Chemistry Reference
In-Depth Information
ity to surface geometry. Since an electric field is present in the space charge layer near
the surface of a semiconductor, the vector of the field varies with the radius of surface
curvature. The surface concentration of charge carriers and the rate of carrier supply,
which are determined by the field vector, are affected by surface curvature. The situa-
tion is different on a metal surface. There exists no such field inside the metal near the
surface and all sites on a metal surface, whether it is curved not, are identical in this
aspect.
Thus, on a perfectly flat semiconductor surface the concentration of charge car-
riers is uniform across the surface because the vector of the field at every spot of the
entire surface is perpendicular to the surface and has the same magnitude. On a curved
surface, on the other hand, the rate of carrier transport to the surface depends on the
radius of curvature. Therefore, the rate of the electrochemical reactions, when is limited
by the supply of charge carriers, is sensitive to the radius of surface curvature. This
sensitivity, as an intrinsic property of semiconductors, determines the distribution of
reactions on the surface and determines the geometric evolution of the surface when
the reaction results in the change of surface morphology. Thus, it plays a critical role
in the formation of pores in silicon and surface roughness. Also, it may be an impor-
tant factor determining the breakdown of passive films of various metals and semi-
conductors.
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This property has generally been observed on semiconductors, for
example, the formation of pores on the electrodes of Ge, InP, GaAs, SiC, etc.
Physically, the sensitivity of reactions to surface curvature can be associated with
the space change layer or the resistance of the substrate. For moderately or highly doped
materials, this sensitivity is only associated with the space change layer because the
ohmic potential drop in the semiconductor substrate is very small. However, for lowly
doped material a significant amount of potential can drop in the semiconductor to cause
the current flow inside semiconductor to be also sensitive to the curvature of the surface.
In this situation, the sensitivity is associated with both the width of space change layer
and the resistivity of the substrate. The radius of curvature required for the occurrence
of the sensitivity caused by space change layer is different from that by substrate resis-
tivity. Therefore, under certain doping and polarization conditions, the current on the
surface can have two different distributions associated with radii of curvature of dif-
ferent scales. Such current distribution can result in the formation of geometric struc-
tures of two different scales such as the formation two-layer porous silicon (see Fig.
8.70).
The distribution of chemical reactions which do not involve charge carriers in the
semiconductor is not affected by surface curvature. Thus, formation of pores does not
occur in KOH solutions where the dissolution of silicon is of almost 100% chemical
nature. Also, the effect of surface curvature is little when the surface is covered with
an oxide film which masks the semiconductor properties of silicon, e.g., during elec-
tropolishing in HF.
The sensitivity depends on the radius of curvature relative to the width of space
charge layer. This effect can be measured by a normalized parameter defined
as relative curvature.
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At i.e., when the radius of curvature is in the order of
the width of space change layer the electrochemical reactions will be significantly
affected (see Fig. 8.64). Thus, for any electrochemical reactions that can cause the
surface geometry to change, either a dissolution or deposition, the surface distribution
