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

Figure 4.4 Different orientations of a Si Crystal. Randomly oriented, planar aligned, and
aligned along the [110] direction. From (4).
When a crystalline sample is aligned to the beam along a crystallographic
direction, the open channels are produced and the beam “sees” only the surface atoms.
Ions undergo large angle scattering from the first layer atoms but the second and
subsequent layer atoms in regular lattice positions are located within the so called
shadow cone, and give rise to no scattering as illustrated in Figure 4.5a). The shadow
cone radius Rc, can be calculated by (4):
1
2 2
⎛ Z1Z
2e
d ⎞
c =
⎜
E ⎟
(4.9)
0
R
⎝ ⎠
where d is the distance between atoms. Ions that enter the channel can undergo small
angle defections with the strings of atoms as shown in Figure 4.5b). There is a critical
angle for channelling, and with 100 keV He + in Si it is ~ 2.5° (1).
When aligned, the scattering yield from deeper atoms is considerably reduced,
by as much as a factor of around 100 compared to scattering from a randomly orientated
sample, greatly increasing sensitivity in the near surface region (25). Less than 5% of
the incoming ions will undergo a hard collision, these ions will be scattered through
large angles with a spread of directions. The rest of the ions will be gently steered into
the open channels, with small angle deflections due to interactions with the channel
walls (1, 5).
If the lattice atoms were stationary then the beam would only see 1 atom per
row. Because of lattice vibrations this is not strictly the case. The lattice vibration time
is much lower than the transit time of an ion so the ion effectively sees atoms frozen in
their thermally displaced positions during its passage (1). The work of Stensgaard
shows that effectively 1.3 – 1.4 atoms per row are left visible to the beam, the net effect
is any atom displaced from a regular lattice site by 0.1 – 0.2 Å gives rise to
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