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

6.2.2.2 Results and Discussion
Figure 6.1 shows the MEIS energy spectra for 2.5 keV As ions, as-implanted at
1.8 × 10 15 cm -2 and after RTA at 600 °C for 10 s, 700 °C for 10 s, 900 °C for 10 s and
1050 °C spike annealing. Depth scales have been added to the figure for As, Si and O.
The as-implanted sample shows that the 2.5 keV As implant amorphises the Si to ~ 9.5
nm, (half height of the back edge of the damage peak). The As profile is centred around
5 – 6 nm. Note that all the Si damage peaks include a contribution from the surface peak
(FWHM 2 nm).
The annealing behaviour is best introduced by first considering the sample
annealed at 600 °C for 10s. The annealing produces a reduction in the width of the
amorphous layer, from 9.5 nm to 6.5 nm (FWHM), due to solid phase epitaxial
regrowth (SPER). An average regrowth rate of ∼ 0.3 nm s -1 is obtained in this region in
which the interface moved through a region with a high As concentration. This value
compares to a value of 1 nms -1 found for intrinsic Si (6, 7). It is known that regrowth
rate is dopant concentration dependent and that a high As concentration causes a
reduction in the regrowth rate (6, 7, 28). Regrowth rates with a changing of As
concentration are examined in more detail in section 6.3.2.1.
MEIS shows that during annealing there is a close correlation between the
reduction in the width of the Si damage peak and the movement of the As implant. The
600 °C anneal has also caused a distinct change in the As distribution. The half height
of the back edge has moved to a depth of 6.5 nm which coincides with the depth of the
crystalline/amorphous interface. It also shows that the As yield in front of this interface
has increased by about 20%. This increase is due to As atoms that apparently could not
be accommodated in the regrown layer, and therefore segregates out and are
“snowploughed” in front of the advancing interface (7). Overall the As peak area
measured by MEIS is reduced by 12 %. SIMS measurements, discussed later, show the
retention of all the implanted As atoms. Hence the “missing” As atoms have moved into
substitutional positions where they are no longer visible to the analysing MEIS beam.
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