS's. The basic principle of collective impact ionization theory is that, at high carrier densities, carrier-carrier scattering will enhance the impact ionization rate. This occurs because these interactions increase the number of carriers with energies above the impact ionization threshold. This generalized breakdown theory uses a rate equation approach to obtain the carrier density or densities which, at a given electric field, result in a steady state or a zero net carrier growth rate. In this approach, the competition between carrier generation (by impact ionization) and carrier recombination (by Auger and defect mechanisms) leads to a steady state condition for the net carrier growth rate. It is the existence of this steady state that governs whether or not electrical breakdown occurs. This approach leads to a definition of the bulk breakdown field as the lowest field for which the injection of an infinitesimally small carrier density will result in a steady state with a large carrier density. It also leads to the definition of the lock-on field as the lowest field for which a stable, steady state carrier density is possible. To implement this theory for PCSS materials, the Ensemble Monte Carlo (EMC) method is used to calculate the carrier distribution function, including the effects of carrier-carrier scattering. This distribution function is used to calculate the impact ionization and Auger recombination rates and thus the steady state carrier growth rate. Since the EMC calculations which include cc-scattering are computationally intense and time consuming, this theory is also implemented using both low and high density approximations for the distribution function. The low density limit is obtained using the EMC method without including cc-scattering. The high density limit is obtained by approximating the distribution function as a steady state Maxwellian. Using this theory, predictions are made for both the lock-on field and the bulk breakdown field in several materials and the results are compared, where possible, with experiment. In this theory, the lock-on effect is a type of carrier-density dependent electrical breakdown which occurs in all insulating materials. Further, it is the difference between the predicted lock-on and the breakdown fields which determines whether or not the lock-on effect will observable as a phenomenon distinct from ordinary breakdown. If the two fields are sufficiently distinct, it is likely that the two phenomena can be distinguished. However, if they are similar, it is likely that they will be difficult to distinguish experimentally.Item Contact effects in light activated GaAs switches(Texas Tech University, 1985-05) Durkin, Peter SNot availableItem Optically controlled diffuse discharges for switching applications(Texas Tech University, 1986-08) Hutcheson, George ZohnDiffuse discharges containing electronegative gases, at present, are the only means capable of fast, repetitive, long-life operation as opening switches. Optical control of diffuse discharge switches has been proposed as both a means of sustaining and of enhancing the performance of such switches. Processes considered in this dissertation are photo ionization. photo detachment. and several approaches to optically enhanced attachment. Ultraviolet (UV) ionization has been used for several years now as a means of preionizing self-sustained diffuse discharges and, recently, has been used as a sustainment mechanism for diffuse discharge opening switches. Time-resolved measurements of the optical emission of a spark array, similar to those used for preionization or sustainment of diffuse discharges, are presented. Results of experiments in atmospheric pressure diffuse discharges containing admixtures of attachers, sustained and initiated by such a spark array, are also presented. Photo detachment is considered as a mechanism which could decrease switch losses and decrease switch closure time by counteracting dissociative attachment. Experimental results are presented demonstrating optically increased current densities, of as much as 900%, in externally sustained and externally initiated, atmospheric pressure, diffuse discharges containing 0^. This process proceeds through photodetachment of 0 by 590 nm light generated by a dye laser (0 +hu -• 0+e). Optically enhanced attachment through the generation of vibrationally excited attachers appears particularly attractive as a means to decrease turn-off or opening times for diffuse discharge switches. For some molecules attachment cross-sections can be increased by orders of magnitude by vibrational excitation. The influence of this effect on the resistivity of a discharge through IR pumping of NH.;^ and CpH^aF by a low power CO^ laser in continuous low pressure discharges is presented. UV enhanced attachment is also shown in gases containing molecules such as C^HpF^. CpHpClp, and CpH-Cl where UV photo dissociation produced vibrationally excited, strongly attaching, molecules (e.g.. C2H2F2+hu-C2H2+HF(v>1)).Item Recombination and transport in hydrogenated amorphous silicon(Texas Tech University, 1996-05) Yi, Seung-HoThis dissertation is the study of properties of a-Si:H using photoluminescence (PL) and photoconductivity (PC) measurements to shed light on understanding of the recombination and transport process of photogenerated carriers. Based on the field dependent PL measurement, we developed a model to explain the field quenching in PL at low temperatures (below 100 K). Our model showed that most radiative recombination occurs between geminate pairs. We determined the distribution of separations of geminate pairs from results of the field quenching in PL. The distribution does not change much with excitation energy. Results of simultaneous PC measurement also indicated the geminate pair recombination, even though it is not so apparent due to, probably, the existence of a fast non-radiative recombination process. Our results imply that there is no significant change in the distance between an electron and a hole in a geminate pair in bandtail states due to thermalization, which is contradictory to a generally accepted model. Field dependent PC measurements at various temperatures confirmed the validity of the concept of effective temperature' invented for explaining field dependent PC in terms of temperature. Results from the measurements showed the existence of safe hole traps (SHTs), which might relate to light degradation. The intensity dependent PC measurements supported this conclusion. We also investigated phosphorous doped a-Si:H with above gap and sub gap excitation in order to study the effect of defects in a-Si:H. The results allow us to develop a possible recombination mechanism for defect related PL.