Verlag des Forschungszentrums Jülich
JUEL-3077
Krzyzowski, Michael Adam
Untersuchungen zum heteroepitaktischen Wachstum im Submonolagenbereich mittels Streuung thermischer Heliumatome
113 S., 1995
Abstract
Heteroepitaxial growth of thin metal films on metal surfaces has been analyzed in
the submonolayer regime by thermal helium atom scattering (TEAS). Experiments have
been performed on two different substrates: the non-reconstructed Pt(lll) surface and
the Au(lll) surface which exhibits a long-range (~ 22 x V3)-reconstruction. Ag was
deposited on the Pt substrate and Co on the Au substrate.
Based on the analysis of the growth of Ag on Pt(lll) it is
found that below T = 45K
the monomer is the stable nucleus. At these low temperatures small islands are formed
with mean size and separation depending on the deposition rate and temperature. It
is shown that these structures are not thermally stable and that their arrangement is
governed by kinetic processes. Upon increasing the deposition temperature the size of the
critical nucleus increases and the growth mode changes from a three dimensional growth
to a layer-by-layer growth based on a "step flow" mechanism. At all temperatures the
growth of the first Ag layer is pseudomorphic (i. e. with the same lattice parameter and
orientation as the Pt(lll) substrate).
Surprisingly, at a deposition temperature of about T = 600 K, however, submonolayer
deposition of Ag again leads to the formation of small islands with an initial diameter of
about 10 A. Furthermore, these islands are arranged in a rather regular pattern. The size
as well as the separation of the islands depends neither on the deposition rate nor on the
deposition temperature. To generate this structure the Pt(lll) surface covered with submonolayer
amounts of Ag only has to be heated once above a critical temperature of about
T = 600 K. It is demonstrated that these quasi-regularly arranged islands represent the
thermal equilibrium configuration. For this phase two distinct structural phase transitions
are observed: (i) Upon increasing the Ag coverage the structure of the Ag islands changes
from a two-dimensional array of compact islands (disk phase) with an average separation
of about 30Aat e = 0.2 NIL to a one-dimensional "striped" pattern at coverages around
e = 0.5 NIL with approximately the same average distance of "" 30Abut no preferential
orientation of the stripes. (ii) An order-/ disorder-transition wjt]; a critical temperature of
about T; = 620 K. At low coverage (disk phase) the regular arrangement of the islands is
destroyed whereas at higher coverage (stripe phase) the one-dimensional structure starts
to meander and to disintegrate into smaller clusters. The existence of disk and stripe
phases as well as the observed transitions can be understood within a simple model based
on elastic interactions (originating from the lattice mismatch of the Ag adlayer and the
Pt(lll) substrate). A main result of this model is that the ratio between the initial size
of an island and the minimum separation between islands (i. e. the minimum periodicity
of the pattern) is 1/3 and that this ratio does not depend on the symmetry of the pattern
(stripes vs. disks). This result is corroborated by the present experiments.
A comparison of the structure with a regular arrangement of Ag islands obtained after
deposition of Ag at low and high temperature, respectively, reveals essential differences.
With several types of experiments it is confirmed that the structures obtained for Ag
on Pt(lll) at temperatures below T = 600K are adsorbed on top of the Pt substrate
while the Ag structures which were annealed (or deposited) above this temperature are
embedded whithin the topmost layer of the Pt(lll) surface. We have abo found that the
reactivity of the surface strongly depends on the location of the Ag-islands with regard
to the Pt surface. For instance, the reactivity of structures with Ag atoms embedded in
the surface for hydrogen adsorption at a surface temperature of T = 38 K is much higher
than that for Ag adsorbed on top of the Pi substrate.
In the case of deposition of Al on the dean (~ 22 x y'3)-reconstructed Au(lll) surface
at temperatures above T = 240 K a new reconstruction of the Au(lll) surface is
observed. This structure is induced by very small amounts of Al, Based on a detailed Hediffraction
analysis of this new reconstruction it is demonstrated that the corresponding
superstructure has a threefold symmetry and a characteristic periodicity of ~ 30Afor
an Al coverage below 0.5 ML. The periodicity decreases with increasing coverage reaching
15Aat nearly monolayer Al coverage. During Al deposition this Al-induced reconstruction
of the Au surface is overgrown by the deposited Al which forms two-dimensional
unreconstructed islands, Furthermore the growth modes at different temperatures were
analyzed. In particular it is shown that the adsorbed Al is likely to diffuse into the Au
bulk at about T = 500 K. This mixing of Al and Au can be suppressed by oxidation of
the Al adlayer.
The deposition of Co on Au(I11) has been analyzed at room temperature. It is found
that the Co nucleates at the so-caned "elbows" of the Au reconstruction lines, in contrast
to Al on Au(lll) where no preferential nucleation is observed. Because of the regular
arrangement of the (~ 22 x y'3) Au reconstruction the arrangement of the Co nudei is
regular (long-range periodic), too. We find strong evidence for the fact that these Co
nudei and the small Co islands of this system initially have monatomic height. However,
once the island size has reached a critical size of about 30 Co atoms, the second layer is
populated and the growth proceeds by formation of a complete double layer. Subsequent
Co layers then grow again as single layers of monatomic height.
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