Medipix2: Processing and measurements of GaAs pixel detectors

For applications with high-energy X-ray photons (30-120 keV) semiconductors with high Z like CdTe and GaAs are required. In comparison with silicon detectors, the absorption of GaAs is higher. GaAs wafers had been prepared and flip-chip bonded to the Medipix2 readout chip at the Freiburger Materialforschungszentrum (FMF) using a low-temperature process. For this work, 300-mu m-thick wafers were chosen for first measurements. The pixel and backside contacts were processed with Ti-Pt-Au to form Schottky contacts. The surface of the samples was passivated with BCB. The pixel pitch was 55 mu m (standard Medipix) and 110 mu m. The performance of these GaAs detectors was studied for different radiation sources and X-ray tubes. The efficiency of the GaAs assemblies is compared to 700-mu m-thick Si assemblies. The number of detected quanta in 300 mu m GaAs is 10 times higher than in 700 mu m Si for gamma-rays at 44 and 60 keV. This can be explained by the higher absorption of the GaAs. (C) 2007 Elsevier B.V. All rights reserved.