Chemistry Reference
In-Depth Information
The individual steps involved in this electrode reaction are illustrated in Fig. 5.62. In
this reaction scheme the surface is considered to be terminated by fluoride. The first
reaction begins at the fluoride-terminated surface. When a field is applied across the
interface, holes move toward the surface. One hole is trapped at the surface and a Si-Si
group
swings away from the corresponding Si atom at which the hole is trapped (step b). This
process is the rate-determining step and can work only if holes are present and the
hole is trapped sufficiently long. The product of reaction (5.11), silicon difluoride, is
not stable and tends to change into the stable tetravalent form by further reacting with
HF:
bond is weakened (step a). A certain thermal activation is required so that the
Reaction (5.12) results in evolution which is of chemical nature and is responsible
for the effective dissolution valence of 2. The Turner-Memming model explains the
overall characteristics of the anodic reactions, that is, two different reaction paths in
HF and in non-HF solutions; passivation by an oxide film at high anodic potentials;
evolution of hydrogen to account for the effective dissolution valence being less than
4. However, it lacks the details to account for phenomena such as surface termination
by hydrogen, current multiplication, and variation of effective dissolution valence.
