Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 73

extra capacity, appearing as a peak, occurs over that expected from the space charge
capacity. The peak, in height, broadness, and potential position, varies with frequency,
illumination, direction of potential sweep, pH, and presence of ions as shown in
Table 2.13. The median energy of the band of active levels from the results of Fig. 2.25
is close to the conduction band edge for n -type silicon and several tenths of a volt from
the valence band edge for p -type silicon. The frequency dependence of the capacitance
indicates that the states have a certain energetic distribution since the farther the surface
states are away from the band edges, the slower is the exchange of carriers in the states
with the bands.
A peak similar to that shown in Fig. 2.25 is observed on a silicon surface with
about 40 Å of thermally grown oxide and the peak becomes higher and broader with
additional anodic thickening. The apparent density depends on light intensity and on
whether the sample is biased cathodically or anodically before the measurement. This
capacitance peak, however, disappears almost completely in the presence of HF, which
dissolves silicon oxide. Thus, the surface states of a silicon electrode in KCl solution,
according to Madou et al ., 716 are physically associated with the unsaturated bonds at
the interface. On the other hand, in similar solutions, Chazalviel 400 found that
surface states, situated at about 0.9eV below the conduction band, are caused by the
adsorbed ions but not the oxide. Thus, for an oxide-covered electrode (e.g., 12nm