Chemistry Reference
In-Depth Information
LEED pattern of the clean (557) surface is seen. It consists of a regular sequence of
small (111) and (112) terraces [
15
]. Marked are the integer order spots of the small
(111) terraces. The multiple spots in-between correspond to a regular array of steps
with a periodicity of 5.7 nm. Also visible are streaky
spots, which are due to
the small (111) oriented terraces, and streaks at half-order positions. These indicate
a period doubling at the step edges.
This step morphology changes dramatically, when Pb is adsorbed at concentra-
tions between 1.2 and 1.4 monolayers (ML, referenced to the Si density of surface
atoms) onto this surface once the Si
(
7
×
7
)
reconstruction on the terraces is destroyed
by an annealing step to 640 K in presence of (at least) 1ML of Pb. Subsequent cov-
erage increments lead to further refacetting at temperatures as low as 180 K. The
result at a coverage of 1.3 ML is shown in Fig.
9.1
b. As seen there, the average step
separation has changed dramatically. From the distance of the spots between those
marked (00) and (01) we deduce an average Pb-induced step separation of 4.67 Si
lattice constants. This corresponds to the formation of a (223) oriented facet, which
has a higher step density than the original (557) surface. Occasional formation of
steps in the opposite direction can compensate for the higher step density in order
to maintain the macroscopic surface orientation. These, however, are not periodic
and are therefore not visible in LEED. This Pb-induced step array is not a minor-
ity species on the surface. Otherwise the surface conductance properties described
below would not be possible. As explicitly tested with LEED, larger (111) terraces
are only a small minority, which do not influence the main conclusions drawn here.
Measurements as shown exemplarily for the Pb concentration of 1.30 ML were
carried out systematically in the coverage range between 1.20 and 1.50 ML. After
carrying out the high-temperature annealing step once, tuning of the facet orienta-
tion and thus of the step density is possible just by increasing the Pb concentration in
small increments. Starting with the (112) orientation at 1.2 ML the (335) orientation
follows at 1.24 ML. At 1.27 ML the (223) facet appears, which turned out to be the
most stable. It exists at Pb concentrations above 1.27 ML up to saturation of the
physical monolayer close to 1.5 ML. The homogeneously stepped (557) surface
turned out to be stable only for annealing temperatures up to 80 K at a Pb concen-
tration close to 1.35 ML. At higher annealing temperatures it irreversibly transforms
into (223) facets.
In Fig.
9.2
we summarize our main results in a phase diagram of the Pb-induced
facetting transformations. The phase diagram includes, apart from the facet orienta-
tion, also the periodic structures seen in LEED parallel to the step edges, as judged
from the spots of the superstructures seen in between the integer order spots. At 1.2
ML the structure on the remaining (111) terraces is given. In all LEED images the
×
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×
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)
2 reconstruction of step edges of the clean Si(111) surface remains, indicating that
the step edges are not covered by Pb up to 1.31 ML when the LEED pattern starts
to change. This leads to new phenomena also in conductance and will be discussed
in a separate section below.
Going back to Pb concentrations close to 1.2 ML, it seems that the smaller size
of Pb compared to Si leads to an increasing tensile stress at the surface with increas-
ing Pb concentration, favoring not only formation of a homogeneously stepped
