Abstract
Three custom-made Al0.2Ga0.8As p-i-n mesa X-ray photodiodes (200µm diameter, 3µm i layer) were electrically characterised and investigated for their response to illumination with soft X-rays from an 55Fe radioisotope X-ray source (Mn Kα = 5.9keV; Mn Kβ = 6.49keV). The AlGaAs photodiodes were shown to be suitable for photon counting X-ray spectroscopy at room temperature. When coupled to a custom-made low-noise charge-sensitive preamplifier, a mean energy resolution (as quantified by the full width at half maximum of the 5.9keV photopeak) of 1.24keV was measured at room temperature. Parameters such as the depletion width (1.92µm at 10V), charge trapping noise (61.7 e−rms ENC at 5V, negligible at 10V) and the electronic noise components (known dielectric noise (63.4 e− rms), series white noise (27.7 e−rms), parallel white noise (9.5 e−rms) and 1/f series noise (2.2 e−rms) at 10V reverse bias) affecting the achieved energy resolution were computed. The estimated charge trapping noise and mean energy resolution were compared to similar materials (e.g. Al0.8Ga0.2As) previously reported, and discussed. These results are the first demonstration of photon counting X-ray spectroscopy with Al0.2Ga0.8As reported to date.
Highlights
Since the first reported detection of X-rays with AlGaAs [1], the material has received increased attention as a promising alternative for X-ray [1,2,3] and beta particle [4,5] detection at room temperature and above
Compared to narrow bandgap materials, wide bandgap detectors can possess superior resolutions at high temperature (» 20 °C) due to lower thermally induced leakage currents [7] and lower parallel white noise [8]. This presents the possibility of reducing cooling requirements for X-ray detectors, with subsequent reduction of mass, volume, power requirements and cost of the instrument; these improvements are important for applications including X-ray fluorescence spectroscopy for future space missions subjected to high temperature and intense radiation conditions, as well as terrestrial applications in industrial instrumentation
The material Al0.2Ga0.8As has been characterised for soft X-ray spectroscopy via a set of three p-i-n (200 μm diameter, 3 μm i layer) photodiodes
Summary
Since the first reported detection of X-rays with AlGaAs [1], the material has received increased attention as a promising alternative for X-ray [1,2,3] and beta particle [4,5] detection at room temperature and above. Compared to narrow bandgap materials, wide bandgap detectors can possess superior resolutions at high temperature (» 20 °C) due to lower thermally induced leakage currents [7] and lower parallel white noise [8]. Previous work on Al0.8Ga0.2As for photon counting Xray spectroscopy detectors has focused on thin i layers, e.g. 1 μm [4,15] and 1.7 μm [2] This is due to the lattice mismatch between Al0.8Ga0.2As and GaAs (the substrate material typically used for AlGaAs growth) which leads to relaxation when the wafer is cooled from growth temperatures. The Authors’ specifications at the EPSRC National Centre for III-V Technologies, Sheffield, UK
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