Abstract
Thin (2 μm active layer) spectroscopic p+-i-n+ GaAs X-ray photodiodes of circular mesa geometry (200 μm and 400 μm diameter; one representative diode of each diameter) have been characterised for their energy response using high-purity X-ray fluorescence calibration samples excited by an X-ray tube, giving energies between 2.1 keV (Au Mα1) and 21.18 keV (Pd Kα1), and an 241Am radioisotope γ-ray source (26.3 keV, 59.5 keV). The photodiodes were operated uncooled at +33°C. The 200 μm diameter device's energy resolution (FWHM) was found to be constant (0.79 keV) and primarily limited by electronics noise at energies between 2.1 keV and 21.18 keV, but it broadened to 0.85 keV at 26.3 keV, and to 1 keV at 59.5 keV. The 400 μm diameter device's energy resolution (FWHM) was constant (1.1 keV) for photon energies between 4.95 keV and 9.89 keV, but increased to 1.15 keV at 16.62 keV, 1.25 keV at 21.18 keV, 1.3 keV at 26.3 keV and 1.66 keV at 59.5 keV. The broadening of energy resolution (FWHM) observed in both cases is greater than can be attributed solely to increasing Fano noise and is hypothesised to be at least in part due to energy dependent charge trapping. However, for both types of device, the peak charge output from the devices was found to be linearly (R2 ⩾ 0.9999) dependent on incident X-ray energy.
Highlights
Thin (2 μm active layer) spectroscopic p+-i-n+ GaAs X-ray photodiodes of circular mesa geometry (200 μm and 400 μm diameter; one representative diode of each diameter) have been characterised for their energy response using high-purity X-ray fluorescence calibration samples excited by an X-ray tube, giving energies between 2.1 keV (Au Mα1) and 21.18 keV (Pd Kα1), and an 241Am radioisotope γ-ray source (26.3 keV, 59.5 keV)
A representative diode of each diameter was connected in turn to a low-noise charge-sensitive preamplifier made at University of Leicester
The effects of deviations from the ideal Gaussian peak shape, for example as would be caused by charge tailing on the low energy side of the peaks, were minimized by measuring the number of counts in each peak between the limits of the peak centroid and right hand limit of the peak’s full width at half maximum (FWHM) only
Summary
GaAs photodiodes photolithographically etched from an MBE-grown wafer (table 1) as part of a joint research programme between University of Leicester and University of Sheffield were mounted in TO-5 packages. The metallisation covers 45 % and 33 % of the active faces of the 200 μm and 400 μm diameter photodiodes, respectively. The 200 μm and 400 μm diameter devices had capacitances at full depletion (10 V reverse bias) before packaging of 1.94 pF and 7.21 pF, as measured at University of Sheffield using an HP 4275 LCR meter with the AC test voltage signal magnitude and frequency set to 50 mV r.m.s. and 1 MHz. The 200 μm and 400 μm diameter devices had capacitances at full depletion (10 V reverse bias) before packaging of 1.94 pF and 7.21 pF, as measured at University of Sheffield using an HP 4275 LCR meter with the AC test voltage signal magnitude and frequency set to 50 mV r.m.s. and 1 MHz After packaging, they were measured at University of Leicester to have leakage currents of 12.2 pA and 12.9 pA, respectively, at reverse biases of 10 V in a dry N2 environment at a temperature of 21◦C. The devices were grown as part of a programme working towards thicker (higher efficiency) devices and monolithic photodiode arrays
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