High resolution x-ray spectroscopy using GaAs arrays

Abstract

We have produced a number of small format gallium arsenide (GaAs) arrays to address the material, electronic, and technological problems that need to be solved in order to develop mega pixel, Fano-limited spectroscopic x-ray imagers. Results will be presented of a series of x-ray measurements carried out on a prototype 5×5 array, fabricated from 40 μm thick epitaxial GaAs. The device has pixel sizes of 200×200 μm2 and pitch 250 μm. As a preliminary investigation of performance, two pixels have been instrumented. Measurements from 5.9 to 98 keV were carried out both in our laboratory and at the Hamburger Synchrotronstrahlungslabor research facility in Hamburg, Germany. Both pixels were found to be remarkably uniform, both in their spectral and spatial response to x-rays. The average nonlinearity in the spectral response is <1% across the energy range 5.9–98 keV. Using a 12 keV, 20×20 μm2 pencil beam, the spatial uniformity was found to be better than 98% over the entire pixel surfaces, consistent with the statistical precision of the measurement. The energy resolution at −40 °C is 400 eV full width at half maximum (FWHM) at 5.9 keV rising to 700 eV FWHM at 98 keV. No difference in energy resolution was found between full area and pencil beam illumination. An analysis of the resolution function has shown that the detector is dominated by electronic noise at low energies and Fano noise at energies above 30 keV. By best-fitting the expected resolution function to the entire data set, we derive a Fano factor of 0.140±0.05, together with a charge transport factor as low as 1.4×10−3. Further improvement in the resolution function has been achieved by replacing the conventional resistive feedback preamplifiers with a new resistorless design, which provides a lower component of electronic noise. In this case, a resolution of 266 eV FWHM at 5.9 keV has been achieved at room temperature (23 °C) and 219 eV FWHM with only modest cooling (−31 °C). The expected Fano noise at this energy is ∼140 eV.

High temperature GaAs X-ray detectors

“…This resulted in an energy resolution not as good as the best results achieved with GaAs p þ -i-n þ diodes with Schottky contacts and guard rings at room temperature (0.266 keV FWHM at 5.9 keV 12 and 0.300 keV FWHM at 5.9 keV 14 ). The dielectric noise was minimised in the detectorpreamplifier system reported by Owens et al 12 and Erd et al 14 by mounting both the detector and input JFET of the preamplifier in close proximity on the same low-loss substrate. It was estimated that if the dielectric noise of the additional dielectrics could be eliminated from the presently reported system, the energy resolution (FWHM at 5.9 keV) at 20 C of both spectrometers S1 and S2 would be reduced to 0.400 keV, respectively.…”

“…9 Compared to Si, GaAs is more radiation hard for c-rays, electrons, low energy protons (<50 MeV 10 ), and neutrons; however, it should be noted that results have been reported showing that GaAs is less radiation hard than Si for high energy hadrons. 11 Owens et al 12,13 reported a planar 5 Â 5 GaAs p þ -i-n þ diode array structure, with a 40 lm epilayer and Au/Pt/Ti Schottky contacts at the p þ layer, which had low dark current densities (<6 nA/cm 2 ) and the best energy resolution (0.266 keV Full Width at Half Maximum, FWHM, at 5.9 keV, at room temperature) reported to date for GaAs X-ray diodes. Later devices with similar structure but thicker epilayer (325 lm) had an energy resolution of 0.300 keV FWHM at 5.9 keV, at room temperature.…”

Temperature dependent characterization of gallium arsenide X-ray mesa p-i-n photodiodes

“…10 Following this, larger devices with a thicker epilayer and similar structure to the detectors reported in Ref. 10 in a 32 Â 32 pixel array have been reported with an energy resolution of 0.3 keV FWHM at 5.9 keV at room temperature. 11 Research has also been conducted on GaAs p þ -i-n þ mesa Xray photodiodes.…”

“…A 5 Â 5 GaAs diode array structure was reported; its ultrapure epitaxial planar layer, p þ -i-n þ structure, Schottky contact at the p þ layer, and the guard ring all contributed to the low dark current densities (<6 nA/cm À2 at room temperature). 10 An energy resolution (FWHM) of 0.266 keV at 5.9 keV at room temperature was reported with these devices, after coupling them to ultra low noise front end electronics. 10 Following this, larger devices with a thicker epilayer and similar structure to the detectors reported in Ref.…”

“…10 An energy resolution (FWHM) of 0.266 keV at 5.9 keV at room temperature was reported with these devices, after coupling them to ultra low noise front end electronics. 10 Following this, larger devices with a thicker epilayer and similar structure to the detectors reported in Ref. 10 in a 32 Â 32 pixel array have been reported with an energy resolution of 0.3 keV FWHM at 5.9 keV at room temperature.…”