30 µm thick GaAs X-ray p+-i-n+ photodiode grown by MBE

A GaAs p+-i-n+ photodiode detector with a 30 µm thick i layer and a 400 µm diameter was processed using standard wet chemical etching from material grown by molecular beam epitaxy. The detector was characterized for its electrical and photon counting X-ray spectroscopic performance at temperatures from 60°C to -20 °C. The leakage current of the detector decreased from 1.247 nA ± 0.005 nA (= 0.992 µA/cm2 ± 0.004 µA/cm2) at 60 °C to 16.0 pA ± 0.5 pA (= 12.8 nA/cm2 ± 0.4 nA/cm2) at -20 °C, at the maximum investigated applied reverse bias, -100 V (corresponding to an applied electric field of 33 kV/cm). An almost uniform effective carrier concentration of 7.1 × 1014 cm-3 ± 0.7 × 1014 cm-3 was found at distances between 1.7 µm and 14 µm below the p+-i junction, which limited the depletion width to 14 µm ± 1 µm, at the maximum applied reverse bias (-100 V). Despite butterfly defects having formed during the epitaxial growth, 55 Fe X-ray spectra were successfully obtained with the detector coupled to a custom-made charge-sensitive preamplifier; the best energy resolution (Full Width at Half Maximum at 5.9 keV) improved from 1.36 keV at 60 °C to 0.73 keV at -20 °C. Neither the leakage current nor the capacitance of the GaAs detector were found to be the limiting factors of the energy resolution of the spectroscopic system; noise analysis at 0 °C and -20 °C revealed that the dominant source of noise was the quadratic sum of the dielectric and incomplete charge collection noise.