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a silicon crystal, abbreviated H-Si(100). Atomically flat, ordered H-termination
is ordinarily achieved by cracking Hydrogen gas, H 2 , into H atoms by collision
with a hot tungsten filament and allowing those H atoms to react with a clean
silicon surface in a vacuum chamber. If the H-termination process is incomplete,
or if an H atom is removed by some chemical or physical means, a DB is created.
H atoms removed by the local action of a scanning tunneling microscopy (STM)
are the focus here. Broadly speaking, the scanning motion of the tip can be halted
to direct an intense electrical current in the vicinity of a single Si-H surface bond
[ 7 - 10 ]. At approximately a 2 V bias between tip and sample it is understood that
multiple vibrational excitations lead to dissociation of the Si-H bond. At near
5 V bias it is thought the Si-H bond can be excited to a dissociative state, as
in a photochemical bond breaking event. Other not well understood factors are
at play, such as a catalytic effect, intimately depending on particular tip apex
structure and composition that might ease the Si-H bond apart as a substantial
H atom-tip bond forms while the Si-H bond lengthens and weakens. The fate of
removed H atoms is unclear though there is substantial evidence, in the form of
H atom donation to the surface that some atoms reside on the STM tip [ 11 , 12 ].
Many details related to exact position of the tip and precise metering of the
energetic bond breaking process so as to create just the change desired and not
other surface alterations will be touched upon in the section on Quantum Silicon
Incorporated and the commercial drive to fabricate atom scale silicon devices.
Figure 2 a shows a model of a H-Si(100) surface. Silicon atoms are yellow.
Hydrogen atoms are white. Note the surface silicon atoms are combined with H
atoms in a 1 to 1 ratio. Note also that the surface silicon atoms deviate from the
bulk structure not only in that they have H atom partners, but also in that each
surface silicon atom is paired-up into a dimer unit. The dimers exist in rows.
Figure 2 b shows a constant current STM image of a H-Si(100) surface. The
dimer units are 3.84 A separated along a dimer row. The rows are separated by
twice that distance, 7.68 A. The overlaid grid of black bars marks the position
of the silicon surface dimer bonds. To reiterate, there is an H atom positioned
at both ends of each dimer unit.
Figure 3 indicates the localized creation of a dangling bond upon action
directed by a scanned probe tip. Figure 3 b shows an STM image of several DBs
so created [ 13 ].
3 The Nature of Silicon Dangling Bonds
Figure 4 shows two silicon surfaces imaged under the same conditions [ 14 ]. Both
surfaces have a scattering of DBs. The left image is of a moderately n-type doped
sample. It has been shown that DBs on such a surface are on average neutral.
The DBs in that case are visible as white protrusions. The right hand image is of
a relatively highly n-type doped sample. In that case each DB has a dark “halo”
surrounding it. These DBs are negative. This results as the high concentration
of electrons in the conduction band naturally “fall into” relatively low-lying DB
surface state to make it fully, that is 2 electron, occupied. This localization of a
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