Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 5017–5020
Part 1, No. 7B, July 2002c2002
The Japan Society of Applied Physics

Pseudo-Real Time Observation of the Dynamics of Phase Defect on Si(100) Surface
Shoji YOSHIDA1 , Osamu TAKEUCHI1 , Kenji H ATA1 , Ryuji M ORITA2 , Mikio YAMASHITA2 and Hidemi S HIGEKAWA1, ∗1 Institute of Applied Physics, CREST, University of Tsukuba, Tsukuba 305-8573, Japan
2 Department of Applied Physics, CREST, Hokkaido University, Sapporo 060-8628, Japan(Received March 11, 2002; accepted for publication April 26, 2002)

The dynamics of a phase defect (P-defect) on the Si(100) surface was studied for the first time by scanning tunneling microscopy
in pseudo-real time below 10K, using the repeated-line-scan technique in which single line scans are repeated on the same pathalong a dimer row. In addition to the pair creation and annihilation of the P-defect, the effect of a step on the motion ofthe P-defect was clearly demonstrated. Since the local barrier height for the migration of the P-defect depends on the local
environment such as strain and electronic structure due to the existence of defects or dopants, the obtained results also indicatethat analysis of the P-defect is promising for local probing of the Si(100) surface. [DOI: 10.1143/JJAP.41.5017]
KEYWORDS: scanning tunneling microscopy, Si(100), defect, low temperature, phase transition, dimer, step

1. Introduction
Top-layer Si atoms of the Si(100) surface form dimers inorder to reduce the surface energy, resulting in a (2 × 1) structure.1) In fact, the first scanning tunneling microscopy (STM)
observation at room temperature showed a symmetric dimer(2 × 1) structure.2) However, as is now well known, dimers
buckle for further energy reduction, and the observed symmetric dimers are considered to represent an averaged view of
the thermally activated flip-flop motion of the buckled dimers.
Along a dimer row, dimers alternately buckled in opposite
orientations. Therefore, two configurations are possible forthe buckled dimer arrangement: one is the c(4 × 2) phasewhich has antiphase arrangement between two adjacent dimerrows, and the other is the p(2 × 2) phase which has in-phasearrangement. Many theoretical calculations indicate that thec(4 × 2) phase is the more stable structure.1) At low temperature (140 K ∼ 80 K), the buckled dimer c(4 × 2) phase
was observed, by STM3) and low energy electron diffraction(LEED),4) to cover the surface. Therefore, c(4 × 2) was concluded to be the ground state of this surface and was considered to remain stable at lower temperature. However, recently,p(2 × 2)/c(4 × 2) mixed,5) anomalous flip-flop (apparent symmetric dimer),6) and symmetric dimer (no flip-flop motion)7)
phases were observed by STM below 20 K, and the c(4 × 2)
phase was not observed. These results again give rise to muchcontradiction concerning this issue. Very recently, more detailed STM investigation clarified the following:8) (1) the stable structure below 10 K depends on the type of dopant; thec(4 × 2) phase remains only for the p-type substrate, and thep(2 × 2) single phase appears for the n-type substrate, (2)mixture of the c(4 × 2) and p(2 × 2) phases and the fluctuation between them is induced by the introduction and dynamics of the P-defect (a phase defect), (3) symmetric dimer
phases caused by bias dependence and STM tip-induced flipflop motion appear at low temperature. However, even considering all these results, the low-temperature issue still remains
an enigmatic subject concerning the Si(100) surface. The Pdefect is a phase defect with a structure similar to that of
the C-defect; it consists of two adjacent dimers which buckle
with the same orientation.5) However, the P-defect can mi∗ E-mail address: hidemi@ims.tsukuba.ac.jp

grate along a dimer row, unlike the C-defect, as one of its twodimers changes orientation (Fig. 1). If a P-defect exists on adimer row, it serves as a phase shifter and induces phase mixing between c(4 × 2) and p(2 × 2). At 80 K, the P-defect tends
to migrate to reduce the p(2 × 2) area because c(4 × 2) is morestable than p(2 × 2) at this temperature.9) On the other hand,
at 5K, fluctuation among c(4 × 2) and p(2 × 2) phases is dueto the weak interactive migration of P-defects.5) Therefore,
an understanding of the dynamics of P-defect is important indetermining the stability of the surface phase. As a result ofa recent increase in the attention paid to nanoscale scienceand technology, the Si(100) surface is expected to be used asa template, in combination with organic materials, to develop
new functional devices. However, it is well known that surface
local structures such as defects, steps and dopants influence
the local electronic and geometric structures of the Si(100)surface.11, 12) From this point of view, it is extremely important to investigate the effect of such local structures. Since
migration of the P-defect is induced by the flip-flop motionof the dimers which reflects the local barrier height,13) andthe barrier height depends, for example, on the local strain
and electronic structures due to the existence of defects anddopants, the P-defect is promising as a probe of the external
field introduced by the local environment. For the reasons described above, real-time analysis of the P-defect is extremely
important and useful. However, until now, detailed dynamics
of the P-defect could not be determined, because the conventional scanning method requires at least several ten seconds
to obtain one image, and it is too slow to follow the dynamics
of the P-defect. In this study, we investigated the dynamics
of the P-defect by the repeated-line-scan technique in which
single line scans are repeated on the same path along a dimerrow.14) By this method, (1) the creation and annihilation of
the P-defect, and (2) the influence of a step on the motion of
the P-defect were clearly observed in real time.
2. Experimental
An As-doped (∼ 0.003 Ω·cm) Si(100) sample was cleaned
by heating to ∼ 1200◦ C after prebaking for one day; the pressure during the heat treatment was kept below 5 × 10−8 Pa.
Measurements were performed at 10 K in ultrahigh vacuum(5 × 10−9 Pa) using a W tip. Sample temperature was moni-

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Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 7Bp(2x2)

S. YOSHIDA et al.

c(4x2)

P-defectp(2x2)

c(4x2)

Fig. 1. Schematics of the P-defect and its movement caused by dimer
flip-flop motion.

tored with a thermocouple.
3. Results and Discussion
3.1 Method of analysis of the P-defect dynamics
Figure 2 shows a typical STM image of the Si(100) surface obtained below 10 K. When defect density is low, thep(2 × 2) phase is dominant on this surface.8) There exist someP-defects at the boundaries between c(4 × 2) and p(2 × 2)
phases as indicated by arrows. Figure 3(a) show an STM image of a P-defect. In the STM image, there exists a dark region in the dimer row indicated by an arrow. Under theseconditions (sample bias: 0.6 V, tunneling current: 0.2 nA), Pdefects appear darker than normal dimers. As can be seen inFig. 3(a), the dimer arrangements on the left and right sidesof the dark region are c(4 × 2) and p(2 × 2) phases, respectively. And this region migrates along the dimmer row. Therefore, the noisy dark region is a movable phase shifter and corresponds to the position of the P-defect. Figure 3(b) is thetime-versus-position pseudoimage obtained along the single
dimer line with the P-defect indicated by an arrow in Fig.
3(a). Here, the same single dimer row was scanned repeat-Fig. 3. (a) STM imge of a P-defect (sample bias: 0.6 V, tunneling current:0.2 nA). (b) Time-versus-position pseudoimage obtained along the dimerrow with a P-defect indicated by an arrow in (a).

Fig. 2. A typical STM image of Si(100) surface obtained below 10 K (sample bias: 0.8 V, tunneling current: 1.0 nA). Some P-defects are indicated byarrows.

edly; the time evolution of the change is shown on the longitudinal axis. The image consists of 800 lines, and each line
was scanned at the rate of 0.25 s/line. Since dimers are alternately buckled in opposite orientations, the periodicity of
2a0 (a0 = 0.38 nm, surface lattice constant) is shown along
the dimer row in Fig. 3(b). Figure 4 shows the schematic of
the observed time-versus-position pseudoimage in Fig. 3(b).
The grid lines in Fig. 4 indicate the periodicity of the surface
lattice constant a0 . Solid circles in Fig. 4 shows the discrete
positions of the P-defect determined from the cross sectionsof the original STM image in Fig. 3(b). This image clearlyshows that migration of the P-defect is caused by the jumping of the P-defect in the step of the surface lattice constant.
By plotting a histogram of this discrete graph, we can analyze the effect of the local environment on the dynamics ofJpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 7B

S. YOSHIDA et al.

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400(a)
100300time(sec)50

200020nm

0(b)

100

Lattice site (a0)0(c)

05101520
lattice(a0)25Fig. 4. Schematic of the observed time-versus-position pseudoimage inFig. 3(b). The grid lines indicate the periodicity of a0 (a0 = 0.38 nm,surface lattice constant). Solid circles shows the discrete positions of theP-defect determined from the cross sections of the STM image in Fig. 3(b).

Fig. 5. (a) STM image of a P-defect near a step edge. (b) Time-versusposition pseudoimage of the P-defect along the dimer row indicated byan arrow in (a). (c) Histogram presenting the residence percentage of theP-defect shown in (b).

served effect in detail. Such analysis is now in progress.
the P-defect.
3.2 Effect of a step on the dynamics of P-defectIt has been suggested that steps and defects have greatly influence the dimer structures of the Si(100) surface.11) In order
to apply our technique to investigating this issue, we observed
the dynamics of a P-defect near a step edge. Figure 5(b) showsa time-versus-position pseudoimage of the P-defect near astep shown in Fig. 5(a). There is a step on the left side in thescan area, which is imaged white since contrast is adjusted tothe lower terrace. As clearly shown in Fig. 5(b), the P-defectmigrates in the narrow region near the step edge on the left
side. In order to examine the characteristic property of the Pdefect in greater detail, a histogram representing the residence
percentage of the P-defect along the dimer row is shown in
Fig. 5(c), as obtained by a method similar to that for Fig. 4.
It is clear that there exists a strong influence of the step edge
on the migration of the P-defect. As mentioned in §1, since
the dynamics of the P-defect reflects the local barrier height
which governs the buckling of the dimer in the P-defect, theP-defect is promising as a local probe which is sensitive tothe external field introduced by the local environment. In thiscase, an attractive interaction exists near the step edge. The
existence of a local effect is clear. However, since the dynamics of the P-defect is influenced by many other factors, suchas the surrounding phase structure, other defects, and the tipeffect, further analysis is necessary for understanding the ob-3.3 Pair creation and annihilation of P-defects
When the surface is covered by a single phase, p(2 × 2) orc(4×2), P-defects are considered to be introduced through thethermal activation of the dimer flip-flop motion.15–17) Since
the P-defect is a phase shifter, its existence induces the mixingof the c(4 × 2) and p(2 × 2) phases, resulting in an increase in
the surface energy. Flip-flop of the only one dimer is enoughto create a P-defect from other defects or steps. However, on
the terraces, since P-defects act as phase shifters, they must
be created in the form of a pair, as shown in Fig. 6. Therefore, at low temperature, they are considered to be generatedmainly from other defects and steps, i.e., through low-energypaths.9) However, when some perturbation with sufficient energy is applied to the surface, P-defects are expected to becreated. Figure 7 shows a time-versus-position pseudoimageof the Si(100) surface. At point A, a pulse bias (0 V to 5 V)
was applied from the STM tip during the scan. It is clear thatfour (two pairs of) P-defects were created by the pulse bias.
Migration of the P-defects was subsequently observed, andpair annihilation occurred at point B. This is the first direct
observation of the creation and annihilation of the P-defect inpseudo-real time. P-defects can be introduced as a local probe
by using the above technique, making it possible to analyze
their dynamics even in a single-phase surface area.

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the repeated-line-scan technique. In addition to pair creation
and annihilation of the P-defect, the effect of a step on themotion of the P-defect was clearly demonstrated. Since thelocal barrier height for the migration of the P-defect depends
on the local environment such as strain and electronic structure due to the existence of defects or dopants, the obtainedresults also indicate that analysis of the P-defect is promising
for local probing of the Si(100) surface.
Acknowledgements
The support of a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports and Culture ofJapan is acknowledged.

P-defectFig. 6. Schematic of the creation of a pair of P-defect.

80

B40A0028nm

Fig. 7. Time-versus-position pseudoimage of the creation and anhilation ofP-defects.

4. Conclusion
Dynamics of the P-defect on a Si(100) surface was studied for the first time by STM in real time below 10 K, using

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