Damage coefficient associated with free exciton lifetime in GaAs irradiated with neutrons and electrons

The lifetime τ of the free exciton at T=17 K in irradiated GaAs has been determined by the reflectance of layers grown by metalorganic chemical vapor deposition and of semi-insulating samples grown by the liquid-encapsulated Czochralski method. The samples had been irradiated with 1 MeV and thermal neutrons, and with 7 MeV and white electrons up to 7 MeV. The linear increase of τ −1 as a function of the fluence is explained by a simple kinetics model, which applies especially well to the epitaxial layers. The damage coefficient kτ associated with this lifetime has been measured. For the defects created by all types of radiation, the radius associated with the cross section for the capture of excitons is three to ten times the radius of the free exciton. The exciton transition energy is found to decrease as its reflectance structure is broadened.