Chemistry Reference
In-Depth Information
Reaction (2.13a) has the smallest activation energy, meaning that the Si-OH bond is
highly unstable in HF solutions and if present will be quickly replaced by other bonds.
Reaction (2.13b), which results in hydrogen termination, is kinetically more favorable
than reactions (2.13c) and (2.13d). This means that fluoride termination tends to be
replaced by hydrogen termination, which, whether in the form of mono-, di-, or trihy-
dride, is kinetically more stable. Similar to HF, the attack by
and
leading to
hydrogen termination has also been proposed.
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Hydrogen termination of the dangling bonds on the surface stabilizes the silicon
surface. Since the electronegativity difference is small between Si-H and Si-Si bonds,
the H-terminated surface is stable due to the lack of polarization of the back Si-Si
bonds.
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Thus, the energy level of the hydrogen-terminated Si surface is identical to
that of the top of the valence band.
99
The fast removal of di- and trihydrides in water
can be explained by the difference in the extent of polarization among the different
hydrides. Trihydride has three polarized Si-H bonds, whereas dihydride has two,
making the back bonds of and weaker than SiH.
635
The hydrogen atoms bonded to the silicon atoms in water, according to Niwano
et al
.,
409
are, however, not completely immobile but undergo a constant hydrogen
exchange reaction with the hydrogen atom of the water molecule, generating negatively
charged surface silicon atoms through the reactions and
The generation of negatively charged silicon atoms could then
be a pathway for other reactions.
2.3.2. Fluoride Termination
The silicon surface after immersion in a HF solution, although terminated pre-
dominantly by hydrogen, may also be terminated by fluorine atoms to various amounts
depending on solution composition. In general, fluorine content on the HF-treated
silicon surface compared to that of hydrogen is very low and is not affected by the
immersion time after the complete removal of the native oxide.
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Higher fluo-
rine contents on the HF-treated surface have, however, been reported. For example, the
(111) surface after immersion in 1.5% HF can contain about 12% fluoride.
635
Fluorine
termination of over half of the surface area is possible using HF at a concentration
exceeding 50%.
563
The amount of surface fluorine tends to increase with HF concen-
tration as indicated in Fig. 2.13.
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It also varies with the pH of ammonium fluoride
solutions as shown in Fig. 2.14; the coverage reaches a peak of about 20% at a pH of
about 6.4.
260
In addition, it varies with the type of solution: The hydrogen-terminated
surface has about 2.5% Si-F bonds when cleaned with 5% HF but this is reduced by
two-thirds in buffered solutions with a minimum of almost zero coverage at a pH of
5.3.
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Submonolayer of F has been reported to be present on silion surface under current
doubling condition.
909
A fluorine-terminated surface is not as stable as a hydrogen-terminated one and
is readily removed by a water rinse.
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For example, the amount of fluorine, ~1%,
