Effects of neutron and electron irradiation on the absorption edge of GaAs

Abstract

Semi-insulating liquid encapsulated Czochralski GaAs wafers were submitted to irradiation with 1 MeV neutrons, thermal neutrons, 7 MeV electrons, and white electrons (up to 7 MeV). The direct absorption edge was then studied by means of transmission (T = 15 K) and thermoreflectance (T = 35 and 51 K) measurements. Thermoreflectance spectra showed that the position of the exciton does not shift after irradiation with 1 MeV neutrons, but that its broadening parameter Γ increases as the fluence increases. Transmission measurements have revealed the presence of two acceptor levels (C and Zn) in the unirradiated samples. The absorption associated with these impurities increased by a factor of 10 after irradiation with 1 MeV neutrons. This effect was not produced by the three other types of radiation. However, an absorption tail extending well below the direct edge is seen after irradiation with all four types of particles. A model proposed by Toyozawa fits this continuum quite well, suggesting that irradiation causes amorphization of the crystal. The rate of introduction of defects and its dependence on irradiation fluence is different for each type of radiation.