GaAs/Al0.8Ga0.2As separate absorption and multiplication region x-ray spectroscopic avalanche photodiodes

A GaAs/Al0.8Ga0.2As separate absorption and multiplication (SAM) x-ray avalanche photodiode (APD) structure was grown by metalorganic vapor phase epitaxy. Mesa photodiodes of different diameters (200 mu m and 400 mu m) were fabricated from the structure. Two of the photodiodes (one of each diameter) were characterized at 20 degrees C for their electrical properties and response to x rays using an Fe-55 radioisotope x-ray (Mn K alpha =5.9keV; Mn K beta =6.49keV) source. An energy resolution of 508eV +/- 5eV full width at half maximum (FWHM) at 5.9keV was achieved at an apparent avalanche gain, M, of 1.1. This is the best energy resolution so far reported for GaAs/AlxGa1-xAs x-ray SAM APDs. The noise components associated with the achievable spectroscopic energy resolutions are reported. Comparisons between the 200 mu m and 400 mu m diameter GaAs/AlxGa1-xAs SAM x-ray APDs and recently studied GaAs p(+)-i-n(+) detectors were made, showing that the inclusion of the avalanche layer improves the achievable energy resolution; energy resolutions of 508eV FWHM at 5.9keV at M=1.1 and 603eV FWHM at 5.9keV at M=1.2 were achieved with the 200 mu m and 400 mu m diameter GaAs/AlxGa1-xAs SAM x-ray APDs, respectively; this is better than was previously reported for similar devices without avalanche layers: 690eV FWHM at 5.9keV and 730eV FWHM at 5.9keV for 200 mu m and 400 mu m diameter GaAs p(+)-i-n(+) detectors, respectively [Lioliou et al., J. Appl. Phys. 122, 244506 (2017)].