Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 3841–3844
Part 1, No. 6B, June 1999c
°1999
Publication Board, Japanese Journal of Applied Physics

Si(111) Surface under Phase Transitions Studied by the Analysis of Inner Layer Structures
Using Bias-Dependent Scanning Tunneling Microscopy
Koji M IYAKE1, ∗1 , Takashi K AIKOH1 , Yan Jun L I1 , Haruhiro O IGAWA1 and Hidemi S HIGEKAWA1,2, ∗2
1 Institute of Applied Physics, Center for TARA, CREST, Japan Science and Technology Corporation (JST),

University of Tsukuba, Tsukuba 305-8573, Japan
2 Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo, Tokyo 113-8656, Japan(Received February 15, 1999; accepted for publication April 5, 1999)

In both cases of quenched and HBO2 -molecule-irradiated Si(111) surfaces, corner holes are observed to exist along theboundaries between 7 × 7 and disordered structural domains. From the analysis of the bias-dependent STM images, it was
found that the corner holes included complete stacking fault and dimer structures in the second layer, i.e., a complete cornerhole. This result strongly indicates that the complete corner holes play important roles in both the formation and stabilization
processes of the dimer-adatom-stacking fault (DAS) structure. In addition, the formation of a structure similar to that of thecorner hole was often observed at the boundaries of three out-of-phase c(2 × 8) structural domains in quenched surfaces, which
may result in nucleuses for the formation of the complete corner hole.
KEYWORDS: complete corner hole, Si(111), underlying layer structure, quench, HBO2

1. Introduction
The Si(111)-7 × 7 reconstructed surface has been extensively studied, and the dimer-adatom-stacking fault (DAS)model proposed by Takayanagi et al.1) is widely accepted for
explaining the structure. The DAS structure consists of three
atomic layers, i.e., the adatom, stacking fault, and dimer layers. The atomic and electronic structures of the 7 × 7 reconstructed surface in the static form have been discussed extensively, and in most part have been clarified. However, the dynamic mechanism behind the formation and stabilization processes of the DAS structure still remains unsolved, because of
the complicated reconstruction involving the first few surface
layers.
In order to clarify these processes, quenched and moleculeadsorbed Si(111) surfaces have been analyzed using scanningtunneling microscopy (STM).2–7) In the case of the quenched
Si(111) surface, the high-temperature “1 × 1” phase is frozen
during the rapid cooling process. Therefore, the analysis of
the quenched surface yields information about the formation
process of the DAS structure. On the other hand, in the caseof the molecule-adsorbed Si(111) surface, STM images show
the destruction process of the 7 × 7 structure induced by theforeign molecules and yield information about the stability of
the DAS structure.
Recent high-temperature STM studies of quenched surfaces revealed that the growth of DAS domains occurred witha single faulted half unit as the building unit and faulted halfunits were formed sharing corner holes.2–4) At the boundaries
between 7×7 and disordered domains, intrusion of the faulted
half units into the disordered region was also observed fromthe corner holes.5, 6) Furthermore, in the destruction processof the DAS structure, corner holes remained at the boundaries between the 7 × 7 and disordered structures.7) These
results strongly suggest that corner holes play important roles
in both the formation and stabilization processes of the DAS
structures.
Some analyses have been carried out within the framework∗1 Corresponding author. E-mail address: miyake@ims.tsukuba.ac.jp∗2 Corresponding author. E-mail address: hidemi@ims.tsukuba.ac.jp

http://dora.ims.tsukuba.ac.jp

of the role of corner hole.7) However, only the atomic structure of the topmost adatom layer was analyzed in previous
studies, in spite of the fact that the information on the atomic
arrangement in the second and third layers is important for theunderstanding of the formation and stabilization processes of
the DAS structures in detail. The lack of information about
the underlying layers is mainly due to the fact that STM images generally yield information only about the topmost layer.
Therefore, in order to clarify the role of corner holes in detail,it is necessary and important to analyze the underlying layer
structures of corner holes.
It is well known that the DAS structure has a characteristic electronic property; charge transfer from the top-layer
adatoms to rest atoms in the underlying layer. The amountof the charge transfer from adatoms to rest atoms depends
on the number of rest atoms which surround the adatoms.8, 9)This characteristic electronic property makes it possible toanalyze the atomic arrangement in the second and third layers of the DAS structure. The structure of underlying layers
can be discussed by considering the redistribution of charge
density on adatoms in STM images at negative sample bias
voltages.10, 11) In fact, we recently succeeded in analyzing
the structures of the 7 × 7 phase boundaries by studying thebias-dependent STM images.10) In consideration of the characteristic electronic property of the DAS structure describedabove, the observed charge redistribution on the top layer
could be comprehensively related to the structural change inthe dimer layer caused by the phase mismatch at the boundary. We applied this technique in the analysis of the role ofcorner holes in the formation and the stabilization processesof the 7 × 7 DAS structure.
In this paper, we present the results of structural analysis performed on quenched and HBO2 -irradiated Si(111) surfaces. We used HBO2 molecules as adsorbates because of
the fact that they preferentially react with the center adatoms
in unfaulted half units,7, 12, 13) making it possible to analyze
the stability of stacking fault structures during the breakdown
process of the 7 × 7 structure.
2. Experiments
Phosphorus-doped n-type Si(111) (ρ = 0.4–1 Ä · cm) substrates were used in this study. The samples were chemically

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6Bcleaned by acetone, and then loaded into the UHV chamber. The samples were outgassed at ∼500◦ C for 12h andthen flashed at about 1150◦ C in order to form the 7 × 7 structure. For the quenching experiments, after confirming the formation of the clean 7 × 7 structure by STM, samples werereheated to ∼1200◦ C, and then quenched by turning off theheating power. On the other hand, for the experiments with
HBO2 molecular irradiation, after confirming the formationof the clean 7 × 7 structure, HBO2 molecules were irradiatedonto the surface. The entire irradiation process was performedby maintaining the substrate temperature at ∼750◦ C in orderto prevent oxygen incorporation. The HBO2 cell temperatureduring irradiation was maintained at ∼800◦ C.
All STM observations were performed at room temperature using an electrochemically etched tungsten tip. The basepressure was ∼ 1 × 10−10 Torr and the pressure during HBO2
irradiation was less than 2 × 10−8 Torr.
3. Results and Discussion
3.1 STM images of quenched and HBO2 -irradiated Si(111)
surfaces
Figure 1 shows the STM images of Si(111) surfaces after(a) quenching (sample bias: Vs = 2.0 V, tunneling current:

Fig. 1. STM images of the Si(111) surfaces after (a) quenching (sample
bias: Vs = 2.0 V, tunneling current: It = 0.3 nA) and (b) HBO2 irradiation(Vs = 2.1 V, It = 0.45 nA).

K. M IYAKE et al.

It = 0.3 nA) and (b) HBO2 irradiation (Vs = 2.1 V, It =0.45 nA). The 7 × 7 and disordered structures are observedon each surface. In the disordered structure of the quenched
Si(111) surface, there exist domains consisting of c(2 × 8)
and other structures as shown in Fig. 1(a). On√ hand,√the othershort-range ordering of adatoms forming ( 3 × 3)R30◦ ,2 × 2 and c(2 × 4) structures are observed in the disorderedstructure on the HBO2 -irradiated Si(111) surface (Fig. 1(b)).
The difference between the atomic arrangement of the disordered structures on the quenched and HBO2 -irradiated surfaces is due to the difference in their origins. In the caseof the quenched Si(111) surface, STM images show a snapshot during the formation process of the 7 × 7 DAS structure.
Therefore, the quenched Si(111) surface yields information
about the formation process of the DAS structure. On theother hand, the STM images obtained for the HBO2 -irradiatedSi(111) surface show the destruction process of the 7×7 structure induced by HBO2 irradiation. In this process, B atoms are
considered to disturb the stability of the DAS structure, andinduce the change into the disordered structure.14)
3.2 Structural analysis model including underlying layers
Prior to the detailed discussion about the quenched andHBO2 -irradiated Si(111) surfaces, we illustrate the structural
analysis model of a simple Si(111) structure. Figure 2 shows
the schematics of Si(111) surface structures with (a) and without (b) the stacking fault in the underlying layer. In Figs. 2(a)and 2(b), the large circles, closed circles and closed ellipses
correspond to the adatoms, rest atoms, and dimers, respectively, as shown in the inset. In the case of the Si(111)7 × 7 structure, electrons transfer from the dangling bondstate of top-layer adatoms to that of the rest atoms in theunderlying layer. The quantity of transferred charge on theadatoms depends on the number of rest atoms that surroundthe adatoms.8, 9) By using these characteristic properties ofSi(111)-7 × 7 structure, we can analyze the atomic arrangement in the second and third layers of the DAS structure. As
shown in Figs. 2(a) and 2(b), the arrangement of adatoms
is the same in both cases, however, the stacking fault structure is formed only in the case of Fig. 2(a). This structuralFig. 2. Schematics of Si(111) surface structure (a) with and (b) without
the stacking fault in the underlying layer. The large circles, closed circles
and closed ellipses correspond to the adatoms, rest atoms, and dimers,
respectively. Gray and white adatoms are considered to become dark andbright, respectively in the filled-state STM image.

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

difference changes the number of rest atoms surrounding theadatoms. In the case of the normal stacking structure shownin Fig. 2(b), four rest atoms exist around one adatom. On
the other hand, when the stacking fault structure exists in theunderlying layer, dimers are formed at the boundary between
unfaulted and faulted regions as shown in Fig. 2(a). The number of rest atoms surrounding an adatom is reduced as a resultof the dimer formation, that is, two rest atoms exist around
one adatom. Therefore, the adatoms on the underlying layer
with normal stacking structure have less charge density thanthose on the underlying layer with stacking fault structure.
As a result, the adatoms on the underlying layer with normal
stacking structure become dark in the filled-state STM image,while the brightness of adatoms does not change for the caswith stacking fault structure. Therefore, we can analyze theunderlying layer structures by using bias-dependent STM images.
In the following sections, we analyze the quenched andHBO2 -irradiated Si(111) surfaces in detail by using the technique described above. From the viewpoint described in the
introduction, we focus on the role of corner holes in the formation and the stabilization processes of the DAS structure.

K. M IYAKE et al.

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ber of corner-hole-like structures in the disordered area, as
indicated by circles in Fig. 4(a) (Vs = 2.0 V, It = 0.3 nA).
A magnified image of this structure and its schematic model
are shown in Figs. 4(b) and 4(c), respectively. These figures clearly show that the corner-hole-like structures exist at
the boundary of three out-of-phase c(2 × 8) domains. This
result suggests the possible formation mechanism of corner

3.3 Analysis of the quenched Si(111) surface
On the quenched Si(111) surface, corner holes clearly exist
along the boundaries between the 7 × 7 and disordered areas,
as previously pointed out.7) We analyzed the boundary structures using the structural analysis model proposed by us.10, 11)
Figure 3 shows a filled-state STM image (Vs = −2.0 V,
It = 0.3 nA) of a boundary region between 7 × 7 and disordered structures. The adatoms of the corner holes are brighteven in the filled-state image as shown in Fig. 3. On the basis of the structural analysis model, these corner holes are
surrounded by complete stacking fault and dimer structures;
charge transfer is suppressed by the formation of underlying
layers.
DAS fragments, which consist of some half units of DAS
structures, were also observed in the disordered area as shownin Fig. 4(a). From the same analysis, a stacking fault structureis determined to be formed under these half units. In addition,
they share the same corner hole. These results strongly suggest that corner holes with complete stacking fault and dimer
structures in the second layer (complete corner holes) play
an important role in the formation process of DAS structures.
Energetic analysis of the DAS formation process in a subunit
level is discussed elsewhere.15)
On the quenched Si(111) surface, there exist a large num-Fig. 3. Filled-state STM image (Vs = −2.0 V, It = 0.3 nA) of a boundary
region between 7 × 7 and disordered structure on a quenched Si(111) surface. The solid and dashed circles indicate the complete and incomplete
corner holes, respectively.

Fig. 4. (a) STM image of a quenched Si(111) surface (Vs = 2.0 V,
It = 0.3 nA). (b) Magnification of the corner-hole-like structure indicatedby the square in (a). (c) Schematic structure of (b).

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

images. It is clear that corner holes exist along the boundary between the 7 × 7 and disordered area as in the case of
the quenched surface. The adatoms surrounding the cornerholes which exist along the boundary are bright even in thefilled-state STM image (Fig. 5(b)). This result indicates thatthe complete stacking fault and dimer structures are formedin the second layer. Namely, the complete corner holes play
an important role not only in the formation process but also inthe stability of the 7 × 7 structure.
4. Conclusions
Bias-dependent STM images of the quenched and HBO2 molecule-irradiated Si(111) surfaces showed that the complete stacking fault and dimer structures are formed in the second layer around the corner holes existing along the boundary
between 7 × 7 and disordered structures. This result indicatesthat the complete corner holes play an important role in boththe formation and the stabilization processes of 7 × 7 DAS reconstruction. In addition, the formation of a structure similar
to a corner hole was often observed at boundaries of three outof-phase c(2 × 8) structural domains in a quenched surface,which may be a nucleus for the formation of corner holes.
Fig. 5. (a) Empty- (Vs = 2.1 V, It = 0.45 nA) and (b) filled- (Vs = −2.1 V,
It = 0.45 nA) state STM images of the boundary between 7 × 7 and disordered structure on a HBO2 -irradiated Si(111) surface. The solid anddashed circles indicate the complete and incomplete corner holes, respectively.

Acknowledgement

holes, namely, the nucleation of corner holes may occur atthe boundary of the three out-of-phase c(2 × 8) domains. Thec(2 × 8) local ordering in the disordered region is formed bysimple adatom arrangement with the c(2×8) ordering over the
rest atom layer, and does not intrinsically include the stacking
fault structures.6) However, the strain induced by the domainboundary may act as a trigger for the introduction of dimers
and stacking fault structures to form complete corner holes.

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3.4 Analysis of the HBO2 -irradiated Si(111) surface
In the disordered structure formed on the HBO2 -irradiatedSi(111)√surface, there exist local atomic arrangements of√( 3 × 3)R30◦ , 2 × 2, and c(2 × 4) structures. In a previous
study, we analyzed their origin, and confirmed that the disordered structure is an intrinsic part of the B-induced reconstruction.14) Namely, the shorter B-Si bond length increases
the tensile stress in the surface layers, resulting in the disturbance of the stability of the DAS structure.14) In fact, in thecase of the HBO2 molecular irradiation, there exist no cornerhole-like structures and DAS fragments that were observed in
the case of the quenched surface as shown in Fig. 1(b).
Figures 5(a) and 5(b) show empty- (Vs = 2.1 V, It =0.45 nA) and filled-state (Vs = −2.1 V, It = 0.45 nA) STM

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