Cesium-induced structural transformation from the Si(113)3 ×2 to the 3×1 surface
C. C. Hwang, K. S. An, S. H. Kim, Y. K. Kim, C. Y. Park, S. N. Kwon, H. S. Song, K. H. Jung, T. Kinoshita, A.
Kakizaki, T.-H. Kang, and B. Kim
Citation: Journal of Vacuum Science & Technology A 18, 1473 (2000); doi: 10.1116/1.582371
View online: http://dx.doi.org/10.1116/1.582371
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Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Cesium-induced structural transformation from the Si„113…3Ã2
to the 3Ã1 surface
C. C. Hwang
Department of Physics and Institute of Basic Science, Sung Kyun Kwan University,
Suwon 440-746, Korea and Beamline Research Division, Pohang Accelerator Laboratory, Pohang
University of Science and Technology, Pohang, Kyungbuk 790-784, Korea
K. S. An
Institute of Material Structure Science, High Energy Accelerator Research Organization, Oho 1-1,
Tsukuba 305-0801, Ibaraki, Japan and Advanced Materials Division, Korea Research Institute of Chemical
Technology, Yusong POB 107, Taejon 305-600, Korea
S. H. Kim and Y. K. Kim
Department of Physics, Sung Kyun Kwan University, Suwon 440-746, Korea
C. Y. Parka)
Department of Physics and Institute of Basic Science, Sung Kyun Kwan University,
Suwon 440-746, Korea
S. N. Kwon, H. S. Song, and K. H. Jung
Department of Physics, Yonsei University, Seoul 120-749, Korea
T. Kinoshita
Tsukuba Branch of Synchrotron Radiation Laboratory, Institute for Solid State Physics,
The University of Tokyo, Oho, Tsukuba, Japan
A. Kakizaki
Institute of Material Structure Science, High Energy Accelerator Research Organization,
Oho 1-1, Tsukuba 305-0801, Ibaraki, Japan
T.-H. Kang and B. Kim
Beamline Research Division, Pohang Accelerator Laboratory, Pohang University of Science
and Technology, Pohang, Kyungbuk 790-784, Korea
共Received 30 September 1999; accepted 10 April 2000兲
Cesium-induced structural transformation from the Si共113兲3⫻2 to the 3⫻1 surface has been
investigated by using low energy electron diffraction and synchrotron radiation photoemission
spectroscopy. We measured the change of work-function, core level (Si 2p,Cs 4d), and valence
band spectra with increasing cesium deposition time. As previously reported, cesium induced the
structural transformation from the Si共113兲3⫻2 to the 3⫻1 surface at the initial stage of adsorption.
Two shoulders (S1,S2) in Si 2p core level and one 共SS1兲 of dangling bond surface states 共SS1, SS2兲
in valence band spectrum from the 3⫻2 surface disappeared with the structural transformation to
the 3⫻1. These results show that surface atoms in sp 2 - and s 2 p 3 -like configurations on the 3
⫻2 surface are changed to s p 3 -like ones during the structural transformation. Therefore, the
cesium-induced structural transformation seems to be related to the reduction of elastic energy by
the donation of valence electrons from cesiums. © 2000 American Vacuum Society.
关S0734-2101共00兲15604-0兴
I. INTRODUCTION
Ideally bulk-truncated Si共113兲 surface is reconstructed at
room temperature 共RT兲 to a 3⫻2 phase, which transforms to
a 3⫻1 at the substrate temperature of about 800 K.1 Various
adsorbates, such as H,Na,Cs,H2O,C2H4 are known to induce
the structural transformation from the 3⫻2 to the 3⫻1 at
RT.2–4 There has been much effort to make clear the structure of the 3⫻2 surface and explain its easy transition to the
3⫻1 by adsorbates.
Up to now, several structural models have been suggested
for 3⫻2 and 3⫻1 surfaces. Ranke proposed the ‘‘adatoma兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
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J. Vac. Sci. Technol. A 18„4…, JulÕAug 2000
dimer 共AD兲’’ model for the 3⫻1 surface.5 It has been suggested that the 3⫻2 surface be induced by the
asymmetrization4,5 of every second dimer or by the
puckering6 of every second tetramer on the AD model. Jacobi and Myler claimed from high resolution electron energy
loss spectroscopy study of the H/Si共113兲 surface that the
phase transition to the 3⫻1-H is attributed to the lifting of
asymmetric dimers by the adsorption of atomic hydrogen.4
Chang et al. reported by using Kikuchi electron holography
that the adsorption of hydrogen causes puckered tetramers to
the symmetric ones.7 On the other hand, Dabrowski et al.
proposed a structural model for the 3⫻2 surface from their
scanning tunneling microscopy 共STM兲 measurements and ab
initio total energy calculation, where subsurface interstitial
atoms were inserted into every second tetramer on the AD
0734-2101Õ2000Õ18„4…Õ1473Õ5Õ$17.00
©2000 American Vacuum Society
1473
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1474
Hwang et al.: Cesium-induced structural transformation
model to explain the pentagon-like image in STM for the 3
⫻2 surface.8 They suggested that the extraction or the insertion of subsurface interstitial atoms results in the phase transition. The origin of the easy transition by adsorbates, however, still seems to be unclear due to the lack of information
about the adsorption behavior of several adsorbates as well
as the structure of the 3⫻2 surface.
In this work, we have studied the adsorption behavior of
cesium on the Si共113兲3⫻2 surface at RT because the adsorption of alkali metal having simple electronic structure will
give information about the easy structural transformation
from the 3⫻2 to the 3⫻1. We report the change of surface
periodicity, work function, core level (Si 2p,Cs 4d), and valence band spectra with increasing cesium deposition time at
RT by using low energy electron diffraction 共LEED兲 and
synchrotron radiation photoemission spectroscopy 共SRPES兲.
II. EXPERIMENT
The experiment was performed in an ultrahigh vacuum
chamber equipped with LEED, X-ray photoelectron spectroscopy 共XPS兲, and SRPES at the beam line 18A of the
photon factory in KEK. A well-defined n-type Si共113兲 wafer
(3.5⫻22⫻0.35 mm3) was etched and preoxidized chemically by Shiraki’s method9 before loading in the chamber.
The heating was done by passing a direct current through the
sample. The temperature was monitored by an optical pyrometer. The sample was carefully degassed at about 1100 K
for about 8 h and resistively flashed to about 1500 K several
times. We observed the 3⫻2 LEED pattern for clean
Si共113兲 surface at RT, in which ⫻2 spots were broader and
weaker than those with the 3⫻1 periodicity. This result is
very consistent with a previous result.1 The surface cleanliness was monitored by XPS and SRPES: no impurity peaks,
such as C1s, O1s, and O2p were observed. The base pressure of this chamber was about 2.0⫻10⫺11 mbar. Cesium
was deposited on the 3⫻2 surface by passing the direct current of 5.5 A through a carefully outgassed SAES getter
source. During the evaporation of cesium, the pressure did
not exceed 1.0⫻10⫺10 mbar. Angle-integrated and resolved
analyzers were used to measure core level spectra 共Si 2p and
Cs 4d兲 and valence band spectra, respectively.
III. RESULTS AND DISCUSSION
Figure 1 shows Cs 4d peak intensity 共䉭兲, work-function
change 共䊏兲, and observed surface periodicity corresponding
to each cesium deposition time on the Si共113兲 surface at RT.
The incident photon energy was about 100 eV for the measurement of Cs 4d peak. The work-function change was
measured by the shift of low-energy cutoff in valence band
spectrum 共normal emission at the photon energy of about
21.2 eV兲 from samples biased negatively by 15 V. As previously reported,3 the adsorption of cesium at RT induced the
structural transformation from the Si共113兲3⫻2 to the 3⫻1
surface at the initial stage of adsorption 共6 min兲. The 3
⫻1 LEED pattern persisted up to 10 min 共the minimum
point of work function兲. The Cs 4d peak intensity increased
and the work-function change decreased 共to about 3.4 eV兲
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FIG. 1. Cs 4d peak intensity, work-function change, and LEED pattern corresponding to each cesium deposition time on the Si共113兲 surface at RT.
linearly, up to 10 min with increasing cesium deposition
time. The Cs 4d peak intensity increased slightly and then
saturated, the work-function change increased up to 3.0 eV,
and a diffuse 3⫻1 LEED pattern was observed for further
deposition of cesium. Relative intensity to the saturation coverage is about 0.48 and 0.83 for 6 and 10 min, respectively.
These results are very similar to those for alkali metal adsorbed low index silicon surfaces.10,11
Figure 2 shows Si 2p core level spectra at normal emission 共a兲 and high polar angles 共25° and 50°兲 共b兲 for several
phases, such as 3⫻2, 3⫻1-Cs, d3⫻1-Cs at RT. The incident photon energy was about 138 eV, which is a very
surface-sensitive energy. Two shoulders, denoted as S1 and
S2, were observed at positions of higher and lower binding
energy 共BE兲 than the bulk peak. Their relative intensities to
the bulk increased at high polar angles. When a small
amount of cesium was adsorbed on the 3⫻2 surface, two
shoulders decreased gradually with increasing cesium deposition time. Two shoulders thus seem to be originated from
surface atoms with dangling bonds. As clearly shown in Fig.
2共b兲, these shoulders almost disappeared and Si 2p core level
spectrum became rather complex along with the formation of
the 3⫻1-Cs surface. A new shoulder (S3) was observed at
lower BE than the bulk peak at the saturation coverage. Considering the electronegativity difference between Si and Cs,
the shoulder would come from the silicon atom bonded to
cesium at RT. When compared to the clean surface, the overall intensity of the Si 2p core level spectrum at the saturation
coverage became relatively weak because the distance for a
photoelectron to escape increased by the adsorption of cesium. Si 2p spectra for the Cs/Si共113兲 surface at RT were
also measured by the photon energy of about 109.8 eV 关Fig.
2共c兲兴. Two shoulders from the 3⫻2 surface, clearly visible at
the photon energy of about 138 eV, were relatively small at
this photon energy due to larger mean free path of photoelectrons. The Si 2p peak from the clean surface shifted toward
lower BE by the adsorption of cesium up to 10 min and then
toward higher BE slightly for further deposition time, which
shows the change of surface band bending caused by the
adsorption of Cs.
Valence band spectra from the Cs/Si共113兲 surface at RT
J. Vac. Sci. Technol. A, Vol. 18, No. 4, JulÕAug 2000
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Hwang et al.: Cesium-induced structural transformation
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FIG. 2. Si 2p core level spectra taken with the incident photon energy of
about 138 eV at normal emission 共a兲 and at high polar angles 共25° and 50°兲
共b兲 and taken with the incident photon energy of about 109.8 eV at normal
emission 共c兲 for several phases, such as 3⫻2, 3⫻1-Cs, and d3⫻1-Cs, at
RT.
are shown in Fig. 3. The incident photon energy was about
21.2 eV and the spectra were taken at 0° 共a兲 and 14° 共b兲
along the 关 1̄01̄ 兴 direction. A surface state 共SS兲 was observed
at about 1 eV for the 3⫻2 surface, as shown in Fig. 3共a兲.
The SS decreased with increasing cesium deposition time at
RT. The SS was still observed after the 3⫻2 surface was
transformed to the 3⫻1. The SS seems to disappear at the
minimum point of the work-function change. Further cesium
deposition resulted in a new state, denoted as MS, slightly
crossing the Fermi level. As shown in Fig. 3共b兲, the SS split
into two states: SS1 and SS2, at the emission angle of about
14° along the 关 1̄01̄ 兴 direction, which was not observed in a
previous article.2 This indicates that SS actually consists of
two surface states with different origins from each other.
They were highly sensitive to the adsorption of cesium at
RT. Accordingly, these states seem to be attributed to the
dangling bonds. A weak structure W was also observed at
about 0.3 eV. The W was not always observed and had a
small dispersion. Therefore, the W may be a defect state.
Considering that a peak with similar binding energy was
observed at an antiphase domain boundary in a previous
STM study,12 the W may be originated from antiphase domain boundaries on the 3⫻2 surface. The SS1 state was
observed to be more sensitive than the SS2 state to the adsorption of cesium. The SS1 state almost disappeared when
the surface periodicity was changed to the 3⫻1. On the other
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1476
Hwang et al.: Cesium-induced structural transformation
FIG. 3. Valence band spectra corresponding to each cesium deposition time
at RT on the Si共113兲 surface taken with the incident photon energy of about
21.2 eV at two emission angles, 0° 共a兲 and 14° 共b兲 along the 关 1̄01̄ 兴 direction.
1476
FIG. 4. Cs 4d core level spectra taken with the incident photon energy of
about 100 eV from the Cs/Si共113兲 surface with increasing cesium deposition
time at RT 共a兲 and for 8, 10, and 16 min after the subtraction of linear
background 共b兲.
J. Vac. Sci. Technol. A, Vol. 18, No. 4, JulÕAug 2000
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1477
Hwang et al.: Cesium-induced structural transformation
hand, the SS2 state decreased gradually with the appearance
of a new state 共N兲 between 8 and 12 min. As shown in Fig.
3共a兲, all remaining dangling bonds seem to be occupied by
cesiums at 10 min. No distinct features were observed for
further cesium deposition.
Figure 4共a兲 shows Cs 4d core level spectra from the Cs/
Si共113兲 surface with increasing cesium deposition time at
RT. The incident photon energy was about 100 eV. As mentioned above, the Cs 4d peak intensity increased linearly
with increasing cesium deposition time and then saturated.
Cs 4d core level spectra for 8, 10, and 16 min after the
subtraction of linear background are redrawn in Fig. 4共b兲 in
order to compare these spectra in detail. Core level spectra
below 8 min were very similar except for different intensity.
However, the line shape changed significantly between 8 and
16 min. An asymmetric broadening appeared at the high BE
side for 10 min and then the intensity increased significantly
at 16 min despite little change in the peak height. The asymmetric broadening is well known to be originated from multielectron excitations in metal.13 In addition, a surface state
共MS兲 in valence band spectrum was observed to slightly
cross the Fermi level at coverages above 10 min. These results show that additional cesiums form a two-dimensional
metallic layer after the initial adsorption of Cs at specific
sites such as dangling bond sites preferentially.
Based on present photoemission spectroscopy 共PES兲 results, we describe the characteristics of the 3⫻2 surface and
the phase transition to the 3⫻1 as follows. As mentioned
above, we observed two shoulders in Si 2p core level spectrum from the 3⫻2 surface. Their BE shift was opposite in
direction but similar in size to each other. Thus there seems
to be a charge transfer between surface atoms contributing to
two shoulders, namely, surface atoms contributing to S1 donate their electronic charges to those contributing to S2. This
implies that surface atoms contributing to S1 and S2 can be
rehybridized from s p 3 - to s p 2 - and s 2 p 3 -like configurations.14 The total energy can be lowered by the rehybridization 共atomic relaxation兲 owing to reduced electronic
energy despite the increase in elastic energy.14 Stage 1, in
which the 3⫻2 surface was transformed to the 3⫻1, is characterized by the disappearance of two shoulders 共S1 and S2兲
in Si 2p core level spectrum from the 3⫻2 surface. This
indicates that silicon surface atoms on the 3⫻1-Cs surface
are in s p 3 -like configuration. This change in the hybridization of silicon surface atoms from s p 2 -and s 2 p 3 -like to
sp 3 -like configuration can be explained in terms of surface
energy containing both electronic and elastic energy.
When cesiums are adsorbed on the 3⫻2 surface and form
the bonding to silicons, they will give their valence electrons
to the empty dangling bond state of the s p 2 -like rehybridized
silicon atom on the 3⫻2 surface. The gain in electronic energy by the rehybridization toward s p 2 -and s 2 p 3 -like configurations is expected to diminish. Consequently, accumulated elastic energy in s p 2 -and s 2 p 3 -like configurations can
cause surface atoms to be in a more s p 3 -like configuration.
From this point of view, it is expected that empty and
1477
filled dangling bond states of sp 2 -and s 2 p 3 -like rehybridized
surface atoms are most sensitive during the structural transformation. As shown in Fig. 3共b兲, the SS1 state was
quenched with the structural transformation. Therefore, the
SS1 in valence band spectrum from the Si共113兲3⫻2 surface
would be due to the filled dangling bond state of s 2 p 3 -like
rehybridized surface atoms.
In this article, we report characteristics of the structural
transformation at RT. However, it seems that further studies
on the atomic structure of the 3⫻2 surface are needed in
order to make clear the adsorbate-induced structural transformation.
IV. CONCLUSION
Cesium-induced structural transformation from the
Si共113兲3⫻2 to the 3⫻1 surface has been investigated by
using LEED and SRPES. Two shoulders (S1,S2) in Si 2p
core level and one 共SS1兲 of dangling bond surface states
共SS1, SS2兲 in valence band spectrum from the 3⫻2 surface
disappeared with the structural transformation to the 3⫻1.
These results show that surface atoms in sp 2 -and s 2 p 3 -like
configurations on the 3⫻2 surface are changed to sp 3 -like
ones during the structural transformation. The cesiuminduced structural transformation thus seems to be related to
the reduction of elastic energy by the donation of valence
electrons from cesium.
ACKNOWLEDGMENT
The authors would like to acknowledge financial support
from the KOSEF 共Korea Science and Engineering Foundation兲 through the ASSRC 共Atomic-Scale Scientific Research
Center兲 at Yonsei University and Basic Science Research
Institute Program of Ministry of Education, Project No.
BSRI-99-2445.
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