Optical Evaluation of the Ionized EL2 Fraction in Proton (24 GeV) Irradiated Semi-Insulating GaAs

Semi-insulating SI GaAs samples from a zone refined crystal were irradiated with high energy protons (24 GeV/c, fluences up to 1.64×1014 p/cm2). Optical spectra in transmittance and reflectance were accurately measured in the energy range of 0.6–1.4 eV to determine, through the absorption coefficient, the concentrations of both neutral and ionized EL2 defects as a function of the proton fluence. Both these concentrations have been shown to increase linearly with the proton fluence; this behavior well explains the remarkable decrease of the charge collection efficiency observed in proton irradiated GaAs detectors at doses associated with high luminosity beams at a new particle collider accelerator (e.g., the LHC at the CERN laboratory).