Phonon localization in ion-irradiated semiconductors

We have carried out real-time Raman measurements on the (100) surfaces of GaAs and Ge under 5 keV He+ irradiation, and investigated the kinetics of the lattice disordering during ion irradiation by estimating the correlation length of the longitudinal optical phonon from the Raman spectra. The phonon correlation length of GaAs has been seen to decrease inversely proportional to the cubic root of ion fluence. The result implies a linear correspondence of the phonon correlation length to the mean distance between the defects in GaAs, and indicates localization of phonon due to defect formation induced by ion irradiation. The Raman spectral change of Ge also suggests phonon localization while under ion irradiation. We have performed real-time Raman measurements on a Si(111) surface under irradiation by 5 keV He+, Ar+, and Kr+ ions. Little change has been observed in the Raman line shape, resulting from the optical penetration depth too large compared with the projected range of ions in the present experimental condition. The intensity of the optical phonon peak has been seen to decrease with increasing ion fluence. The reduction of the phonon line intensity is interpreted as an increase in the absorption of the light in the damaged region near the surface, being dominated by the total number of the defects within the optical penetration depth. (author)