Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 58

In addition to water rinse, certain other procedures also produce flatter surfaces.
It has been reported that a BHF-treated surface is flatter than one treated with HE. 641 A
photocurrent-assisted etching followed by a dark current transient has been found to
reduce microscopic roughness. 1036 A multistep process, involving formation of an
anodic oxide, dissolution in 0.2 M at pH 4 and then at pH 4.9, produced a com-
pletely monohydride-terminated (111) surface. 743
In organic solvents, the hydrides of HF-treated surface maintain their structure
but the bond energy tends to vary with solvent type. 619 The amount of hydrides
decreases with immersion time due to oxidation of the hydrogenated surface by the
small amounts of water present in the organic solvents. There is a marked difference
of the oxidation rates for (111) and (100) surfaces.

The silicon surface terminated by hydrogen after etching in HF is rather stable
at room temperature, reacting only very slowly with oxygen or water. A change of less
than 10% of the surface M concentration is found over a period of 1 month in an ultra-
high vacuum. 895 The sticking probability of oxygen on H-terminated surface is many
orders magnitudes smaller than that of a clean silicon surface. 1096 In water, whereas a
bare silicon surface is quickly oxidized, a hydrogen-terminated surface is replaced by
a monolayer of oxide only after immersion for more than 100 min as shown in Fig.
325,663 In particular, the ideally hydrogen-terminated Si (111) surface, that is, mono-
hydride, is extremely stable in both highly acidic and highly basic solutions (e.g., from
HC1 to
Hydrogen termination also serves to passivate grain boundaries. 571 Terrace monohy-
dride has different stability from step monohydride. 1109 As a result of hydrogen passi-
vation, HF-treated silicon surface exhibits a very low density of surface states in various
acids over a wide concentration range. 648

2.11.

574 1096

Polyhydrides are found to be more stable than monohydride.

Mechanistic Aspects. The energy of the hydrogen-silicon bond is in the range
of 3-4eV, which is similar to the silicon-silicon bond. 229,914,980 On the other hand, the
fluorine-silicon bond has a much higher bond energy, 6-7eV. Thus, thermodynamically,
Si-F is the more stable bond. F-terminated silicon complexes are unstable in HF