Damage formation and annealing studies of low energy ion implants ...

andom direction. For He it has been found that R is energy dependant and has a
maximum near the maximum in the dE/dx curve. The crystal orientation also has a
strong influence on R. For He in Si R is ≈ 0.93 near to the maximum. This is
reduced to ~ 0.9 in the energy range 100 – 200 keV. For the Si directions R is ≈
0.83 at the maximum (25).
2.3.5 Correlation between stopping and screening
As the electronic and nuclear stopping occurs at the same time in close collisions
and the screening of the nucleus is due to the electrons it would appear that there should
be some correlation between the elastic and inelastic energy loss. However it is
justifiable to ignore these correlations and regard the electronic stopping as a continuous
process (26).
2.3.6 Example of relevant nuclear and electronic stopping powers
In Figure 2.4 the nuclear and electronic stopping powers for He and As in Si are
plotted. These values are obtained from SRIM 2003. There is a wide energy range for
both, displaying the trends described previously. The relevant energy regimes used for
MEIS and low energy implantation are indicated between dashed lines for the He and
As plots, respectively. As mentioned previously it is electronic stopping that dominates
for the medium energy light He ions. The same is also true for RBS measurements in
the 1 – 2 MeV energy range. Nuclear stopping is negligible, apart from the actual
backscattering event. On the other hand for low energy heavy ions used for
implantation, nuclear stopping dominates.
25