Chemistry Reference
In-Depth Information
unchangeable. Actually at this temperature gold eliminates from the surface due to
sublimation and/or dissolution into the bulk due to diffusion processes.
The phenomenon of the step instability under small amount (less 0.1ML) of gold
deposition was observed previously. Latyshev et al. [
28
,
29
] attributed this effect to
the reversing of an effective charge of adsorbed silicon atoms during gold deposition
on the silicon (111) surface. Minimal critical surface concentration of gold atoms
for this transition was estimated as 0.008-0.016ML. Later it was proposed that this
transition is caused by changing of atomic step properties, namely, due to the change
of permeability of the atomic steps for silicon adatoms from the impermeable steps
to the permeable ones [
30
,
59
-
62
]. Minoda et al. [
31
] reported that step bunching at
increased gold coverage and the step-down DC direction occurred due to changing
a free energy of Si(111) and this step bunching caused by thermodynamics. While
the observation of this transition was reported, the investigations of impact of the
current direction and increased gold atom concentrations (higher 0.1ML) on the
silicon surface morphology have not been done.
By increasing the gold coverage above 0.24ML, the surface morphology was
changed by causing the second transformation of the silicon surface morphology
by means of gold-induced step bunching (Fig.
11.4
c). We found that bunches
formed by gold adsorption were stable in the range of gold coverage between 0.24
and 0.42ML. The process of gold-induced step bunching at these gold coverages
occurred faster than one at clean silicon (111) surface.
Increasing gold coverage above 0.42ML caused the third transition of the silicon
surface morphology: from step bunches to regular steps (Fig.
11.4
d). Further gold
deposition (above 0.75ML) on the silicon surface caused the formation of three-
dimensional particles, probably gold-silicide islands on the surface similar to the
one shown in Fig.
11.3
c. Some of the moving particles were pinned at step bunches
in agreement with [
63
].
We also have investigated the silicon surface transformations during gold depo-
sition in the case when heating DC flowing in the step-up direction. Similar to the
case of DC-induced step bunching on clean surface, the silicon surface transfor-
mations under gold deposition were found to be dependent on the DC directions.
Figure
11.4
e-h represents the surface relief at 900
◦
C during gold deposition when
heating direct current flows in the step-up direction. Figure
11.4
e shows initially
clean silicon surface at 900
◦
C with regular atomic steps. The system of regular
distributed atomic steps on the clean silicon surface was stable at these conditions.
When gold was deposited, the several transitions of regular steps to step bunches and
vice versa were occurred on the surface in agreements with one for the step-down
DC direction assuming that regular step morphology was changed on step bunches
and vice versa. Thus, the morphology of the silicon (111) surface depends crucially
on the direction of the heating current and amount of deposited gold atoms.
Figure
11.5
schematically represents the surface morphology dependence on
gold coverage for different directions (step-up and step-down) and types (DC and
AC) of the heating electrical current at 900
◦
C. Dark areas correspond to the step
bunched morphology whereas white areas relate to the regular distributed atomic
steps. There are four gold coverage ranges which can be characterized by alternative
