Influence of substrate on the performances of semi-insulating GaAs detectors

A study of the carrier transport mechanism, the charge collection efficiency and the energy resolution has been carried out on semi-insulating GaAs X-ray detectors realised on substrates with concentrations of acceptor dopants Na, varying from 1014 to 1017 cm−3. The electron collection efficiency (ECE) and the reverse current were found to decrease with increasing Na, while the resistivity of the material was found to increase. At room temperature, the best collection efficiency (95%) and the best energy resolution (13.7 keV FWHM) for 59.5 keV X-rays of the 241Am source, have been achieved with the less doped detectors (Na∼1014 cm−3). The concentrations of ionised EL2+, determined by optical measurements in IR regions, was shown to increase with Na and to be quasi-inversely proportional to the ECE values. This behaviour strongly supports the hypothesis that the EL2 defects play a main role in the compensation of the material and in the limitation of the detection properties.