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

ackscattering (27). This fact forms the basis of the sensitivity of MEIS to lattice
damage and implanted dopants in non lattice sites.
a) b)
Figure 4.5 a) Shadow cone produced by a surface atom. b) Ions steered into the open
channel. From (5).
If the sample is aligned to the beam on its inward path, this situation is called
shadowing. Additionally data can be recorded along a direction of a crystallographic
axis on the exit path and this is called blocking, it gives a further increase in surface
sensitivity. The configuration of combined shadowing and blocking is known as double
alignment.
Channelled particles can become dechannelled when undergoing deflections
larger than the critical angle. Displaced atoms within a channel, e.g. point defects,
stacking faults, dislocation loops and amorphous zones, can cause such deflections of
the ions (forward scattering), which then cause the ion to be backscattered from deeper
within the sample. Figure 4.6a) illustrates dechannelling, in comparison to channelling
and backscattering. Amorphous layers cause dechannelling of the collimated beam, as
illustrated in Figure 4.6b). Dechannelling results in a higher background scattering
yield.
a) b)
Figure 4.6 a) Schematic illustrations of particles undergoing dechannelling, channelling and
backscattering. b) Illustration of the divergence of a collimated beam through an amorphous
layer, which results in a high level of dechannelling. From (4).
74