Synchrotron characterization of deep depletion epitaxial GaAs detectors

We report the results of a series of synchrotron characterizations of two epitaxial GaAs detectors of active areas 2.22 mm(2) and thicknesses 40 and 400 mu m. In spite of an order of magnitude difference in depletion depths, the detectors were found to have comparable performances at similar to-40 degrees C, with energy resolutions of similar to 1 keV full width at half maximum (FWHM) at 7 keV rising to similar to 2 keV FWHM at 200 keV and noise floors in the range of 1-1.5 keV. At the lower energies, the energy resolution was dominated by leakage current and electromagnetic pickup. At the highest energies, however, the measured resolutions appear to approach the expected Fano limit; e.g., similar to 950 eV at 200 keV. Both detectors were remarkably linear, with average rms nonlinearities of 0.2% over the energy range of 10-60 keV. By raster scanning the active areas with 20x20 mu m(2) monoenergetic photon beams, it was found that the nonuniformity in the spatial response of both detectors was less than 1% and independent of energy. The material used to fabricate the detector is extremely pure. For example, low temperature photoluminescence measurements indicate that the density of the As antisite defect (EL2) is of the order of 10(12) cm(-3), which is similar to 2-3 orders of magnitude lower than that generally reported. This indirect measurement of material purity is confirmed by Monte Carlo simulations of the detector x-ray response, which show that in order to reproduce the observed energy-loss spectra, electron and hole trapping cross-section/density products must be much less than 1 cm(-1). (C) 1999 American Institute of Physics. [S0021-8979(99)01620-5].