PHYSICAL REVIEW B, VOLUME 65, 113306

Characteristic adsorption of Xe on a Si„111…-„7Ã7… surface at low temperature
Y. J. Li, O. Takeuchi, D. N. Futaba, H. Oigawa, K. Miyake,* and H. Shigekawa†
Institute of Applied Physics, CREST, University of Tsukuba, Tsukuba 305-8573, Japan

Y. KukDepartment of Physics and Center for Science in Nonometer Scale, Seoul National University, Seoul 151-742, Korea
共Received 16 August 2001; published 8 March 2002兲
Site-dependent adsorption structures of Xe atoms on a Si(111)-7⫻7 surface were studied by scanningtunneling microscopy at 8 K. Xe atoms initially adsorb as dimers on the site between a rest atom and its twoneighboring center adatoms, and then near the corner adatom site. Similar structures were formed at both thefaulted half 共FH兲 and the unfaulted half 共UH兲 of the 7⫻7 unit. The observed structure is in excellent agreementwith that estimated by the rigid ball model, however, site-dependent bias dependence and different stabilities
were observed for them, indicating the existence of different interactions between Xe and Si atoms in the 7⫻7 unit despite the stable electronic structure of Xe. Based on the results with thermal desorption spectroscopy, the adsorption energies are estimated to be 200 and 220 meV for FH and UH units, respectively.
DOI: 10.1103/PhysRevB.65.113306

PACS number共s兲: 68.43.⫺h, 68.35.Bs

The interaction among rare-gas atoms on various surfaces
has been widely studied to understand the characteristics ofideal gas systems since they have simple and stable electronic structures.1,2 Recently, scanning tunneling microscopy
共STM兲 studies have revealed intriguing phenomena.3– 6 For
example, Xe atoms on Pt共111兲 and Cu共111兲 surfaces exhibit
completely different behaviors at step edges.3–5 Xe atoms on
the Pt共111兲 surface are preferentially adsorbed at the upper
step edges, while Xe atoms on the Cu共111兲 surface are preferentially adsorbed at the lower step edges and form onedimensional chains. The following growth processes of them
are completely different from each other. The measurementof the change in the dispersion of the Cu surface state with
Xe adsorption is a good example to demonstrate the complex
nature of Xe adsorption.7 These phenomena indicate the existence of the complicated interaction between rare gases andsubstrate metal surfaces despite the stable structure of therare gases. Namely, the adsorption of rare gases modifies thesubstrate electronic structures, and the subsequent rare gases
can probe the change in the electronic structure, resulting in
the complicated adsorption processes.
In order to understand the interesting and important raregas behavior comprehensively, understanding of Xe growthmode on the surface with well-defined electronic structure isessential. In this paper, we report the adsorption processes of
Xe on a Si(111)-7⫻7 surface by STM at 8 K. The
Si(111)-7⫻7 surface has been used as a model surface tounderstand chemical reactions, because it consists of several
atoms with well-defined electronic structures: namely, corner, center, and rest atoms in faulted half 共FH兲 and unfaulted
half 共UH兲 units. Therefore, characteristic features of the Xeadsorption process can be analyzed directly. We report unusually complex adsorption structures on Si(111)-7⫻7.
As-doped, n-type Si共111兲 samples with a conductivity of
0.37–0.625 ⍀cm were flash-cleaned at ⬃1500 K for 30 sec,
and then annealed at ⬃1100 K for 30 sec before cooling. Thebase pressure was 7⫻10⫺9 Pa, and was remained below 2⫻10⫺8 Pa during the cleaning process. After a completed
7⫻7 structure was obtained, the samples were cooled to 8

K. Xe dosing was performed using an electronically controlled pulse valve. An electrochemically etched tungsten tip,
was retracted from the sample during the Xe gas exposure,and STM imaging was performed over the area. The sample
temperature was monitored using an Au⫹0.07% Fe-Cr thermocouple located near the sample.
Figure 1 shows a typical structure of adsorbed Xe atoms
observed at various locations on the same surface. Xe atoms
were not stable during STM measurement, but sitedependent structures were clearly visible. As shown in Figs.
1共a兲–1共c兲, in the FH unit at low coverage area, adsorbed Xe
atoms were imaged as elliptical bumps and three types ofadsorption structures were distinguished, as named 共a兲 1D,
共b兲 2D, and 共c兲 3D structures, respectively. Figure 2共a兲shows a magnified image of an elliptical bump, and the crosssection along A-B in Fig. 2共a兲 is shown in Fig. 2共c兲. Thereexist two small peaks in the graph. Since the separation isclose to the value of the Xe diameter, 4.3 nm, we consider
that the elliptical bump is a Xe dimer.
At the high coverage area, three additional types of adsorption structures were observed, as shown in Fig. 1共d兲;
namely, 3D⫹1C, 3D⫹2C, and 3D⫹3C structures. Here,C means the additional bump observed near the corner adatom sites. Those bumps were not stable, and when they
moved during STM scanning, white-stripe noises were imaged, as indicated in Fig. 1共d兲 by the red arrows. The each of
those bumps is considered to be a Xe atom adsorbed on the
Xe dimer and corner adatom. In fact, when the corner Xeatom moves away under STM scanning, a Xe dimer in thefirst layer appears. For comparison, some of the structuresare schematically depicted in Fig. 1共e兲. Green and yellow
circles indicate the Xe dimer and corner atoms, respectively.
As is evident in Fig. 1共d兲, similar structures are observedin the UH unit, which consists of corner Xe adaoms on the
3D structure. The Xe dimer structures without the upper
corner Xe atoms were, however, not observed in the UH unit
even with low Xe coverage, and only the 3D⫹1C, 3D⫹2C, and 3D⫹3C structures were observed stably. For example, 3D⫹2C and 3D⫹3C structures are shown in Fig.

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FIG. 2. 共Color兲 Magnified STM images of 共a兲 1D ⬘ in FH and共b兲 3D⫹2C in UH (Vs⫽⫺2 V, It⫽0.3 nA). 共c兲 Cross sections
along A-B in 共a兲 and C-D in 共b兲.

FIG. 1. 共Color兲 共a兲–共d兲 Structures of Xe atoms absorbed on
Si(111)-7⫻7 surface at 8 K (Vs⫽⫺2 V, It⫽0.3 nA). 共e兲 Schematic models of 3D, 3D⫹3C, and 1D⫹1C.

1共d兲, as indicated by yellow triangles. Figures 2共b兲 shows amagnified STM images of 3D⫹2C structures in the UH
units. The cross section along C-D in Fig. 2共b兲 is similar tothat of A-B as shown in Fig. 2共c兲, indicating the existence ofthe dimer structure in the case of the UH unit, too. Although
the structures described above represent the majority, trimer
structures that consist of 1D⫹1C 关Fig. 1共e兲兴 were observedin some FH units, when the neighboring UH unit is occupied
by Xe atoms. An example is shown in Fig. 1共d兲 as indicatedby the green arrow.
It must be pointed out that our STM images were obtainedon Xe adsorbed surface and Xe atoms are mobile during the
STM scan. Therefore, the actual growth process was not observed in real time. However, the adsorption process can bederived from the observed structures as follows: 共1兲 Xe atoms are adsorbed in a dimer structure, resulting in the hexagonal shape formed by three dimers, as shown in Fig. 1共c兲
关green circles in Fig. 1共e兲兴. 共2兲 After formation of the hexagonal structure consisting of three Xe dimers, Xe atoms areadsorbed one by one on the three corner sites contacting the
Xe dimers 关yellow circles in Fig. 1共e兲兴. Some information,such as whether UH or FH units are occupied by Xe atoms
first, could not be obtained from our results, due to the mobility of Xe during the STM scan.

With further exposure, the 3D⫹3C structure covered theentire Si(111)-7⫻7 surface. Based on analysis of the crosssections, this structure was confirmed to consist of only asingle adsorption layer of the 3D⫹3C structure. No further
adsorption of Xe atoms on the complete adsorption layer wasobserved in our measurement, which may be due to the instability of the structure under STM measurement.
In order to confirm the present structural model we adopt
a very simple rigid ball model with the atomic radii of Xeand Si atoms to be 0.217 and 0.117 nm, respectively, and theXe atoms are assumed to be located at high coordinationsites. When two Xe atoms were located at the positions indicated by the red arrows in Fig. 3共a兲, contacting the blue
colored Si atoms, a rest atom, its neighboring center adatoms, and one of the Si atoms to which the center adatom isbonded, the distance between the two Xe atoms became approximately 0.43 nm, which is almost equal to the diameterof the Xe atom. Subsequently, each of the corner Xe atoms
was placed in contact with a corner Si atom and a Xe dimer
adsorbed in its neighboring site. The positions of the Xeatoms in our model are summarized in Fig. 3共b兲. The heightsof the Xe atom positions are measured from the top of thecorner adatom in the FH unit. In the UH unit, the values
measured from the corner adatom in the UH unit are shown
together in the brackets. Note that the atomic structure in the
FH and UH units are almost same, reflecting the fact that thefirst two Si layers of DAS model have almost the same inboth half units.8
Figure 3共c兲 shows the cross sections obtained along thelonger diagonal of the 7⫻7 unit with and without adsorbed
Xe atoms. The thermal drift was corrected before the crosssection analysis. The red and black cross sections are obtained for the 3D⫹3C and bare 7⫻7 structures, respec-

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FIG. 4. 共Color兲 STM images taken at Vs⫽⫺1 and ⫺4 V (It⫽0.3 nA).

FIG. 3. 共Color兲 共a兲 Adsorption sites used for calculation. 共b兲
Calculated structures of the adsorbed Xe with the assumption of therigid ball model. 共c兲 Cross sections experimentally obtained along
the longer diagonal of the 7⫻7 structure.

tively. The blue cross section in the FH unit was measured on
a 3D structure without upper Xe atoms. Numerous crosssections were examined. The position of the corner hole wasdetermined from the threefold rotational symmetry of thesurface structure to compare the peak position with the calculated atom positions, which are indicated by arrows withthe distance from the half unit boundary. It is evident that theobserved peak positions in the cross sections are in excellent
agreement with our structural model that employs simple
assumptions. As is well known, the STM peak position does
not always corresponds to the actual atom position. In thepresent system, however, the Xe has a closed shell structure
and the interaction of rare-gas atoms with neighboring atoms
can be expected to be so small as to modify it negligibly.
Thus, we consider the agreement seen here to be strong evidence of the validity of our structural model.
One must consider why Xe atoms adsorb in this specificmanner, although many other high coordination sites exist inSi(111)-7⫻7 units. Unless the Xe atoms prefer to form
dimers initially and those in the second layer prefer to beadsorbed on corner adatoms, many other configurations canbe imagined. Thus, the adsorption process must be a sign ofthe characteristic interaction between Xe and Si. In the caseof C60 on Si(111)-7⫻7,9 single molecules are also adsorbed
at the high coordination sites, in agreement with weak andnoncovalent or chemical bonding between C60 and Si. Instead, small amount of charge transfer from Si to C60 is believed to exist. That results in a weak electrostatic bonding
and produces the site preference. The site immediately above
rest atom, which has the highest electron density, is the most
preferential sites as expected, but the corner hole site and thesites on the dimer chain are also occupied with C60 with lessprobability. On the other hand, in the case of Xe, the preferential sites are located between a rest atom and its neighboring center adatom with much higher selectivity. Center adatoms are the sites with less electronic density.
In the 7⫻7 units, charge transfer takes place from adatoms to rest atoms, and there exist induced dipolelike electronic structures between atoms in a unit. Since center adatoms are less charged compared to corner adatoms, astronger dipole between rest atoms and center adatoms may
interact with Xe atoms. The positions determined by the calculation are the high coordination sites for the formation of
Xe dimers. Once a Xe dimer is formed, the sites between the
Xe dimer and its neighboring corner adatom also become
high coordination sites for Xe, and the dipole between acorner adatom and a neighboring rest atom may be attractive
for the subsequent Xe atoms. As observed in the cases of Xe
adsorption on metal surfaces, Xe atoms adsorbed at the stepedges induced repulsive interaction with the subsequent Xeatoms.3–5 However, since no single Xe atom was observed inthis case, the dimerization of Xe atoms is necessary for thestability of the first layer structure. In order to explain theadsorption structure, it is necessary to understand the electronic structure of the adsorbed Xe atoms. An excellent
agreement was obtained for the model and experimental results regarding the lateral positions of Xe atoms; however, as
shown in Fig. 3, the apparent heights of the Xe atoms do notcoincide with that obtained with our structural model. In addition, although the final structures are the same in FH and
UH units, Xe atoms at the different sites showed different
stability under STM scan, indicating the difference in thestability of the structures with different electronic structures.
The difference and similarity between the electronic structures of Xe atoms at different sites were confirmed at various
bias voltages. STM images taken at ⫺1.0 and ⫺4 V areshown in Fig. 4. At ⫺1.0 V, in comparison to the image at⫺2 V, Xe dimers become darker and, in contrast, the apparent height of the corner Xe atoms was emphasized. Further-

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more, in the dimer structure, the central part is particularly
dark, and its two Xe atoms are clearly separated, as shown in
Fig. 4共a兲. The observed change was similar in both units. On
the other hand, at ⫺4 V, the central part of the dimers becomes brighter, and the dimers appear to have a beanlike
shape. According to a local density approximation calculation, a hybrid electronic structure is formed between the Xeatom and the substrate. In the case of a metal substrate, hybrid state is formed near the Fermi level. The observed bias
dependence at higher voltage is considered to be due to theposition of the Si surface states. This local density of state
observed at the center of a dimer in the deep filled statemight be an indication of the bonding state between the two
Xe atoms.
In addition, the three dimers in the FH unit are more
separately imaged from each other than those in the UH unit,
indicating that the interaction reflects the difference in theelectronic structure between FH and UH units. As shown inthis paper, the growth model has several steps, indicating theexistence of corresponding stable structures in the processes.
In fact, based on the comparison of STM images, the firstand second layers show different site-dependent electronic
structures.
In the previous thermal desorption spectroscopy 共TDS兲
measurement of Xe/Si(111)-7⫻7, several different adsorption states with different desorption energies were observed,
five peaks at ⬃67 K 共D兲, ⬃80 K (C ⬘ ), ⬃87 K 共C兲, ⬃93 K
共B兲, and ⬃97 K 共A兲.10 The peak at ⬃67 K 共D兲 with thelowest energy was attributed to the multilayer adsorbates.
The ratio of the two major peaks at ⬃80 K (C ⬘ ) and ⬃87 K
共C兲 was 1:1 and the amount of the other two peaks was muchsmaller. There are four different types of Xe atoms in our
structural models, which are Xe dimer and corner atoms inboth FH and UH units. The atom numbers are 3:6:3:6 for关UH corner兴: 关UH dimer兴:关FH corner兴:关FH dimer兴. Thus, theonly possible assignment is to attribute the entire 3C⫹3D
structure in FH and UH units to the two major peaks of (C ⬘ )and 共C兲. The difference in the first and second layer in eachhalf unit is assumed to be unresolved. Since UH units are
more stable under STM measurement, the structure in the
UH unit is assigned to the higher temperature peak. The
remaining two peaks of 共B兲 and 共A兲 may be from the defector step-related adsorption sites. In consideration of thesefindings, the binding energies of Xe atoms in FH and UH
units are estimated to be 200 and 220 meV, respectively.
The obtained site-dependent bias dependence in STM images directly indicates the existence of the interaction between Xe atoms and the Si substrate. This interactionchanges not only the electronic structure of Xe, but also thatof the substrate, although the change in the electronic structure of the substrate could not be observed in the STM images. The change in the electronic structure of the substrateis probed by the Xe adsorption process itself. In fact, asshown in Fig. 1共d兲, the unit cells with the Xe atoms exhibit atendency to cluster, which suggests the existence of an indirect interaction between adsorbed Xe atoms through modulation in the Si substrate electronic structure. Note that Xe
atoms in neighboring half units are too far apart to interact
directly. In addition, as mentioned previously, the 1D⫹1C
structure formed in UH units was observed only when Xe
atoms were present in the neighboring FH units. Althoughfurther theoretical study is needed, the observed interaction
between the rare gases and the substrate is considered to play
an important role in the characteristic interactions of raregases adsorbed on the transient metals.
In conclusion, site-dependent adsorption of Xe atoms on aSi(111)-7⫻7 surface were studied by STM at 8 K. The
adsorption sites were in excellent agreement with those calculated by the rigid ball model. However, from the analysisof the bias dependence, the site-dependent interaction between Xe atoms and the Si substrate was confirmed. The
existence of the indirect long-range interaction between therare-gas atoms through the modulation of the substrate wasalso observed. This is an observation of the interaction between Xe atoms and the substrate with well-defined electronic structure at the atomic scale.

*Present address: Institute of Mechanical Systems Engineering,5

National Institute of Advanced Industrial Science and Technology
共AIST兲, Tsukuba 305-8564, Japan.
†
Corresponding
author.
Emailaddress:
hidemi@
ims.tsukuba.ac.jp., URL http://dora.ims.tsukuba.ac.jp
1
B. Poelsema, L. K. Verheij, and G. Comsa, Phys. Rev. Lett. 51,
2410 共1983兲.
2
K. Kern, R. David, P. Zeppenfeld, and G. Comsa, Surf. Sci. 195,
533 共1988兲.
3
J. Y. Park, S. J. Kahng, U. D. Ham, Y. Kuk, K. Miyake, K. Hata,
and H. Shigekawa, Phys. Rev. B 60, 16 934 共1999兲.
4
P. Zeppenfeld, S. Horch, and G. Comsa, Phys. Rev. Lett. 73, 1259
共1994兲.

S. Horch, P. Zeppenfeld, and G. Comsa, Appl. Phys. A: Mater.
Sci. Process. 60, 147 共1995兲.
6
Y. J. Li, K. Miyake, O. Takeuchi, D.N. Futaba, M. Matsumoto,
T. Okano, and H. Shigekawa, Jpn. J. Appl. Phys. 40, 4399
共2001兲.
7
J. Y. Park, U. D. Ham, S.-J. Kahng, Y. Kuk, K. Miyake, K. Hata,
and H. Shigekawa, Phys. Rev. B 62, 16 341 共2000兲.
8K. D. Brommer, M. Needels, B. Larson, and J. D. Joannopoulos,
Phys. Rev. Lett. 68, 1355 共1992兲.
9
X. D. Wang, T. Hashizume, H. Shinohara, Y. Saito, Y. Nishina,
and T. Sakurai, Jpn. J. Appl. Phys. 31, L983 共1992兲.
10
J. W. Bartha, U. Barjenbruch, and M. Henzler, J. Phys. C 19,2459 共1986兲.

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