published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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The photoconductive properties of undoped hydrogenated amorphous silicon have been extensively studied. Measurements of optical absorption, dark d.c. conductivity, steady state photoconductivity, step response transient rise photoconductivity and impulse response flash decay photoconductivity have been made. In addition, computer simulation has been used to give an insight into the physical processes involved in the photoconductivity experiments.Two materials were used in the study, to provide a comparison. All the above measurements, except the transient measurements were made on sputtered material prepared by the author. a-Si:H prepared elsewhere by the glow discharge decomposition of silane was measured by all the above experiments, except optical absorption.The results obtained from the flow discharge material were interpreted as due to recombination in distributed states, which are restricted in energy, extending from the dark Fermi level upwards to 0.6eV below Ec. The capture cross section of these states was of a value expected for the neutral dangling bond, so they have been denoted as Do states. States outside this energy were seen to be ineffective as recombination centres. The model thus has features intermediate between a simple 2-trap system and a distributed density of states. Chapter 2 presents a detailed analysis of steady state photoconductivity for the case of a single correlated defect level, and demonstrates its near equivalence to a simple 2-defect one electron system. This partly justifies the subsequent use of one electron states in computer modelling. Computer simulation successfully predicts the form of the experimental step response. The flash decay required the existence of an extra discrete state at 0.4eV, but gave a better quantitative fit.The sputtered material appeared to have the recombination in the steady state controlled by discrete states 0.6eV below Ec, but inconsistencies remain between the interpretations of different measurements on this material.

This dissertation presents results pertaining to the mathematical modeling of semiconductor photoconductors and includes the formulation, analysis, and solution of photoconductive device model equations. The fundamental semiconductor device equations of continuity and transport are derived for the case of a material which contains a large density of deep-level impurities. Electron and hole trapping on deep-level impurities is accounted for by trapping-kinetics rate equations. The coupling between carrier drift and the electric field is completed through Poisson's equation. Simple, nonlinear model equations are presented for bulk-material response based on the dynamics of electron and hole trapping and recombination on deep-level impurities. The characteristics of the solution to these model equations are observed to depend strongly on the excitation intensity. These model equations qualitatively reproduce observed experimental behavior of an iron-doped indium phosphide photoconductor. A theory of the effect of deep-level centers on the generation-recombination noise and responsivity of an intrinsic photoconductor is presented. It is shown that the deep-level centers can influence the generation-recombination noise and responsivity in three main ways: (i) they can shorten the bulk carrier lifetime by Schockley-Read-Hall recombination; (ii) for some values of the capture cross sections, deep-level densities, and temperature, the deep-level centers can trap a significant fraction of the photogenerated minority carriers. This trapping reduces the effective minority carrier mobility and diffusivity and thus reduces the effect of carrier sweep out on both generation noise and responsivity; (iii) the deep-level centers add a new thermal noise source, which results from fluctuations between bound and free carriers. The strength of this new noise source decreases with decreasing temperature at a slower rate than band-to-band thermal generation-recombination noise. Photoconductive device model equations based on time-dependent, convective/diffusive transport equations are presented. The system of model equations is solved numerically with boundary conditions that represent ideal ohmic contacts. Computed results are presented for different photoconductor lengths and bias voltages with spatially uniform, rectangular light-pulse illumination.

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Thesis (Ph.D.)--New Jersey Institute of Technology and Rutgers, The State University Of New Jersey-Newark, Federated Physics Department, 2000. / JT20011108 MBT20100106 ANT20100113 Typescript. Includes bibliographical references (leaves 108-115). Also available via the World Wide Web.

Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references.

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