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

attributed to the interference between the air/SiO2 surface and the interface at the high
density, subsurface layer to which the As and any Si interstitials are swept by the
regrowing crystalline front. For the as-implanted sample the As depth distribution is
broad without sharp interfaces, as can be seen by the MEIS results shown in Figure 6.3
and 6.4a). As a consequence SR reflectivity appears to be quite similar as the nonimplanted
wafer where only one long-period oscillation is observed resulting from the
thin native oxide. The resulting electron density profiles from a fit of the data are given
in Figure 6.6 b).
a) b)
Reflectivity
10 1
10 -1
10 -3
10 -5
10 -7
10 -9
10 -11
PAI as-impl.
PAI 600
PAI spike
Non-implanted
Fits
0.0 0.2 0.4 0.6 0.8
q [Å z -1 ]
SR does not possess specific mass sensitivity, unlike MEIS which can
unambiguously distinguish between scattering off different masses. Through a
comparison with the MEIS profiles (Figure 6.3 and 6.4), regions in the SR density
profiles (Figure 6.6b) can be attributed to the different layers and features of the
samples, i.e. the SiO2 layer and the As rich segregated layer, these are indicated in
Figure 6.6b). A contribution to the As rich peak from Si interstitials swept in front of
the a/c interface during SPER may also occur. An advantage of the SR measurements is
that in some instances the thickness of layers may be inferred with more accuracy than
with MEIS. With MEIS it is unavoidable that for very thin layers some broadening
occurs due to the system energy resolution and energy straggling.
Returning to the results and considering first the spike annealed sample, the SR
density depth profile shows a segregated As rich peak with a width of 0.7 nm. The SR
result of 0.7 nm agrees well with the theoretical result of 1ML segregated As (8). This
shows up a significant difference in the As layer thickness between MEIS and SR, with
ρ el [Å -3 ]
130
0.8
0.7
0.6
0.5
SiO 2
Third layer
PAI as-impl.
PAI 600
PAI spike
As-rich
layer
Si bulk
0 30 60 90
Depth [Å]
Figure 6.6 a) Specular reflectivity curves for the PAI sample series and non-implanted Si with
corresponding fits. b) Depth profile of the density as derived from the fits. From (19, 20).