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

Further models are based on effects such as the Fermi level, vacancies or
dangling bonds. The models differ in the assumptions on the defect responsible for
SPER. Also it is not clear if defects have to migrate from the Si layer or from the
amorphous layer to the interface, or whether it can be determined by local concentration
at the interface. There is not sufficient experimental evidence to back up any one
particular model (39).
3.3.3 Random Nucleation and growth (RNG)
When amorphous Si has no crystalline seed to act as a template for regrowth the
behaviour upon annealing is to form random nucleates resulting in polycrystalline Si.
Since as mentioned previously the free energy of the crystalline state is lower than that
of the amorphous one, upon annealing there is a strong tendency to start to form the
four-fold coordination and tetrahedral bond angles of the minimum free energy, which
leads to the formation of small recrystallised nuclei from which crystallites grow from.
The orientation of the nuclei will not be orientated with respect to other nuclei so a
single crystal structure will not form. As the crystallites grow larger eventually all the
amorphous material will be converted into crystalline grains. High impurity
concentrations that inhibit SPER may also cause RNG (38).
3.3.4 Annealing ambient considerations
Annealing is usually carried out in either an N2, or N2 5% O2 environment. The
addition of some O2 to the environment maintains or leads to growth of the surface
oxide layer. This has the effect of trapping any segregated dopant under the oxide,
hence all the implanted dopant is retained. The presence of O in Si may also lead to
oxygen enhanced diffusion. Annealing in a pure N2 environment can lead to dopant loss.
3.4 Defects after annealing and evolution during annealing
3.4.1 Classification of Defects
Defects arising during annealing in ion implanted Si are dependent on
implantation and annealing conditions and have been classified in five different
categories (36, 41). MEIS is not sensitive to the type of individual defects and as such
specific defects have not been studied in great detail. The five categories will only be
briefly summarised here, taken from the reviews in (36, 41) and important defects will
be discussed later. Defect evolution during annealing has an important impact on
diffusion of dopants discussed in section 3.5.
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