Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 3837–3840
Part 1, No. 6B, June 1999
c°1999Publication Board, Japanese Journal of Applied Physics

Structure Transformation of the C Defects Observed at Low Temperature (80 K)
Kenji H ATA ∗1 , Ryuji M ORITA1 , Mikio YAMASHITA1 and Hidemi S HIGEKAWA ∗2
Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST),
University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8573, Japan
1 Department of Applied Physics and CREST, Japan Science and Technology Corporation (JST),
Hokkaido University, Sapporo 060-8628, Japan(Received February 19, 1999; accepted for publication March 17, 1999)

The dynamical characteristics of the C defect at low temperature (80 K) were studied by sequential scanning tunnelingmicroscopy observations. We found that the C defect frequently transforms into a another type of defect termed as the CLT
2LTdefect. In addition, the reversal CLT
2 to C defect structural transformation was observed which implies that the C2 defect is ametastable state of the C defect. The observed structural transformation was completely different from that observed at roomtemperature. We interpret that structural transformation associated with a change in the phase of the defect is suppressed at low
temperatures because it disturbs the ordering of the surrounding buckled dimers.
KEYWORDS: scanning tunneling microscopy, scanning tunneling spectroscopies, surface electronic phenomena, silicon, surface defects

Defects are one of the most serious problems of the Si(001)surface. The first scanning tunneling microscopy (STM) observation of a clean Si(001) surface carried out by Tromp etal.1) at room temperature, showed the existence of a rather
large number of atomic-scale defects on the surface. Subsequent, STM observations of clean Si(001) surfaces consistently showed a certain amount of defects, which were classified into three types, termed “A”, “B”, and “C” defects byHamers and Kohler.2)
Among these, the C defects are the most important. Whenthe filled states are probed, the C defects appear to be two adjacent Si atoms missing along the dimer row direction and areobserved as bright protrusions in the empty states.2, 3) Other
reported characteristics of the C defects are as follows. Tunneling I − V measurements on the C defects show a metallicfeature.2, 4–6) Regarding the rather high concentration of C defects, they were supposed to be the prominent cause of Fermi
level pinning.2) Furthermore, it has been reported that the Cdefects influence the site of adsorption. For example, it isreported that the C defects are sensitive to oxidation.4, 7)
The C defect is also known to strongly influence the configuration of the surrounding dimers.8) At room temperature, themain part of the dimers are observed in a symmetrical configuration in the STM images,1) though dimers adjacent to theC defects are frequently observed as buckled.1, 2) At room
temperature, buckled dimers are induced by the C defects.
When the temperature of the sample is lowered below 200 K,
a symmetric⇔buckled phase transition occurs and the majority of dimers are observed in a buckled configuration.9) It is
generally accepted that the flip-flop motions of the dimers arefrozen at low temperatures below 200 K and buckled dimers
are observed in the STM images.9) In this article, low temperature means a temperature below 200 K. Whether the surrounding dimers are buckled or not is a factor which requires
serious consideration. Tochihara et al.8) have shown that the Cdefects influence the configuration of the surrounding dimers
differently at low temperatures. They showed that dimers at
one side of the C defects appear buckled while those at theother side appear symmetrical at low temperatures (140 K).8)
Symmetrical dimers are induced by the C defects at low tem∗1 E-mail address: hata@ims.tsukuba.ac.jp, http://dora.ims.tsukuba.or.jp∗2 E-mail address: hidemi@ims.tsukuba.ac.jp

peratures.
Recently, Zhang et al.3) reported an interesting dynamical
characteristic of the C defects at room temperature. They reported the observation of a frequent structural transformationof the C defect into another 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. Appearance of the C2 defect in the STM image is similar to one of
the individual protrusions of the C defect transferred to theopposite side of the dimer row. Foregoing studies have shownthat the reverse transition, a C2 →C structural transformation
exists, which suggests that the C2 defect is a metastable andexcited state of the C defect.10) It should be noted that thesestructural transformations were observed at room temperature, and at low temperatures the C defects might show different dynamic features as was the case for their static characteristics.6)
In this article, we report the observation of the structuraltransformation of the C defects by STM at low temperature(80 K) which yields new insights to our understanding of theC defects. A completely different structural transformationfrom that reported at room temperature was observed at low
temperature which we interpret is induced by the differencein the configuration of the surrounding dimers at room and at
low temperatures.
Standard procedures were employed to fabricate the cleansurface. Si samples were phosphorus-doped at a conductivityof 0.005 Ä cm. After the sample was prebaked at ∼700◦ C for
one night, it was flashed once to 1250◦ C for 30 s, followed byannealing at 700◦ C for another 30 s. The base pressure waskept below 5×10−8 Pa during the flashing. Electrochemicallyetched tungsten tips were used for the STM measurements.
Through continuous observations of identified C defects
over a long period, we studied the structural transformationof the C defect in detail. A set of subsequent STM images
is displayed in Figure 1, showing the structural transformation of the C defects at 80 K, a temperature far below thecritical temperature of the symmetric⇔buckled phase transition. Figures 1(a) and 1(b) show the filled (Vs = −0.8 V) andempty (Vs = +0.6 V) states of the initial surface, respectively.
Note that the surface bias (Vs = +0.6 V, −0.8 V) is significantly lower than the typical bias (−2 V). The surface bias
is a very important factor which should be treated with care3837

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

Fig. 1. A series of STM images showing the structural transformation of the C defect at 80 K. The sequence of events is(a)→(c)→(e)→(f)→(g)→(h). These images were taken at a surface biasof −0.8 V, thus they are images of the filled states, except (b) and (d) taken
at +0.6 V, and show the empty states of (a) and (d), respectively. The time
displayed indicates the duration in minutes and seconds from the end of
scanning Fig. 1(a) to the end of scanning each image. Two C defects areobserved in the images, as indicated by the arrows. Each circle in the images represents a single structural transformation of the C defects. Scale:
10 nm × 10 nm. Tunneling current=1 nA.

when studying the structural transformation of the C defect at
low temperatures as discussed below. Two C defects are observed in these images, as indicated by the arrows. Since thesurface bias is lower than usual the C defects appear in thefilled states as two protrusions that is slightly brighter than
the surroundings with a depression in the other side, a resultthat is in accordance with the intense local density of statesnear the Fermi energy of the C defects.4–6) After 2 min, thefirst structural transition occurred and both of the C defects
transformed into different structures, which we term as CLT
2defects, as indicated by the circles in Fig. 1(c), an STM imageK. H ATA et al.

of the filled state. Figure 1(d) is the corresponding STM image of the empty state (Vs = +0.6 V). The CLT
2 defect appears
as a single very bright protrusion in the low-biased filled-state
STM image, similar to the appearance of the C defect in theempty states. It should be noted that the appearance of theCLT
2 and C defect in the empty-state STM images are practically indistinguishable, hence the structural transformation
would not be noticed only if the empty states were continuously observed. After 27 min, the second transition was observed, as shown in Fig. 1(e). A reversal procedure of the firsttransition occurred and the CLT
2 defect in the upper region ofFig. 1(c) transformed into a C defect, as indicated by the circle. Observation of the reversal CLT
2 →C defect transition isvery important, since it directly shows that the C→CLT
2 defect transition is not an irreversible and one-way structuraltransformation but instead implies that the CLT
2 defect is ametastable and excited state of the C defect at low temperLT
ature. Both the C→CLT
2 and the reversal C2 →C transitionof defects were observed frequently, as shown in the subsequent STM images where each circle indicates an individual
structural transformation. During the observation, the C defect in the upper region in Fig. 1(e) was again excited into aLTCLT
2 defect (Fig. 1(f)), and the C2 defect in the lower regionin Fig. 1(e) recovered to a C defect (Fig. 1(g)) and was again
excited to a CLT
2 defect (Fig. 1(h)). The results show that theC defect is easily excited into a metastable CLT
2 state. Notedefectobserved
exhibited
thesamefeatures inthat every CLT
2the STM images.
We found that the appearance of the CLT
2 defect in the STM
images has a very interesting and complex dependence on thesurface bias. One striking feature is that the appearance of theCLT
2 defect is not confined to a single dimer row but crosses
over to the adjacent dimer rows in the filled-state STM images at typical surface biases. A series of STM images of asingle CLT
2 defect, taken with different biases at 80 K are displayed in Fig. 2. At −0.8 V (Fig. 2(a)), the CLT
2 defect appears
as a white protrusion, although a close inspection reveals thatthe shape of the protrusion is irregular when compared to thewhite protrusion observed in the empty state (Fig. 2(e)), andhas a small projection which is indicated by an arrow. This
projection grows larger as the surface bias is increased, as
shown in Fig. 2(b), an image taken at Vs = −1.0 V. Here, thesize of the original protrusion in dimer row (2) has decreased
while the projection in the adjacent dimer row (1) has growninto a new protrusion. At Vs = −1.2 V, this feature becomes
clearer, and the new protrusion in dimer row (1) and the original one in dimer row (2) become comparable in intensity andsize. At this stage, the appearance of the defect is not confinedin a single dimer row but crosses over two dimer rows. At
higher voltages the situation becomes more complicated andat Vs = −1.5 V, a surface bias close to the typical value used,
only the new protrusion in dimer row (1) remains, while theoriginal protrusion in dimer row (2) transforms into a structure which resembles a zigzag buckled dimer having a slightlystronger intensity than the surroundings. Note that the protrusion observed in the empty states is always located in dimerrow (2). Such a complicated dependence of the appearance
on the surface bias has rarely been reported and implies thatthe electronic structure of the CLT
2 defect is very complicated.
The observed structural transformation at a low temperature is similar to that observed at room temperature in theJpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6BFig. 2. A series of STM images showing the peculiar voltage dependenceof the appearance of the CLT
2 defect at 80 K. The surface bias is displayedin the upper-right region of each image. Note that (a) to (d) are images ofthe filled state while (e) is an image of the empty state. Dimer rows (1) to(3) are numbered. Tunneling current=1 nA.

sense that the C defect is excited into a particular structure(CLT
2 and C2 defect) which can also reverse the C defect. However, the appearance of the excited structures is completelydifferent. At room temperature, the C2 defect is similar to a
zigzagged buckled dimer which has a slightly stronger intensity than its surroundings as shown in Fig. 3(b), and shows nobias dependence. For comparison, we have presented an example of the C→C2 and the reversal C2 →C defect transitionin Fig. 3. In the following, we present a possible explanation
for the difference in the excited states observed at room andlow temperatures.
Even though the practical atomic structure of the C defectis not known, schematically it can be considered to represent
two adjacent dimers buckled in the same direction (ferromagnetic ordering), thus disorganizing the antiferromagnetic ordering of the buckled dimers at low temperature. Thus, theC defect acts as a phase shifter in a complete set of buckled
dimers at low temperatures, although it does not at room temperature, because high-ordering does not exist among dimers
at room temperature. Tochihara et al.8) have proposed that
the C defect influences the configuration of the surrounding dimers at low temperatures because it is a phase shifter.
Whether a particular structure acts as a phase shifter or not isa factor which has to be treated with great care at low temperatures although it is not so important at room temperature.

K. H ATA et al.

3839

Fig. 3. A series of STM images showing the structural transformation of
the C defect at room temperature. Tunneling conditions Vs = −1.0 V,
I = 1 nA. The C defect, indicated by an arrow in (a), changes into a
C2 defect in (b) and reverses to the C defect in (c). Schematics in themiddle row show the stick and ball structures of the C and C2 defects. The
black circles represent the C and C2 defects, the gray circles the dimers
which flipped to the opposite configuration during the transition. Notethat the C defect is a phase shifter while the C2 defect is not. The phaseshifter ingredient is carried in and out by a P defect, and the gray circles
show the direction of migration because the dimers will flip to the opposite
configuration when a P defect migrates over them (ref. 11). Accompanying
the structural transformation of the C→C2 defect, a P defect was released
and migrated downwards. On the other hand, accompanying the structuraltransformation of the C2 →C defect, a P defect was absorbed in the defectwhich came from upward as schematically shown in the right-hand sidefigures.

In this sense, it is important to note that the C2 defect is
not a phase shifter. Therefore, accompanying the C→C2 andthe reversal C2 →C defect transition, the C defect switchesfrom a phase defect to a defect which is not a phase shifter,and vice versa. The phase shifter ingredient is carried out/infrom the C/C2 defect by another phase shifter termed as the P
defect. The P defect has been defined as two adjacent buckled
dimers aligned in the same direction, thus it is, by definition, aphase shifter.11) When a P defect migrates along a dimer row,
the buckled direction of the dimers switches to the opposite
configuration. Taking this into consideration, we can deducethe direction of migration of the P defect, as shown in theright-hand schematics in Fig. 3 for the structural transitions
displayed in Fig. 3.
At low temperatures, the birth and migration of a P defectalong the dimer row will cause a dramatic change in the configuration of the surrounding buckled dimers. Indeed, when aC defect is artificially perturbed by the STM tip at low temperatures, its phase shifter ingredient is converted into a P defectand accompanied to its migration, a large scale phase transition of the surrounding buckled dimers is induced.12) Such aLT
dynamic change accompanied with the C→CLT
2 and C2 →C3840

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

defect transition was not observed in our experiments. We
interpret that the observed difference in the structure of theexcited states of the C defect between low and room temperature is caused by the difference in whether or not the excitedstates can easily damp out the phase shifter ingredient into a P
defect to the surrounding dimers. In other words, at low temperature, the C defect cannot easily switch between a phaseand a non-phase defect whereas at room temperature this ispossible.
In conclusion, we have observed the structural transformation of the C defect at low temperature (80 K). Frequently,
the C defect transforms into a different defect, termed as theLT
CLT
2 defect. As well, a reversal C2 →C structural transformation occurs, which implies that the CLT
2 defect is a metastableand excited state of the C defect. Appearance of the CLT
2 defect has a very unique dependence on the surface bias in thefilled-state STM images, although it remains similar to the C
defect in the empty-state images. The difference between thestructural transformation observed at low and room temperature illustrates the close and complex relationship between theC defects and the configuration of the surrounding dimers.
This work was supported by the Shigekawa Project ofK. H ATA et al.

TARA, 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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