PHYSICAL REVIEW B 70, 235411 (2004)

Probe effect in scanning tunneling microscopy on Si„001… low-temperature phases
Shoji Yoshida,1 Tomohiko Kimura,1 Osamu Takeuchi,1 Kenji Hata,1,* Haruhiro Oigawa,1 Toshihiko Nagamura,2Hiroshi Sakama,3 and Hidemi Shigekawa1,†
1Institute of Applied Physics, 21st Century COE, NANO project, CREST, University of Tsukuba, Tsukuba 305-8573, Japan
2Unisoku Co. Ltd., Hirakata, Osaka 573-0131, Japan
3Department of Physics, Sophia University, Tokyo 102-8554, Japan(Received 18 February 2004; published 9 December 2004)
The probe effect, the effect of parameters in scanning tunneling microscopy (STM) measurement, on the
Si共100兲 surface with two competing phases in delicate balance, was investigated systematically by reexamining
its influence on the Si共100兲 dimer flip-flop motions at 5 and 80 K. On the basis of the results, the complex
array of the phenomena of the Si共100兲 surface structures was comprehended. The phase transition betweenc共4 ⫻ 2兲 and p共2 ⫻ 2兲 structures below ⬃40 K was studied by STM, as well as by low-energy electron diffraction, and the appearance of the p共2 ⫻ 2兲 structure at a reduced probe effect was confirmed. In these investigations, a phase with long-range ordering of the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 structures was observed.
DOI: 10.1103/PhysRevB.70.235411

PACS number(s): 68.35.Rh, 61.14.Hg, 68.35.Bs, 68.37.Ef

I. INTRODUCTION

All experimental measurements inevitably influence theresults obtained. Scanning tunneling microscopy (STM) is avery powerful method for the analyses of physical andchemical phenomena because of its high potential to probe
the local electronic structures of materials. However, as has
been pointed out, STM parameters, such as bias voltage, tunneling current, and tip-sample distance, influence the sample
characteristics.1–8 For example, as was recently clarified in
the photovoltage measurement, tunneling of a small amountof the photoexcited carriers influences the local band bending and results in the scattering of the results depending on
the measurement conditions.1,2 Inelastic-tunneling spectroscopy is one of the promising techniques included in STM,
and when the bias voltage is adjusted to the selected vibrational mode, even chemical reactions can be induced at lessthan a few volts.3,4 This effect may critically influence theresults obtained.
These effects have generally not been taken into account,when the surface structures of semiconductors and metals are
the target, because of their stable structures under ordinary
experimental measurement conditions. However, recently,
STM analysis of the Si共100兲 surface has been recognized as
one of the cases in which the effect of the experimental processes is apparent.5–8 Namely, despite its great importance,
the ground state of the Si共100兲 surface is still one of the mostcontroversial issues in surface science. Since the Si共100兲 surface has two competing phases in delicate balance, the perturbations by STM measurement are considered to play anessential role in the chaotic situation.
On the Si共100兲 surface, pairs of top-layer atoms form
dimers, which gives rise to a 共2 ⫻ 1兲 structure;9,10 however,
the 共2 ⫻ 1兲 phase is not the Si共100兲 ground state, and different reconstructions within similar dimer structures are possible. The energetically favored alternation of dimer bucklingalong the dimer rows leads to the two phases. One is thec共4 ⫻ 2兲 reconstruction, in which alternating buckling angles
are also present between two neighboring dimer rows. An1098-0121/2004/70(23)/235411(11)/$22.50

other possible phase is the p共2 ⫻ 2兲 phase, in which the buckling angle is the same between two neighboring dimer rows.
The difference between the energies of the p共2 ⫻ 2兲 andc共4 ⫻ 2兲 reconstructions obtained by theoretical calculations
is only of the order of 1 meV per dimer,10 which further
complicates the issue.
From low-energy electron diffraction (LEED) measurements, the reversible second-order phase transition between
the disordered 共2 ⫻ 1兲 and ordered c共4 ⫻ 2兲 phases is foundto occur at ⬃200 K.11–13 The most stable phase was concluded to be the c共4 ⫻ 2兲 phase. Through low-temperature
STM studies at ⬃140 K, the c共4 ⫻ 2兲 structure was also confirmed to be the most stable arrangement of the buckled
dimers.14,15 The 共2 ⫻ 1兲 phase observed by STM at room
temperature was attributed to the quick flip-flop motion of
the buckled dimers.16,17 These results seemed to have settled
the controversy; however, in subsequent intensive studies on
the low-temperature phase of Si共100兲 by STM and atomic
force microscopy, c共4 ⫻ 2兲, p共2 ⫻ 2兲 / c共4 ⫻ 2兲 (coexisting),and symmetric dimer phases were observed to exist below
40 K, which gave rise to further controversy regarding the
ground state of Si共100兲.6–8,18–25
Recently, in order to solve this issue, STM measurement
conditions, such as the effect of the external electric field andtunneling current, were reconsidered.5–8,26–29 In addition, thesurface was studied by LEED to reduce the probe effect.31
However, the p共2 ⫻ 2兲, c共4 ⫻ 2兲, and disordered phases withbuckled dimers were identified again for the respective
cases.6,7,30
These results indicate that there still exists a controversy
regarding the ground state, which, however, is due to thedifficulty in comprehending the essential point of a complex
array of the phenomena. For example, the c共4 ⫻ 2兲 phase, thedominantly observed structure by STM at low bias voltage,changed into the p共2 ⫻ 2兲 phase with increasing bias voltage.
Similar structural change, from c共4 ⫻ 2兲 to p共2 ⫻ 2兲, was also
observed upon decreasing the tunneling current with the bias
voltage held constant. With the latter case left unexplained,
the c共4 ⫻ 2兲 phase was concluded to be the ground state, and235411-1©2004 The American Physical Society

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the appearance of the p共2 ⫻ 2兲 phase was considered asa bias-voltage-induced artificial structure.7 However, when
the two results are taken into consideration together, thep共2 ⫻ 2兲 phase appears when the tip-sample distance is increased, namely, under the condition of a reduced probe effect. Even for the LEED measurement, the effect of theprobe electrons has been pointed out.31 Therefore, it is important to analyze the phenomena systematically to gain
comprehensive understanding.
In this paper we present our results of the reexaminationof the STM probe effect, the key issue that has been causing
the current controversy. Systematic study of the Si dimer
flip-flop motion proved the existence of the probe effect in
STM measurement both at 80 and 5 K and the appearance ofthe p共2 ⫻ 2兲 phase under a reduced probe effect condition. In
these investigations, a phase with a long-range ordering of
the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 structures was observed.
II. EXPERIMENT

After prebaking for ⬃5 h at ⬃700 ° C under a vacuum
pressure below 5.0⫻ 10−9 Pa, the Si共100兲 samples (P and As
doped: 0.01– 0.001 ⍀) were heated at 1200 ° C for 10 s. After the samples were cooled to 5 K, an additional flash heating was carried out to reduce the defect density and the effectof the adsorbates. Sample temperature was measured with athermocouple or a Si-diode sensor attached to the sample
holder.
III. RESULTS AND DISCUSSIONSIn order to understand the STM probe effect comprehensively, its influence on the dimer flip-flop motion was analyzed systematically. First, the flip-flop motion induced athigh bias voltage was examined. Next, the probe effect at
low bias voltage was examined by observing the dynamics of
P defects. The details will be described later. Then, the phasetransition between c共4 ⫻ 2兲 and p共2 ⫻ 2兲 structures below⬃40 K was studied by LEED as well as by STM. Thereby,
the appearance of the p共2 ⫻ 2兲 structure at a reduced probe
effect was confirmed well. Through these investigations, aphase with a long-range ordering of the c共4 ⫻ 2兲 and p共2⫻ 2兲 structures was observed.
A. Analysis of the STM probe effect
1. Inelastic tunneling in high-bias-voltage region at 5 K

Figures 1(a)–1(f) show the STM images obtained in theconstant current mode at the same area with six different bias
voltages. Symmetric and buckled dimers were imaged athigh [(a) and (d)] and lower [(c) and (f)] bias voltages, respectively. These characteristics are in good agreement withthose reported in other papers.6–8 However, the noteworthy
point is that, in the middle-bias range (−0.95 V and 1.45 V),
the image of the dimers is rather noisy compared to those at
higher bias voltages. The difference is clearer in the line
profiles shown in Fig. 1(g), which were taken along thewhite lines in Figs. 1(a) and 1(b).

FIG. 1. (a)–(f) Topographic images obtained at −150, −0.95,−0.50, +1.30, +1.45, and +1.60 V 共It = 0.1 nA兲. (g) Line profiles
along the lines in (a) and (b) from their current images. (h) Timedependence of the tunneling current obtained at a Si atom of adimer as shown in Fig. 2(a).

To examine this point further, the tunneling current wasmeasured with the STM tip being held above a Si dimer witha weak feedback.32 As shown in Fig. 1(h), the tunneling current changes between two well-defined values, indicating theflip-flop motion of the dimer [Fig. 2(a)], as was observed in
the previous studies.32
These characteristics are similar to the change in thetemperature-dependent dimer images between 80 K androom temperature (RT), as was described in the Introduction.
Namely, the symmetric dimers at high-bias voltage are considered to be caused by the averaged image of the fast flipflopping motion of dimers, instead of the stable symmetricone.
In order to clarify the probe effect on the flip-flop motion,
the bias dependence of the flip-flop rate was observed withthe current held constant at 0.2 nA. For the flip-flop rate
shown in Fig. 2(b) as a function of the bias voltage, we findthresholds (⬃−0.90 V and ⬃ + 1.45 V兲 for the process. On
the other hand, the flip-flop rate increased with increasing
tunneling current, as shown in Fig. 2(c). Here bias voltage
was held at −0.90 V, just above the threshold, to measure thechange clearly. Saturation at high voltage is due to the suppression caused by the tip-sample interaction, which will be
discussed in the following section.
These results clearly indicate the existence of mechanisms
such as inelastic tunneling for the flip-flop motion instead ofa simple bias effect in this region, which results in the observed symmetric dimer structure. As will be discussed laterin Sec. III A 2 (d), this effect was found to appear even at

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FIG. 2. (a) A Schematic of the measurement set up. Flip-floprate as a function of applied bias voltage (b) and tunneling current(c).

80 K. Therefore, first of all, in order to analyze the groundstate, we must observe the surface at the bias voltage below
the threshold values to remove the inelastic-tunneling process.
In the following section, bias dependence of the probe
effect in the low-bias-voltage region will be examined indetail.
2. Low-bias-voltage region(a) Bias-voltage dependence of STM images at 5 K.
Figure 3(a) shows a STM image at 5 K of the change in
the Si共100兲 surface low-temperature phase obtained bychanging the sample bias voltage from –0.45 to +0.45 V
then to +0.60 V during one scan. Magnified images of thesquare areas in Fig. 3(a) are shown in Fig. 3(b)–3(d). Figure3(e) is the schematic of the square area in Fig. 3(c). At−0.45 V, the c共4 ⫻ 2兲 / p共2 ⫻ 2兲 phase was observed, where

FIG. 3. (a) STM images obtained at −0.45, +0.45, and 0.6 V
共It = 0.1 nA兲. (b), (c), and (d) Enlarged images of the square areas
indicated by (b)–(d) in (a). (e) Schematic of the P defects in (c).

bright and less bright regions correspond to the p共2 ⫻ 2兲 andc共4 ⫻ 2兲 structural areas, respectively [Fig. 3(b)]. The details
will be discussed in Sec. III A 2 (c). On the other hand, at+0.45 V, the dominant structure was the c共4 ⫻ 2兲 phase,
which changed into the p共2 ⫻ 2兲 phase with increasing bias
voltage, as clearly seen in the change from Fig. 3(c) to 3(d).
These phenomena are similar to those observed in the previous study.7
As a noteworthy point, at +0.45 V, there are many black
regions which look like missing dimers. These are the P
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that of the C defect [Fig. 3(e)].16,18,33–41 The C defect consists of two adjacent dimers which buckle with the sameorientation. It appears as two protrusions along the dimerrow direction on one side of the dimer row and a depression
on the other side. However, the P defect can migrate along a
dimer row, unlike the C defect, as one of its two dimerschanges orientation. Since the P defect changes the orderingof the buckling orientation of dimers along a dimer row, as
can be seen in Fig. 3(e), the P defect becomes the boundary
between the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 structures, inducing the
mixture of the structures.
As is shown in Fig. 3(a), P defects are introduced when
the bias voltage is changed from −0.45 to +0.45 V. If the P
defects are introduced at random positions, the ratio between
the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 structural area is considered to remain the same. However, the dominant structure is c共4 ⫻ 2兲at the bias voltage of +0.45 V. Namely, the P defects are
introduced in order to change the surface structure fromc共4 ⫻ 2兲 / p共2 ⫻ 2兲 to c共4 ⫻ 2兲. The P-defect array shown in
Figs. 3(c) and 3(e) is observed widely over the surface, resulting in the reduction of the p共2 ⫻ 2兲 area. These results
indicate that the c共4 ⫻ 2兲 structure is more stable than thep共2 ⫻ 2兲 structure under this condition.
It is significant that although the P defects were fixed at
the same places at +0.45 V, they began to fluctuate at+0.60 V with the introduction of the p共2 ⫻ 2兲 phase area.
Namely, movement of the P defects strongly reflects the influence of the probe effect. Therefore, it is important to investigate the influence of the effect on the mobility of the P
defect, which will be discussed in the following section.
(b) Tip-sample interaction studied through dynamics of P
defect for the positive-bias-voltage region at 5 K.
The probe effect at low bias voltage was examined byobserving the dynamics of P defects. Figure 4(a) shows examples of the time-versus-position pseudoimage37 at 5 K ofa P defect obtained at the bias voltages of +0.7 and +0.8 V.
P defects can be identified easily since they appear darker
than normal dimers. Here, the same single dimer row along
the longitudinal axis was scanned repeatedly; the time evolution of the change is shown on the horizontal axis. Since
dimers are alternately buckled in opposite orientations, theperiodicity of 2a0 共a0 = 0.38 nm兲 arises when a P defectmoves one step, which was well confirmed.
The hopping rate as a function of the bias voltage for thetunneling current of 0.5 nA is summarized in Fig. 4(c). The
hopping rate increases with the bias voltage, namely, withincreasing the tip-sample distance.
In order to discriminate the two effects, a similar experiment as above was performed, but with the tunneling current
being changed at constant bias voltages. Tunneling-currentdependent movements of the P defect are schematicallyshown in Fig. 4(d). As summarized in Fig. 4(d) (example of
0.6 V), the hopping rate increases with decreasing tunnelingcurrent, namely, with increasing tip-sample distance.
From these results, it can be concluded that the surface isinfluenced by the tip-sample distance rather than the bias
voltage in this region. The p共2 ⫻ 2兲 phase appears under thecondition of reduced tip-sample interaction.

FIG. 4. (a) Time vs position pseudoimage of a P defect observed at the bias voltages of +0.7 and +0.8 V. (b) Tunnelingcurrent dependence of the time vs position pseudoimage of a P
defect. Lattice unit in the vertical axis is 4a0. Hopping rates of theP defects as a function of the bias voltage (c) and tunneling current(d).

(c) The c共4 ⫻ 2兲 / p共2 ⫻ 2兲 phase observed for the lownegative-bias voltage at 5 K.
Next, let us examine the details at negative-bias voltage.
As is shown in Figs. 3(a) and 3(b), a complex structure consisting of the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases was found with anappropriate choice of the measurement conditions: between−0.9 and −0.4 V. The structure shows a different contrast
between the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases, resulting in theobserved stripe pattern, while they are imaged with almost
the same brightness at positive-bias voltage, for example, as
shown in Fig. 3(d).
In order to clarify the electronic structure of the surface,
we carried out tunneling spectroscopy of the c共4 ⫻ 2兲 andp共2 ⫻ 2兲 phases on the surface. Figures 5(a) and 5(b) show atypical I-V curve and corresponding STS spectra, respectively. The measured range was limited between −0.6 and+0.8 V to prevent the flip-flop motion of dimers due to inelastic tunneling which appears at higher bias voltages. The

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both phases, and be held at the same distance during the STS
measurement.
From the STS spectra, it is clear that the difference in theelectronic structure is dominant in the low-bias region in thenegative-bias side. Figure 5(c) shows a dI/dV image at several bias voltages over the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phase area.
The difference in the electronic structure between the twophases is clear. Namely, the p共2 ⫻ 2兲 area is more conductive. Since the characteristic is identified at low temperature,
the effective contribution of the surface states may be dominant. The observed difference may be caused by the difference in the local band bending due to charge transfer in thesurface layer,42 and may play a role in stabilizing the structures. Theoretical analysis is necessary.43
Although the induced structure is completely different
from that observed for the positive-bias region, the distancedependent probe effect was similar; the p共2 ⫻ 2兲 area tends todecrease with the bias voltage, namely, with decreasing tipsample distance. As shown in Fig. 5(c), when the tip-sample
distance was held at the same point, contrary to the results
shown in Fig. 3, the surface structure did not change even
if the bias voltage was changed. These results clearly indicate that the tip-sample distance plays an essential role in
the observed change of the ratio between the c共4 ⫻ 2兲 andp共2 ⫻ 2兲 phases. Details will be discussed in Sec. III B.
In order to see the stripe structure consisting of thec共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases, we reinvestigated the bias dependence of the surface structure at 80 K, the details of
which will be discussed in the following section.

FIG. 5. (Color) I-V (a) and dI/dV (b) curves obtained over thec共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases. (c) A dI/dV image at −0.6, −0.2, +0.2,
and 0.6 V. (d) Spectra obtained by averaging over the c共4 ⫻ 2兲 andp共2 ⫻ 2兲 phases.

setpoint bias voltage and tunneling current were +0.8 V and
2.5 nA, respectively, with which the c共4 ⫻ 2兲 and p共2 ⫻ 2兲phases are imaged with almost the same height. Thereby, thetip-sample distance can be the same for the measurements on(d) The c共4 ⫻ 2兲 / p共2 ⫻ 2兲 phase observed for the lownegative-bias voltage at 80 K.
To further understand the probe effect in the negativebias-voltage region in more detail, we investigated the bias
dependence of the surface at 80 K. Until now, only the c共4⫻ 2兲 phase has been observed at 80 K for both n- and p-typesamples, and the c共4 ⫻ 2兲 phase is considered to be the stable
one at this temperature. In fact, in our sample, we also observed the c共4 ⫻ 2兲 phase by LEED measurement.
However, at −0.8 V, we observed the emergence of thep共2 ⫻ 2兲 phase for n-type samples, as shown in Fig. 6(a), in
which a magnified image is also shown. As can be seen in
Fig. 6(a), the p共2 ⫻ 2兲 phase was imaged brighter than thec共4 ⫻ 2兲 phase, which is similar to the result obtained at 5 K.
As shown in Fig. 6(b) of the line profile along the red line in
Fig. 6(a), there seems to be a certain periodicity on the surface for the ordering of the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phase areas.
The noteworthy point is that even when the surface structure
fluctuates, the periodicity remains.
Figure 6(c) shows the magnified image of the rectangulararea in Fig. 6(a). The scan direction is from left to right andbottom to top side. The line profiles obtained at the points
A – D in Fig. 6(c) are shown in Fig. 6(d). At each point, theperiodic structure fluctuates along the scan line. However,the periodicity clearly remains in each line, indicating thatthe stripe structure of the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases are
merely shifted perpendicular to the dimer row with the periodicity being retained. Namely, there is a strong long-range

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FIG. 7. (Color) (a) Two-dimensional autocorrelation obtained
for the double-size topographic image of Fig. 6(a) 共250⫻ 50 nm2兲.
(b) Line profiles of the autocorrelations obtained for several bias
voltages.

FIG. 6. (a) The c共4 ⫻ 2兲 / p共2 ⫻ 2兲 phase observed for thenegative-bias voltage at 80 K. (b) Line profile of A-B in (a). (c)Magnified image of the square area in (a). (d) Line profiles along
the A-D lines in (c).

interaction on the surface.
In order to understand the point further, we analyze
the periodic structure. Figure 7(a) is the two-dimensional autocorrelation obtained by STM of the surface. The STM
image used for the analysis is the double-size topographic
image of Fig. 6(a) 共250⫻ 50 nm2兲. The calculation was
performed usingc共⌬x,⌬y兲 ⬀

1 1
2X 2Y

冕 冕
+X

+Y

−X

−Y

z共x,y兲z共x + ⌬x,y + ⌬y兲dx dy

with 2000 pixels⫻ 400 pixels, where 1 nm= 8 pixels. And
c共0 , 0兲 is located at the center of the image.44
The line profiles of the autocorrelations obtained for several bias voltages are summarized in Fig. 7(b). As can beseen in the profile of −0.9 V, it is clear that there are twocomponents (␭ = 0.76 nm and 4 nm). The shorter one corresponds to the periodicity of the dimer row. The longer one is
the periodicity which appeared in Fig. 6(a) as a long-rangeordering consisting of the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases. A
similar result was obtained at 5 K, and the existence of thesame periodicity was confirmed. A noteworthy point is thatthe structure is very stable, and fluctuation does not occur at
5 K, as was shown in Figs. 1(c), 3, and 5(c).
For low bias voltages, the contrast of the periodicity
decreases, which is due to the fact that the area of thec共4 ⫻ 2兲 phase increases with decreasing tip-sample distance.
An example of the corresponding change in the STM images
is shown in Fig. 8. Figures 8(a) and 8(b) show the changewith bias voltage and Figs. 8(c) and 8(d) show the changewith the tunneling current. These results indicate that even in
the negative-bias-voltage region, the tip-sample distance effect increases the extent of the c共4 ⫻ 2兲 structure on thesurface.
When the bias voltage was increased, the dimers began tobe imaged as symmetric ones, which is similar to those obtained at 5 K. The bias-voltage-dependent STM images areshown in Fig. 9. The c共4 ⫻ 2兲 / p共2 ⫻ 2兲 structure begins to beunstable on increasing the bias voltage, as shown in Fig.
9(c), which is similar to the characteristic observed at 5 K(Fig. 1). The other electronic state between two adjacentdimer rows are imaged at +2.4 V [Fig. 9(e)], as was pointed
out in the previous work.45,46 For the positive-bias voltages,
flip-flop motion has not yet been identified clearly.
A noteworthy point is that the c共4 ⫻ 2兲 structure remains
at −1.0 and +2.4 V, which was pointed out in previous
works,5 which is due to the fact that the c共4 ⫻ 2兲 structure isstable at 80 K. The result is clearly shown in Fig. 9(d), whichis the magnified image of the square area in Fig. 9(a), with a

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FIG. 9. Bias-voltage dependence of the STM images at
80 K.

stability of the c共4 ⫻ 2兲 phase is not a tip effect but atemperature-dependent characteristic. We discuss the point indetail in the following section.

B. Comparison of the phase diagrams at 80 and 5 K

FIG. 8. Bias-voltage [(a) and (b)] and tunneling-current [(c) and(d)] dependences of the c共4 ⫻ 2兲 / p共2 ⫻ 2兲 structure at 80 K.

high contrast. Namely, the c共4 ⫻ 2兲 structure appears under
the condition of reduced tip-sample interaction which depends on the tip-sample distance. Since the probe effect observed in the negative-bias-voltage region is similar to thatobserved at 5 K, while the c共4 ⫻ 2兲 phase remains for the
positive-bias voltage, the c共4 ⫻ 2兲 structure is more stable forthe positive-bias voltages at 80 K. However, the p共2 ⫻ 2兲
structure appears under the condition of a reduced probe effect at 5 K. These results indicate that the change in the

We have seen several probe effects at 5 and 80 K. Fromthe standpoint of the bias voltage and the tip-sample distancedependence, the following occur.
(1) Symmetric dimers appear in the high-bias voltage region.
(2) The stripe structure consisting of the c共4 ⫻ 2兲 / p共2⫻ 2兲 complex phases with the long-range periodicity appears
in the low-negative-bias region at both 5 and 80 K.
(3) The c共4 ⫻ 2兲 phase area tends to increase in the stripe
structure at both 5 and 80 K with decreasing tip-sample distance, namely, under the condition of tip-sample interaction(the characteristic was clearer at 80 K).
(4) When the tip-sample distance is held at the same
point, the surface structure does not change even if the bias
voltage is changed for the STS measurement.
(5) The p共2 ⫻ 2兲 phase appears at the intermediate
positive-bias-voltage region at 5 K.

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FIG. 10. (Color) Phase diagrams obtained at 80 and 5 K, withthe tunneling current of 0.1 nA.

These bias dependences are summarized as phase diagrams for 80 and 5 K, as shown in Fig. 10. The bias voltages
to determine the characteristics of the surface structures maychange slightly, for example, in the order of 0.1 V, depending on the difference in the STM-tip conditions such as theshape of the tip apex used, which, however, does not make

FIG. 12. (Color) Typical LEED patterns obtained at (a) ⬃50 K
and (b) ⬃20 K. (c) Temperature dependence of the LEED spot
intensities for the (1 / 2, 1 / 2)-p共2 ⫻ 2兲 and (3 / 4, 1 / 2)-c共4 ⫻ 2兲 spots.

FIG. 11. (Color) Typical STM images (1.0 V, 0.1 nA) obtainedat (a) 29 K, (b) 43 K, and (c) 50 K. (d) Magnified current image
obtained for the square area in (b). (e) Temperature dependence ofthe ratio of the c共4 ⫻ 2兲, p共2 ⫻ 2兲, and flip-flopping dimer areas.

any essential changes in the understanding of the obtained
characteristics.
As can be seen, the characteristics of the 80 and 5 K
diagrams are similar to each other, in the negative-biasvoltage and +0.4 to +0.5 V regions. At high-bias-voltage regions, the results indicate that the c共4 ⫻ 2兲 phase is stable at
80 K, even with the flip-flop motion.
It was noted in the previous paper that the p共2 ⫻ 2兲 phaseis the tip-induced structure.7 However, it is curious that theprobe effect has a temperature dependence only for positivebias voltage (shaded area in Fig. 10). Therefore, first of all,there exists a temperature effect that changes the relative
stability between the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases, which
determines the surface structure, between 80 and 5 K.
Namely, from the results including those in the past, thec共4 ⫻ 2兲 structure becomes unstable at 5 K. Then at higher
bias voltage, symmetric dimers are induced by such effects
as the inelastic-tunneling mechanism. In the bias-voltage region lower than the threshold of inelastic tunneling, there is abias-voltage effect which induces the stripe structure appearing at negative sample bias voltage. The reason is not clear at
this stage, but can be related to the polarity of the bias voltage. Namely, there also exists a bias-voltage effect, as has
been pointed out recently.29,47,48 Since STS spectra change
between the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 areas, charge transfer may
play a role in the stabilization of the structure.
As was pointed out earlier, we must consider the tipsample interaction in addition to the influence of the bias
voltage. This interaction becomes stronger when the tip-

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sample distance is reduced. In fact, as has been shown, this is
the dominant effect. In order to clarify the mechanism of thiseffect, experiments of, for example, atomic force microscopy
and theoretical calculation are necessary; these we leave for
future work. However, since the c共4 ⫻ 2兲 phase increases
with decreasing tip-sample distance, the previously reported
results mentioned in the Introduction6–8 can be explained
comprehensively. As was pointed out, when the tip-sample
distance was kept constant, the ratio between the c共4 ⫻ 2兲and p共2 ⫻ 2兲 phases did not change even if the bias voltage
was changed (Fig. 5).
As has been shown, the probe parameters such as bias
voltage, tunneling current, and tip-sample distance in STM
measurement were clarified by investigating their influenceon the dimer flip-flop motions of the Si共100兲 surface. Therefore, in order to understand the low-temperature phase transition at temperatures below 50 K, we must observe the surface adopting conditions under which we can reduce theSTM measurement effects, namely, the bias voltage of
0.5– 1.5 V, and a low tunneling current such as 0.1 nA. We
performed analysis taking account of the conditions. For
comparison, we also performed LEED measurements of thesurface with which we can examine the STM probe effect inmore detail.
C. Low-temperature phase transition studied by STM andLEED
1. Observation by STM

Here, let us examine the phase change at low temperature
in detail. Taking the phase diagram in Fig. 10 into consideration, structural change was observed as a function of thetemperature in order to clarify the phase transition below
40 K, under the conditions at a reduced tip-sample interaction (1.0 V, 0.1 nA). Figures 11(a)–11(c) show typical STM
images at 29, 43, and 50 K, respectively, where c共4 ⫻ 2兲(red), p共2 ⫻ 2兲 (blue), and unstable (yellow) dimer areas are
seen. In the current image, unstable dimers were observed as
white dots, which indicates that the dimers are flip flopping,as shown in Fig. 11(d) of the magnified image acquired overthe square area in Fig. 11(b). As shown in Figs. 11(a)–11(c),the structural change is clear. Namely, at 29 K, the main
structure is the p共2 ⫻ 2兲 phase, but the c共4 ⫻ 2兲 structure
emerges and covers almost all of the surface at 50 K, similar
to that at 80 K. A noteworthy point is that around the phasetransition temperature, a disordered phase due to the mixture
and fluctuation of the three phases appears, as shown in Fig.
11(b), indicating the instability of the surface. The percentages of the c共4 ⫻ 2兲, p共2 ⫻ 2兲, and flip-flopping dimer structures were counted over a wide area and are plotted as afunction of temperature in the range of 30– 50 K in Fig.
11(e). Although the tip-sample distance tends to increase
with increasing temperature due to the increase in the conductance, the amount of the c共4 ⫻ 2兲 phase increases, contrary to the characteristic shown in Figs. 3 and 8. The resultclearly indicates the surface characteristic at a reduced probe
effect condition, namely, the existence of the phase transition
caused by the change in the temperature.

2. Observation by LEEDIn order to confirm the observed temperature dependence,the structural change was detected by LEED without theSTM probe effect. LEED patterns were recorded using avideo-rate charge-coupled device camera (30 frames/s), andsequences of 30 frames were integrated to obtain images forthe analysis of the change in the LEED pattern. Figures 12(a)and 12(b) show LEED patterns obtained at ⬃50 and ⬃20 K,
respectively. At 50 K, only the one-fourth-order spot wasobserved, which clearly indicates that the c共4 ⫻ 2兲 phase isdominant at this temperature. On the other hand, at ⬃20 K,
the (3 / 4, 1 / 2)-c共4 ⫻ 2兲 spot intensity decreased and the(1 / 2, 1 / 2)-p共2 ⫻ 2兲 spot intensity became apparent, asshown in the figure. In order to examine the details of thechange, the temperature dependence of both spot intensities
is summarized in Fig. 12(c). The diffuse background was
subtracted from the integrated spot intensity in the analysis.
The p共2 ⫻ 2兲 phase increased below ⬃30 K. The temperature dependence of the phase transition is in good agreementwith the result obtained by STM measurement. The slightly
lower transition temperature and the saturation of the p共2⫻ 2兲 spot intensity below 20 K observed in LEED measurement is probably due to the fact that the sample is notshielded against radiation in the LEED setup. In addition,
since the surface structure becomes unstable at low temperature, the probe electron energy of ⬃50 V for the LEED study
may cause the flip-flop motion, as was pointed out recently,31
and may result in the perturbation of the surface structure
instead of remaining the single p共2 ⫻ 2兲 phase. This is supported by our results shown in Fig. 2. However, the appearance of the p共2 ⫻ 2兲 phase without such perturbations was
confirmed by STM at a reduced probe effect condition asshown in the preceding section.
Since the intensity of the LEED spot of the c共4 ⫻ 2兲 has amaximum at ⬃40 K, which is time independent and reproduced well, the existence of the temperature-dependent stability of the c共4 ⫻ 2兲 phase is clear. The result suggests thechange in the potential of the flip-flop motion below ⬃40 K.
The detail will be discussed in the following section.
3. Discussion about the low-temperature phase transition

The c共4 ⫻ 2兲 and p共2 ⫻ 2兲 structures are dominant at ⬃80
and ⬃10 K, respectively, and they gradually change into thep共2 ⫻ 2兲 and c共4 ⫻ 2兲 phases, respectively, via an unstable
disordered phase with increasing temperature, as shown in
Figs. 11(e) and 12(c). Since a second-order phase transition,
disordered 共2 ⫻ 1兲 to c共4 ⫻ 2兲, occurs at ⬃200 K, some additional mechanism is necessary to explain the appearance ofthe second phase transition at low temperature.
At high temperatures, since the c共4 ⫻ 2兲 phase is morestable than the p共2 ⫻ 2兲 phase, the potential depth of thec共4 ⫻ 2兲 structure is greater than that of the p共2 ⫻ 2兲 structure. Since the c共4 ⫻ 2兲 phase is stable up to ⬃200 K, thebarrier height from the c共4 ⫻ 2兲 to the p共2 ⫻ 2兲 phase ishigher than the thermal energy of this temperature, and thepotential height for the opposite path should be sufficiently
low. Furthermore, at room temperature, the symmetric dimer
structure must appear with the flip-flop motion of the dimers.

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FIG. 13. Schematics of the potential-energy landscape for thec共4 ⫻ 2兲 and p共2 ⫻ 2兲 buckled dimer structures at ⬃80, ⬃45, and⬃10 K. Bars on the right side present the thermal energy at eachtemperature.

Namely, the barrier heights for both paths are sufficiently
small compared to the thermal energy at room temperature.
On the other hand, at ⬃10 K, p共2 ⫻ 2兲 covers almost theentire surface and fewer c共4 ⫻ 2兲 and symmetric dimer structures are observed,49 therefore, the total energy of p共2 ⫻ 2兲 islower than that of c共4 ⫻ 2兲, and the energy barrier heightfrom p共2 ⫻ 2兲 to c共4 ⫻ 2兲 is higher than the thermal energy.
Also, the potential height for the opposite path must be sufficiently low.
On the basis of these analyses, in order to explain thesecond phase transition at lower temperature, it is necessary
to consider a temperature-dependent change in the potentialenergy landscape for the buckled dimer structures. A possible
schematic model of the potential-energy landscapes for the
buckled dimer structures at ⬃80 and ⬃10 K are shown in
Fig. 13.
At the phase-transition temperature, the c共4 ⫻ 2兲 or
p共2 ⫻ 2兲 phase merges with the p共2 ⫻ 2兲 or c共4 ⫻ 2兲 phasearea, respectively, with the introduction of P defects, which
are movable phase defects.16,18,36–41 In addition, there appears to be a high proportion of flip-flopping dimer areas, as
shown in Fig. 11(b). The appearance of the flip-flopping
dimers during the phase transition directly indicates the instability of the buckled dimer structures at the phasetransition temperatures. The existence of the disorderedphase due to mixing of and fluctuation between the c共4⫻ 2兲 and p共2 ⫻ 2兲 phases indicates that the mutual structuralchange is easy and the two phases have almost the sameenergy at the phase-transition temperature, ⬃43 K.
According to the analysis of the unstable structures including the flip-flopping dimers at 40– 45 K, similar to thoseat higher and lower temperatures, the barrier height for the
dimer flip-flop motion may be sufficiently low compared tothe thermal energy at this temperature, as shown in Fig. 13.
Another possible mechanism is that since macroscopic quantum tunneling becomes effective at low temperatures, theobserved instability and fluctuation of the buckled dimer
structures may be induced by the quantum fluctuation be-

tween the two phases.41 In this case, if the potential depths
for the c共4 ⫻ 2兲 and p共2 ⫻ 2兲 phases are similar, mutual tunneling occurs even if the barrier height is higher than thethermal energy. When the potential depth for the p共2 ⫻ 2兲phase becomes greater than that of the c共4 ⫻ 2兲 phase at low
temperature, the transition from c共4 ⫻ 2兲 to p共2 ⫻ 2兲 becomesdominant, which also results in the p共2 ⫻ 2兲 phase.
Since the ratio between c共4 ⫻ 2兲 and p共2 ⫻ 2兲 changes
gradually at the phase-transition temperatures, as shown in
Figs. 11(e) and 12(c), the potential-energy landscape related
to the structures is also expected to undergo a continuouschange. How does this temperature-dependent potentialchange come about? It is well known that the dimer structures are stabilized with a delicate balance between the electronic and elastic interactions of dimers.40 Therefore, a possible explanation may be the relationship of the phenomena
to the change in the strain due to negative thermal expansion,as was previously pointed out.19 Also, since the carrier density changes effectively at low temperature, it may play animportant role in the change in the interaction. In order toclarify this in detail, further experiments on, for example, thedopant effect on the phase-transition temperature, and theoretical calculations including aspects such as the effect of thedopants and the thermal expansion effect are needed.
The effective macroscopic quantum tunneling process at
low temperatures may play a role in the observed instability
and fluctuation of the buckled dimer structures around thephase-transition temperature.30,41 In consideration of the difference between the energies of the p共2 ⫻ 2兲 and c共4 ⫻ 2兲reconstructions obtained by theoretical calculations, 1 meV
per dimer,10 the stability of the c共4 ⫻ 2兲 phase at around
100 K may need to be reconsidered.
IV. CONCLUSION

The effects of STM parameters such as bias voltage, tunneling current, and tip-sample distance in STM measurement
were examined by investigating their influence on the dimer
flip-flop motions of the Si共100兲 surface, which has two competing phases in delicate balance. Referring to the results, thecomplex array of the Si共100兲 surface phenomena was comprehended. The phase transition between c共4 ⫻ 2兲 and p共2⫻ 2兲 structures below ⬃40 K was studied by STM, as well
as by low-energy electron diffraction, and the appearance ofthe p共2 ⫻ 2兲 structure under a reduced probe effect was confirmed. Since the reversible phase transition between the disordered 共2 ⫻ 1兲 and the ordered c共4 ⫻ 2兲 structures at⬃200 K is well known to be a second-order phase transition,
the result suggests that potential energy for Si dimer buckling is temperature dependent.
ACKNOWLEDGMENT

This work was supported by a Grant-in Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.

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53, 4553 (1996).

*Present address: AIST Tsukuba, Tsukuba 305-8565, Japan.

27 K. Cho and J. D. Joannopoulos, Phys. Rev. B†http://dora.ims.tsukuba.ac.jp

28 H. Kawai and Osamu Narikiyo, J. Phys. Soc. Jpn. 73, 417 (2004).

1 O. Takeuchi, S. Yoshida, and H. Shigekawa, Appl. Phys. Lett. 84,

29 K. Seino, W. G. Schmidt, and F. Bechstedt, Phys. Rev. Lett.

3645 (2004).
2
Ch. Sommerhalter, Th. W. Matthes, J. Boneberg, P. Leiderer, and
M. Ch. Lux-Steiner, J. Vac. Sci. Technol. B 15, 1876 (1997).
3 B. C. Stipe, M. A. Rezaei, and W. Ho, Science 279, 1907 (1998).
4
Yousoo Kim, Tadahiro Komeda, and Maki Kawai, Phys. Rev.
Lett. 89, 126104 (2002).
5 T. Mistsui and K. Takayanagi, Phys. Rev. B 62, R16 251 (2000).
6 K. Hata, S. Yoshida, and H. Shigekawa, Phys. Rev. Lett. 89,
286104 (2002).
7 K. Sagisaka, Daisuke Fujita, and Giyuu Kido, Phys. Rev. Lett.
91, 146103 (2003).
8 D. Riedel, M. Lastapis, M. G. Martin, and G. Dujardin, Phys.
Rev. B 69, 121301(R) (2004).
9
R. J. Hamers, R. M. Tromp, and J. E. Demuth, Phys. Rev. B 34,
5343 (1986).
10 A. Ramstad, G. Brocks, and P. J. Kelly, Phys. Rev. B 51, 14 504(1995).
11 T. Tabata, T. Aruga, and Y. Murata, Surf. Sci. 179, L63 (1987).
12 M. Kubota and Y. Murata, Phys. Rev. B 49, 4810 (1994).
13 J. Ihm, D. H. Lee, J. D. Joannopoulos, and J. J. Xiong, Phys. Rev.
Lett. 51, 1872 (1983).
14 R. A. Wolkow, Phys. Rev. Lett. 68, 2636 (1992).
15 H. Tochihara, T. Amakusa, and Masashi Iwatsuki, Phys. Rev. B
50, 12 262 (1994).
16 H. Shigekawa, K. Hata, K. Miyake, M. Ishida, and S. Ozawa,Phys. Rev. B 55, 15 448 (1997).
17 J. E. Northrup, Phys. Rev. B 47, 10 032 (1993).
18
H. Shigekawa, K. Miyake, M. Ishida, K. Hata, H. Oigawa, Y.
Nannichi, R. Yoshizaki, A. Kawazu, T. Abe, T. Ozawa, and T.
Nagamura, Jpn. J. Appl. Phys., Part 2 35, L1081 (1996).
19
T. Yokoyama and K. Takayanagi, Phys. Rev. B 61, R5078 (2000).
20 Y. Kondo, T. Amakusa, M. Iwatsuki, and H. Tokumoto, Surf. Sci.
453, L318 (2000).
21 T. Uozumi, Y. Tomiyoshi, N. Suehira, Y. Sugawara, and S.
Morita, Appl. Surf. Sci. 188, 279 (2002).
22 Y. Yamashita, S. Machida, M. Nagao, S. Yamamoto, Y. Kakefuda,
K. Mukai, and J. Yoshinobu, Jpn. J. Appl. Phys., Part 2 41,
L272 (2002).
23 G. LeLay, A. Cricenti, C. Ottaviani, P. Perfetti, T. Tanikawa, I.
Matsuda, and S. Hasegawa, Phys. Rev. B 66, 153317 (2002).
24 M. Ono, A. Kamoshida, N. Matsuura, E. Ishikawa, T. Eguchi, and
Y. Hasegawa, Phys. Rev. B 67, 201306 (2003).
25 L. Perdigao, D. Deresmes, B. Grandidier, M. Dubois, C. Delerue,
G. Allan, and D. Stievenard, Phys. Rev. Lett. 92, 216101(2004).
26 K. Cho and J. D. Joannopoulos, Phys. Rev. Lett. 71, 1387 (1993).

93,036101 (2004).
30
M. Matsumoto, K. Fukutani, and T. Okano, Phys. Rev. Lett. 90,
106103 (2003).
31 S. Mizuno, T. Shirasawa, Y. Shiraiashi, and H. Tochihara, Phys.
Rev. B 69, 241306(R) (2004).
32
K. Hata, Y. Saino, and H. Shigekawa, Phys. Rev. Lett. 86, 3084
(2001).
33 M. Nishizawa, T. Yasuda, S. Yamasaki, K. Miki, M. Shinohara,
N. Kamakura, Y. Kimura, and Michio Niwano, Phys. Rev. B 65,
161302(R) (2002).
34 M. Z. Hossain, Y. Yamashita, K. Mukai, and J. Yoshinobu, Phys.
Rev. B 67, 153307 (2003).
35 K. Hata, S. Ozawa, and H. Shigekawa, Surf. Sci. 441, 140(1999).
36 H. Kawai, Y. Yoshimoto, H. Shima, Y. Nakamura, and M.
Tsukada, J. Phys. Soc. Jpn. 71, 2192 (2002).
37
S. Yoshida, O. Takeuchi, K. Hata, R. Morital, M. Yamashita, and
H. Shigekawa, Jpn. J. Appl. Phys., Part 1 41, 5017 (2002).
38 K. Hata, M. Ishida, K. Miyake, and H. Shiekawa, Appl. Phys.
Lett. 73, 40 (1998).
39 H. Shigekawa, K. Miyaka, M. Ishida, and K. Hata, Jpn. J. Appl.
Phys., Part 2 36, L294 (1997).
40 Y. Yoshimoto, Y. Nakamura, H. Kawai, M. Tsukada, and M. Nakayama, Phys. Rev. B 61, 1965 (2000).
41 Y. Yoshimoto and M. Tsukada, in Proceedings of the 25th International Conference on the Physics of Semiconductors, editedby N. Miura and T. Ando (2002), pp. 277-278 (Springer Proceedings in Physics 87).
42 Y. Kuk, R. S. Becker, P. J. Silverman, and G. P. Kochanski, Phys.
Rev. Lett. 65, 456 (1990).
43
Z. Zhu, N. Shima, and M. Tsukada, Phys. Rev. B 40, 11 868(1999).
44 H. A. McKay, H. Chen, R. M. Feenstra, and P. J. Poole, J. Vac.
Sci. Technol. B 21, 18 (2003).
45 K. Hata, S. Yasuda, and H. Shigekawa, Phys. Rev. B 60, 8164
(1999).
46 K. Hata, Y. Shibata, and H. Shigekawa, Phys. Rev. B 64, 235310(2001).
47 J. Nakamura and A. Natori (private communication).
48 Y. Takagi, Y. Yoshimoto, K. Nakatsuji, and F. Komori, J. Phys.
Soc. Jpn. 72, 2425 (2003).
49 Recently, appearance of the p共2 ⫻ 2兲 phase under the condition
without any tip effects was confirmed by freezing the original
surface structure with Kr gas adsorption. T. Kimura, S. Yoshida,
O. Takeuchi, E. Matsuyama, H. Oigawa, and H. Shigekawa, Jpn.
J. Appl. Phys., Part 2 43, L990 (2004).

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