Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 3811–3814
Part 1, No. 6B, June 2000
c°2000
The Japan Society of Applied Physics

Origin, Cause, and Electronic Structure of the Symmetric Dimers of Si(100) at 80 K
Kenji H ATA, Tomohiko K IMURA, Osamu TAKEUCHI and Hidemi S HIGEKAWA ∗Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,Tennodai 1-1-1, Tsukuba 305-8573, Japan(Received February 9, 2000; accepted for publication March 14, 2000)

The characteristics of the apparent symmetric dimers observed in the scanning tunneling microscope (STM) images at low
temperatures (20K–200K), far below the symmetric ⇔ buckled phase transition temperature, were investigated by utilizing
the technique applied to fabricate an almost defect free Si(100) surface, the art of atomic manipulation, and current imaging
tunneling spectroscopy. We show that the symmetric dimers are observed at metastable regions caused by the surrounding
defects, and they appear symmetric as a result of flip-flop motions of buckled dimers.
KEYWORDS: scanning tunneling microscope, scanning tunneling spectroscope, Si(100), dimer, defect

1. Introduction
Even though the Si(100)dimer is one of the most simple
surface reconstructions, it has turned out to be a source ofever lasting controversy. A great deal of research has been
devoted to elucidate its atomic configuration, optical properties, and electronic structure. In particular, scanning tunneling microscope (STM) have considerably contributed to enlighten our understandings.1–17) At room temperature, mostof the dimers of Si(100) appear in a symmetric configuration in the STM images.1) The observed apparent symmetricdimers in the STM images were first considered to support
the concept of symmetric dimers. However, many experimental2) and theoretical3) studies revealed the buckled dimers to
be more stable than the symmetric dimers, and at present, it isbelieved that the buckled configuration is the ground state ofthe dimers. Indeed, below 200 K, Wolkow showed that mostof the dimers are observed in a buckled configuration in theSTM images.2) In order to understand the apparent symmetric dimers observed in the STM images at room temperature
within the framework of buckled dimers, the concept of flipflop motion was introduced. It is implied that the buckled
dimers flip-flop much faster than the scanning rate of STM
at room temperature, providing apparent symmetric dimers in
the STM images, while the flip-flop motion is frozen at low
temperatures (below 200 K), hence the dimers appear buckled.
However, even at temperatures (e.g., 80 K) far below thephase transition temperature of the symmetric ⇔ buckled
dimers (200 K), not all of the dimers are observed in a buckled configuration. STM images published so far (in the rangeof 20–200 K) always show a certain ratio of dimers appearing in a symmetric configuration. What makes the situation
more complicated is the report of the reappearence of thesymmetric dimers at temperatures below 20 K.4) Concerning
the symmetric dimers observed at low temperatures far below
the symmetric ⇔ buckled phase transition temperature (hereafter, low temperature indicates the range of 20 K–200 K),
there are several issues which remain to be elucidated. (1)Are the symmetric dimers observed at room temperature, thesame as the symmetric dimers observed at low temperatures?
(2) What is the origin of the symmetric dimers observed at
low temperatures; are they intrinsic or induced by external
factors? (2) What is the electronic structure of the symmet∗ E-mail address: hidemi@ims.tsukuba.ac.jp, http://dora.ims.tsukuba.ac.jp

ric dimers, is it different from the electronic structure of thebuckled dimers?
In this article we address these issues by utilizing the technique to fabricate an almost defect free Si(100) surface, theart of atomic manipulation, and current imaging tunnelingspectroscopy (CITS). We show that the symmetric dimers observed at low temperatures are metastable states of the surface
caused by the defects on the surface. Note that we exclude
symmetric dimers observed below 20 K in this article because
they might have a completely different origin.4) We show thatthe effect of a single defect extends over a long range. Scanning tunneling spectroscope (STS) measurements show thatthe electronic structure of the symmetric dimers resembles
the average electronic structure of the upper and lower atomsof the buckled dimer. From these results, we conclude thatthe low temperature symmetric dimers appear as a result offlip-flop motions of buckled dimers, and are identical to thesymmetric dimers observed at room temperature.
2. Experimental
N-type Si samples phosphorus-doped with a conductivityof 0.1 Ä·cm were used. The base pressure of the vacuum
chamber was 3×10−9 Pa. After ultrasonic cleaning for 10 min
in acetone, the Si(001) sample was loaded into the vacuum
chamber and prebaked at ∼600◦ C for 12 h with a vacuum
pressure below −1.0 × 10−7 Pa. After prebaking, the sample
was once flashed to 1200◦ C for 30 s to remove the oxidizedlayers. After the sample and sample holder are completely
cooled, an additional flashing was carried out at 1200◦ C for avery short time (∼5 s) to reduce the density of defects.
3. Results and Discussion
A typical STM image of the clean Si(100) surface at 80 K
is shown in Fig. 1 to demonstrate the coexistence of the symmetric and buckled dimers. In this and the following figures,the symmetric dimers are colored dark. In the middle region,
the dimers appear in a symmetric configuration, while in theupper-left and lower-right regions the dimer rows appear as
zigzagged chains. The zigzagged chains reflect the antiferromagnitic alignment of the buckled dimers, since only theupper or lower atom of the dimer is observed by STM depending on whether the filled or empty state is probed (STMimages shown in this article are those of the empty states).
This ordering provides a surface reconstruction of c(4 × 2).
Frequently, at the boundary of the buckled-symmetric dimer
domains, defects are observed, as shown in a typical example

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K. H ATA et al.

Fig. 1. (a) STM image of the clean Si(100) surface at 80 K showing thecoexistence of the symmetric dimer and buckled dimer domains. The symmetric dimers are colored dark. Vs = +0.6 V, It = 1 nA. (b) A C defect atthe boundary of a symmetric and buckled dimer domain.

in Fig. 1(b). Various STM studies have shown that the defects
on the Si(100) surface significantly influence the configuration of the surrounding dimers.5–14) For example, at 200 K,
Yokoyama and Takayanagi have shown that the A defects5)
serve as growth nuclei of buckled dimer domains. Also,
Tochihara et al. have shown that the C defects act as phaseshifters in a complete set of buckled dimers.6) Frequently, theC defects are observed at the boundaries of buckled and symmetric dimer domains.2, 6) Taking into consideration these results, it might be supposed that the boundaries, shapes, andsize of the symmetric and buckled domains are regulated bythe defects.
In order to investigate the influence of the defects, we observed the distribution of the symmetric and buckled dimer
domains on various surfaces with different defect densities asshown in a set of STM images in Fig. 2. On a surface withhigh defect densities, the surface consists of a random mixtureof symmetric and buckled dimer rows (Fig. 2(a)). As the defect density decreases, symmetric and buckled dimer domains
emerge and grow in size (Figs. 2(b) and 2(c)). On the surface
with a rather low defect density, the surface is covered withlarge symmetric and buckled dimer domains (Fig. 2(d)). This
result demonstrates that the defect density is the main factor
that determines the configuration of dimers at low temperatures. However, the following issues remains to be addressed,“would these symmetric domains be observed on a defect freesurface?”
Recent progress in the fabrication methodology of Si(100)clean surfaces makes it possible to fabricate an almost defectfree Si(100) surface.15) An STM image of a clean Si(100)surface with extremely low defect density (−0.1%) at 80 K
is shown in Figs. 2(e) and 2(f). Almost no symmetricdimers were observed, and the surface is covered with large
buckled dimer domains. These experimental results suggestthat the symmetric dimers observed at low temperatures aremetastable regions induced by the surface defects.
The influence of individual defects (C defects) can be investigated by a unique method.16) When the STM tip is positioned above a pre-selected C defect and a controlled electricfield is applied by a regulated ramped bias, the C defect canbe demolished. In many cases, accompanying the destructionof the C defect, an avalanche type dynamical symmetric ⇔buckled transformation of the configuration of the surround-Fig. 2. (a)-(d) A set of STM images showing the influence of defects on
the configuration of the surrounding dimers. The defect density decreases
from (a) to (d). Vs = +1.0 V, It = 1.0 nA. The symmetric dimers are colored dark. (e), (f) STM images of a clean Si(100) surface with extremely
low defect density. Temperature = 80 K. Vs = +0.6 V, It = 1.0 nA.

ing dimers was observed. Two examples are shown in theset of STM images in Fig. 3. For the case displayed in
Figs. 3(1-a) to 3(1-d), we destroyed the four C defects enclosed by the white circles one after another in the order indicated by the figures written next to the circles. Accompanying each destruction, a symmetric ⇔ buckled transformationof the configuration of the surrounding dimers was observed.
For example, accompanying the destruction of the C defectnumber 1, the symmetric dimer domain observed in the initialstate in Fig. 3(1-a), diminished in size and changed its shape
as shown in Fig. 3(1-b). An illustrative symmetric ⇔ buckled
transformation was observed after destruction of the C defect numbered 4. In this case, a new symmetric dimer domain emerged after the destruction in the lower-right region
as shown in Fig. 3(1-d), which was initially a single buckled dimer domain as shown in Fig. 3(1-c). It should be notedthat the configuration of the dimers in between the destroyed
C defect number 4 and the newly emerged symmetric dimer
domain did not change but remained in a buckled configursation as can be seen from Figs. 3(1-c) and 3(1-d). This result shows that the influence of a single defect on the configuration of the surrounding dimers extends to a long distance. Figures 3(2-a) and 3(2-b) show another example. In
this case, we destroyed seven C defects enclosed with the cir-Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 6B

Fig. 3. (1-a) to (1-d) Four C defects enclosed by white circles were perturbed and destroyed in an order indicated by the figures. (1-a) The initialstate before destruction. (1-b) After destruction 1. (1-c) Before destruction 2, 3, and 4 (1-d) After destruction 2, 3 and 4. (2-a) Initial state before
destruction. (2-b) After destruction of the seven C defects enclosed by thecircles. Vs = +0.6 V. It = 1.0 nA. Temperature = 80 K.

cles. Fig. 3(2-a) shows the initial state of the surface, andFig. 3(2-b) shows the final state of the surface after all destructions were executed. By comparing the location, size,and shape of the symmetric dimer domains observed in theupper-right region of Figs. 3(2-a) and 3(2-b), it is easy to seethat the influence of the destruction has extended to a coupleof ten nanometers,
The surface morphologies before and after destruction canbe regarded as representing the equilibrium surface configuration with and without the C defect. Therefore, comparing the configurations before and after destruction will give aunique opportunity to study the influence of the C defects on
the configuration of the surrounding dimers. The drastic longrange transformation of the configuration of the surrounding
dimers means that the influence of individual defects extends
to a long range. From these experimental results, we conclude that the low-temperature symmetric dimers emerge inmetastable regions caused by the surrounding defects.13, 14)
As the symmetric dimers are caused by external factors,
they might have a different structure from those observed atroom temperature. In fact, a real symmetric dimer is predicted
to have a metallic characteristic3) while the buckled dimers

K. H ATA et al.

3813have a semiconductive feature due to charge transfer from thelower atom to the upper atom. In order to clarify this point,
we carried out a CITS measurement of the symmetric andbuckled dimers to resolve the electronic structure of the symmetric dimers observed at low temperatures, and to compare
it with that of the buckled dimers. The normalized tunnelingconductivities (STS spectra, (d I /d V )/(I / V ) versus V ) are
numerically calculated from the tunneling I–V spectra (notshown) extracted from the CITS data. I–V spectra show thatthe buckled and symmetric dimers both have a similar semiconductive feature with a surface band gap of ∼0.5 V, similar
to that of the symmetric dimers at room temperature. TheFermi level is at the edge of the conduction band because thesample is n-type doped. Figure 4 shows the STS spectra of
the upper and lower atom of the buckled dimer with the STS
spectra taken on both sides of a symmetric dimer. The shaded
region represents the energy window of the surface band gap
where the tunneling noise is emphasized. Immediately it isclear that the buckled dimers and the symmetric dimers havevery similar electronic structures. The similarity between theSTS spectrum of the buckled dimer at low temperature andthat of the dimers obtained at room temperature by Hamers.
et al,1) ensures that obtained STS spectra are not devaluedby any possible electronic structure of the tunneling tip. The
main peaks in the filled and empty states are attributed to theπ and π ∗ surface states of the dangling bonds of the buckled
dimers, respectively. The upper atom of the buckled dimer
has a stronger intensity at the π surface state than the loweratom, while the lower atom has a stronger intensity at the π ∗surface state than the upper atom. This result confirms thecharge transfer from the lower atom to the upper atom (chargetransfer from the π ∗ to π surface state) that was predicted bytheoretical calculations and the general law of chemical bonding.
As mentioned above, theoretical calculations predict a real
symmetric dimer to have a metallic feature,3) quite different
from the measured semiconductive feature. Hence, we conclude that the apparent symmetric dimers observed by STM
are not truly symmetric but buckled. Generally, the electronic structure of the symmetric dimer can be considered toFig. 4. STS spectra of the lower (dashed line) and upper (dashed-dottedline) atoms of the buckled dimer with two STS spectra (real lines) takenfrom both sides of the symmetric dimer. All of the data was takenwith the same tip apex. The shaded region represents the energy window of the surface band gap where the tunneling noise is emphasized.
Temperature = 80 K.

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be close to the average electronic structure of the upper andlower atoms of a buckled dimer. This result strongly indicatesthat the apparent symmetric dimers observed at low temperatures are also caused by flip-flop motions of buckled dimers,
identical to the case at room temperature.
4. Conclusion
We have investigated the characteristics of the apparent
symmetric dimers observed in STM images at low temperatures (20 K–200 K) far below the symmetric ⇔ buckledphase transition temperature. We conclude that the cause of
the symmetric dimers at low temperatures is the defects on
the surface. Experimental results suggest that the symmetricdimers emerge in metastable regions caused by the surrounding defects. STS measurements show that the electronic structure of the symmetric dimers resembles the average electronic
structure of the upper and lower atoms of the buckled dimer.
This suggests that symmetric dimers observed at low temperatures are not really symmetric dimers but appear to be so as aresult of flip-flop motions of buckled dimers, and are identicalto the symmetric dimers observed at room temperature.
The support of a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports and Culture ofJapan is acknowledged.

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