AlGaAs/GaAs avalanche detector array -- 1 Gbit/s x-ray receiver for timing measurements

The authors report on the first realization of 2x2 detector arrays based on Aluminum Gallium Arsenide/Gallium Arsenide (AlGaAs/GaAs) heterostructure avalanche photo diodes. These structures consists of a GaAs absorption layer and an AlGaAs/GaAs avalanche layer which acts as a multiplication region. The samples were grown by Molecular Beam Epitaxy (MBE) and processed into pin diodes of different diameters. Dark current densities were as low as 200 pA/mm2 at 90% of the breakdown voltage as determined by I-V measurements. The avalanche gain of the devices have been measured with optical pulses. Gains up to a factor of M = 1,000 have been determined before breakdown. Additionally the excess noise factor F(M) has been derived for gains between M = 1 and M = 300. The ionization rates ratio of the structure is k = α/β = 3.4 ± 0.3. In connection to a fast electronic readout chain the time response of the detectors to 14.4 keV X-ray photons has been tested at the ESRF (Grenoble). The time resolution was found to be 200 ps (FWHM) using standard timing electronics