SCANNING VOL. 20, 398–402 (1998)© FAMS, Inc.

Received May 13, 1998
Accepted with revision May 26, 1998

How the Down Step Edges Influence Formation of the 7×7 Structure
KENJI HATA, SHUICHI OKAWA, KOJI MIYAKE, HIDEMI SHIGEKAWA
Institute of Materials Science and Center for Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba, Japan

Summary: A surface that has wide terraces was fabricatedby utilizing the step bunching phenomenon to study the effectof the down step edges on the formation of the 7×7 reconstruction. The surface with wide terraces was quenchedthrough the 1×1–7×7 phase transition, freezing the formation process of the 7×7 reconstruction. On this surface, it
became possible to investigate the influence of the down stepedges on the formation of the 7×7 reconstruction because
the influence of the upper step edge, which is located at theother side of the terrace, did not reach to the down step edge.
A considerable decrease in the existence probability of the
7×7 reconstruction was observed near the down step edges.
This decrease cannot explained by electromigration and steps
advancement caused by the difference in adatom density
between the 7×7 and disordered structure. Instead, we propose that the decrease in existence probability at down stepedges is well explained by a simple topographic model
assuming corner holes as the growth nuclei of the 7×7
domains.
Key words: Si(111), steps, 7×7, scanning tunnelingmicroscopy, quench

Introduction
The formation process of the 7×7 structure of Si(111) isof interest both scientifically and technologically; therefore,
extensive research has been carried out to study how and why
the 7×7 reconstruction forms and stands as a stable structure(Kumamoto et al. 1996). Even though the complete atomic
formation process remains unclarified, many studies showthat the formation of the 7×7 structures is influenced by manyfactors, such as preannealing treatment, miscut degree of thesubstrate, and the residual gas of vacuum (Endo et al. 1995).
To exemplify, it is reported that the 1×1–7×7 phase transition temperature is subject to the oxygen concentration in
the surface region (Ohdomari 1992).

Probably the best known factor that influences the formation of the 7×7 structure is the upper step edge. The upper
step edge is well known to serve as a growth nucleus for theformation of the 7×7 reconstruction (Phaneuf et al. 1988).
High temperature in situ scanning tunneling microscopy(STM) observations have revealed how the 7×7 domains
form and grow from the upper step edge (Kitamura et al.
1991, Tokumoto and Iwatsukui 1993). This is explained byan easier formation of a corner hole at the upper step edge
compared with the normal terrace, because the formation of
the corner hole is the rate-limiting process for the formationof the 7×7 structure.
On the other hand, the influence of the down step edge on
the formation of the 7×7 reconstruction has not been
researched. This is partly because on a usual vicinal surface,
once the 7×7 domain emerges at the upper step edge, it quickly grows through the terrace and reaches the down step edge,
making it impossible to discriminate effects of the down stepedge from other factors.
In this paper we report the manner in which the down stepedge influences the formation of the 7×7 reconstruction. A
surface with wide terraces exceeding 200 nm in width was
fabricated by utilizing the step bunching phenomenon (Latyshev et al. 1989). The surface was quenched through the
1×1–7×7 phase transition temperature, freezing the formation process of the 7×7 reconstruction. We were able to study
the influence of the down step edge on the formation of the
7×7 domain, because the influence of the upper step edge
only reached 40 nm into the terrace (< 1/5 of the width of theterrace). A considerable decrease in the existence probability of the 7×7 reconstruction was observed near the down
step edges. This decrease cannot be understood by electromigration and advancement of step caused by the differencein adatom density between the disorder and reconstructedarea (Yang and Williams 1994). Instead, we report that thegeneral trend toward decrease in probability could be understood by a simple topographical model in which corner holes
are considered to be the nuclei of the 7×7 domains (Miyake
et al. 1997).

Address for reprints:

Experiments and Results

Kenji Hata
Institute of Materials Science and Center
for Tsukuba Advanced Research Alliance
University of Tsukuba
Tsukuba 305, Japan

Flat N-type Si(111) samples (As-doped, resistance =0.375–0.625 Ω cm) were cleaned ultrasonically for 10 min
by acetone, prebaked for 10 h, followed by flashing to1250˚C, and cooled down to room temperature. The flash-K. Hata et al.: Down step edges influencing 7 × 7 structure

ing temperature and the direction of the current flow weresuch that step bunching occurred (Latyshev et al. 1989).
Scanning tunneling microscopy observation confirmed theformation of step bunching as well as the fact that the surface is completely covered with the 7×7 reconstruction. This
substrate was flashed again at 1250˚C and was quenched byturning off the power supply.
The inset in Figure 1 shows a typical STM image of thesurface of a quenched Si(111) surface.The small triangles

399

observed in the inset, which has six adatoms each, are thehalf unit of the 7×7 reconstruction. These half units form atriangle-shaped 7×7 domain. Many 7×7 domains areobserved surrounded by the disordered 1×1 areas. Even
though many small 2×4, 2×2, and 2×8 phases are observedin the disordered region, the atomic structures of the disordered area remain unclear. They have a strong tendency toappear bright and like a single or a cluster of adatoms in thefilled state STM images, although they appear dark in the

20 nm

Step up7 × 7 reconstruction
Disorder area
300 nmFIG. 1 Set of combined scanning tunneling microscopy images showing a wide region of the quenched surface. Step bunching has formed and theterrace widths exceed 200 nm. The black regions are reconstructed to 7×7. The inset shows a close-up of the quenched surface. Tunneling voltage–1V, current 0.3 nA. The triangle-shaped regions are reconstructed to 7×7.

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Scanning Vol. 20, 5 (1998)

empty state STM images (Miyake 1995, Miyake et al. 1995).
This aspect has many implications concerning the formation process of the 7×7 domain. It is natural to assume thateach 7×7 domain has grown out from a nucleate core (Miyake
1995). The latest studies report that the nucleation of a corner hole is the rate-limiting process for the formation of the
7×7 structure (Miyake 1998). This implies that the key
process for the development of the 7×7 domains is the formation of an isolated single corner hole in the disorderedregion and that the 7×7 domain grows centering the cornerhole. This point is a basic assumption we employ hereafter
regarding the formation process of the 7×7 domains.
A wide region of the surface observed by STM is shownin Figure 1. The dark and light gray regions represent the
7×7 and disordered regions, respectively. All of the steps–
observed were misoriented in the <11 2 > direction. At thefirst look at Figure 1, it is clear that step bunching has
occurred on the surface, and wide terraces exceeding 200
nm in width are observed. Also, an intense increase in theexistence probability of the 7×7 reconstruction at the upper
step site is clearly notable. From Figure 1, the existence probabilities of the 7×7 structure versus the distance from theupper and down step edge were measured, as shown in Figure 2. As reported before, the upper step edge shows a highexistence probability that gradually decreases to a constantvalue (the existence probability of the terrace) as the distancefrom the upper step edge increases (Tokumoto and Iwatsukui1993). On the other hand, we observed a considerable
decrease in the existence probability of the 7×7 reconstruction at the down step edge. The existence probability gradually increases to a constant value as the distance from thelower step edge increases. Since the existence probabilities
from the upper and down step edge recover to the same constant value, which is the existence probability of the 7×7
reconstruction on the terrace, we can conclude that thedecrease in this existence probability at the down step edge
is not influenced by the upper step edge bounding the otherside of the terrace and reflects the characteristics of the down
step edge.

Discussion
Several factors can be considered to be the origin of theobserved decrease in the existence probability. First we consider whether or not the electromigration phenomena(Yasunaga 1991) and the step motion caused by the difference in adatom density between the 7×7 reconstruction anddisordered 1×1 phase can explain the observed decrease inprobability. It is known that the disordered phase has a higher density of adatoms compared with the 7×7 phase (Yangand Williams 1994). Therefore, we assume that areas covered with high density of adatoms will have a high probability to remain as disordered during the quenching process.
Disordered areas can be regarded as regions where the superabundant adatoms created during the formation of the 7×7
phases were dumped.
Fluctuation of steps caused by fluctuation between thenumber of adatoms incorporating into and detaching fromthe step edge might be the origin of the observed decrease in
the probability of 7×7 near the down step edges. Such step
fluctuations have been directly observed by in situ high temperature STM experiments (Tokumoto et al. 1992). If thefluctuation is large, then the down step sites will have a small
chance to form the 7×7 reconstruction. This is because whensteps recede as a result of fluctuation, the newly exposed
regions at the down step edge have a lower possibility to forma growth nucleus and growth in the 7×7 domain, in this case
20 nm: the distance the down step edge influences the existence probability of the 7×7 phase and represents the char-

1.0
0.8

Probability

0.4

Probability

0.5

0.33

0.6
0.4
0.34

0.3
0.2
0.2
0.1
0.0
00

0.15(theory)

0.0

80

40

00

–
[112 ]

0.11(ex.)40

80

Wide terrace (exceeds 200 nm)

FIG. 2 The existence probability of the 7×7 reconstruction from theup (right) and down (left) step edge versus distance calculated from thescanning tunneling microscopy image of Figure 2. The averaged existence probability on the terrace is 0.34 and 0.33 (the regions which are
influenced by the step edges were excluded in the averaging).

FIG. 3 Schematic showing how the down step edge influences thetotal area of the 7×7 domains. The black-triangles represent the 7×7
domains. The white dot in the center is the growth nucleus of thedomain. Light gray triangles are 7×7 domains which would exist if thestep did not exist, but do not when the step exists.

K. Hata et al.: Down step edges influencing 7 × 7 structure

acteristic length of the fluctuation of the steps at the temperature where the kinetics of steps movement were frozen.
A similar explanation applies when the steps recede as aresult of sublimation, even though sublimation is negligible
at the temperatures at which the 7×7 structure forms. As mentioned above, the disordered 1×1 phase has a higher density of adatoms than the 7×7 phase (Yang and Williams 1994).
This means that as the total area of 7×7 increases, increasingly more excess adatoms come into being on the surface.
Steps will serve as sites to absorb these excess adatoms andconsequently they will move forward. In such a situation,
the above discussion is not valid because steps do not recede.
Electromigration is a phenomenon in which an electricalcurrent applied to the sample exerts a force on the surface
adatoms to migrate (Yasunaga 1991). This phenomenon isthe direct origin of the step bunching phenomenon utilized
to fabricate the wide terraces in this study (Latyshev et al.
1989). Recently it was elucidated that Si adatoms on the surface are positively charged and migrate toward the directionof the electrical current flow (Hibino et al. 1996). It is possible that surface adatoms diffuse across the terrace by electromigration and accumulate at the down step edges, because
they feel that a large activation energy diffuses over the stepedge or is incorporated into the step edges (Schwoebel andShipsey 1966). An accumulation of adatoms at down stepedges will give a lower 7×7 existence probability. However,during quenching, no direct current is applied to the sub–[112 ]yUpper step(a)

C
Lower step

(c)

A

C′
D

x

A′
(d)
B
D′
E

(b)
B′
(e)
Remain
D

E

C
E′

B
A

Eliminate

A′
B′
C′
D′

E′

FIG. 4 Series of schematic displayed to visualize the procedure toestimate the existence probability of the 7×7 domains at the down stepedge. See text for details.

401

strate. The power supply is cut off at 1250˚C and 7×7 forms
only when the surface has cooled down below the 7×7–1×1
phase transition temperature around 840˚C; thus, there exists
no electric field when the 7×7 reconstruction forms on thesurface, and we exclude electromigration as the cause of thedecrease in existence probability of the 7×7 reconstruction.
Another simple model is proposed by which it is possible to explain the decrease in existence probability of the
7×7 reconstruction. The model assumes corner holes as
growth nuclei of the 7×7 domain (Miyake 1998, Miyake etal. 1997) as schematically shown in Figure 3. Within theframe of this model, the existence probability of the 7×7
domain must definitely decrease near the down step sites.
This is because sites close to the down step edge have a small
number of nuclei in the vicinity when compared with sites
on the terrace. While sites on the terrace are surrounded by
nuclei in every direction, those nearby the down step edges
lack the nuclei beyond the down step edge. This aspect isshown in Figure 3. In this schematic, the black triangles represent the 7×7 domains and the white spot in the center of
each domain the nuclei. As the steps are misoriented in the–<11 2 > direction, the triangle-shaped 7×7 domains are positioned so that one of the sides of the triangles is parallel tothe step edge. The light gray triangles are the 7×7 domains
which exist if the down step edge did not exist, althoughthese 7×7 domains do not exist when the step exists. The
down step edge would experience a lower existence probability of the 7×7 reconstruction than the terrace because ofthis elimination. The average domain width was 25 nm andthis is consistent with 20 nm: the distance the down stepedges influence the existence probability of the 7×7 reconstruction. We can easily estimate the existence probability(compared with the terrace) exactly at the down step edge
predicted by this model, as shown in Figure 4. Consider aplane. The y-axis represents an imaginary step edge and thepositive x-region the terrace. The ratio of decrease of the
7×7 domain at the step edge can be estimated by locating a
growth nucleus at the center of each 7×7 domain (same size)
at every point on the x-axis and counting the total eliminated 7×7 area by the step edge against the remaining 7×7 areaat the step edge. This counting process can be visualized byconsidering a triangle-shaped 7×7 domain with a growth
nucleus at the center approaching the step edge from theinfinite as displayed in Figure 4. Figure 4 (a) and (b) showscases in which the 7×7 domain remains while Figure 4 (c),(d), and (e) shows cases in which the domains are eliminated by the step. Each contribution to the area (eliminatedor remaining) is different, as marked in each figure as
AA′–EE′. These contributions are transformed into another triangle, as displayed in Figure 4 (f). Now it is easy tounderstand that the shaded regions are eliminated and thewhite area remains. Thus, at the down step edge, our model predicts that the 7×7 existence probability is 4/9 of thatof the terrace (0.33), which gives 0.11. The actual existence
probability was 0.15.
The small deviation between the model and experimentcan be attributed to many factors. It is possible that a distor-

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Scanning Vol. 20, 5 (1998)

tion in structure at the down step edge suppresses the formation of the growth nucleus, or the growth rate of the 7×7
domain is slow compared with that of the terrace because of
the presence of an elastic strain field induced by the down
step edge, and the critical size of the 7×7 domain is larger
compared with that of the terrace.

Conclusion
We have fabricated a surface with wide terraces by utilizing the step bunching phenomenon to study the effect of thedown step edges on the formation of the 7×7 reconstruction.
The surface with wide terraces was quenched through the
1×1–7×7 phase transition, freezing the formation process ofthe 7×7 reconstruction. At the down step edge we observed
a considerable decrease in the existence probability. Thisdecrease cannot be explained by electromigration and bysteps advancement caused by the difference in adatom density between the 7×7 and disordered structure. We show thata simple topographic model, considering a corner hole as a
growth nucleus of the 7×7 domain, explains the prime characteristics of the observed decrease in probability.

Reference
Hibino H, Homma Y, Ogino T: `1×`1 to (7×7) phase transition on
Si(111) under heating current. Surf Sci 364, L587–L590 (1996)
Kitamura S, Sato T, Iwatsuki M: Observation of surface reconstruction on silicon above 800°C using the STM. Nature 351, 215–217(1991)

Kumamoto K, Hoshino T, Kokubun K, Ishimaru T, Ohdomari I:
Dynamic growth steps of n × n dimer-adatom-stacking-fault
domains on the quenched Si(111) surface. Phys Rev B 53,
12907–12911 (1996)
Latyshev AV, Aseev AL, Krasilnikov AB, Stenin SI: Transformations
on clean Si(111) stepped surface during sublimation. Surf Sci 213,
157–169 (1989)
Miyake K, Ishida M, Hata K, Shigekawa H: Role of corner holes inSi(111) 7×7 structural formation studied by HBO2 molecular irradiation and quenching. Phys Rev B 55, 5360–5363 (1997)
Miyake K, Shigekawa H, Yoshizaki R: Electronic structure ofSi(111)–7×7 phase boundary studied by scanning tunnelingmicroscopy. Appl Phys Lett 66, 3468–3470 (1995)
Miyake K: (to be published)
Ohdomari I: An hypothesis on the role of oxygen in the Si(111)1×1→7×7 phase transition. Surf Sci 271, 170–178 (1992)
Phaneuf RJ, Williams ED, Bartelt NC: Temperature dependence ofvicinal Si(111) surfaces. Phys Rev B 38, 1984–1993 (1988)
Schwoebel RL, Shipsey EJ: Step motion on crystal surfaces. J Appl
Phys 37, 3682–3686 (1966)
Tokumoto H, Iwatsukui M: Scanning tunneling microscopy of clean
silicon surfaces at elevated temperatures. Jpn J Appl Phys 32,
1368–1377 (1993)
Tokumoto H, Miki K, Morita Y, Sato T, Iwatsuki M, Suzuki M, Fukuda T: Real-time observation of step motion on Si(111) surface byscanning tunneling microscopy. Ultramicroscopy 42-44, 816(1992)
Tsukui K, Endo K, Hasunuma R, Hirabayashi O, Yagi N, Aihara H,
Osaka T, Ohdomari I: Changes in transition temperature of theSi(111)1×1–7×7 phase transition. Surf Sci 328, L553–560 (1995)
Yang Y-N, Williams ED: High atom density in the 1×1 phase and origin of the mestastable reconstructions on SI(111). Phys Rev Lett
72, 1862–1865 (1994)
Yasunaga H: Atomic layer control by electromigration on semiconductor surfaces. Appl Surf Sci 60/61, 64–70 (1992)