Chemistry Reference
In-Depth Information
where is the initial current, that is, the limiting current of an oxide-free surface,
and are two constants which vary with pH. The relatively large and its variation
with pH and doping concentration are attributed to the change in Schottky barrier
height. It can be described by
where is the field strength at the surface and is exchange current when the field
is zero. Thus, increasing doping concentration would result in a reduction of the barrier
height and thus an increase of the initial limiting current. Increasing solution pH is also
found to reduce the barrier height and increase the limiting current.
When HF is present in the solution, the current does not decay to near zero
as shown in Fig. 5.24 but reaches a steady-state value.
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The time required to reach
the steady-state value decreases with increasing concentration from 100 s at 1 M HF
to 10 s at 10 M HF. The decay of current in HF solutions is due to hydrogen termina-
tion of the silicon dangling bonds, unlike in non-HF solution where formation of an
oxide film is responsible for the current decay. The initial limiting current in fluoride
solutions depends on pH as shown in Fig. 5.25.
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At pH below 5.3, the dark currents
reach a current peak followed by a plateau; however, at pH 5.3 the current increases
slowly to a plateau value without a current peak prior to the plateau current. Surface
analysis indicates that hydrogen termination is associated with the surface at pH below
5.3. The hydrogen adsorption process begins after the current maximum and is com-
pleted when the current levels out. No hydrogen can be detected at pH values above
