Phonon shifts in ion bombarded GaAs: Raman measurements

When 15 MeV ions bombard single crystal GaAs (111), (100), and (110) faces, they leave a strained, crystalline, surface layer with many defects (as well as a buried amorphous layer). Using Raman spectroscopy, we measure the shits and line widths of the optic phonons of these strained crystalline layers. Using simple models, the possible sources of the phonon shifts are quantitatively considered. We conclude that the strains, and a change in the ionic plasma frequency (LO-TO splitting) due to a ratio of interstitials, or antisites, to atoms in the crystals of ≈ 2% − 3% account for the major portion of the phonon shifts. These effects have been ascribed previously to phonon confinement.