Subsurface structure of epitaxial rare-earth silicides imaged by STM
C. Rogero,* J. A. Martín-Gago, and J. I. Cerdá
Instituto de Ciencia de Materiales de Madrid CSIC, 28049 Madrid, Spain
Received 1 March 2006; revised manuscript received 27 June 2006; published 21 September 2006
We combine scanning tunneling microscopy STM images, density functional theory total energy calcula-
tions, and STM simulations to conclusively determine the surface structure of the Y
3
Si
5
0001 silicide epi-
taxially grown on Si111. We observe, for the same sample, two different types of atomic resolution images
exhibiting either p3m or p6 symmetry, in analogy with previous works on similar rare-earth silicide surfaces.
We elucidate the long-standing controversy regarding the interpretation of these images by showing that they
are mainly related to the registry of the surfacemost Si bilayer with respect to the Si vacancy network located
two layers below the surface and, therefore, to the existence of two different buried structural domains. Our
results demonstrate an unsual STM depth sensitivity—up to 5 Å—for metallic systems.
DOI: 10.1103/PhysRevB.74.121404 PACS numbers: 68.55.Ln, 68.37.Ef, 68.43.Bc
The scanning tunneling microscope STM depth
sensitivity—i.e., its ability to image buried defects—strongly
depends on the system under study. For semiconductor sur-
faces, and due to the weak screening, impurities located even
down to the third layer are well resolved in the images.
1
In
the case of metallic surfaces, where the screening is more
efficient, one would expect a strong decay of the defect sig-
nal with its normal distance to the surface. Nonetheless, there
exist several works reporting STM images where defects
buried under a metallic surface could be imaged, such as
interstitial impurity light atoms at the Pd111 surface
2
or
transition metal atoms buried under noble metal surfaces.
3
A
deeply buried two-dimensional 2D ordered material may
also be imaged via the development of stationary waves be-
tween the surface and the material.
3,4
Furthermore, the as-
cription of a specific feature in the STM image to a buried
defect is not generally a trivial task and has led to many
controversies when trying to determine structural models.
5
In
this paper we apply the combination of experimental with
simulated atomic resolution STM images for solving a struc-
tural problem open in the last decade and not addressable by
other techniques: understanding the atomic termination of
rare-earth silicides epitaxially grown on Si111. We will
take advantage of the depth sensitivity of a STM image to
detect buried vacancies below the third layer and show how
they determine the final aspect of the STM images.
The family of metallic thin films of heavy rare-earth RE
silicides R =Er,Gd,Dy,Y,etc. epitaxially grown on
Si111 substrates present very similar atomic and electronic
structures.
6,7
They have been broadly studied in recent years
because of their appealing properties: unusually low values
of the Schottky barrier height 0.3 eV for n-type Si Ref. 8,
a small lattice mismatch e.g., 0.0% for Y, -1.2% for Er, and
0.83% for Gd silicides,
6
and an abrupt interface. The bulk
structure of the RE silicides has been well described from the
beginning of the 1990s;
9
they exhibit an AlB
2
-type atomic
structure, consisting of a hexagonal layered arrangement in
which Si and RE planes alternate with each other. In this
configuration the Si-Si nearest-neighbor distance is smaller
than in the diamond bulk structure 2.2 and 2.35 Å, respec-
tively, leading to a compressive strain. To release the strain,
an ordered network of Si vacancies is formed in the Si
planes. These Si vacancies induce a relaxation of the sur-
rounding atoms, leading to a Th
3
Pd
5
structure and to a RSi
1.7
stoichiometry.
10,11
As a result of the periodic arrangement of
the vacancies, a
3
3R30
pattern appears in low- and
high-energy electron diffraction LEED and HEED.
7,12,13
However, and in spite of their technological relevance, the
surface structure of the RE silicides is still an open issue.
Although it is well accepted that these surfaces are termi-
nated in a bulklike Si bilayer without vacancies,
14
two dif-
ferent structural models have been proposed based on two
independent STM studies on the ErSi
1.7
surface epitaxially
grown on Si111. Roge et al. found a hexagonal arrange-
ment of protrusions p6 symmetry in their STM images,
15
whereas Martín-Gago et al. observed a triangular grouping
of the protrusions with a p3m symmetry.
16
Both groups in-
terpreted the protrusions in their images as arising from the
upper Si atoms Si
up
in the bilayer but, in order to explain
the observed symmetries, they arrived at different registries
for the bilayer with respect to the Si vacancies at the third
layer. Roge et al. concluded that one of the three Si
up
within
the
3
3R30
cell was located on top of a vacancy Si
up
v
with an outward relaxation rendering this atom brighter than
the other two Si
up
p6 model in Fig. 1.
15
On the other hand,
Martín-Gago et al. assumed it was a lower Si atom in the
bilayer, Si
down
v
, the one in registry with the vacancies, and
invoked lateral relaxations of the three Si
up
atoms toward the
Si
down
v
in order to account for the threefold symmetry
16
p3m
model in Fig. 1.
Subsequent work on these surfaces pointed toward the
p3m model. The ab initio calculations of Magaud et al.
found it more stable than the p6 model although the total
energies for both were very similar.
11
In a LEED study Rog-
ero et al. attained a good agreement with experiment for the
p3m case.
12
On the other hand, Duverger et al.
17
were able to
reproduce theoretically the p6-type images, although hardly
any reference was made to the p3m phase.
In this Rapid Communication we reconcile the two mod-
els by presenting STM images where both the p3m and p6
phases coexist. We have additionally performed ab initio
density functional theory DFT calculations and STM simu-
lations in order to confirm the correspondence between the
experimental images and the two structural phases. It turns
out that the symmetry apparent in the images is solely deter-
mined by the relative registry of the Si
up
atoms with respect
PHYSICAL REVIEW B 74, 121404R2006
RAPID COMMUNICATIONS
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