AlGaAs/GaAs avalanche detector array -1 GBit/s X-ray receiver for timing measurements

We report on the first realization of 2×2 detector arrays based on an aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) heterostructure avalanche photodiodes. These structures consists of a GaAs absorption layer and an AlGaAs/GaAs avalanche layer which acts as an multiplication region. The samples were grown by molecular beam epitaxy (MBE) and processed into p-i-n 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=1000 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