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Space charge and high field effects in thin amorphous films Shousha, Abdel Halim Mahmoud

Abstract

The present thesis is concerned mainly with space charge and high field effects on the electrical properties of thin amorphous films. A theory of space charge contribution to the polarization current in thin dielectric films is proposed. The transient current on short-circuiting a thin dielectric film is believed to consist of two components, one due to the dielectric polarization and the other due to trapped space charge. The space charge contribution is investigated using a model for a film containing distributed traps. Computed results seem to be consistent with experimental results on Ta/Ta₂O₅/Au diodes, so that space charge effects are more important at low preapplied fields. The applicability of step response techniques to determine low frequency dielectric losses is discussed and the effect of space charge on the dielectric losses is analysed. The theory of thermoluminescence and thermally stimulated currents is extended to the case of traps with distributed binding energies to investigate the possibility of distinguishing between distributed and discrete trap levels. It seems possible to distinguish experimentally between distributed and discrete traps by using different doses of optical radiation to obtain initially different amounts of trapped charges, and by varying the frequency of optical excitation over a suitable frequency range to allow only certain energy levels to be occupied by excited electrons. High field electronic conduction through very thin films sandwiched between two metal electrodes is analysed. In view of the fast tunneling time of electrons through very thin films, MIM structures can be used for microwave detection. It is shown that the maximum responsivity-bandwidth product of such detectors is obtained when they are biased at a voltage equal to the anode work function (in volts), and that the presence of invariant positive space charge increases the magnitude of this maximum. In considering high field switching in thin films of semiconducting glasses, it is suggested that Joule heating, which could account for the delay times observed experimentally, serves only to initiate an electronic switching mechanism. A model for current-controlled negative resistance due to space charge formation is proposed and its dc characteristics are computed. Carrier injection from the electrodes is taken to occur either by Schottky thermionic emission or a Fowler-Nordheim tunneling mechanism. The injected carriers develop space charge regions near the electrodes by impact ionization. The position dependent generation-recombination rate is discussed. The small ac signal equivalent circuit of the model is given. The formation of current filaments is analysed. Memory devices are discussed in terms of filament formation and phase change mechanisms due to excessive heating. Filamentary breakdown has been observed in anodic films grown on Ta, Al, Nb and Ti. A detailed experimental study of film growth and the effects of growth conditions, film thickness, counterelectrodes and temperature on breakdown strength has been carried out. A possible mode of breakdown, in which breakdown can result from thermal effects following a non-destructive electron avalanche, is proposed and its limitations are pointed out. It is concluded that breakdown in thin anodic films would occur due to disruption of the chemical bonds as the applied field approaches the formation field. The product of the molecular dissociation and the presence of energetic electrons could start an accumulative process which might end with the formation of a highly conducting channel. The injected electrons, field distortion and thermal runaway could assist in the channel development. Once the channel is developed, the sample's stored energy starts to dissipate through the channel. The voltage collapse has been found experimentally to occur in a time of less than 200 nanoseconds.

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