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

2.2.3 Energy transferred in elastic collisions
The amount of energy that can be transferred in an elastic collision is dependent
on the masses of the particles and the impact parameter. The maximum energy transfer
results from a head on collision and is given by (14):
T
4M
M
E
1 2
= 2 0
(2.11)
( M1
+ M 2 )
The energy transferred will decrease with increasing impact parameter and the average
energy transferred is half that of the maximum value. The path of each ion depends on
the individual collisions that it encounters and hence the final depth of an implanted ion
will be subject to statistical variations. The nuclear stopping power (dE/dx)n gives an
average rate of energy loss of the ions through elastic collisions. The nuclear stopping
power can be related to the scattering cross section. The rate of nuclear energy loss
affects both the range of an implanted ion, discussed in section 2.4, and the damage to
the crystal, discussed in chapter 3.
2.3 Inelastic energy loss / Electronic stopping
The interaction of an energetic ion with the electrons in the target results in
inelastic energy loss of the ion. As the ion passes by, each atom will be individually
perturbed, leading to electron excitation within each participating atom, in some cases
ionisation will occur. Electron excitation and ionisation lead to internal energy
consumption in the colliding atom system. The energy absorbed in the excitation
process will not generally be restored to the kinetic or potential energy of the system
and this leads to inelastic energy loss. An average of the individual interactions gives
the rate of inelastic energy loss, called the stopping power or the specific energy loss,
termed (dE/dx)e, and in this thesis expressed in units of eV/Å.
As an ion travels through a solid it would undergo many interactions and as such
it is not possible to know the precise details of these interactions. Several models exist
to describe the interactions and provide estimates for the stopping powers. Only a brief
summary of important models is given here. Further explanation is beyond the scope of
this thesis. Further details are given in (5, 8, 17). Accurate values for the inelastic
energy loss rates as a function of energy are essential for the correct depth scale
calibration of the MEIS results. This is discussed further in section 4.2.3.1.
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