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Title: Conductivity, drift and diffusion in amorphous semiconductors
Author: Clark, John David
ISNI:       0000 0001 3554 4540
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1980
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Drift mobility experiments in amorphous semiconductors frequently show a particular pattern of non-Gaussian, anomalous carrier pulse propagation (ACPP) characterised by a power-law decay of transient current and super- linear dependence on the ratio length/electric field. Such behaviour is predicted whether the transport mechanism be hopping, trap-controlled hopping or trap-controlled band transport. A new theory of hopping ACPP is developed, based upon regarding the hopping as a random walk on a set of sites with both random positions and random energies. In contradistinction to earlier theories no artificial regular lattice is introduced. This new theory has the advantage of simplicity. A macroscopic equation of motion of the pulses is derived which shows how such ACPP is governed by the ac mobility: its existence may therefore be directly tested by comparison with ac conductivity measurements. Extensive calculations of the pulse-shape are performed. The theory is found to be consistent with computer simulations of hopping pulses. Hopping does not appear to be the dominant ACPP mechanism in chalcogenide glasses. Evidence is emerging that it may be the principal ACPP mechanism in certain doped organic polymers. A new formulation of trap-controlled hopping is proposed. It is shown that all three proposed mechanisms lead to the same macroscopic pulse propagation equation with an Einstein-like relation and with suitable physical interpretation of involved quantities. This is in contrast to the findings of other authors. Continuous-time random walk theory of hopping conductivity is reviewed. It is argued that this theory is sound provided it is physically interpreted as a random walk among sites whose random locations are rerandomized immediately after each hop. Reasons for this are discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics