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Fig. 6.
(a) STM micrograph (10
×
10 nm, 2 V, 0.2 nA) of a H-Si(100) surface with two
DBs. The distinct dark halo indicates each DB is negatively charged. (b) An additional
DB is created at a site near DB2 causing both the new DB3 and DB2 to appear very
differently.
atoms sharing in three ordinary Si-Si bonds and containing one dangling bond
have a special mixed character. Like 4-coordinate silicon atoms, such atoms are
very strongly bonded to the lattice and have an intimate role in the disper-
sive bands that delocalize electrons. At the same time, 3-coordinate atoms have
one localized state, approximately of
3
character. This state is localized because
it is in the middle of the band gap and mixes poorly with the valence and con-
duction band continua. The DB-containing atom is odder still in that the DB
is partly directed toward the vacuum where it has a relatively limited spatial
extent but is also partially contained within the silicon crystal where, because
of dielectric immersion, is somewhat larger in spatial reach.
It was stated above that a DB state is like a deep dopant. Whereas a typical
dopant has an ionization or anity energy of several tens of meV, the DB has
corresponding energies an order of magnitude larger. Consistent with that dif-
ference, the spatial extend of the DB state within the solid reaches several bond
lengths, much less than the size of a common dopant atom [
21
].
The zero dimensional character of the DB, combined with the capacity to
exhibit several (specifically 3) charge states leads us to think of the DB as
a quantum dot. This may at first seem a bit odd as a quantum dot is often
described as an artificial atom whereas we have a genuine atom, actually one
part of an atom, forming our dot. But if a quantum dot is most fundamentally a
vessel for containing and configuring electrons then, as subsequent examples will
show, the ASiQD naturally and ably fits the definition, especially as the ease
and precision of fabrication allows complex interactive ensembles of identical
quantum dots to be made.
sp
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