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

The above observation by MEIS of the correlation between the planar growth of
the amorphous layer and the movement of the As peak to greater depth with increasing
dose, implies that most of the dopant that initially came to rest beyond Rp, migrates
towards, on average, shallower depths, back into the growing, highly disordered/
amorphous layer in which it is more readily accommodated than in the still crystalline
material. This movement is possibly initiated or occurs during the quenching stage of
the cascade and is likely to be point defect mediated.
The combined damage evolution and dopant movement behaviour observed for
shallow As implants is, in most respects, replicated for the even heavier Sb ion,
implanted at 2 keV into Si at room temperature. Sb with a mass of 121 amu compared
to As 75 amu, is expected to show reduced straggle and should in principle give better
defined implant profiles. Whereas As is believed to diffuse either through an interstitial
or vacancy assisted mechanism, Sb diffuses exclusively through the latter (28).
Figure 5.4 shows the dependence of the MEIS spectra on the Sb dose, over the
range from 1×10 14 cm -2 to 5×10 15 cm -2 . All implants were again carried out at room
temperature. As before the spectrum for the virgin Si surface and the random spectrum
are shown for reference and the peaks due to scattering off implanted Sb, surface Si and
O are marked. The expected reduced straggling shows itself in the steeper down slope
of the deep edge of the Sb implant profile compared to the equivalent region of the As
profile shown in Figure 5.1. The development of the Sb scattering peak as a function of
dose indicates a peak shift to greater depths, similar to that seen for As. The lowest
implanted Sb dose of 1×10 14 cm -2 has already produced a fully amorphised near surface
layer as demonstrated by attainment of the Si random level in the MEIS peaks. For
increasing Sb dose this layer shows a planar growth to greater depth, again akin to that
found above for As. The growth in width of the near-surface amorphous layer also
causes the increased dechanneling level seen in the spectra for high Sb doses. For the
highest dose of 5 × 10 15 cm -2 a dilution of the Si peak is observed that manifests itself in
a dip in the displaced Si signal. This is caused by the unusual growth of the oxide
thickness, in combination with the high Sb concentration. The oxide layer grows from a
width of 2 nm at a dose of 3×10 15 cm -2 to 4 nm at 5 × 10 15 cm -2 . This observation and
other MEIS results (not shown) appear to indicate that a high Sb implant concentration
at shallow depths promotes the growth of the oxide layer to depths well beyond that of
the native oxide.
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