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|Title:||An evaluation of Gallium arsenide for detector applications in x-ray astronomy|
|Authors:||Short, Alexander David Thurnall.|
|Presented at:||University of Leicester|
|Abstract:||Forthcoming, imaging X-ray telescopes cannot detect photons with energies above ~10KeV due to the limitations of current optics and silicon detectors. However, there are known to be X-ray emissions at higher energies from neutron stars, pulsars, black hole candidates, supernova remnants, active galactic nuclei and galactic clusters. In order that future telescopes may spectroscopically image these emissions, research is being conducted into new optics, and the use of new semiconductors for detector arrays. This thesis covers work to assess gallium arsenide (GaAs) as an X-ray detection medium.;Detector fabrication is discussed and results are presented demonstrating the effect of carrier trapping on spectral resolution. By measuring charge collection efficiency, the activation energy of the dominant electron trap is calculated to be ~0.73eV. This trap may be 'frozen in' at temperatures below -85Â°C giving a resolution of 2.4keV FWHM at 59.5keV. Trapping is modelled, and a Monte-Carlo simulation is developed. By spectral fitting, the activation energy of the dominant hole trap is calculated to be ~0.2eV. These energy levels are associated with the native EL2 defect. Proton spectra demonstrate that the depletion region width varies linearly with bias at room temperature which is consistent with the existence of a quasi-neutral region.;A series of large scale Liquid Phase Epitaxial (LPE) growth trials was conducted to assess this potential source of high purity material. Layers in excess of 100microm thick were grown, but the resultant devices could not be operated as detectors.;It is concluded that the density of native EL2 defects in bulk GaAs is still too high to allow effective room temperature high resolution spectroscopy, and epitaxial material is difficult and expensive to produce. New sources of material will be sought and other semiconductors such as CdTe will be investigated, as will methods of operating simple high Z detector arrays.|
|Rights:||Copyright © the author. All rights reserved.|
|Appears in Collections:||Theses, Dept. of Physics and Astronomy|
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