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

essentially a “zero dose” profile in a randomised solid, agreement with a low dose
implant profile may be expected, provided that the Si matrix is amorphised. Figure 5.1
shows that this is the case for doses above 9 × 10 13 cm -2 , as evidenced by the Si damage
peak reaching random level close to the surface, however this is not the case for the
lowest dose investigated of 3 × 10 13 cm -2 .
The development of depth profiles of displaced Si in Figure 5.2 suggests that the
Si damage build up involves two distinct phases that partly overlap, i.e. an initial growth
of disorder near to the oxide /Si interface until a ∼4 nm wide amorphous layer centred at
a depth of ∼3.5 nm is completed, followed by the planar growth of the amorphous layer
from the surface inwards, until for doses above 1.5 × 10 15 cm -2 the amorphous layer
approaches a saturation depth of ∼10 nm. The increase in the thickness of the
amorphous layer also causes the dechannelling level to rise, as is seen in Figure 5.1. The
observed damage evolution behaviour cannot be explained in terms of the
heterogeneous nucleation model of amorphous phase formation at room temperature (7-
10). In this model the energy deposition in the collision cascade is sufficiently dense to
produce small amorphous zones (either in a single collision cascade or via cascade
overlap) and these zones once they spatially overlap at a certain dose, effect a lattice
phase change. Accordingly, the build up of displaced atoms would be expected to
follow the shape of the energy deposition function as is given by vacancy and interstitial
profiles produced by the implant ion. These profiles, again calculated by TRIM, are
added to the top of Figure 5.2 showing the Si damage distributions, using arbitrary units
but the same logarithmic scale. The two profiles are spatially close together and have
their maxima at shallower depths than the TRIM calculated As profile in the bottom of
Figure 5.2, as expected. It is clear that the growth of the disordered / amorphous Si layer
does not coincide with the maximum in the energy deposition rate; in fact the damage
profile for the lowest As implant dose is appreciably shallower than the calculated
profiles. It appears that for a shallow As implant, initially at low doses, a highly
damaged/amorphous 4 nm wide (FWHM) near-surface layer builds up, nucleated at the
oxide/Si interface. No deeper damage is observed at this stage and this suggests that a
fraction of the point defects produced at greater depths annihilate in dynamic annealing
processes during the cascade. This behaviour is similar to that observed in previous
MEIS studies for ULE B + implants into Si (19, 20) which showed the development of a
3–4 nm wide near-surface, highly damaged/amorphised layer, independent of implant
energy up to 2 keV at room temperature. The formation of this layer was ascribed to the
action of the oxide / Si interface as a sink for interstitials that are generated along the
110