Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 416

The formation of macro PS on p- Si was initially thought by Propst and Kohl 248
to relate to the chemistry of the organic solvents because some of the electrochemical
reactions are fundamentally different from aqueous solutions, e.g., no hydrogen evo-
lution occurred during PS formation. With the findings on the formation of macro PS
in aqueous solutions, Wehrspohn et al. 1086,1061 provided a model based on an analysis
of the resistance of the electrolyte, interface, and substrate and of the stability of current
against perturbation. They suggested that a necessary condition for formation of macro
PS on low-resistivity p is that the resistivity of the substrate is higher than that of

the electrolyte. The micro PS, which was considered to follow the same field intensi-
fication mechanism, acts as a precursor for the formation of macro PS.

-Si

The model of Wehrspohn et al. was soon proven to be invalid as macro PS was
also found to occur in electrolytes having much higher resistance than the silicon sub-
177,1027 Alternatively, Lehmann and Ronnebeck 1027 postulated that the formation of
macro PS on lowly doped p- Si is due to the dominant effect of thermionic emission
which is sensitive to barrier height rather than barrier width. However, this does not
explain how the two PS layers with a difference of several orders of magnitude in pore
size could be determined both at the same time by the space charge layer nor what
governs the dimension of the macropores.
Miscellaneous Hypotheses. In many studies, the presence of a surface layer has
been suggested to be essential for the formation of PS. For example, selective reaction
of the hydrogen-bonded species has been suggested by Allongue et al. to be a neces-
sary condition for the formation of PS. 794,1108 Similarly, surface roughening caused to
the formation of surface hydrides and the associated dissolution of the silicon has been
considered to be related to the formation of PS by Rappich and Lewerenz. 775 On the
other hand, the observation of a very thin surface layer that is more dense than the bulk
micro PS led Unagami 40 to suggest that the formation of PS is promoted by the depo-
sition of a silicic acid on the pore walls which hinders the dissolution of the walls and
results in the directional dissolution at the pore tips. Alternatively, Parkhutik et al . 41
suggested that a passive film composed of silicon fluoride and silicon oxide covers the
wall and the bottom of the pores and that the formation of PS is similar to that of porous
alumina where a barrier layer exists on the base of pores.
Some of the hypotheses have been highly mathematically developed. For
example, a theoretical modeling based on charge transfer kinetics for PS morphology
has been attempted by Jaguiro et al. 1170 Similar theoretical modeling considering the
transport phenomena of carriers in the semiconductor, ions in the electrolyte, and
surface tension has been proposed by Valance. 1056 A theory based principally on ther-
modynamical arguments has been offered by La Monica et al . 1163

strate.

Integration of Models. An attempt has been made recently by Lehmann
12,71,1027,1084 to unify the theories on the formation mechanisms of all types of PS.
The fundamental assumption of the unified theory is that all of the surface of PS struc-
ture except for the pore tips is passivated due to the depletion of carriers. Different
mechanisms are involved in the depletion zone between the pores: depletion by
quantum confinement for the nano-scale pores and depletion by formation of the space
charge layer for all other PS having a pore size larger than a few nanometers.
As summarized in Fig. 8.62, the depletion by the space charge layer is further divided
into four groups according to the size of pores in relation to the doping type and

et al .