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

A GaAs p(+)-i-n(+) photodiode detector with a 30 mu m thick i layer and a 400 mu m 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 degrees C to -20 degrees C. The leakage current of the detector decreased from 1.247 nA +/- 0.005 nA (= 0.992 mu A/cm(2) +/- 0.004 mu A/cm(2)) at 60 degrees C to 16.0 pA +/- 0.5 pA (= 12.8 nA/cm(2) +/- 0.4 nA/cm(2)) at -20 degrees 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 x 10(14) cm(-3) +/- 0.7x 10(14) cm(-3) was found at distances between 1.7 mu m and 14 mu m below the p(+)-i junction, which limited the depletion width to 14 mu m +/- 1 mu m, at the maximum applied reverse bias (-100 V). Despite butterfly defects having formed during the epitaxial growth, Fe-55 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 degrees C to 0.73 keV at -20 degrees 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 degrees C and -20 degrees C revealed that the dominant source of noise was the quadratic sum of the dielectric and incomplete charge collection noise.