AlGaAs/GaAs SAM-avalanche photodiode: An X-ray detector for low energy photons

An AlGaAs/GaAs avalanche photodiode with separate absorption- and multiplication regions (SAM-APD) has been realized as an X-ray detector. We report on the growth (MBE) and the processing of the detector as well as on results obtained by measurements with radioactive sources. Gains higher than 20 could be achieved with a staircaselike multiplication region. Typical dark currents of 100 pA at 90% of the breakdown voltage were measured for devices with an active area of A = 320 × 450 μm2. Tests with X-rays of an 241Am source showed an energy resolution of 1.95 keV (FWHM) at 13.96 keV without avalanche gain. At an optimal working point the detector reached an energy resolution of 0.9 keV (FWHM) for the same energy with an internal gain of M = 4.1. This corresponds to a relative energy resolution of ΔEE = 6.5% at room temperature. The calculated efficiency was approximately η = 23%. Additionally three X-ray escape peak doublets related to the 13.96 keV, 17.8 keV and 20.8 keV peaks of 241Am could be resolved.