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Development of low-pressure vapour-phase epitaxial GaAs for medical imaging

Bates, RL and Manolopoulos, S and Mathieson, K and Meikle, A and O'Shea, V and Raine, C and Smith, KM and Watt, J and Whitehill, C and Pospisil, S and Wilhelm, I and Dolezal, Z and Juergensen, H and Heuken, M (1999) Development of low-pressure vapour-phase epitaxial GaAs for medical imaging. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 434 (1). pp. 1-13. ISSN 0168-9002

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Abstract

A summary is given of progress accomplished with the development of low-pressure vapour-phase epitaxial GaAs as a material for X-ray detectors. As the III–V concentration ratio is altered from Ga-rich to As-rich, the material is shown to improve from p-type, to n-type with compensation via deep levels, to n-type with a doping density of 1.7×1014 atoms cm−3. The measured barrier height is 0.8 V, as expected for the Ti contact used. Overdepletion was obtained before breakdown, enabling a layer thickness of m to be deduced for the final sample. For the later samples, charge collection for 60 keV Am-241 gammas was bias independent at a value of 100±8%. Spectra were also obtained from Sr-90 electrons. The most probable value of the charge collected as a function of the bias reached a plateau and from this value a depletion width of m was found for the final sample, equal to the epitaxial layer thickness. Results from detailed alpha and low-energy proton spectroscopy are shown for diodes fabricated from this material. A charge collection efficiency of 100% was obtained when the diode could be depleted sufficiently. The concept of a charge collection depth was introduced, since a significant amount of charge was collected without bias. The minimum depth of such a region was shown to be m at 0 V reverse bias, far greater than the m predicted for the depletion depth. Charge coupling between the guard ring and the pad was observed and successfully modelled.