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|Title:||The electrical and optical properties of orthorhombic sulphur crystals.|
|Authors:||Adams, Alfred R.|
|Presented at:||University of Leicester|
|Abstract:||The orthorhombic form of sulphur is a typical molecular crystal and is composed of a structure of S8 rings. Specimens have been grown from solution in CS2 both in a thin platelet habit and in the form of thicker double pyramids. Drift nobility techniques have been used to study the charge transport between 180K and 370K. The results show that the charge is carried by the generated holes having a room temperature lattice mobility of about 10 cm2 sec-1v-1 and a lifetime of 10-5 sec. At the lower temperatures the hole drift mobility is an activated process, and during transit the generated holes interact with a level of centres 0.19 ev above the valence band. With increasing temperature a transition to a lattice controlled transport takes place. On the basis of the infra-red transmission spectrum it is suggested that the above centres are associated with solvent molecules. The range of generated electrons is estimated to be less than 10-8 cm2/volt; a predominant electron trap is situated 0.9 ev below the conduction band. Measurements on the photoconductivity in orthorhombic Sulphur revealed for the first time a response in the ultra-violet. The photocurrent reaches a peak at 2,550 A.U. with unit quantum efficiency and it is likely that this is an intrinsic photo- conductive effect. The photoconductivity in the visible region, which has been reported before, (15-16) has a maximum at 4,600 A.U. with a quantum efficiency several orders of magnitude less than that in the ultra-violet. It is suggested that this photocurrent is generated by the interaction between molecular excitons. The absorption spectrum has a sharply rising edge at 4,500 A.U. reaching an absorption coefficient of 2 x 103 /cm at 3,600 A.U. There is a slight window in the ultra-violet with a minimum at 2,700 A.U. The absorption edge shows distinct dichroism, moving to longer wavelengths as the E-vector becomes parallel to the c-axis. The total shift of the edge corresponds to an energy difference of 0.05 ev.|
|Rights:||Copyright © the author. All rights reserved.|
|Appears in Collections:||Theses, Dept. of Physics and Astronomy|
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