Synchrotron characterization of deep depletion epitaxial GaAs detectors

We report the results of a series of synchrotron characterizations of two e pitaxial GaAs detectors of active areas 2.22 mm(2) and thicknesses 40 and 4 00 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 d egrees C, with energy resolutions of similar to 1 keV full width at half ma ximum (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 resolut ion was dominated by leakage current and electromagnetic pickup. At the hig hest energies, however, the measured resolutions appear to approach the exp ected Fano limit; e.g., similar to 950 eV at 200 keV. Both detectors were r emarkably linear, with average rms nonlinearities of 0.2% over the energy r ange of 10-60 keV. By raster scanning the active areas with 20x20 mu m(2) m onoenergetic photon beams, it was found that the nonuniformity in the spati al response of both detectors was less than 1% and independent of energy. T he 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 si milar to 2-3 orders of magnitude lower than that generally reported. This i ndirect measurement of material purity is confirmed by Monte Carlo simulati ons of the detector x-ray response, which show that in order to reproduce t he observed energy-loss spectra, electron and hole trapping cross-section/d ensity products must be much less than 1 cm(-1). (C) 1999 American Institut e of Physics. [S0021-8979(99)01620-5].