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

oadening of the Si peak at the base occurs due to Si isotopes, the system resolution,
and any overlap of the Si peak with the dopant peak. This prevented the base of the Si
peak from being used as a reference point for alignment. The gradient of the leading
edge of Si peaks varies between samples, primarily because of a dilution for different
oxide layer widths, causing a marginal error if a set height on the peaks was chosen. The
best point on the spectra to align the peaks to was near the base but above the isotope
tail. Even so there was a lack of reproducibility in this method and an error, estimated to
be of up to 0.2 nm could be introduced.
A fitting routine developed at Daresbury laboratory that attempted to find the
best fit of beam energy to the peaks was trialled but did not give sufficient reliability to
be used routinely.
Finally it was concluded that the best solution was to deposit a very small
amount of Au onto the surface of each sample using a k-cell evaporator. This was used
as a reference to determine the beam energy, from the kinematic factor and the energy at
the centre of the Au peak. The presence of considerably less than a monolayer of Au
would have no appreciable affect on the rest of the spectrum. Using the k-cell could
produce a consistent Au layer on each sample. Knowing the beam energy accurately for
each sample meant that the relationship between the energy scale and scattering depth
could be calculated, referenced to the actual beam energy used for each sample.
4.2.3.3 Numerical solution to depth scale
In order to utilise the knowledge of the beam energy a method of calculating the
depth scales quickly was needed. A computer program was written to calculate these
depth scales using numerical methods. There are important differences in principle
compared to the surface approximation method, which uses a fixed rate of energy loss.
The numerical method calculates the depth scale in a manner which mirrors the actual
situation. Within the program the instantaneous rate of energy loss is calculated. The
stopping power is given as a function of the beam energy by a third order polynomial
(given in Figure 4.18) which gives an accurate fit to the values obtained from SRIM.
Given the starting energy of the incident beam, the instantaneous rate of inelastic energy
loss is calculated. After travelling a fixed distance (set at 0.1 nm), a reduced “beam
energy” is calculated from the inelastic energy lost, and from this “beam energy” a new
rate of inelastic energy loss is determined. From the path length the number of repeat
cycles in the calculations is determined. Once at the scattering depth, the reduced “beam
energy” is multiplied by the kinematic factor, to account for scattering. The same
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