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

elevant dopants later. For equal concentration of n and p type dopants, the regrowth
rate is reduced to that characteristic of intrinsic amorphous Si (38).
For increasing dopant concentrations, of the order 1 × 10 20 cm -3 , processes such
as impurity segregation and precipitation can occur, which alter the crystallisation
kinetics. These processes are strongly influenced by the growth temperature and the rate
at which dopant diffuse in the amorphous Si layer. Precipitation e.g. dopant clustering
has been observed to retard the epitaxy. Segregation or “Push out” of the dopant occurs
for slow diffusing species (As, Sb), which do not precipitate prior to epitaxy.
Segregation occurs at the a/c interface and the effect is to “snowplough” excess dopant
in front of the advancing a/c interface (14, 38). Segregation is not observed below a
certain critical impurity concentration, which is species dependant and is always greater
than the equilibrium solid solubility, i.e. the maximum concentration that can be
accommodated through diffusion at a particular temperature. Note that the term solid
solubility or just solubility is often used to refer to the concentration that can take up
substitutional positions during SPER. At concentrations below the solid solubility all
dopants present can take up substitutional positions. For higher concentrations above
the solid solubility excess dopant segregates. At these concentrations the regrowth rate
is observed to appreciably slow down. For much higher dopant concentrations
polycrystalline growth has been observed, associated with random nucleation and
growth (38), as described in section 3.3.3.
B has the highest solid solubility of the elements in group III (36, 44). For
concentrations from 2 × 10 19 cm -3 to 2 × 10 20 cm -3 B enhances the SPER rate (42). The
amount of enhancement increases with increasing dose within this concentration range.
Above 2 × 10 20 cm -3 a saturation in rate occurs. The saturation level can be increased
with increasing annealing temperature. This effect is believed to be due to the limited
solid solubility of B in Si, and the assumption that only dissolved B increases the SPER
rate.
BF2 is most commonly used to implant B. BF2 contains both rate enhancing B
and rate retarding F in the same layer. The effect of BF2 is simply a combination of rate
enhancement from B effects and retardation effects from F. F retardation is strongly
temperature dependent therefore rate retardation due to F in BF2 sample should be less
pronounced as the temperature is increased. During annealing F has been observed to
both segregate to the surface and diffuse to the EOR region (45-48).
When As is present in amorphous Si there is a temperature dependant interplay
between processes such as SPER, random crystallisation, impurity diffusion, defect
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