Chemistry Reference
In-Depth Information
β(
√
3
×
√
3
67 nm
2
a67
×
stepped area with initially the low coverage
)
phase
(
θ
=
1
/
3ML). This phase first transforms into a non-well-annealed version of the
α(
√
3
×
√
3
1ML and the ring transforms a
four-layer island to a five-layer island which is the preferred height on this interface.
The presence of rings on top of metal islands has been noted in earlier growth
experiments that have studied the static film morphology, but no association was
made to the mobility of the surrounding wetting layer; or no measurements were
made of the speed with which the rings are completed. Rings were seen during
growth of Ag on Si(111) and the observation of island shape oscillations [
41
],
although the rings were not discussed. These experiments were carried out at a
relatively high temperature of 500 K. Rings were also seen in the same system at
similar temperature, but the coadsorption of In to act as a surfactant speeds up the
growth of the Ag islands to larger sizes and makes the rings on their top more distinct
[
42
]. Large Pb(111) islands were grown on Si(100) at room temperature and imaged
with LEEM and STM [
43
]. These larger islands have multiple height rings (up to
10ML) with the inside of the island resembling a “depression” and at the sidewall
the rings forming an ascending sequence of stepped terraces. These islands with
“depressions” form after rapid diffusion and after an initial wetting layer of density
1.5ML is present (as a
c
)
phase (4/3ML) after the deposition of
∼
(
4
×
4
)
superstructure). Both the rapid diffusion and the
higher density of the
c
are reminiscent of the conditions for the Pb/Si(111)
wetting layer to become more mobile and build the islands. Ring morphologies on
Pb/Si(111) have been seen with STM on large Pb islands at room temperature after
they are completed and stop evolving [
44
,
45
]. Rings were found on much taller
islands of varying height (because these “mesas” were grown over several substrate
steps). The islands have heights tens of monolayers so QSE differences between
areas on the “mesa” of different heights are reduced, since the spacing and number
of confined electronic levels become closer together as height increases). The cur-
rent work, which was carried out at lower temperatures and for smaller height Pb
islands, while the rings are forming, clarifies the crucial role of the wetting layer in
building the islands. In addition in [
44
,
45
] the ring formation was attributed to the
interaction of the electric field between the tip and the substrate that apparently can
trigger mass transport to the island top. The ring formation and the relative fractions
of flat vs. ring areas of the island were discussed either in terms of energetics, i.e.,
the system phase separates into the two areas driven by QSE energy differences for
stable vs. unstable heights [
44
], or purely in terms of kinetics (the faster process
of moving atoms to the rings is triggered by the tip and the slower completion of
the island top is controlled by vacancy diffusion [
45
]). The numerous differences
between the current experiments and the previous work (i.e., lower temperatures,
lower height islands, no tip effects, selective flow of Pb only toward unstable height
islands, the role of the critical coverage
(
4
×
4
)
θ
c
of the wetting layer, and the similarity
between ring morphologies on both interfaces (7
×
α
-phase) point to the intrin-
sic role the dense wetting layer plays in transferring efficiently the mass to the stable
islands and not to extreneous effects due to the electric field.
Finally we note that this unusual control of the Pb film growth and the possi-
bility to select the film morphology has also been exploited to answer questions
in other physics problems, i.e., what is the nature of superconductivity in reduced
7) vs.
