Arsenic antisite defects and other paramagnetic centres in neutron irradiated and annealed GaAs

GaAs samples have been irradiated with fast neutrons up to doses of 1*10²⁰ n⁰ cm^-2. EPR measurements revealed the four-line spectrum attributed to As_Ga defects and the singlet spectrum previously ascribed to gallium vacancies. The dose dependences of these spectra imply that the paramagnetic 'singlet' defects are located in the core regions of disordered material whereas the As_Ga centres must be located both in the cores and the surrounding shells of less-heavily damaged material. Isochronal anneals up to 550 degrees C of undoped or n-type samples showed that the As_Ga centres were still present after the singlet spectrum had annealed at 500 degrees C. The spin-lattice relaxation time for the former defect increased during this latter stage. Some comments are made about the identity of the defects giving the singlet spectrum.