Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 72

The surface of silicon treated in HF solutions is generally associated with a very
low density of surface states because the dangling bonds are saturated by hydrogen ter-
648,940,1035 The Si-H bonding, which has an energy similar to that of Si-Si, gives
rise to electronic states far into the bands and removes the dangling bond states from
the band gap. A certain density of residual surface states may exist on a hydrogen-
terminated surface due to defects and other inhomogeneities. It has been shown that a
good H-terminated surface, when covered with a thermal oxide, is characterized by a
U-shaped interstate state distribution with a low density of states less than
at midgap. 1035 Typically, a clean silicon electrode has a density of surface states on the
order of at least

mination.

which is of the same order as the surface defects and conta-

131,488 Large

mination of metals and organic species after a normal cleaning procedure.

amounts of surface states occur when the silicon surface has a significant amount of
adsorbed solution species, of an oxide film, or of reaction intermediates.
According to Memming and Schwandt, 61 surface states are always present on
silicon electrodes in acidic aqueous solutions. The energy levels depend on whether
fluoride ions are present; the surface states in acidic fluoride solutions are associated
with the dissolution of the silicon. The quantity of the surface states depends on the
type of silicon and the illumination intensity. 39,61 When fluoride ions are present in the
solution, the n- Si surface, being oxide free and terminated by hydrogen, exhibits a low
density of surface states. 940
As an example, Fig. 2.25, reported by Madou et al., 716 shows the capacitance as
a function of voltage on p- and

n -type silicon in a saturated KCl solution at pH 4. An