Improved performance of GaAs radiation detectors with low ohmic contacts

Gallium arsenide (GaAs) offers an attractive choice for room temperature Xand γ-ray detectors. However performance of SI LEC bulk GaAs detectors is at present limited by the short carrier lifetime and by the diode breakdown occurring as soon as the electric field reaches the back ohmic contact. We have shown that ohmic contacts based on ion implantation allowed us to go far beyond the bias voltage necessary to achieve a fully active detector. However the conventional thermal treatments (850°C, 30 s) required to anneal the damage induced by ion implantation strongly reduces the charge carrier lifetime in the detector. Alenia S.p.A. has developed two improved processes (RA and RE) which avoid high temperature annealing and the consequent charge carrier lifetime reduction. With the new detectors, in pixel (200×200 μm²) configuration, a charge collection efficiency (cce) of 90% for 59.5 keV X-rays and a FWHM of 3.35 keV have been achieved at room temperature. These features, thickness, applied voltage, cce and FWHM are suitable for application of GaAs pixel detectors in medical imaging. Results obtained with α particles and X-rays at different temperatures and in a wide range of applied bias in detectors made with standard, implanted and improved processes are presented, compared and discussed