IMPROVED PERFORMANCE OF GAAS RADIATION DETECTORS WITH LOW-TEMPERATUREOHMIC CONTACTS

Gallium arsenide (GaAs) offers an attractive choice for room temperatu re X- and gamma-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 th e back ohmic contact. We have shown [1] that ohmic contacts based on i on implantation allowed us to go far beyond the bias voltage necessary to achieve a fully active detector. However the conventional thermal treatments (850 degrees C, 30 s) required to anneal the damage induced by ion implantation strongly reduces the charge carrier lifetime in t he detector. Alenia S.p.A, has developed two improved processes (RA an d RB) which avoid high temperature annealing and the consequent charge carrier lifetime reduction. With the new detectors, in pixel (200x200 mu m(2)) 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 imagin g. Results obtained with a particles and X-rays at different temperatu res and in a wide range of applied bias in detectors made with standar d, implanted and improved processes are presented, compared and discus sed.