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

a Si atom will suffer little angular deflection in a collision and will continue into the
lattice making further collisions with Si atoms along its path. For heavy ions in the
energy range of interest, the cascade from a single ion will produce a small amorphous
zone.
Considering the behaviour of light ions, such as B, the mean energy transfer will
be lower than with a heavy ion. The primary Si recoils produced from light ions will
have on average less energy and consequently fewer secondary recoils will be produced.
Secondary cascades will be small and may consist of only a few displaced atoms. For a
light ion, the collision cross section σ is much lower. From equations 3.1 (and 2.6) it is
obvious that the mean free path between collisions is much greater with a light ion than
a heavy ion. This results in a much lower energy deposition density for light ions. As
the small cascades will be created much further apart they will not overlap, so
amorphous zones will not be produced (18). As a light ion travels a substantially greater
distance between collisions a higher proportion of its energy will be lost inelastically,
than with heavy ions. However this effect is less important at the low energies of
interest where electronic stopping is low. A light ion has more likelihood of being
deflected through large angles than heavy ions and so it will tend to take a zigzag
pathway. The overall effect is that small pockets of damage are formed at locations
along the path as shown schematically in Figure 3.5b). A single implanted light ion will
not produce an amorphous zone.
In summary the distribution of displacements around an ion track depends
strongly on the ion mass. Heavy ions have a much higher energy deposition density
resulting in small amorphous zones. A light ion has a much lower energy deposition
density resulting in damage which is sparse and created in pockets along a zigzag ion
path. Important models of how these damage distributions produce a continuous
amorphous layer are described in the following sections. The models are described with
reference to the type of ion which is most appropriate.
38