Optical Absorption and Modification of Band Edges in Irradiated GaP

GaP crystals were irradiated by 10 MeV electrons to a maximum dose of 3.5×10 19 electrons/cm 2 or by fast neutrons to investigate the effects of radiation-induced defects on the band edges. The “below-gap” and “near-edge” absorptions of the irradiated GaP increase with the electron or neutron dose. The optical gap shrinkage between the parabolic bands is directly proportional to the 2/3 power of the radiation defect density. The photoconduction gap was obtained as the onset of transitions between delocalized states from the photoconduction spectra. The defect-induced photoconduction gap shrinkage is (20±5) meV, while the optical gap shrinkage is ∼67 meV for a dose φ=1×10 17 electrons/cm 2 . Thus the deeper states of about 2/3 of the parabolic bands penetrating into the original gap by the defects are localized.