Surface Science 441 (1999) 140–148
www.elsevier.nl/locate/susc

Metastable and excited states of the C defects of Si(001)
K. Hata, S. Ozawa, H. Shigekawa *Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba, Tennodai 1-1-1,
Tsukuba 305-8573, Japan
Received 8 January 1999; accepted for publication 30 July 1999

Abstract
Dynamic characteristics of the C defect were studied by consecutive scanning tunneling microscopy observations.
Not only the transformation of the C defect into a C2 defect but also the reverse transition, a C2 to C structuralchange, and a more complicated C to C2–C2 transformation were observed. All of the experimental results suggestthat the C2 defect is a metastable and excited state of the C defect that has a similar electronic structure to the C
defect. Extensive scanning tunneling spectroscope measurements reveals that there exists several (at least two) typesof C defect, which we believe to represent metastable states of the C defect. Our interpretation is that there are two
atoms (probably silicon) in the top layer of the C defect which are weakly bound to the layer beneath, and possibly
take several configurations. Each of the configurations is observed as a metastable state of the C defect. © 1999
Published by Elsevier Science B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopies; Silicon; Surface defects; Surface electronic phenomena

1. Introduction
The first scanning tunneling microscopy (STM )observation of a clean Si(001) surface, carried out
by Tromp et al. [1] at room temperature, not only
gave the first direct evidence of dimerization of
the top-layer silicon atoms, but also showed that
there exists a rather large number of atomic-scale
defects on the surface. Subsequent STM observations of the clean Si(001) surface repeatedly
showed certain amounts of defects. Hamers andKohler classified these defects into three types,
termed ‘A’, ‘B’ and ‘C’ defects [2].
Among these, the C defects are the most important. The C defects appear as two protrusions* Corresponding author. Fax: +81-0298-55-7440;
Internet: http://dora.ims.tsukuba.ac.jp.
E-mail address: hidemi@ims.tsukuba.ac.jp (H. Shigekawa)

along the dimer-row direction on one side of the
dimer row and as a depression on the other side.
The appearance of the protrusions is dependent
on the surface voltage: they appear brighter than
the surrounding dimers in empty-state STM images
and darker in filled-state images taken at typicalsurface biases (ca. −2 V ) [2,3]. Other reported
characteristics of the C defects are as follows.
Tunneling current–voltage (I–V ) measurements on
the C defects show a metallic feature [2,4,5].
Regarding the rather high concentration of the C
defects, they were supposed to be the prominent
cause of Fermi-level pinning [2]. Frequently it isreported that the C defects influence the site ofadsorption. For example, it is reported that the C
defects are sensitive to oxidation [4,6,7]. Thesefeatures make it clear that the C defects play aleading role in characterizing the surface [2], andtherefore it is of critical importance to understandthe characteristics of the C defect.

0039-6028/99/$ – see front matter © 1999 Published by Elsevier Science B.V. All rights reserved.
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K. Hata et al. / Surface Science 441 (1999) 140–148

Some interesting features regarding the dynamic
characteristics of the C defect have been reported
by Zhang et al. [3]. They observed a frequent
structure transformation of the C defect intoanother defect in two sequentially acquired STM
images. The new defect was termed the C2 defectand was considered to be a variation of the C
defect. The appearance of the C2 defect in the
STM image is like one of the individual protrusionsof the C defect being transferred to the opposite
side of the dimer row.
In this paper we report our detailed study ofthe structural transformation of the C defects by
STM and scanning tunneling spectroscope (STS)measurements that yield new insights into our141understanding of the C defects. During the sequential STM observation of a single C defect, weobserved not only the transformation of the C
defect into a C2 defect but also the reverse transition, a C2 to C structural change, and a more
complicated C to C2–C2 transition. Observationsof both the CC2 and the reverse C2C transitions suggest that the C2 defect is a metastableand excited state of the C defect. At each stage,
before and after the transition, we carried out STS
measurements of the defects to resolve theirelectronic structures. We found that the C2 defecthas a very similar electronic structure to the C
defect even though their topographical appearance
is quite different in the STM images, a result which

Fig. 1. A set of STM images showing the structural transformation of a C defect. Insets show stick and ball models of the structureof the C defect (black circles) and the surrounding dimers (white circles). (a) The initial surface. Two C defects are observed. The C
defect indicated by the arrow showed the structural transformation. (b) An STM image taken 36 min later. The C defect has
transformed into a C2 defect. (c) An STM image taken 57 min later. The C2 defect has recovered to a C defect. (d) An STM image
taken 78 min later. A C to C2–C2 structural transformation was observed. Tunneling conditions were surface bias=−1.0 V, current
1 nA. During the long intervals between the images shown, basically the same region was continuously scanned with a fresh rate of
30 s per image.

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K. Hata et al. / Surface Science 441 (1999) 140–148

implies that the C2 defect is a close variation of
the C defect. A simple structure model of the C
defect is proposed based on the observed featuresof the C and C2 defects.

2. Experimental
Standard procedures were employed to fabricate
the clean surface. Silicon samples were phosphorus-doped, with a conductivity of 0.005 V cm.
After the sample had been prebaked at ~700°C
overnight, it was flashed once to 1250°C for 30 s,
followed by annealing at 700°C for another 30 s.
The base pressure was kept below 5×10−8 Paduring flashing. Electrochemically etched tungsten
tips were used for the STM and STS
measurements.

3. Results
3.1. Observation of the dynamic structuraltransition of the C defectBy continuous observations of identified C
defects over a long time we studied the structuraltransformation of the C defect in detail. A set ofsubsequent STM images showing three consecutive
structural transformations of a single C defect,
which occurred in the course of a 1.5 h sequential
observation, is displayed in Fig. 1 as an example.
Fig. 1a shows the initial surface. Two identical C
defects are observed, as indicated by the arrows.
The first structural transformation occurred 36 minlater, the C defect on the right transformed into a
C2 defect as indicated by the arrow in Fig. 1b.
This C to C2 structural transition was accomplished by transformation of the right protrusionof the right C defect to the opposite side of the
dimer, in a fashion similar to that previously
reported by Zhang et al. [3]. Note that the C2
defect appears brighter in the STM image at thissurface bias than the C defect, which can be seen
by comparing the C2 defect with the C defect on
the left.
After 57 min, the second transition wasobserved as shown in Fig. 1c. A reverse procedureof the first transition was observed and the C2
defect transformed into a C defect, a structuraltransition reported here for the first time. Carefulanalysis reveals that the recovered C defect is notexactly the same as the original C defect. This canbe realized from two points: (1) the two protrusions of the original C defect were located on theright side of the dimer, while those of the recovered
C defect are located on the left side; and (2) the
intensity of the recovered C defect in the STM
images is stronger than that of the original C
defect. This can be understood by comparing theintensities of the recovered C defect and the original C defect. The recovered C defect appears
brighter than the surrounding dimers, while theintensity of the original C defect is similar to thatof the dimers. We frequently observed the C to C2transition and the reverse C2 to C transition. As
discussed later, observation of the C2 to C transition is very important to understand the characteristics of the C2 defect.
The final transition observed took place at
78 min, and the most complicated transformation
was observed as shown in Fig. 1d. This transition
occurred when we were carrying out an STS measurement on the recovered C defect; thus webelieved that this transition was artificiallyinduced. Indeed, this type of transition was notobserved again. During the course of the transition, the recovered C defect transformed into two
C2 defects. There is one buckled dimer in between
the two C2 defects, at the location where theprotrusion of the C defect was located. No transition was observed after this.
3.2. Electronic structure of the C and C2 defects
We have measured the tunneling I–V characteristics (as shown in the insets of Fig. 2) of thedefects at each stage before and after the transitions displayed in Fig. 1 to investigate theelectronic characteristics of the defects. Not only
every C defect but also every C2 defect observed
here showed a similar metallic feature. Each time
the tunneling I–V characteristics of the dimers
were measured with the same tip apex and showeda semiconducting feature with a band gap of−0.5 V, a result in accordance with previous

K. Hata et al. / Surface Science 441 (1999) 140–148

results. Tunneling spectra of the dimers were taken
to confirm that the STS measurements were notinfluenced by the electronic structure of the tips,
as mentioned in the following.
In the range from −2 V to +2 V, the normalized tunneling conductivities [STS spectra,(dI/dV )/(I/V ) versus V ] were calculated numerically from the tunneling I–V curves. The corresponding STS spectrum of each structure of the C
defect shown in Fig. 1 is displayed in Fig. 2. Fig. 2ais the spectrum taken on top of the original C
defect (C defect on the right side in Fig. 1a),Fig. 2b is the spectrum of the C2 defect in Fig. 1b,
Fig. 2c the spectrum of the recovered C defect in
Fig. 1c, Fig. 2d is a spectrum of another C defect(not shown) measured with the same tip apex, andthe spectra of the dimers taken before ( Fig. 2e)and after (Fig. 2f ) the measurements of the defects
are given for comparison and to justify the reliability of our STS measurements.
The normalization procedure of the tunnelingcurrent considerably emphasizes the tunnelingnoise (ca. several pA) around the Fermi level,
preventing observation of the intrinsic electronic
structure near the Fermi level. The shaded regionsin Fig. 2 represent the energy window in which no
reliable STS signals could be obtained because of
the emphasized tunneling noise. We have relied on
the tunneling I–V curves shown in the insets ofFig. 2 and the non-normalized spectra (the dI/dV
versus V curves, not shown) to determine whether
the measured target is metallic or semiconducting.
The original C defect showed a metallic
electronic structure in a similar fashion as reported
before. Its electronic structure is characterized by
one wide state located above the Fermi level spanning from −1 V to ~1 V with an intensity above~3 (the exact intensity cannot be estimatedbecause it is located just above the Fermi level ).
This state is the origin of the appearance of the C
defects as bright protrusions in STM images taken
at low biases.
Surprisingly, the electronic structure of the C2
defect is almost the same as that of the C defect.
It shows a metallic feature, and is characterized
by one wide state located above the Fermi level.
When the difference in their appearance in STM
images is considered, this is surprising. The sim-

143

ilarity in the electronic structure suggests that theC2 defect is a variation of the C defect having asimilar atomic structure.
Close inspection of the electronic structures ofthe original C defect and the recovered C defectshows that some differences exist between them inthe filled states. While the original C defect showsa dip in intensity of the states around −1 V, theintensity of the states of the recovered C defectremains approximately flat in the filled states in
the regime of −2 to −0.5 V. This difference isreadily observable in the STM images of Fig. 1
taken at a surface voltage of −1 V (the bias atwhich the difference is most eminent), and therecovered C defect appears brighter than the original C defect, in accordance with the STS measurements. Fig. 2d shows an STS spectrum of another
C defect located 20 nm to the right of the C defects
displayed in Fig. 1. The electronic structure of thisC defect is exactly the same as that of the recovered
C defect, a result which suggests that C defects ofthis type are not peculiar and unique. This observation implies that there exists several (at least two)types of akin C defects having a very similarelectronic structure. The difference in intensity
between the original and recovered C defects might
be related to the fact that the two defects act asdifferent phase defects on the surrounding dimers.
Finally, we address the reliability of the STS
measurements. It is well known that STS measurements are subject to the condition of the tunnelingtip. A given tip apex will provide reproducible STS
spectra, although a different tip apex might provide
quite different spectra [8,9]. Regarding this point,
each time (before and after the transitions) STS
measurements were carried out on the dimers andwe checked that each STS spectrum had a similar
shape to the spectrum reported by Hamers et al.
[10]. Representative spectra of the dimers measured before and after the structural transitions
are displayed in Fig. 2e and f, respectively. Notethat they are almost identical and have a similar
shape to that reported by Hamers et al. [10]showing a semiconducting feature. The spectrumof the dimers obtained by Hamers et al. [10] (andthus ours) is consistent with many experimental
results and is believed to represent the trueelectronic characteristics. By this procedure we can

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K. Hata et al. / Surface Science 441 (1999) 140–148

Fig. 2. STS spectra of the C defects. (a) STS spectrum of the C defect indicated by the arrow in Fig. 1a. (b) STS spectrum of the C2
defect in Fig. 1b. (c) STS spectrum of the recovered C defect in Fig. 1c. (d ) STS spectrum of another C defect not shown in the STM
images, taken during the measurements with the same tip apex. (e) STS spectrum of the dimers taken before the measurements of(a)–(d). (f ) STS spectrum of the dimers taken after the measurements. Insets in the images show the tunneling I–V curve in thevicinity of the Fermi level in the range of −1 V to +1 V. The C defects show a metallic characteristic while the dimers are
semiconducting.

K. Hata et al. / Surface Science 441 (1999) 140–148

safely confirm that all of the STS measurements
are carried out under the same tip conditions andthat the experimental results are not devalued by
the possible electronic structure of the tips.

4. Discussion
All of the experiments can consistently beunderstood if we interpret that the C2 defect is ametastable and excited state of the C defect. Direct
observation of the C2C defect transition is mostimportant, since it excludes the possibility of theCC2 defect transition being an irreversible andone-way structural transformation. Also, several
other observations support this point: (1) when aparticular surface area is scanned by STM for the
first time, usually no or only a small number of
C2 defects is observed in the area; and (2) the
number of C2 defects observed in the scanned area
increases gradually with repeated scanning.
The characteristics of the observed dynamictransition of the C defect provide many important
implications concerning the atomic structure of
the C defect. In this section we discuss this point,
and propose a simple structure model of the C
defect. High-resolution STM images of the C2
defect are displayed in Fig. 3 to show the dependence of the appearance of the C2 defect on surface
bias. Fig. 3a is a filled-state image and Fig. 3b an

145

empty-state image. Since the surface bias is relatively low (−1 V and 0.6 V ) the states near the
Fermi energy is probed, and thus the two protrusions of the C2 defect appear brighter than thesurrounding dimers, in accordance with their metallic feature. Immediately it is apparent from Fig. 3that the two bright protrusion of the C2 defect are
observed exactly at the same location in the sameway in the filled- and empty-state images. Thismeans that the two bright protrusions do not
originate from electronic modulation but are real
topographic protrusions, and we assume each ofthem to be single atoms. Since these protrusion
atoms easily move to the other side of the dimerrow which always appears dark in the STM images,
we conclude that the dark side is a real topographic
vacancy. Considering this discussion we propose a
simple structure model of the top layer of the C
defect. Top layer of the C defect is composed of
two atoms aligned side by side in the dimer row
direction which appear as bright protrusions in
the empty-state STM image, and the other sidewhich always appears dark in the STM images isa real vacancy. This concept is consistent with the
non-contact atomic force microscopy observationsof Si(001) taken by Kitamura et al. [11], where
the C defects were observed as two atoms missing
along the dimer row. Since the top two atoms
easily move to the opposite of the dimer row, we
assume that they are somewhat weakly bound to

Fig. 3. High-resolution STM images of the C2 defects. (a) An STM image of the filled states; surface bias=−1.0 V, tunneling current
1 nA. (b) An STM image of the empty states; surface bias=+0.6 V, tunneling current 1 nA.

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K. Hata et al. / Surface Science 441 (1999) 140–148

Fig. 4. (a) Empty-state STM image of a suface with a high density of C defects. Surface bias=+0.6 V, tunneling current 1 nA. (b)In situ empty-state STM image at approximately 200°C (estimated value from the power supply). The substrate was annealed
by direct current and, as a result, the tunneling gap voltage is larger than the surface bias. Surface bias=+0.6 V, tunneling current
1 nA.

the second layer. Many other experimental observations are consistent with the notation of weakly
bound top-layer atoms: (1) the top atoms areeasily removed by perturbing the C defect with the
STM tip [12]; and (2) vibration of the top two
atoms induced by the tunneling tip was observed[12]. Our interpretation is that a metastable C
defect is observed when these weakly bound atomsof the top layer move and are trapped in otherconfigurations than the ground state. At theextreme case, one of the atoms slides to the opposite side of the dimer row inducing a structuraltransformation and the C defect is observed as a
C2 defect.
Direct evidence of weakly bound top-layer
atoms comes from an in situ observation of asurface with a high density of C defects at hightemperatures. Fig. 4a shows an empty-state STM
image of an Si(001) surface which has a highdensity of C defects. White protrusions observedin this image were confirmed to be C defects by
an STM observation of the filled states (notshown). This surface was annealed at approximately 200°C and an in situ STM observation was
carried out as shown in the empty-state STM
image of Fig. 4b. The C defects have disappeared
and instead islands with monolayer height andmany dark vacancies are observed. The islands areelongated in the direction perpendicular to thesurface dimer rows. A close inspection of theislands shows that they are composed from dimers
with a ×2 periodicity. Islands formed by deposition of silicon on Si(001) show the same feature[13,14] and we assume that the islands observedon the annealed surface are also composed ofsilicon. The density of the dark vacancies observedon the annealed surfaces and that of the C defects
on the initial surface are the same, thus we conclude that the dark vacancies are remnants of theC defects. Regarding this experiment, we interpretthat the element of the two atoms in the top layer
is silicon, and that they are weakly bound to thesecond layer.
The vacancies observed in the in situ STM
image of Fig. 4b might not be real vacancies as
they were not buried at high temperature. We
assume that there is an extrinsic atom at the second
layer beneath the top two silicon atoms which isobserved as the vacancy at high temperature. The
extrinsic atom can be regarded as the origin of theC defect. We reached this conclusion by observing
that the density of the C defects increases with
exposure time in vacuum, as shown in a set of
STM images of a surface taken at different durations after flashing in Fig. 5. Fig. 5a–c are emptystate STM images taken 30 min, 4 h and 8.5 hafter flashing, respectively. Fig. 5d and e show thedependence of the STM image of the surface

K. Hata et al. / Surface Science 441 (1999) 140–148

147

Fig. 5. A set of STM images of a surface taken at different times after flashing. (a), (b), and (c) are empty-state STM images taken
30 min, 4 h and 8.5 h after flashing, respectively. Surface bias=+0.6 V, tunneling current 1 nA. (a) to (c) are not a result of asequential observation, and each image was taken at a different location on the surface. (d) Empty (surface bias=+0.6 V, current
1 nA) and (e) filled (surface bias=−1.5 V, current 1 nA) state STM images of the same location on the surface 8.5 h after flashing.

exposed to vacuum for 8.5 h on surface bias, taken
to check that the protrusions observed in theempty-state STM image of Fig. 5a–c show thecharacteristics of the C defects in the filled-state
images. It is clear that the density of the C defectincreases with time after flashing. Zhang et al.
have observed an occasional generation of a new
C defect during STM imaging, and have suggested
that the C defects could be induced from someremaining stress generated by the preceding flashing treatment [3]. Since we observed that the rateof increase of the C defects with time depends
strongly on the vacuum pressure, where a highbase pressure gives a lower increase rate, we con-clude that the C defects are not induced by thestress but are induced by extrinsic atoms which
exist in typical vacuum conditions. Further support
for the concept of an extrinsic atom beneath the
top layer comes from an observation that perturbing the C defect by the tunneling tip never left aclean and complete silicon surface, although theremnant was almost always a vacancy that was
not able to be removed [13].
In Fig. 6 we summarize the above discussions
and show a rough sketch of the proposed structureof the C defect. An extrinsic atom exists in thesecond layer, and two atoms (most likely to besilicon) are weakly bound to this extrinsic atom.

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K. Hata et al. / Surface Science 441 (1999) 140–148

defect is a metastable and excited state of the C
defect. We propose a rough sketch of the stillunidentified atomic structure of the C defect. Bymeans of this model, it is possible to explain many
experimentally observed features of the C defect.

Acknowledgements
This work was supported by the Shigekawa
Project of TARA, University of Tsukuba. The
support of a Grant-in-Aid for Scientific Researchfrom the Ministry of Education, Science andCulture of Japan is also acknowledged.
Fig. 6. A schematic model of the proposed structure of the C
defect. The two top and black circles represent the top two
atoms that are likely to be silicon and the oval shape represents
the extrinsic atom which lies in the second layer. See text for
details.

By this simple though reasonable structure it ispossible to explain many observed characteristicsof the C defect. Further studies, especially theoretical calculations, are required to elucidate the element of the extrinsic atom and to elucidate thedetails of the atomic structure of the C defect.

5. Conclusion
In conclusion, the dynamic features of the C
defect were studied by consecutive STM/STS
observations. Both the C to C2 transition and thereverse C2 to C transition were observed. STS
measurements revealed that the electronic structureof the C2 defect is almost identical to that of theC defect, even though their topographical structures are quite different. We assume that the C2

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