At cryogenic temperatures, the accumulation of vacancy-interstitial pairs in
Al2O3 from atomic displacements associated with ion
implantation produces amorphization. At room temperature, these pairs
recombine, and amorphization occurs only at high doses. X-ray reflectivity
measurements show that amorphization of the surface of
Al2O3 implanted at room temperature with 160 keV
Cr+ ions is preceded by a progressive reduction in
near-surface density. Monte Carlo simulations show that this density
reduction can be accounted for by high-energy-transfer collisions which
knock atoms deep into the target, leaving widely separated vacancies and
interstitials, which do not recombine. Electron Microscopy shows that at
least some of these vacancies condense into voids. We propose that this
reduction in near-surface density can lead to amorphization at high doses.
We present simple approximations for the density reduction expected for
different ions and targets.