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-μm-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 μm (standard Medipix) and 110 μm. 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-μm-thick Si assemblies. The number of detected quanta in 300 μm GaAs is 10 times higher than in 700 μm Si for γ-rays at 44 and 60 keV. This can be explained by the higher absorption of the GaAs.