Characteristic structures of the Si„111…-7Ã7 surface step
studied by scanning tunneling microscopy
K. Miyake, S. Okawa, O. Takeuchi, D. N. Futaba, and K. Hata
Institute of Applied Physics, CREST (JST), University of Tsukuba, Tsukuba 305-8573, Japan

R. Morita and M. Yamashita
Department of Applied Physics, CREST (JST), Hokkaido University, Sapporo 060-8628, Japan

H. Shigekawaa)
Institute of Applied Physics, CREST (JST), University of Tsukuba, Tsukuba 305-8573, Japan

共Received 9 October 2000; accepted 5 February 2001兲
Contrary to previous results concluding that all 7⫻7 boundaries in the down step edge werecomplete, 5⫻5 faulted half units were found to be introduced in part when the transition region was
narrow. In addition, the formation characteristic of the 5⫻5 faulted half units was strongly
influenced by the type of step configuration 共nF or nU兲 and by the structure of the upper step edge
共complete or incomplete兲. Even when the step configuration was the same, the difference in theupper step edge structures strongly affected introduction of 5⫻5 units. © 2001 American Vacuum
Society. 关DOI: 10.1116/1.1359538兴

I. INTRODUCTION
The Si(111)-7⫻7 structure has been extensively studied
and the dimer-adatom-stacking 共DAS兲 fault model is widely
accepted to explain the structure.1–3 However, since it has acomplicated structure, the study of its stability and phasetransition mechanism is still one of the most important targets of recent researchers.4–10 Since nucleation and growthof the 7⫻7 phase is governed at the step,11–14 analysis of thestep structures gives information about the stability of DAS
structures upon phase transition. In addition, analysis of
atomic structures of steps on the surfaces is important bothscientifically and technologically because they play an important role in, for example, chemical reactions and crystal
growth by molecular beam epitaxy. From this point of view,
atomic structures of the steps on the Si(111)-7⫻7 surface
have been studied by using scanning tunneling microscopy
共STM兲.
Becker et al. reported the first STM image of the steps on
Si共111兲7⫻7. They observed only one type of step structure,
namely, both the upper and lower terrace edges were perfectly matched at a step by forming 7⫻7 units.15 However,as Tochihara et al. pointed out, taking into consideration thephase mismatching between the DAS structures formed on
upper and lower terraces at a step edge, 14 different typestructures are possible. First, there exist two possibilities:
whether the upper terrace is terminated by the edge of thefaulted half or of the unfaulted half triangles as shown in Fig.
1.16 They were named the F step and the U step, respectively. Then, seven different step types are possible for each
F and U step, as determined by the distance L 共Fig. 1兲 between the 7⫻7 phase boundary on the lower terrace and thestep. The distance L can be represented as 关 (3n⫹4)/21兴 a for
the F step and 关 (3n⫹2)/21兴 a for the U step. Here, n is ana兲

Author to whom correspondence should be addressed; electronic mail:hidemi@ims.tsukuba.ac.jp; http://dora.ims.tsukuba.ac.jp1549

J. Vac. Sci. Technol. A 19„4…, JulÕAug 2001

integer between 0 and 6 and a is the length of the 7⫻7 halfunit triangle 共Fig. 1兲. According to the value of n, steps arecalled nF or nU steps. For a value of n from 1 to 6, a region
the size of which is shorter than the length of the 7⫻7 half
unit, a, is introduced. The area is called the transition region.
Several years ago Wang and Tsong performed a systematic study of the structures of atomic steps of theSi(111)-7⫻7 surface,17 and they observed all of the 14 different type steps described above. According to their results,
in most cases, the effect of phase mismatching at the step
was limited in the transition region. That is, U and F steps
were terminated by complete 7⫻7 units, and the boundaryof the 7⫻7 phase on the lower terrace was also complete.
The transition region was a disorder phase and a smaller
DAS structure such as a 5⫻5 one did not appear eventhough there was enough space for the formation of thestructure such as 5U and 6U steps. In some cases, 0U, 1U,
4F, and 5F upper step edges did not coincide with the complete 7⫻7 unit boundary. However, the boundary of the 7⫻7 phase on the lower terrace was complete even in thecases.
As is known well, the presence of a step reduces the surface symmetry and increases the number of dangling bonds.
The local structures around a step are determined by thecompetition between the increase in the strain energy and the
reduction of the number of dangling bonds, similar to surface
reconstruction. Therefore, in spite of the results of Wang andTsong,17 various kinds of structures are expected to beformed around steps, such as the introduction of faulted
halves of smaller size. In fact, Tochihara et al. reported that5⫻5 faulted half units were formed on the lower terrace inthe case of 5U steps.16 A similar structure, the introductionof 5⫻5 units, was observed in the phase boundaries on theterrace to stabilize the structure.18
In this article, from the standpoint described above, we
reexamined the structure of Si(111)-7⫻7 steps. For a wide0734-2101Õ2001Õ19„4…Õ1549Õ4Õ$18.00©2001 American Vacuum Society

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FIG. 1. Illustration of a step structure. F and U represent the faulted andunfaulted half units of a 7⫻7 structure, a: Length of the 7⫻7 half unit
triangle; L: width of the transition region.

transition region, there was no faulted half unit of any size
within the regions, which agrees well with the results ofWang and Tsong.17 However, we found that 5⫻5 faulted
half units were partly formed in 7⫻7 faulted half units in theboundary on the lower terrace when the transition region was
narrow.
II. EXPERIMENTS
Phosphorus-doped n-type Si共111兲共␳⫽0.375–0.625 ⍀ cm兲
substrates were used in this study. The samples were chemically cleaned by acetone before being loaded into the ultrahigh vacuum 共UHV兲 chamber. The samples were outgassed
at ⬃500 °C for 12 h. Subsequently, the samples were flashedat about 1250 °C for 15 s in order to form the 7⫻7 structure
following slow cooling. The base pressure was less than 8⫻10⫺9 Pa and the pressure during the flashing process was
kept below 2⫻10⫺8 Pa. All STM observations were performed at room temperature using an electrochemicallyetched tungsten tip and all STM images shown here wereobtained in the constant current mode.
III. RESULTS AND DISCUSSION
Figure 2 shows STM images of 1F and 2U step edges ofa Si(111)-7⫻7DAS structure. The sample bias voltages are⫹0.5 V 共empty state兲 for Figs. 2共a兲 and 2共c兲 and ⫺0.5 V
共filled state兲 for Figs. 2共b兲 and 2共d兲. The steps observed arestraight, and the upper terraces of the steps are terminated by
the almost complete boundaries of the 7⫻7 unit cells at thestep edge 共complete step edge兲.
Next, let us focus on the structure of the down steps. In
the case of the 1F step, 7⫻7 unfaulted half units form thestraight boundary shown in Figs. 2共a兲 and 2共b兲. There is atransition region between the 7⫻7 boundary and the stepedge. We cannot find peculiar results differing from thestructures observed in previous work.
On the other hand, a peculiar structure, which has a characteristic adatom arrangement differing from the normal 7⫻7 adatom arrangement, was found in the case of the 2U
step, and it is shown by triangles in Figs. 2共c兲 and 2共d兲.
Figures 3共a兲 and 3共b兲 show a magnified STM image of theobserved characteristic structure of the empty state and thefilled-state, respectively. The N and C in Fig. 3 represent the
J. Vac. Sci. Technol. A, Vol. 19, No. 4, JulÕAug 2001

FIG. 2. STM images of 共a兲, 共b兲 1F and 共c兲, 共d兲 2U steps. The sample bias
voltages are ⫹0.5 V for 共a兲 and 共c兲 and ⫺0.5 V for 共b兲 and 共d兲. Setting
tunneling current is 0.3 nA. The triangles in 共c兲 and 共d兲 indicate the characteristic structure. The upper step is out of contrast.

normal and the characteristic half units, respectively. The
arrows in the filled state STM image 关Fig. 3共b兲兴 indicate thatthe adatoms observed were darker than those in the normal
7⫻7 arrangement. The brightness of the adatoms in the 7⫻7DAS structure in the filled-state STM image reflects theirsecond and third layer structures, i.e., the number of rest
atoms around the adatoms.8,18,19 Therefore, the adatoms indicated by arrows in Fig. 3共b兲 are surrounded by more rest
atoms than normal 7⫻7 adatoms. On the basis of this, we

FIG. 3. Magnified STM images of the characteristic structure in Fig. 2. The
sample bias voltage and setting current are 共a兲 ⫹0.5 V and 0.3 nA and 共b兲⫺0.5 V and 0.3 nA. N and C represent the normal and the characteristic halfunits, respectively. The arrows indicate the adatoms that were observed to
be darker than those in the normal 7⫻7 arrangement. The upper step is outof contrast.

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FIG. 4. Schematic model of the characteristic structure in Fig. 3.

can conclude that the characteristic structures observed consist of 5⫻5 faulted half units, as shown in Fig. 4 schematically. Therefore, the 5⫻5 structure is introduced in the step
region as expected.
Next, we consider the correlation between the conformation of the upper step edge and the emergence of the characteristic structure. Figure 5共a兲 shows an empty state STM
image of a step structure. Here, straight and zigzag structures
coexist at the step edge. There are two phase boundaries,
indicated by arrows in Fig. 5共a兲, and magnifications of them

FIG. 5. 共a兲 An empty state STM image of a step structure. Two phaseboundaries are indicated by arrows in the image. White triangles show the5⫻5 faulted half units introduced. The dashed lines indicate the phaseboundaries. 共b兲, 共c兲 Magnified STM images of phase boundaries A – B andC – D in 共a兲. In 共b兲 and 共c兲, the vertical lines through the phase boundaries
show the phase shift between two 7⫻7 domains.
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are shown in Figs. 5共b兲 and 5共c兲. Phase mismatching at theboundary C – D is in the direction of 关 1̄10兴 , which does not
influence the step configuration between lower and upper
steps. On the other hand, the phase boundary A – B has an
influence on the step configuration because the phase mismatch is in both directions, 关 1̄10兴 and 关 112̄兴. As a result, thestep types changed across the phase boundary, namely, from
5U on the upper side to 0U on the lower side.
It is clear that the structure of the upper step edge changes
due to the change in step type. At the 5U step, the upper stepedge coincides with the complete 7⫻7 unit boundary similar
to in the previous results.16,17 On the other hand, at the 0U
step, there are two types of structure of step edge. One is theupper edge which consists of the 5⫻5 unfaulted half unit
separated by open 7⫻7 faulted subunits 共incomplete 0U
step兲, which is the same structure as that obtained by Wang
and Tsong.17 The other is the structure where the upper stepis terminated by the complete 7⫻7 unfaulted unit cell 共complete 0U step兲, corresponding to the STM image obtained byBecker et al.,15 that is, the structure of the upper step edgechanges even when the type of step configuration is thesame. Therefore, the structure of the lower step edge alsochanges, possibly depending upon the structure of the upper
step edge. Let us discuss this point next.
In the case of the 5U step, the boundary of the 7⫻7phase on the lower terrace is complete. The transition region
is a disorder phase and any smaller size DAS such as the 5⫻5 structure did not appear even though there was enoughspace for formation of the structure. This result agrees withthe result of Wang and Tsong.17 However, in the case of the
0U step, a new structure appeared on the lower terrace. The
5⫻5 faulted half units, which are represented by triangles inFig. 5, were introduced between the complete 7⫻7 faulted
half units along the step. The 5⫻5 faulted half units were
frequently observed especially when the upper edge has thesame structure as that observed by Wang and Tsong: a 5⫻5 unfaulted half unit separated by open 7⫻7 faulted subunits. This tendency is clearly seen in Fig. 6. In Fig. 6, aphase boundary exists on the upper terrace, and the step
types across the phase boundary have changed from 0U on
the upper side to 3U on the lower side. At the complete 0U
step along the dotted lines, there was a wide transition region
instead of the appearance of any size of the faulted half units,even when the transition region was wide enough to form
5⫻5 and/or 7⫻7 faulted half units. On the other hand, 5⫻5 faulted half units were frequently observed on the lower
terrace at the incomplete 0U step, with the 5⫻5 structure
indicated by the triangles.
It is noted that 5⫻5 faulted half units are introduced only
when the transition region is narrow, such as in the case of
0U – 2U steps. They were not observed in the wide transition
region such as in the case of 3U – 6U steps as shown in Figs.
5 and 6. These facts indicate that the width of the transition
regions primarily affects the introduction of 5⫻5 faulted
half units. Then, the introduction of 5⫻5 faulted structures
may reduce the strain energy further depending upon the
structure of the upper step edge.

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joined to each other, namely, there exists a certain space
between the 5⫻5 structures, as clearly seen in Fig. 5. An
optimum structure must exit.
Finally, the existence probability of 5⫻5 faulted halfunits is considered to be different from each other because
the structure of an upper step edge is different. Figure 7
shows an example of the existence probability of 5⫻5
faulted half units at the lower step edge counted from the
STM images. No characteristic structure was observed at thelower F step edge. On the other hand, in the case of the U
step, the existence probability at 2U and incomplete 0U
steps are 0.13 and 0.56, respectively. These results clearly
indicate as expected that the introduction of 5⫻5 faulted
half units is affected by not only the width of the transition
region but also by the structure of the upper step edge.

FIG. 6. 共a兲 Filled state STM image of a step structure. A phase boundary is
indicated by arrows. 共b兲 Magnified STM image of the phase boundary in 共a兲.
The vertical lines through the phase boundary show the phase shift between
two 7⫻7 domains.

When the transition regions are wide, the strain will be
relaxed in the transition region. As a result, it has no use forfurther formation of 5⫻5 faulted half units. On the other
hand, when the transition region is narrow, the structure of
the step affects the stability of the dimers existing along thestep edge on the lower terrace. If dimers are formed on boththe upper and lower step edges as in the case of the complete
0U step, the configuration of the densely formed dimers is
considered to be unstable. As a result, dimers on the lower
terrace become broken and a wider transition region isformed instead. Furthermore, when the upper edge of the 0U
step consists of the 5⫻5 related structure 共incomplete 0U
step兲, the structure makes the transition region narrower
compared to the complete 0U step. Namely, the width of thetransition region becomes virtually equal to that of the 2U
step. Then, the 5⫻5 faulted half units may possibly be introduced on the lower edge of the incomplete 0U step. It isremarkable that the 5⫻5 faulted half units are not formed

FIG. 7. Existence probability of 5⫻5 faulted half units at the lower stepedge counted from the STM images.
J. Vac. Sci. Technol. A, Vol. 19, No. 4, JulÕAug 2001

IV. CONCLUSION
We found for the first time that the formation of 5⫻5
faulted half units strongly depends on both the width of thetransition region and the structural integrity of the upper stepedge. This result indicates that the structures of the upper andthe lower step edges are determined by the energetic competition between the formation of transition regions and thestructural integrity of the upper step edge as expected.

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