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?µm and 400?µm) were fabricated from the structure. Two of the photodiodes (one of each diameter) were characterized at 20?°C for their electrical properties and response to x rays using an 55Fe radioisotope x-ray (Mn Ka?=?5.9?keV; Mn Kß?=?6.49?keV) source. An energy resolution of 508?eV?±?5?eV full width at half maximum (FWHM) at 5.9?keV 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?µm and 400?µm 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 508?eV FWHM at 5.9?keV at M?=?1.1 and 603?eV FWHM at 5.9?keV at M?=?1.2 were achieved with the 200?µm and 400?µm diameter GaAs/AlxGa1-xAs SAM x-ray APDs, respectively; this is better than was previously reported for similar devices without avalanche layers: 690?eV FWHM at 5.9?keV and 730?eV FWHM at 5.9?keV for 200?µm and 400?µm diameter GaAs p+-i-n+ detectors, respectively [Lioliou et al., J. Appl. Phys. 122, 244506 (2017)].