Isotope effects for ion-implantation profiles in silicon

Abstract

Isotope shifts of ion-implantation profiles for 6Li and 7Li, 10B and 11B, 16O and 18O, and 116Sn and 124Sn in silicon have been investigated at energies between 10 and 400 keV. Monte Carlo simulations as well as numerical calculations applying Boltzmann's transport equation yield a reverse isotope shift at high enough energies, i.e., the implantation energy of the heavier isotope should be lower than that of the lighter one to obtain identical depth profiles. This effect is attributed to a larger electronic stopping cross section S e for the lighter isotope at a given energy in the range where S e is roughly proportional to the ion velocity ν and dominates the slowing-down process. Moreover, also at low enough energies where the stopping is predominantly nuclear and screening effects are important, a small reverse isotope shift is predicted since the nuclear stopping cross section S n is approximately proportional to ν a with a > 0 for energies below the maximum of S n. Experimental support for the theoretical predictions is provided by range data for Li and B obtained by secondary-ion mass spectrometry (SIMS). SIMS has proven to be a suitable analysis technique with respect to isotope effects because of its good depth and mass resolution, and a reverse isotope shift is clearly resolved for both Li and B at energies above 100 keV.