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

Bragg’s rule (21). Although this rule is reasonably accurate, measured values of
stopping of ions in compounds can deviate from this, usually by less than 20%. Bragg’s
rule ignores changes in orbital and excitation structure between elements and
compounds. Any bonding changes may alter the charge density, thus changing the
strength of the interaction of the ion with the target medium (20). Data outputs from
SRIM are available for many compounds based on measurements and theory.
In every sample studied in this thesis there was an oxide layer on the surface of
the sample. The stopping power of He in SiO2, as obtained from SRIM, is also shown in
Figure 4.1. The stopping power is approximately 1 eV/Å lower than that in Si for the
beam energies used. Oxide layer widths are usually 1.5 – 2 nm which equates to a 0.1-
0.2 nm depth deviation in the depths scale from using the values of pure Si.
A small deviation arises of the stopping powers in an open channel compared to
that in a random direction (22, 23, 24, 25). The level of deviation depends on the
orientation and energy, e.g. for the direction the channels are more open than the
channel and hence the stopping powers are ~ 20% lower (22). For He in Si in the
direction the stopping powers have been found to be ~ 93% of that of the random
direction (22). For the experiments in this thesis most work is primarily concerned with
damaged or amorphous layers and hence changes in stopping powers do not need
correcting for.
4.2.1.3 Conversion of energy scale to depth
Different methods can be applied to calculate the depth scales in MEIS (4). Two
approaches have been used in this project and these are described in section 4.2.1.4 and
later in section 4.2.3.3. The first method uses a constant value for the inelastic stopping
power along the inward path and a different constant value along the outward path
following scattering. There are several places to choose the stopping power values and
in this project values were chosen at the surface and directly after scattering. This
method is termed the surface approximation. Variations on this method also exist,
choosing for example, some average value of the stopping power at some depth within
the sample. These types of method are valid as the changes in stopping powers are small
over the depths used in MEIS. They are convenient and provide the easiest starting
point to explain how the depth scale may be calibrated.
The other approach used in this project is the use of a numerical solution using a
computer program, and this was written as part of the project. In this method the
instantaneous rate of inelastic energy loss is recalculated along the ion path. It
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