PHYSICAL REVIEW B

VOLUME 60, NUMBER 11

15 SEPTEMBER 1999-I

Reinterpretation of the scanning tunneling microscopy images of Si„100…-„2ⴛ1… dimers
Kenji Hata,* Satoshi Yasuda, and Hidemi Shigekawa*
Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,
Tennodai 1-1-1, Tsukuba 305-8573, Japan
共Received 4 May 1998; revised manuscript received 1 April 1999兲
We revisit and refine the interpretation of the scanning tunneling microscopy 共STM兲 images of the Si共100兲
dimers, based on results from an extensive set of STM observations carried out at low temperature 共80 K兲 andtotal-energy calculations of Si共100兲 surfaces. The interpretation addresses some unresolved questions andbrings much experimental and theoretical research into unanimous agreement. We show that tunneling fromsurface resonances and bulk states seriously contributes to the STM images within usual tunneling conditions.
In the empty state, tunneling from these states overwhelms tunneling from the localized ␲ * surface state,
which STM is generally believed to observe. 关S0163-1829共99兲05935-4兴

I. INTRODUCTION

Even though the dimer of Si共100兲 is one of the most
simple surface reconstructions, it has turned out to be asource of neverending controversy. A great deal of researchhas been devoted to elucidate its atomic configuration, optical properties, and electronic structure.1–6 In particular, scanning tunneling microscopy 共STM兲 has considerably contributed to enlighten our understandings.1–6 Generally, it isinterpreted that STM observes the surface states localized at
the dangling bonds of the dimers.1,2 Interpretation of the
STM images of the dimers from this standpoint seems to be
simple and in accordance with what is expected from thegeneral laws of chemical bonding.1 However a careful analysis of the existing data reveals that our understanding is notcomplete, and in the following we point out two problems
which require further consideration.
The first question is concerned with the interpretation of
the filled-state STM images obtained at room temperature up
to now. Previous room-temperature STM studies show
dimers as bean-shaped protrusions which have a maximum
at the center of the Si-Si dimer bond in the filled states.1,2 We
nominate them as bean-type dimers in the following. In contrast, when the empty states are probed, two round shape
protrusions are resolved,2 which we refer to as protrusiontype dimers. These STM images are easily explained by assuming symmetric dimers,1 though it is now well establishedthat dimers are buckled.3 Indeed at temperatures below ⬃200
K 共low temperatures兲, a majority of the dimers is observed ina buckled configuration.3 Buckling induces a charge transferfrom the lower to upper atom of the dimer, and the filled ␲and empty ␲ * surface states are localized at the dangling
bonds of the upper and lower atoms of the buckled dimer,
respectively. Generally, it is assumed that the ␲ and ␲ * surface states1,2 are observed in the STM images as dimers. In
order to interpret the apparent symmetric dimers observed by
STM at room temperature within the framework of buckled
dimers, the concept of flip-flop motion was introduced: buckled dimers are flip-flopping far faster than the scanning of
STM, thus dimers appear in an apparent symmetric configuration. In this scheme, STM images obtained at room temperature must reflect the average of STM images of the buck0163-1829/99/60共11兲/8164共7兲/$15.00

PRB 60led dimers at low temperature in the two possible
configurations. It is easy to interpret the empty-state images
at room temperature: the two protrusions observed in the(2⫻1) unit cell are both the ␲ * surface state localized at thelower atom of the buckled dimer in the two possible configurations. On the other hand, it is puzzling why similar
protrusion-type dimers are not observed in the filled state
instead of the bean-shaped dimers. Indeed, in Fig. 1 we show
gray scale images of the simulated spatial distribution of theflip-flopped filled ␲ and empty ␲ * surface states calculated
by first-principle methods. Methodology of the first-principle
calculations is presented in Sec. III C. Flip-flop motion was
simulated by adding the electronic structure of the surface
states in the two possible configurations of the buckled
dimer. It is a surprise to notice that the images of the flipflopped ␲ and ␲ * surface states are very similar with a small
node in between the dimer. A similar result has been obtained by Owen et al., where they simulated STM images ofthe flip-flopping buckled dimers following Tersoff and Hamman’s formalism. Their simulation shows a protrusion-type
STM image of the dimers in the filled states 关Fig. 2共a兲 in Ref.
7兴.
The second problem is the mismatch between the typicalsurface bias used to observe the surface in the past and theenergy window where the ␲ and ␲ * surface states are localized. A survey of previous STM observations of the Si共100兲

FIG. 1. Cross sections of the electronic and atomic structure of
the simulated flip-flopped buckled dimers. 共a兲 Simulated filled ␲
surface state and 共b兲 ␲ * empty surface state. Cross sections weretaken along a plane which includes the buckled dimer. The small
black and large gray circles represent the position of the upper andlower atom of the buckled dimer, respectively. The electronic structure was calculated by first-principles methods and the flip-flop motion was simulated by adding the two possible configurations.
8164©1999 The American Physical Society

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REINTERPRETATION OF THE SCANNING TUNNELING . . .

surface reveals that most of the STM images are of the filledstates taken with a surface bias as large as ⫺2 V.1–5 However, the typical bias of ⫺2 V is out of the range of wherethe ␲ surface state is located. We can see this by considering
the surface band gap and the width of the surface states.
Typical I-V curves of the dimers show a surface band gap of⬃0.5 V with the Fermi level located in the midgap.4 The
surface band gap of ⬃0.5 V is also supported from otherexperiments.8 Theoretical9 and photoemission10,11 studies
show that the bandwidths of the ␲ and ␲ * states are 0.6–0.8
V each. When combined with the surface band gap of ⬃0.5
V, this means that the ␲ and ␲ * surface states are roughly
localized in the range of ⬃⫾1 V from the Fermi level. In atypical STM image taken at ⫺2 V, a significant part of thetunneling current must come from states other than the surface ␲ and ␲ * states.
In this paper, we provide results from an extensive set of
STM observations at room and low temperature 共80 K兲, andpresent a refined interpretation of the STM images of dimers.
In our interpretation, at typical tunneling conditions used in
the past, a serious part of the tunneling current comes fromstates other than the ␲ and ␲ * surface states. The new interpretation addresses the aforementioned unresolved questions
and brings results of much experimental and theoretical research into unanimous agreement.
II. EXPERIMENTDefects are the major reason why high biases 共⬃⫺2 V兲
were used in the past. It is believed that surfaces of Si共001兲
inevitably contain a significant density of defects. When one
attempts to lower the bias, these defects become very brightbecause many defects are metallic, making it difficult to observe the dimers clearly. We have overcome this problem byfabricating a surface with very low defect density. After the
sample was prebaked at ⬃700 °C for one night, it was
flashed to 1200 °C for several seconds. The pressure was
kept below 1⫻10⫺8 Pa during flashing 共in most times
around 5⫻10⫺9 Pa). We found that keeping this extremely
good vacuum pressure during flashing is crucial to have asurface with low defect density.12 By this procedure, a clean
Si共100兲 surface with a small ratio of defects lower than 0.2%
was repeatedly made on any provided sample. N-type Si
samples phosphorus-doped with a conductivity of 0.1 ⍀ cm
were used. Tunneling I-V measurements of the surface showthat the Fermi level is at the upper edge of the conduction
band, which agrees with the n-type doping, and a width of
the surface band gap was ⬃0.5 V.
III. RESULTS AND DISCUSSION
A. STM results at room temperature

Surfaces free from defects make observation of the dimers
possible at any desired surface bias. Figure 2共a兲 shows an
STM image of the filled state probed with a low surface bias
just below the surface band gap 共⫺0.6 V兲 at room temperature. Not a bean-type but a protrusion-type dimer is observed. We could observe the protrusion-type dimers in thefilled states at low surface biases with the same frequency asin the empty state. The protrusions are observed at the same
location where the empty-state protrusions are observed, thus

8165

FIG. 2. STM images of dimers at room temperature. 共a兲 Low
bias filled state 共⫺0.6 V兲. 共b兲 The bias was gradually decreased
from ⫺0.9 to ⫺1.7 V. The bean⇔protrusion transition of the
dimers occurred around ⬃⫺1 V. The reason why the bean-shaped
dimers are not clearly visible at high biases might be because thisimage was taken simultaneously with the protrusion-type dimers
observed at low biases, which show stronger contrast than the beantype dimers. 共c兲 Empty-state image. The surface bias was switchedfrom a low 共⫹0.6 V兲 to a high 共⫹1.5 V兲 bias in the middle.

we attribute them to be observing the ␲ surface state locatedat the dangling bond of the upper atom of the dimer. Everytime the bias is increased, the dimers would gradually revert
to the bean type at ⬃⫺1 V as shown in Fig. 2共b兲, in areversible fashion. This indicates that the protrusion-type
dimers observed here are not induced by some peculiar tipsurface interaction.
The empty-state STM images show a more complicateddependence on bias. When the surface bias is above ⫹1.4 V,
regions between the dimer rows are observed. This is highlighted in Fig. 2共c兲, where the bias was switched from a low
共0.6 V兲 to a high bias 共1.5 V兲 in the intermediate of scanning.
Immediately it is apparent that the bright rows observed at
low 共0.6 V兲 and high biases 共1.5 V兲 are completely out ofphase. We attribute the bright rows observed at low biases tothe dimers, since they are in-phase with the dimer rows observed in the filled state. This means that regions between the
dimers are observed at high biases. The phase shift occurs at
a bias around 1.4 V. The phase shift was reported
previously1 and was interpreted as an extension of the nodeof the antibonding state of a symmetric dimer with bias
which was lately supported by theoretical studies.13 However, as mentioned before, dimers are buckled, and the ␲ *surface state of buckled dimers is calculated not to showsuch an extension with bias.13 Considering these points, weinterpret that at high biases, the main part of the tunnelingcurrent comes from states localized between the dimer rows,
overwhelming tunneling from the ␲ * surface state.
Results obtained at room temperature are summarized.
Protrusion-type dimers are observed both in the empty andfilled states at low biases. On the other hand, at high biases,
bean-type dimers are observed in the filled states while theregion between the dimer rows is observed in the empty
states. We interpret the results as the following. 共i兲 At low
biases, STM observes the ␲ or ␲ * surface states. The
protrusion-type dimers are observed as a result of the flipflop motion of the buckled dimers. The range of surface biases 共⫺1 V to ⬃⫹1.4 V兲 where the protrusion-type dimers isobserved is in good accordance with the energy window
where the ␲ and ␲ * bands are localized. 共ii兲 At high biases,

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KENJI HATA, SATOSHI YASUDA, AND HIDEMI SHIGEKAWA

PRB 60

FIG. 3. STM images 共1 nA兲 of the filled states of the dimers at
surface biases ranging from ⫺0.8 to ⫺2.0 V at 80 K. Scale4⫻4 nm.

tunneling from other states starts to participate in, and in theempty states overwhelm, tunneling from the ␲ * surface
state. The spatial distribution of the other states is localized
between the Si-Si dimer bond in the filled states and between
the dimer rows in the empty states, respectively. The new
STM images and interpretation presented here address thetwo problems mentioned in the Introduction. Moreover,many experimental results—the widths of the ␲ and ␲ *bands, surface band gap obtained from I-V curves, and thevoltage dependence of the STM images—come into solid
coincidence within the familiar notion of flip-flopping buckled dimers.
B. STM results at low temperature „80 K…

In order to reinforce our assertions, an extensive set of
STM observations was carried out at a low temperature 共80
K兲 where the flip-flop motion of dimers is frozen. We showthat the low-temperature STM images of the dimers unanimously coincide with the complementary room-temperature
results further supporting our interpretation.
First we provide results of the observation of the filledstates. Figure 3 shows some typical filled-state STM imagesof the dimers with different biases ranging from ⫺0.8 to⫺2.0 V to show the dependence of the STM images on surface bias. STM images taken at low bias 共⬃⫺1.0 V兲 show
clear zigzag chains forming a c(4⫻2) phase,3 which weinterpret to reflect tunneling from the ␲ surface state localized at the upper atom of the dimer. The upper atom of the
dimers and the dimer rows are aligned in an antiferromagnetic order providing the observed c(4⫻2) phase. Based on
this understanding, the locations of the upper atoms of the
dimers are assigned as black circles in the STM image taken
at ⫺0.8 V. As the bias is increased, the c(4⫻2) zigzag
component fades and instead a (2⫻1) component emerges
and grows in intensity as shown in the STM images taken at

FIG. 4. Average cross section of the filled-state STM images
taken at 共a兲 a high bias 共⫺2.0 V兲 and 共b兲 a low bias 共⫺0.8 V兲. 共c兲
Cross section of the electronic structure of the filled ␲ surface state.
Two types of minima indicated by A and B in the cross sections
exist aligned alternatively. HD is defined as the height difference
between A and B, and Corr as the average corrugation of the dimers.
共d兲 Dependence of HD/Corr on surface bias. Also HD vs surface
bias is displayed in the inset.

high biases above ⫺1.5 V. At ⫺2 V, the STM image resembles a c(4⫻2) zigzag component overlapped with a(2⫻1) component. In order to investigate this fading effectmore quantitatively, average cross sections of the dimers in
the STM images taken at high 共⫺2.0 V兲 and low 共⫺0.8 V兲
biases are shown in Figs. 4共a兲 and 4共b兲, respectively. Cross
sections are taken at the locations displayed as dashed boxes
in Fig. 3. In addition, we display the cross section of theelectronic structure of the ␲ surface state calculated by firstprinciple methods in Fig. 4共c兲 for comparison. We registeredFig. 4共c兲 against Figs. 4共a兲 and 4共b兲 by attributing the global
minimum of the cross sections to the middle of the dimerrows where the lower atoms of the dimers are at the adjacent.
By comparing Figs. 4共b兲 and 4共c兲, we can understand that
STM is not observing the atoms of the dimers and the corrugation in the STM images reflects the global corrugation of
the electronic structure with some diminished spatial resolution. From Fig. 4共b兲, it is apparent that there exists two typesof minima aligned alternatively in the cross sections. Minima
A and B defined in Fig. 4 are both located in the middle
between the dimers, though they are different because theupper 共lower兲 atoms are located at the immediate adjacent ofminimum A (B). This difference is due to the ⫻4 periodicity of the reconstruction in this direction and gradually decreases as the bias is increased as shown in Fig. 4共a兲. We
define the height difference between the two minima A and B

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REINTERPRETATION OF THE SCANNING TUNNELING . . .

8167

FIG. 5. An STM image 共1 nA兲 of the empty states at 80 K. 共a兲
The bias was switched from a low 共0.6 V兲 to a high 共1.5 V兲 bias in
the middle.

as HD, and the average corrugation of the dimers as Corr as
shown in Figs. 4共a兲 and 4共b兲. HD divided by the corrugationof the dimers 共Corr兲 is assumed to serve as an indicator of
the strength of the c(4⫻2) zigzag component versus the(2⫻1) component. The dependence of HD/Corr on the surface bias is plotted in Fig. 4共d兲. Also HD versus surface bias
is displayed in the inset of Fig. 4共d兲. HD/Corr remains constant below ⫺1.4 V and gradually decreases with increased
biases. It should be noted that even at biases as high as ⫺2
V, HD/Corr does not drop to zero, thus the remainingc(4⫻2) zigzag component should have been observed andreported as buckled dimers in previous studies.3,14–16 We interpret the experimental results as the following. 共i兲 Tunneling from the ␲ surface state is observed at low biases near
the Fermi edge. 共ii兲 A decrease of the c(4⫻2) zigzag component at high bias reflects an opening of a new tunnelingchannel from other states which are mainly localized in between the Si-Si dimer bond and have an almost (2⫻1) periodicity. 共iii兲 As the bias is increased, tunneling from otherstates becomes important and the c(4⫻2) zigzag component
fades. Tunneling from the other states at high biases shouldbe the cause of the protrusion⇔bean-type transitions of theappearance of the dimers observed in the filled states at room
temperature.
Next, we shift to the observations of the empty states at
low temperature. Figure 5 shows an STM image at low temperature 共80 K兲 where the bias was switched from a low 共0.6
V兲 to a high bias 共1.5 V兲 in the intermediate. Again, thephase shift is observed, and areas between the dimer rows
are observed at the high bias. Dimers appear different in the
STM images taken at low and high biases. At the low bias,
STM images show rows of zigzag chains in a c(4⫻2)
phase,3 which reflects tunneling from the ␲ * surface state
localized at the lower atom of the dimer. In contrast, at thehigh bias, a bright row similar to that observed at room temperature is obtained, though a bright and dark (2⫻1) unit

FIG. 6. STM images of the empty states at surface biases ranging from 0.8 to 1.9 V at 80 K. Tunneling current 0.6 nA. Scale5⫻5 nm.

align alternatively along the dimer-row direction. The
brighter units are registered against the c(4⫻2) region
scanned with low bias, from which we attribute the lower
atom of the dimer to a circle as shown in Fig. 5. It shows thatthe brighter unit corresponds to the location of the upperatom of the dimer. This cannot be explained by an extensionof the wave function of the ␲ * state localized at the lower
atom with bias, but means that tunneling from other states
becomes important at high biases. In order to investigate thedetails of the phase shift with bias, we have carried out STM
observations of the dimers on a defect-free c(4⫻2) buckled
dimer domain scanned with different biases ranging from 0.8
to 2.1 V with an increase rate of 0.1 V as shown in Fig. 6.
Also a set of cross sections of the STM images of Fig. 6
along the dimers are displayed in Fig. 7. In the lower sectionof Fig. 7, the cross section of the calculated electronic structure of the ␲ * surface state is displayed for clarity. Registry
was done in the same fashion employed to display Fig. 4共c兲.
The same phase shift presented in Fig. 5 is evident. At low
biases below 1.4 V, two types of minima in the cross sections are clearly observable, which reflects the c(4⫻2) zigzag component, likewise the case of the observations of thefilled states. We interpret that tunneling from the ␲ * surface
state localized at the lower atom of the dimer is observed atthese low biases. As the bias increases and approaches 1.4 V,
the corrugations of the dimers gradually decrease, whichshould reflect an opening of tunneling channels from otherstates. It is a surprise to observe an almost flat STM image at
the transition bias of 1.4 V. As the bias is increased above

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KENJI HATA, SATOSHI YASUDA, AND HIDEMI SHIGEKAWA

PRB 60

FIG. 8. An STM image 共1 nA兲 showing both the filled andempty states at 80 K. The bias was switched from positive 共⫹0.6 V兲to negative 共⫺0.8 V兲 in the intermediate of scanning at the locationof the black line. The stick and ball schematics are assigned to thec(4⫻2) zigzag components of the STM image, which reflect tunneling from the ␲ * and ␲ surface states.

served in the empty and filled states. This is emphasized bymatching a c(4⫻2) phased stick and ball pattern to thec(4⫻2) zigzag component of the STM image. Obviously
the stick and ball patterns observed in the empty and filledstates are out of phase, giving other evidence that the ␲ and␲ * surface states localized at the opposite side of the dimer
are observed in the STM images taken at low biases. FurFIG. 7. Cross section of STM images of Fig. 6 along the dimers.
The inset in the bottom shows the cross section of the electronic
structure of the ␲ * surface state.

1.4 V, a new corrugation emerges which is out of phase withthat observed at low biases. The new corrugation observed athigh biases is characterized by two types of maxima aligned
alternatively forming a ⫻4 periodicity in contrast to the two
types of minima observed at low biases. Again, the lowtemperature results are fully consistent with the roomtemperature observations.
Subsequently, we register the filled-state STM images
against the empty-state images taken at low biases to emphasize their similarity and to show that band bending does notaffect our interpretation. Figure 8 shows an STM image ofthe dimers at low temperature 共80 K兲 where the bias was
switched from positive to negative in the intermediate ofscanning at the location indicated by the black line. Hence,
the upper half is an empty-state image while the lower half is
an image of the filled state both taken at low biases. Similarity between the dimers in the STM images of the empty
and filled states is striking, which agrees with the protrusiontype image of dimers obtained at room temperature both in
the empty and filled states at low biases. Moreover, what ismore important is that the opposite side of the dimer is ob-

FIG. 9. Spatial distribution of the electronic density of states ofthe buckled dimers within ⫾2 V of the Fermi energy except the ␲and ␲ * bands. 共a兲 Filled states. 共b兲 Empty states. D and U indicate
the lower and upper atoms of the buckled dimer, respectively.

PRB 60

REINTERPRETATION OF THE SCANNING TUNNELING . . .

thermore, we can definitely state that the filled-state STM
images taken at low bias are really observing the filled states,
not the empty states. This is important because it excludes
the possibility that the appearance of protrusion dimers observed at room temperature in the filled states at low biases isdue to observing the empty states at negative surface bias asa result of band bending.
Variation of the tip-surface distance with bias is anotherpossible explanation for the dependence of the STM images
on bias. All of the STM images in Figs. 3 and 6 were taken
at a constant current, thus an increase in bias means an increase in the tunneling barrier height and the tip-surface distance. Generally, an increase in the tip-surface distance results in a decrease of resolution, which one might suspect tobe the cause of the protrustion⇔bean 共room-temperature兲,
zigzag⇔bean transitions 共low temperature兲 of the appearance of the dimers observed in the filled states. However, by
this mechanism the phase shift observed in the empty statescannot be explained. Furthermore, STM images taken at constant barrier height (1⫻109 ⍀) also show an identical transition shown in Figs. 3 and 6 at the same bias in the same
fashion. Strictly speaking, constant barrier height does notmean a constant tip-surface distance. Since the feedback of
STM is regulated to keep the tunneling current constant at aparticular bias, it is difficult to scan the surface with the same
tip-surface distance at different biases. Considering thesefactors, we have compared STM images taken with a large
barrier height 共far from the surface兲 at low biases 共⫺1 V, 100
pA兲 and with a small barrier height 共close to the surface兲 athigh biases 共⫺2 V, 64 nA兲. The STM images were basicallythe same, though somewhat of an increase of the zigzagged
chain component was observed at high bias. We did not observe complete zigzag chained dimers at high biases, thus werule out the variation of tip-surface distance as the maincause of the dependence of STM images on bias.
C. Theoretical calculations

STM observations consistently show that tunneling fromstates other than the ␲ and ␲ * surface states becomes important at high biases. In this section we briefly discuss thepossible states observed at high biases. As the ␲ and ␲ * arethe only localized surface states of the buckled dimers, wehave to consider the more extended surface resonances andthe bulk continuum states. We refer to the calculations carried out by Kruger et al.,17,18 where they have employed aself-scattering theoretical method based on first principles
which is suited to calculate surface resonances extending along range into the bulk. In their calculations, the back bond
(B 1 ) and dimer bond (D 1 ,D *i ) states were shown to lie
close to the ␲ and ␲ * surface states in energy 共look at Fig. 5
in Ref. 18兲. We assign them as the states observed by STM
at high voltages.

*Electronic address: hata@ims.tsukuba.ac.jp,
hidemi@ims.tsukuba.ac.jp
World Wide Web: http://dora.ims.tsukuba.ac.jp
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8169

In order to give a rough idea of the spatial distributions ofthese states, the spatial distribution of overlapped bands
within ⫾2 V of the Fermi energy in the filled and empty
states, except the ␲ and ␲ * bands, was calculated. Standard
formalism of first-principle calculations was employed withthe local-density approximation and plane-wave-based nonlocal pseudopotentials with a cutoff energy of 10 Ry. Calculations were carried out on a c(4⫻2) supercell containing aslab of silicon ten layers thick geometrically optimized by
Ramstad9 with a vacuum region of five layers thickness. It
must be noted that the supercell technique is not suited totreat surface resonances which extend into the bulk. Hence,in this paper, the aim of our calculation is to roughly estimate whether or not the spatial distribution of bands locatednear to the Fermi energy, except the ␲ and ␲ * bands, issimilar to what was observed in the STM images at highbiases. Figure 9 shows the spatial distribution of states near
the Fermi energy except the ␲ and ␲ * bands. A strong intensity in the middle of the Si-Si dimer bond in the filledstates 关Fig. 9共a兲兴 is observed, while they are localized mainly
between the dimer rows in the empty states 关Fig. 9共b兲兴, incoincidence with the STM observations. In order to give amore detailed analysis, we have to calculate the surface resonances with a more proper method or use a very deep cell inwhich the effect of the thickness of the cell becomes negligible. Also we have to simulate the STM images by combining the calculated electronic structures with the tunnelingprobabilities. Matching STM images and theoretical calculations would be an interesting subject for future work.
IV. CONCLUSIONSThis study has two important implications for future STM
studies on Si共100兲. First, careful attention must be paid when
one observes the empty states and attempts to investigate thelocations of absorbates; if the surface is probed at high biases
unintentionally one would make an erroneous attribution.
Also, it is important to use a low surface bias when observing the filled states, particularly when studying absorbates; insome extreme cases we have observed a surface which mimics a perfect surface at ⫺2 V, but shows many defects at ⫺1
V. These defects might influence the sites of adsorption or
the electronic characteristics. If the absorbates only influencethe ␲ and ␲ * states, they might be even invisible at highbiases.
ACKNOWLEDGMENTS

This work was supported by the Shigekawa Project ofTARA, University of Tsukuba. The support of a Grant-inAid for Scientific Research from the Ministry of Education,
Science and Culture of Japan is also acknowledged.

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