Surface Science 447 (2000) 156–164
www.elsevier.nl/locate/susc

Electronic structure of the C defects of Si(001)measured by scanning tunneling spectroscope at roomand low temperature (80 K )K. Hata, S. Ozawa, Y. Sainoo, K. Miyake, 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 May 1998; accepted for publication 12 November 1999

Abstract
We have studied the electronic characteristics of the C defects of Si(001) by scanning tunneling microscopy (STM )and spectroscopy at 80 K and room temperature. At room temperature, the C defects had a metallic feature asreported before. However, at 80 K, the C defects that do not act as phase shifters were found to have a metallic
feature similar to those at room temperature, while the C defects that act as phase shifters have a semiconductive
feature with a band gap of −0.5 V. This indicates that the buckled dimers surrounding the C defects influence andchange the electronic structure of the C defects at low temperature. When compared with the buckled dimers, thesemiconductive C defects have a state with strong intensity located 0.5 V above the Fermi level and the metallic C
defects have a prominent state just above the Fermi level. These states are the origin of the appearance of the C
defects as bright protrusions in the empty state STM images. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopies; Silicon; Surface defects; Surface electronic phenomena(work function, surface potential, surface states, etc.)

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 Si atoms, but also showed that there
exists a rather large number of atomic-scale defects
on the surface. Subsequent, scanning tunnelingmicroscopy (STM ) observations of clean Si(001)* Corresponding author. Institute of Applied Physics,
University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305, Japan.
Tel.: +81 298 536469; fax: 81 298 557440;
http: //dora.ims.tsukuba.ac.jp.
E-mail address: hidemi@ims.tsukuba.ac.jp (H. Shigekawa)surface also showed certain amounts of defects.
Hamers classified these defects into three types, A,
B, and C defects [2].
Among these, the C defects are the most interesting [2–8]. Whereas the A and B defects are surface
dimer vacancies called missing dimer defects, theC defects appears as two protrusions along thedimer row direction on one side of the dimer row
and a depression on the other side. The appearanceof the protrusions is dependent on the surface
voltage: they appear brighter than the surrounding
dimers in the empty state STM images while as
darker in the filled state images taken at usual
surface biases (~−2 V ) [3] as shown in the STM
images and schematically in Fig. 1. Defects which0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0 0 39 - 6 0 28 ( 99 ) 0 11 6 6 -8

K. Hata et al. / Surface Science 447 (2000) 156–164

Fig. 1. (a) Filled state STM image (V=−1.5 V, I=1 nA) of theC defect. (b) Corresponding empty state STM image (V=+0.6 V, I=1 nA). (c) Stick and ball structure model of theC defect.

show these characteristics in the STM images aredefined as the C defects in this study. Other
reported characteristics of the C defects are as
follows. Tunneling I–V measurements on the C
defects show a metallic feature [2,4,5]. Regarding
the rather high concentration of C defects, they
are considered to be the prominent cause of Fermi
level pinning [2]. Also, the C defects are frequently
observed to influence the adsorption sites; forexample, the C defects are sensitive to oxidation[4,6 ]. The C defects are reported to show a complicated structural transformation which differs atroom temperature [3,7] and low temperature(80 K ) [8]. These features make it clear that theC defects play a leading role in characterizing thesurface [2].

157

The configuration of the surrounding dimers
is another factor which must be considered.
Nowadays, it is generally accepted that STM
images at room temperature predominantly show
symmetric dimers because STM is imaging thetime average of the flip-flop motions of the dimers.
Indeed, at low temperatures, the flip-flop motion
is frozen and the dimers appear as buckled in the
STM images [9–11]. Whether dimers are buckled
or not is a factor which has to be consideredseriously. In a previous paper, we have shown that
A and B defects can be, at least, divided into three
sub-categories at low temperatures depending on
the fashion of buckling of dimers adjacent to thedefects [12]. Turning to the C defects, they seemto have a contradictory influence on the surrounding dimers at room and low temperatures. At room
temperature, dimers adjacent to the C defects areobserved as buckled [1,2]. Buckled dimers are
induced by the C defects at room temperature. In
contrast, at low temperature (140 K ), dimers at
one side of the C defects appear as buckled while
those at the other side as symmetric [11]. This isdue to an inherent nature of the C defect, i.e., aphase shifter [11]. Schematically the C defect canbe considered to be two adjacent dimers buckled
in the same direction (ferromagnetic ordering),
hence disorganizing the antiferromagnetic orderingof the buckled dimers at low temperature. In thissense, it would be interesting to study and compare
the properties of the C defects at room and low
temperatures.
In this paper, we present details of the electronic
structure of the C defects measured by scanningtunneling spectroscopy (STS ) at room and low
temperatures (80 K ) where dimers are observed asbuckled. At room temperature, all of the C defects
measured showed a metallic behavior as reported
before. In contrast, at low temperature (80 K ), we
found C defects showing a semiconductive behavior as well as the metallic C defects. The orderingof the surrounding buckled dimers seems to bevery important, and characterizes the electronic
structure of the C defect. In addition, we measured
the spatial distribution of the electronic structuresof the semiconductive C defects at low temperature, showing that the electronic structure of theC defects possesses a plane symmetry perpendicular to the dimer row direction.

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K. Hata et al. / Surface Science 447 (2000) 156–164

2. Experimental
Si samples were phosphorus doped with a conductivity of 0.005 V cm. After the sample had been
prebaked at ~700°C for one night, it was flashedonce to 1250°C for 30 s, followed by annealing at
700°C for another 30 s. The base pressure was
kept under 5×10−8 Pa during flashing. Electrochemically etched tungsten tips were used for the
STM and STS measurements.

3. Results
Tunneling spectroscopic measurements at low
temperature (80 K ) show that the C defects havedifferent electronic characteristics at low and room
temperatures.
First we present results obtained at room temperature for comparison. Before measuring thetunneling I–V characteristics, identification of theC defects was done by scanning the target defectwith both positive and negative bias voltage, andconfirming that the target defect shows the characteristics of the C defect. The full line in Fig. 2ashows a typical tunneling I–V spectrum of the C
defects at room temperature. Tunneling I–V
spectra of the Si(001) dimers were taken with thesame tip and are displayed as the dashed lines.
They were taken to confirm that the measurements
are not influenced by the electronic structure of
the tips as mentioned in details in the following.
The Fermi level is at the edge of the conduction
band, because the sample is n-type. As reported
before, the C defects at room temperature always
show a metallic I–V characteristic [2,4,5] while thedimers show a semiconductive behavior.
In contrast, at low temperature, the situation
becomes different. As mentioned in Section 1, an
isolated C defect must act as a phase shifter in acomplete set of buckled dimers, an example shownin Fig. 3a. However, STM observations carried out
at low temperature show that not all of the C
defects act as phase shifters, an example being
shown in Fig. 5. (It should be noted that STM
images shown in this article are those of the empty
states.) In these cases, usually, there exists a peculiar
structure adjacent to the C defects which absorbs

Fig. 2. (a, b) An I–V curve and STS spectra of the C defects atroom temperature. (c, d) An I–V curve and STS spectra of theC defects at 80 K that are not phase shifters. (e, f ) An I–V
curve and STS spectra of the C defects at 80 K that are phaseshifters. The dashed lines in each figure show the I–V curves
and STS spectra of the dimers taken with the same tip apex
with which the corresponding C defect was measured.

the effect of the C defect to act as a phase shifter
as shown in Fig. 5. We found that the two types ofC defects have very different electronic structures.
Fig 2c and e shows typical tunneling I–V spectra of
the C defects that are not and are phase shifters at
low temperature, respectively. Tunneling I–V
spectra of the Si(001) dimers were taken with thesame tip and are displayed as the dotted lines. It iseasy to see that in each case the dimers show asimilar semiconductive behavior with a band gapof ~0.5 V. The C defects that are not phase shifters

K. Hata et al. / Surface Science 447 (2000) 156–164

159

Fig. 3. (a) An STM image of a C defect on which the spectroscopic measurements were performed. Tunneling voltage 1 V, current
1 nA. The inset shows the stick and ball atomic structure of the dimers and the C defect. Note that in the inset the lower atom of
the buckled dimer is displayed as a circle because the surface bias of the STM image is positive, and hence the lower atom of thebuckled dimer is observed. The black circles in the inset shows the locations where the STS measurements were carried out. (b) A
set of tunneling spectra (dI/dV )/[(I/V ) versus V ] taken at the center of the C defect, curves (1)–(4), and the averaged spectrum,
curve (Average), with an offset applied. Spectrum (4), full line, is overlapped with spectrum (3), dashed line.

show a metallic I–V characteristic similar to the C
defects at room temperature, while those that act
as phase shifters show a semiconductive I–V characteristics similar to the dimers.
The normalized tunneling conductivities (STS
spectra, (dI/dV )/(I/V ) versus V ) are numericallycalculated from the I–V curves as displayed inFig. 2b, d and f. Reliability of the STS measurements was addressed by the following procedure.
By analyzing many independent STS runs, wefound that a given tip will provide reproducible
STS spectra, though a different tip apex mightprovide quite different spectra. In some extreme
cases, even a metallic dimer was observed. It isclear that the STS spectra are influenced by theelectronic structure of the tip as reported previously [13,14]. We also found that the STS spectraof the dimer are rather place insensitive, i.e., every
dimer gives similar STS spectra. Therefore, wemeasured the STS spectra of the dimers with thesame tip apex each time we carried out the STS

measurements on the C defects, and checked thateach STS spectrum of the dimers had a similar
shape to that which is believed to reflect the trueelectronic structure of the dimers, i.e., the STS
spectrum reported by Hamers et al. [15]. Tips
which do not provide this ‘right dimer spectra’
were abandoned. The dotted lines in Fig. 2b, dand f are the measured STS spectra of the dimers
which we have classified as the ‘right dimer
spectra’. The ‘right dimer spectrum’ is characterized by one pronounced peak in the filled state
between −0.7 and −0.8 V with an intensity of 5–8, a bandgap of ~0.5 V, and some peaks in theempty states with an intensity of around 3–4 (notalways clearly resolved). The overall shapes of thethree ‘right dimer spectra’ in Fig. 2 show goodcoincidence, which means that the STS measurements are carried out under the same conditions,
i.e., the tips had a similar electronic structure ineach case. There exist some discrepancies among
the small peaks in the ‘right dimer spectra’ in160

K. Hata et al. / Surface Science 447 (2000) 156–164

Fig. 2 which sets the upper limit of the reliabilityof the STS measurements. In particular, the intensity of the peaks of the empty state strongly
depends on the tips. Therefore, in the following
we will concentrate on and discuss only the overall
trend of the spectra.
The normalization procedure of the tunnelingcurrent seriously emphasizes the tunneling noise(several pA) around the Fermi level, preventing
the observation of the intrinsic electronic structurenear the Fermi level. The shaded regions in Fig. 2band d represent the energy window in which no
reliable STS signals could be obtained because of
the emphasized tunneling noise. We have to investigate the tunneling I–V curves and theun-normalized spectra (the dI/dV versus V curves,
not shown) to determine whether or not any states
are located in this region.
We conclude that the C defects at room temperature and the C defects that do not act as phaseshifters at low temperature have a similar metallic
electronic structure. Their electronic structure ischaracterized by one wide state located above theFermi level spanning from −1 to 1 V with anintensity above ~3 (the exact intensity cannot bemeasured because it is located above the Fermi
level ). This result was confirmed both at room andlow temperature with different tips and defects. The
overall intensity of the STS spectra of the metallic
C defects is lower than that of the dimers in thefilled states. This agrees with the darkening of theC defects in the STM images taken at high voltages.
The metallic C defects have a very different
electronic structure from the dimers. In contrast, C
defects that act as phase shifters at low temperatureshow a different semiconductive electronic structure
which resembles that of the dimers. When comparedwith the dimers, the semiconductive C defects havethe same bandgap, lower intensity in the filled state,
and a higher intensity for the peak located justabove the bandgap (around ~0.5 V ) in the empty
state as shown in Fig. 2f. This result was also
confirmed several times.
We have succeeded in resolving the spatial
electronic structures of the semiconductive C defectby a set of extensive STS measurements. STM
images of the target C defect and its surroundings
is displayed in Figs. 3a and 4b ( Figs. 3a and 4b

are STM images of the same C defect). Locations
where we have carried out spectroscopic measurements are indicated by an arrow. All of the STS
measurements shown in Figs. 3 and 4 were takenwith the same tip apex. The surrounding dimers
were completely buckled. Therefore, the surface
reconstruction is c(4×2) on the right side of theC defect and p(2×2) on the left; the C defect is aphase shifter. Tunneling I–V spectra (not shown)show that the C defect is semiconductive with abandgap similar to that of the dimers.
Fig. 3b shows four different tunneling spectra
taken on top of the C defect, and the average
spectrum (Average). No smoothing is applied tothe spectra. Overall coincidence of the five spectra
is excellent. The locations, height, and shape ofthe main peaks are very similar. The good coincidence among spectra highlights the high reproducibility of the STS measurements within a given tip.
The electronic characteristics of the semiconductive C defect is summarized. In the filled stateregion, the edge of the valance band (around−0.5 V ) is followed by the main peak (occupied
main peak) at −0.6 V. In contrast, in the empty
state, the edge of the conduction band (around
0 V ) is followed by the main peak (unoccupied
main peak) at 0.2 V. The existence of this state is
the origin of the appearance of the C defect as abright protrusion when probing the empty states.
In the higher voltage region (above 0.6 V ) there
exists a set of small peaks which are distributed
somewhat regularly (oscillation structure). The
origin of these regular small peaks will be discussed later.
Fig. 4b shows a set of tunneling spectra taken
at different lateral separations from the C defect,
and tunneling spectra, curves (8) and (9), of the
dimers adjacent to the C defect which belong to
the p(2×2) and c(4×2) phases, respectively. Also
for comparison, we show a tunneling spectrum(Ideal ) of a dimer taken at the middle of a wide
complete c(4×2) domain (ideal surface) with thesame tip. As we move from the center of the C
defect, the tunneling spectrum quickly changes its
shape to that of the ideal surface, and theunoccupied/occupied main peak reduces/increases
its height. (spectra 5–7). Two spectra of the dimers
adjacent to the C defects in the c(4×2) andK. Hata et al. / Surface Science 447 (2000) 156–164

161

Fig. 4. (a) The same STM image of the C defect shown in Fig. 3. The inset shows the stick and ball atomic structure of the dimers
and the C defect. Note that in the inset the lower atom of the buckled dimer is displayed as a circle because the surface bias of the
STM image is positive; hence the lower atom of the buckled dimer is observed. The black circles in the inset shows the locations
where the STS measurements were carried out and the black circle labeled A represents the mean position where the measurementsof Fig. 3 were carried out. (b) Tunneling spectrum of the center of the C defect (Average) with a set of tunneling spectra taken at
different lateral separations from the center of the C defect, curves (5)–(9), and tunneling spectra of dimers adjacent to the C defectwhich belong to the p(2×2) phase (curve (8), dashed line), and c(4×2) phases (curve (9), real line), respectively, with an offset
applied. For comparison a spectrum taken at the dimer located in the middle of a c(4×2) domain is shown (Ideal ).

p(2×2) reconstructions (spectra 8 and 9) aresimilar, although they are different from that of
the ideal surface (a smaller peak at 0.7 V ). Thus,
the C defects influence the electronic structure of
the adjacent dimers. Another feature of the C
defects is that the electronic structure is highly
symmetric regarding the plane parallel to thedimers (the mirror plane is display in Fig. 4a).
This is highlighted by the similarity of two spectra,
curves (6) and (7), taken on the right and leftedges of the C defect. Regarding this high symmetry in the electronic structure, we claim that thereal atomic structure of the C defect also possesses
this mirror plane.

4. Discussion
In order to understand why some C defects aremetallic while others are semiconductive at low

temperatures, it is necessary to consider how theC defects influence the ordering of the surrounding
buckled dimers. Experiments imply that whether
or not the C defects act as phase shifters is thefactor that determines the electronic characteristics. Isolated C defects will act as phase shifters
when they are placed in a complete set of buckled
dimers. These C defects will be highly stressed by
the surrounding buckled dimers because they disturb the order of the dimers.
In contrast, we have shown that a certain
number of C defects do not act as phase shifters
at low temperatures. A typical STM image of the
non-phase shifter C defect is displayed in Fig. 5a.
It is easy to realize that the C defect is surrounded
by a set of buckled dimers with a complete c(4×2)
ordering; thus this C defect does not disturb theordering of dimers. An inspection of the surface
in the vicinity of the C defect reveals the existenceof a peculiar atomic structure adjacent to the C

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K. Hata et al. / Surface Science 447 (2000) 156–164

Fig. 5. (a) An STM image (V=+0.6 V, I=1 nA) of the C defect which does not act as a phase shifter surrounded by a buckled
dimer domain with complete c(4×2) ordering. (b) Stick and ball atomic structure of the dimers and the C defect shown in the STM
image of (c). The two white atoms represent the C defect, and the gray atom indicated by an arrow indicates the peculiar feature.
(c) A high resolution STM image (V=+0.6 V, I=1 nA) of the C defect and the peculiar structure indicated by an arrow.

defect indicated by the arrow in Fig. 5c. The
peculiar atomic feature looks like a dark atom
shifted a little towards the next dimer row in theempty states as indicated by the arrow in theimage. The stick and ball structure of the C defectwith the peculiar feature is displayed in Fig. 5b.
At this stage, we do not understand the atomic
structure of the peculiar feature. However, as the
combination of the C defect and the peculiarfeature does not act as a phase shifter, the peculiarfeature must represent a defect which acts as aphase shifter (it should be noted that a combination of two phase shifters do not act as a phaseshifter). We assume that the combination of the C

defect and the peculiar feature is a close combination of a C defect and a P defect. P defects are
defects in the ordering of the buckled dimers (twodimers aligned in a ferromagnetic order), and havebeen directly observed at 6 K [16 ]. The P defect isa kind of a domain boundary along a dimer row.
A close combination of the C defect with a P
defect can be understood as a reaction of thesurface to reduce the total area of the p(2×2)
area which is energetically unfavorable comparedwith the c(4×2) phase at this temperature [17–19]. Since the P defect is also a phase shifter, acombination of a C defect and a P defect will notact as a phase shifter because each effect as a

K. Hata et al. / Surface Science 447 (2000) 156–164

phase shifter cancels the other out. What is more
important, the combination of the C and P defects
will not be stressed by the surrounding buckled
dimers because they do not disturb the orderingof the buckled dimers.
In contrast, at room temperature, the buckled
dimers are flip-flopping and not static. If the flipflop motion is such that the correlation length of
the buckled dimers is short-ranged, then the C
defect will not be stressed from the surrounding
dimers. Indeed, at room temperature the dimers
adjacent to the C defects are frequently observed
as buckled: the C defects stress the adjacent dimers
at room temperature. We assume that the C defectis not stressed by the dimers at room temperature
and appears as metallic, which we assume to be
the natural characteristics of the C defect.
The experimental results might be explained byassuming that the elastic stress field induced by
the surrounding buckled dimers influences andchanges the electronic structure of the C defects
at low temperatures. We assume that, when the C
defects do not disturb the ordering of the surrounding buckled dimers, the C defects are not stressedby them, thus remaining as metallic close to theirnatural characteristics. In contrast, when the C
defects act as phase shifters at low temperature,
we assume that the stress imposed by the surrounding dimers causes the state at the Fermi level ofthe metallic C defects to split and result in semiconductive C defects. This means that not only do thedefects influence the ordering of the buckled dimers[11,12], but also the electronic (possibly theatomic) structure of the defects is influenced by
the ordering of the surrounding buckled dimers.
The C defect is not stable when compared withthe dimers [20,21]. Frequently STS measurements
would result in a destruction of the C defect. Mostof these STS spectra resemble an accumulation of
the spectra of the C defect and the dimer withdifferent weighting, overlapped with a regularoscillation similar to that observed in Figs. 3 and4. We never observed such a perfect set of equally
spaced peaks in the STS spectra (Fig. 3b) of the
dimers. Therefore, we maintain that the regularoscillations are not due to artifacts such as mechanical oscillation of the tip, but instead that they are
due to an oscillation of the uppermost atoms of163

the C defect induced by the STS measurements.
In the extreme cases, this perturbation results indestruction of the C defect.
Finally, we will discuss implications of theresults obtained here for the structure models
proposed so far. Uda et al. [22] have proposed astructural model of the C defect based on anassumption that the C defects originate from a
removal of one atom in the second layer. The
calculated local density of states (LDOS ) of theirstructure model ( Fig. 3 in Ref. [22]) predicts a
semiconductive C defect. However, in their calculation the two types of the C defects (phase shifteror not) were not discriminated and further studies
are required to check whether the observed semiconductive–metallic transition occurs when theordering of the surrounding buckled dimers is
considered.

5. Conclusion
In conclusion, we have carried out a detailed
STS study to elucidate the characteristics of C
defects at low temperatures where most of the flipflop motions of the dimers were frozen out. In
addition to the well-known fact that the C defects
influence the ordering of the buckled dimers, wefound that the electronic structure of the C defects
is also strongly influenced by the surrounding
buckled dimers. When the C defects are notstressed they show a metallic behavior which weinterpret as reflecting the natural characteristics of
the C defects. In contrast, when the C defects
disturb the ordering of the surrounding buckled
dimers they show semiconductive behavior.

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.

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