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

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 1.7×10 14 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 41 μ 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 40 μ 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 10.8 μ m at 0 V reverse bias, far greater than the 1.1 μ m predicted for the depletion depth. Charge coupling between the guard ring and the pad was observed and successfully modelled.