Al_0.2Ga_0.8As X-ray photodiodes for X-ray spectroscopy

Three custom-made Al_0.2Ga_0.8As p-i-n mesa X-ray photodiodes (200 µm diameter, 3 µm 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 = 5.9 keV; Mn Kβ = 6.49 keV). 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 5.9 keV photopeak) of 1.24 keV was measured at room temperature. Parameters such as the depletion width (1.92 µm at 10 V), charge trapping noise (61.7 e⁻ rms ENC at 5 V, negligible at 10 V) 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 10 V 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. Al_0.8Ga_0.2As) previously reported, and discussed. These results are the first demonstration of photon counting X-ray spectroscopy with Al_0.2Ga_0.8As reported to date.