Phonon shifts relating to the defect structure in neutron-transmutation-doped semi-insulating GaAs

Phonon shifts relating to the defect structure in neutron-transmutation-doped semi-insulating GaAs have been studied using Raman-scattering and x-ray-diffraction methods. The defect structure is discussed for the two cases of vacancy interstitials and antisites using a simple model of the LO-TO-phonon frequency splitting (Delta omega). It is suggested that the slight reduction of Delta omega originates from the vacancy-interstitial clusters rather than the antisite defects, considering the volume expansion, the antisite defect concentrations, and the displacement atoms in neutron-irradiated samples. The clusters are associated with a volume expansion of about 0.4% observed in the neutron-irradiated samples.