Al0.6Ga0.4As x-ray avalanche photodiodes for spectroscopy

Two circular Al0.6Ga0.4As p+-i-n+ 2 µm i layer spectroscopic x-ray avalanche photodiodes (one 200 µm diameter and one 400 µm diameter) were made from a structure produced by metalorganic vapour phase epitaxy. The capacitances and currents of the detectors as functions of applied bias were measured, and 55Fe x-ray (Mn Kα = 5.9 keV; Mn Kβ = 6.49 keV) spectra were accumulated at 20 °C (293 K). Improved energy resolutions (measured as the full width at half maximum of the 5.9 keV peak) with increased applied reverse bias were observed with both detectors. In part, the improvement was attributed to avalanche multiplication. Energy resolutions of 630 eV ± 40 eV and 730 eV ± 50 eV were achieved with the 200 µm detector at an applied reverse bias of 38 V and the 400 µm detector at an applied reverse bias of 40 V, respectively. It is the first time Al0.6Ga0.4As has been demonstrated as capable of photon counting x-ray spectrometry. Measurements to determine the average electron-hole pair creation energy in Al0.6Ga0.4As were made; the results suggested a value of 4.97 eV ± 0.12 eV at 25 °C ± 1 °C (298 K ± 1 K). This value was then used to refine the apparent relationship between bandgap energy and electron-hole pair creation energy as defined by the Bertuccio–Maiocchi–Barnett relationship. AlxGa1-xAs x-ray photodiodes of this type are anticipated to be of benefit for future space missions, including those to explore the surfaces of the inner planets (e.g. Mercury and Venus) and the moons of Jupiter and Saturn.