PHYSICAL REVIEW B

VOLUME 55, NUMBER 23

15 JUNE 1997-I

Origin of the symmetric dimers in the Si„100… surface
H. Shigekawa,* K. Hata, K. Miyake, M. Ishida, and S. Ozawa
Institute of Materials Science, and Center for TARA, Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba 305, Japan~Received 2 January 1997; revised manuscript received 17 March 1997!
A phase defect consisting of a phase boundary in a dimer row was observed to exist and migrate in thesymmetric dimer region of a Si~100! surface at about room temperature. When the phase defects migrate
rapidly compared to the timescale of scanning tunneling microscopy ~STM!, it results in a symmetric image ofdimers. In this case, since dimer flip-flop motion is limited to the domain boundaries of the dimer rows, mostof the surface remains unchanged without the destruction of the 2x anticorrelation of the buckled dimers along
the dimer rows. Considering the obtained results and the fact that the electronic structures obtained by photoemission spectroscopy at room temperature agree well with the theoretical results calculated for a surface withasymmetric dimer structures, the symmetric dimer structure observed at room temperature is concluded to becaused by the characteristic properties of the phase defects. @S0163-1829~97!01924-3#

The geometric and electronic structures of the Si~100!
surface have been studied extensively because of their importance, from both scientific and technological points ofview. Top-layer atoms form dimers, which are imaged byscanning tunneling microscopy ~STM! as a symmetric configuration at room temperature. However, it is widely accepted that dimers are buckled even at room temperature andswitch back and forth between the two possible orientations
very quickly compared to the time scale of STM measurement, which results in the observed symmetric configurationin the STM images.1–7 In fact, the electronic structure of theSi~100! surface obtained by the photoemission spectroscopy
at room temperature agrees with the theoretical results calculated for a surface with alternately buckled dimers.2
However, when the dimers flip-flop without correlation,
the flip-flop motion implies a completely disordered surface,
and the observed agreement in the electronic structure is notexpected. To yield consistent agreement between the photoemission data and theoretical calculations, the 2x anticorrelation of dimer buckling along a dimer row would have topersist up to T;300 K. Such strongly anisotropic coupling
between adjacent dimers was also predicted by the results of
low-energy electron diffraction measurement.7 One possible
mechanism to account for this is that the dimer switchingoccurs sequentially as a domain boundary, where two adjacent dimers are buckled in the same orientation.2 When thephase defect at the boundary migrates rapidly compared tothe time scale of STM, it results in the symmetric imaging ofdimers in STM measurements. In this case, since dimer flipflop motion is limited to the domain boundaries of the dimerrows, most of the surface remains in the 2x anticorrelation
along the dimer rows.
Although the idea explains the results well, the existenceof such a defect was just an assumption. Recently, the existence of such a phase defect, called a type-P defect, wasobserved on a Si~100! surface by using STM at ;6 K. 8 The
type-P defect consists of two adjacent dimers that are buckled in the same orientation and was observed to migratealong a dimer row due to dimer flip-flop motion at the phaseboundaries. Therefore, the symmetric dimer structure of the0163-1829/97/55~23!/15448~4!/$10.00

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Si~100! surface described above may be explained by thecharacteristic properties of the type-P defect.
In this paper, we present an analysis of the formation
mechanism of the symmetric dimers in the Si~100! surface
based on our STM results. A phosphorus-doped(;0.01 Vcm) Si~100! sample surface was used. All images
in this paper were taken in the constant-current mode.
Figure 1 shows an STM image taken at room temperature.
The sample bias Vs and tunneling current were 22.0 V and
1.0 nA, respectively. Similarly to the previous roomtemperature STM results, some dimers near defects are buckled; however, most dimers look symmetric. In order toclarify the structure of the symmetric dimer areas, we firstexamine the surface at ;6 K, where dimer flip-flop motion
is almost frozen.
Figures 2~a! and 2~b! show STM images obtained continuously at ;6 K. Each area in both images shows thechange taking place during a one-minute scan. Both c(432) and p(232) structures coexist, as previously reported.8However, at some domain boundaries between the twophases, noisy flickering structures are seen. The c(432), thep(232), and the noisy flickering areas are colored green,blue, and gold, respectively.
At domain boundaries between the c(432) ~green! andp(232) ~blue! areas, one phase shifter, such as a type-C ora type-P defect, exists on a dimer row. From a comparison
between the two images, some phase boundaries are seen tohave moved, for example, from A 1 ;C 1 in Fig. 2~a! to A 2;C 2 in Fig. 2~b!. Phase defects that have moved during thescan and the dimers of which the buckling angle changed
according to the movement of the type-P defects are indicated by the red dots and open circles in Fig. 2, respectively.
On the other hand, boundaries that did not move during thescan are indicated by . in Fig. 2. Since type-C defects do
not change their positions, while type-P defects do, theboundaries that did and did not move during a scan areformed by type-P and type-C defects, respectively. In fact,
the phase defects indicated by . show a brighter contrast
than do other dimers, which is a characteristic property of thetype-C defect.
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FIG. 1. ~Color! STM image of Si~100! surface obtained at room
temperature ~Vs522.0 V, Is51.0 nA!. For a comparison, phasein the symmetric dimers on a row sandwiched between two fixedbuckled dimer regions is indicated by yellow dots and bars.

At boundary A, five dimers changed their buckling angle
to the opposite direction, and the type-P defect moved fromA 1 to A 2 . In most areas, the dimer flip-flop motion was
suppressed and the isolated motion of the type-P defect wasobserved similar to that at boundary A. However, somesmall domains with unstable dimers were observed as indi-FIG. 3. ~Color! ~a!, ~b! Change in the structure of Si~100! surface during 1 min scan at room temperature ~Vs522.0 V, Is
51.0 nA!. The dimers of which the buckling angle changed during
the scan are indicated by open circles in ~b!.

FIG. 2. ~Color! STM images of Si~100! surface obtained continuously at ;6 K ~Vs51.5 V, Is51.5 nA!. Each scan took 1 min.
The c(432), the p(232), and the noisy flickering areas are colored green, blue, and gold, respectively. . indicates type-C defects.
The dimers of which the buckling angle changed during the scan
are indicated by open circles in ~b!.

cated by gold areas in Fig. 2. Let us look at the relationshipbetween the symmetric dimer structure and the type-P defect.
There was one type-P defect at the boundary B 1 , as indicated by red dots in Fig. 2~a!. It moved to the right duringa scan of about one minute and formed an unstable domain
B 2 , as shown in Fig. 2~b!. On the other hand, at boundary
C 1 , only one dimer was unstable, as shown in Fig. 2~a!,which stabilized and formed a phase defect C 2 , as shown inFig. 2~b!.
In general, at the noisy unstable dimer domains, C 1 ,S 1 , and S in Fig. 2~a! and B 2 and S 2 in Fig. 2~b!, the phaseof the buckled dimers changes from one side to the other ofthe domains. Namely, these domains are sandwiched between rows of dimers that have c(432) ~green! and p(232) ~blue! arrangements with respect to their adjacent dimer
rows. Therefore, at least one type-P defect exists in the unstable dimer areas.
As shown here, the dynamics of the dimer flip-flop motion caused by the type-P defect is related to the apparently
symmetrical structure at 6 K. In this case, as describedabove, most of the surface area remains in the 2x anticorrelation structure along the dimer rows, which agrees with theexperimental result obtained by photoemission spectroscopy.
In order to elucidate the characteristics of the mechanism inmore detail, we examine the dimer structures at room temperature.

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FIG. 5. ~Color! ~a!, ~b! Schematics of the typical structures inFig. 4~b!. ~c! A potential model for the type-P defect in Fig. 4~b!.

FIG. 4. ~Color! ~a! STM image of Si~100! surface obtained at;200 K ~Vs522 V, Is51.0 nA!. ~b!, ~c! Magnifications of theareas indicated by squares A and B in ~a!, respectively. The five
STM images in both ~b! and ~c! were obtained continuously. The
time difference between each image is about 1 min. Fixed dimers,
symmetric dimers, and the dimers that changed the buckling angle
during the scans are indicated by solid circles, bars, and opencircles, respectively.

Figures 3~a! and 3~b! show the STM images obtained forthe same area in continuous scans at room temperature.
Since it is difficult to analyze the structure of a completely
symmetric dimer area, we show an area with a rather highatomic defect density here. By comparing Figs. 3~a! and3~b!, we see that the buckling angle of the dimers near theatomic defects indicated by the red arrows changed duringscan that lasted about one minute, which may be related to
the formation process of type-P defects. Here, stable dimer
flip-flip motion was observed at room temperature because
the motion is suppressed near the atomic defects.
Let us examine whether the type-P defect exists in thesymmetric dimer regions, an example of which is shown in
Fig. 1. Buckled dimers fixed by defects are indicated by solid

circles, and a yellow zigzag line is overlaid on the symmetricdimer region to help us compare the phases of the fixedbuckled dimers indicated by the arrows at either end of thesymmetric dimers. The phase of the fixed buckled dimers in
the two regions is out of phase, and at least one type-P
defect must exist in the symmetric dimer domain sandwiched
between the two fixed buckled dimer regions. Therefore, theobserved symmetric dimer structure along the dimer row inFig. 1 is considered to be caused by the migration of thetype-P defect, taking into account the results obtained at 6 K.
In order to study the characteristic properties of the typeP defect in the symmetric dimer region further, we observed
the surface at a slightly lower temperature (;200 K). As isshown in Fig. 4~a! ~Vs522 V, It51.0 nA!, the buckled
dimers extend further from the surface atomic defects thanthose at room temperature did, however, most of the dimers
still remain symmetric at this temperature, similarly to theprevious results.5Figures 4~b! and 4~c! are magnifications of the areas indicated by squares A and B in Fig. 4~a!. Acoustic noise fromliquid nitrogen lowered the resolution, but the buckled
dimers are clearly recognizable. In Fig. 4~b!, a symmetricdimer region exists on a dimer row sandwiched between two
buckled dimer regions, similarly to that in Fig. 1. Fixeddimers, symmetric dimers, and the dimers that changed thebuckling angle during the scans are indicated by solid
circles, bars, and open circles, respectively. By comparing
the phase of the buckled dimers on either side of the symmetric dimer area, we see that there is one type-P defect inthe symmetric dimer region, similarly to the case in Fig. 1.
However, since the temperature is lower and the dimer flipflop motion is more suppressed in this case, the symmetricdimers are limited to a smaller area, as expected. As isshown in Fig. 4~b!, the symmetric dimer area changed its
position with time due to the migration of the included typeP defect. The observed results agree well with the mechanism described above; the symmetric dimers are caused by
the migration of the phase defects of dimer rows.
As is shown in Fig. 4~b!, the symmetric dimer regionchanges its position according to the migration of the typeP defect, but is located only around the sides of the dimer
domain and never appears in the central part. Figures 5~a!
and 5~b! show the schematics of two typical STM images.
The observed results suggest the potential shape shown inFig. 5~c! for the type-P defect, as a function of its position
along the dimer row. The observed result may be related tothe tendency for the 2x ordering of the dimer arrangement
along a dimer row, however, in order to clarify the detailed
mechanism, further experiment is necessary.
Finally, let us investigate how the type-P defects can be55

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introduced. Figure 4~c! shows an STM image near a type-A
defect, i.e., a vacancy of a dimer.3 In the first image, there isone type-P defect on the left side of the type-A defect asindicated by an arrow. In consideration of the buckling angleof the dimers, the type-P defect disappeared through thetype-A defect in the second image. As shown in the following STM images, another type-P defect is introduced in thefourth image, but it disappears again in the fifth image. Thisis one of the possible mechanisms by which type-P defects
are introduced into the surface. At room temperature, similar
process is seen in Fig. 3, and creation and annihilation of thetype-P defects are also expected.8
In conclusion, a phase defect of dimer rows was observed
to exist and migrate in the symmetric dimer regions of theSi~100! surface at around room temperature. When the phase
defects migrate rapidly compared to the time scale of STM

measurement, symmetric dimers can be formed withoutcompletely destroying the 2x anticorrelation arrangement,
which agrees well with the fact that the electronic structure
obtained by photoemission spectroscopy at room temperature is consistent with the theoretical result calculated under
the assumption of ordered asymmetric dimers. Considering
these results, the symmetric dimers in the Si~100! surface areconcluded to be caused by the characteristic properties of thetype-P phase defect of the dimer rows.

*Corresponding author.

4

Electronic address: hidemi@mat.ims.tsukuba.ac.jp
http://www.ims.tsukuba.ac.jp/lab/shigekawa/index.html
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2
J. E. Northrup, Phys. Rev. B 47, 10 032 ~1993!.
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R. Hamers, R. Tromp, and J. Demuth, Phys. Rev. B 34, 5343
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5This work was supported by the Shigekawa Project ofTARA, 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. One ofthe authors ~K.M.! would like to thank Japan Society for thePromotion of Science ~JSPS! for financial support.
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