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 x 1019 electrons/cm2 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 phi = 1 x 1017 electrons/cm2. Thus the deeper states of about 2/3 of the parabolic bands penetrating into the original gap by the defects are localized. (author)