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

3.2.1.4 Dynamic annealing
A specific consequence of the migration of vacancies and interstitials is that
damage can be removed by the annihilation of vacancies and interstitials during the
collision cascade caused by implantation. This process is called dynamic annealing and
the rate is dependant upon implant parameters, principally the rate of defect generation
and the substrate temperature. An increased defect migration at higher implantation
temperatures causes correspondingly higher levels of defect annihilation. A high
implant temperature could stop amorphous layers forming depending on the implant
species and fluence. (See section 3.3 for a description of the amorphisation process.) If
however implantation is carried out at liquid nitrogen temperatures the majority of
defects will be frozen in. It is worth mentioning there is a careful balance of effects
occurring within implantation to the extent that under certain conditions it is possible to
re – crystallise amorphous layers by ion implantation (14-16) in a process called ion
beam induced epitaxial crystallisation, through the energy supplied in the implantation.
There is a delicate balance between crystallisation and the damage formation (14).
3.2.2 Amorphisation
3.2.2.1 Structure of amorphous Si
Amorphous Si differs from crystalline Si by having no long range order or
periodicity. It can be described as having a frozen liquid structure or as a continuous
random network of Si atoms. Generally each Si atom is four fold coordinated bonded to
four other Si atoms but with large average distortion of bond angles and lengths that
incorporate 5 and 7 ring structures as well as the 6 ring structure for the crystalline, as
shown in Figure 3.4. Additionally amorphous Si has been found to contain defects
without four fold coordination, such as dangling bonds and vacancy type defects (4).
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