Chemistry Reference
In-Depth Information
concentrations. The models for the formation mechanism of each of the groups have
been described in the previous sections. (One may note that although the groups of
theories organized in Fig. 8.62 seem to cover all mechanisms, they are disconnected
and are not a coherent and integral body of theories.)
Current Burst Theory.
The current burst theory of Foll
e
al
.
1087,1135,1140,1141
has
been developed most recently and is perhaps the most comprehensive model on the for-
mation of different types of PS. This is so far the only model that considers, on the one
hand, the relationships of morphological features to formation conditions, and on the
other hand, to the specific electrochemical reactions under these conditions.
This model has a basic assumption that the electrochemical reactions involved in
the dissolution of the silicon surface operate in microscopic units. These reaction units
have a temporal and a spatial distribution in number and in the state of activity. The
formation of pores is due to the synchronization of these operational units at certain
time and geometrical scales. It is further assumed that the state of reactivity of these
units oscillates by the same mechanism as the oscillation involved in the oxide forma-
tion-dissolution in HF at anodic potentials (see Section 5.10). Due to the nature of the
oscillation the unit on any specific position of the electrode surface can be silent or
burst into action resulting in an increase in current. More specifically, Foll
et al.
assumed the system to have the following features:
Current flow is always spatially and temporally inhomogeneous, that is, it
occurs by local current bursts.
Current flow induces either direct dissolution or oxide formation and dissolu-
tion which always follows a current burst.
The band bending of oxide-free surfaces cannot be changed by applied poten-
tial because the Fermi level is pinned in the midband gap due to the large
surface state density. The surface tends to be terminated by hydrogen resulting
in the passivation of the surface states and unpinning of the Fermi level. As a
result of hydrogen termination and de-termination, the position of the Fermi
level and thus the width of the space charge layer also oscillates with time.
t
