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

On the basis that by creating an interstitial a vacancy is also created and that
when an interstitial and vacancy combine they are annihilated, then there will be an
excess of interstitials in the ion implanted Si, due to the number of implanted ions
which is known as the plus one model (11). This only holds in damaged crystalline Si.
In amorphised Si the plus 1 model holds in the area beyond the amorphous/crystalline
interface, with the ions that reach that depth.
Point defects can become electrically active if they accept or lose electrons
during their formation and may have electron energy states that fall within the Si band
gap. In Si the presence of a vacancy results in the neighbouring atoms having
unsatisfied bonds. Unsatisfied bonds reconfigure themselves to accommodate the defect
in the lattice. This can create different charge states. Vacancies can have one of 5 charge
states V 2- V - V 0 V + V 2+ (12). They are predominantly double negative in n-type material
and neutral in p-type. An interstitial, having valence electrons not involved in bonding,
exhibits donor like properties. The lattice reconstruction around a point defect depends
on its charge state. Charge states can effect the migration of the defect.
3.2.1.3 Point defect migration and secondary defects formed prior to annealing
Depending on lattice temperature and charge state, point defects become able to
migrate at different temperatures. Vacancies and interstitials are mobile at low
temperatures, ~ 150K (12). Divacancies are stable up to ~ 500K and therefore are the
dominant vacancy complex found in room temperature implants. If a vacancy is bound
to an impurity they can become stable to 400 – 500 K (13).
A variety of outcomes are possible with mobile point defects. For example a
mobile vacancy could recombine with an interstitial, or vice versa. Bonding to impurity
atoms can occur, forming for example an impurity-vacancy pair. Mobile interstitials can
be trapped at various defect traps and sinks, for example the SiO2 – Si interface, the
surface, pre-existing damage, amorphous zones, and amorphous/crystalline interfaces.
Secondary defects are formed from the agglomeration of several point defects to form
clusters. They can be created during the quenching stage of the implantation, or post
implantation through the migration of mobile defects, or during annealing. Secondary
defects act as precursors for extended defects that are formed during annealing. There is
a lower free energy for conglomerations of secondary and extended defects compared to
point defects. These issues are discussed further in sections 3.4 and 3.5.
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