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

they are small compared to the differences due to the scattering cross sections. With the
random yield typically 375 counts, representing a concentration of 5 × 10 22 cm -3 Si
atoms, then displaced Si can be detected down to ~ 1 × 10 20 cm -3 and As to 1×10 19 cm -3 .
A correction has to be applied to account for the different energy widths that peaks of
the different elements have on the energy scale. The heavier the element, the greater
energy range that equates to a fixed depth. Stated differently ∆E/∆z is larger for higher
masses and hence energies. The variation is usually less than 10% but needs to be
accounted for (4). It can be most convenient to calculate the number of dopant atoms on
the basis of the depth scale.
The effect of neutralisation should be considered when using an electrostatic
analyser, as neutrals cannot be detected (1). Analysis of data taken at the MEIS facility
at the FOM institute (NL), carried out at Daresbury Laboratory, would suggest that
there was little variation in neutral fraction for different elements at the energies used in
MEIS (48), which was contrary to other findings (44). The measured results showed
that the neutral fraction varied with the velocity of the ion leaving the sample, being
lower at lower energies. The amount of variation was small over the energy range of a
MEIS spectrum (48). The implications for the dose calculation would therefore appear
to be not particularly significant. This is supported by a comparison of the results of
MEIS dose calibration with results of RBS measurements carried out at the University
of Salford, using a surface barrier detector, which detects neutrals. Good agreement to
within 3% was obtained.
4.2.1.6 Crystallography – Channelling, shadowing, blocking and dechannelling
By changing the orientation of a Si crystal the number of atoms “visible” to the
beam can be altered. Alignment along crystallographic planes produces so called open
channels. Atoms deeper within the crystal effectively become invisible, as illustrated in
Figure 4.4. The number of visible atoms is reduced from the initial random orientation,
to a planar alignment, and further reduction as the model is axially aligned along the
[110] direction on the right.
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