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

complex formation, precipitation and segregation. At low annealing temperatures ~
600 °C crystallisation is strongly dependent on ion fluence. There is not a simple rate
enhancement, which tracks the As concentration profile as seen with B. The maximum
concentration before retardation occurs is ~ 1.5 × 10 21 cm -3 (49). The formation of
dislocations maybe directly related to As clustering and precipitation. Solubility
increases with temperature and therefore fewer defects occur with high temperatures. It
is suggested that As vacancy clusters can form at the a/c interface and contribute to the
rate retardation (38). Severe retardation of the SPER rate is observed at high
concentrations.
During SPER dopants may become substitutional and electrically active in
concentrations well exceeding the equilibrium solid solubility (39). Substitutional
concentrations incorporated into an amorphous regrowth can exceed the equilibrium
solid solubility by as much as 560 times (41). The high substitutional concentration of
impurities observed after SPER can be attributed to trapping by the rapidly moving
interface (39).
3.3.2.3 Models for SPER
A number of models adopting several differing approaches have been developed
to describe the mechanisms of the SPER process. Atomistic models consider how the
atoms on the amorphous Si side of the interface are rearranged onto the crystal template.
Electric field models consider the electronic properties of the structure at the amorphous
/ crystalline interface and consider more closely the effects of the dopant on the SPER
process. Finally kinematic models attempt to identify the rate limiting step of the SPER
process and derive equations that can predict the SPER rate as a function of temperature
and dopant concentration (17). The most important models are briefly mentioned here.
Starting with the atomistic approach models, the focus is to consider the
behaviour at the a/c interface and the structure of the amorphous Si, which can be
described by a continuous random network (CRN). Consideration is made as to how
atoms from the amorphous side might reorder themselves onto the crystalline template.
A model was first created for Si (111) (50). The a/c interface is viewed to be fully
coordinated, i.e. there are no free bonds, so it would be in the lowest energy
configuration. The difference in energy between amorphous and crystalline Si is due to
bond angle distortion (50, 51). In the model, crystallisation is viewed as a simple bond
rearrangement process which starts with the breaking of a single bond and continues as
the defect propagates along a crystal ledge to minimise bond angle distortion and
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