PHYSICAL REVIEW B

VOLUME 55, NUMBER 8

15 FEBRUARY 1997-II

Role of corner holes in Si„111…-737 structural formation
studied by HBO2 molecular irradiation and quenching
Koji Miyake, Masahiko Ishida, Kenji Hata, and Hidemi Shigekawa*
Institute of Materials Science, and Center for TARA, Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba 305, Japan~Received 30 September 1996!
By using the characteristic of the HBO2 molecules, which initially react preferentially with the centeradatoms in the unfaulted half units of the 737 structure, stability of corner holes in Si~111! structure wasstudied by scanning tunneling microscopy. The 737 structure began to break from the areas below step edges.
Along the boundaries between 737 and the disordered areas including the A33 A3 phase, the structure of thecorner hole in the 737 structure was observed to be maintained. A similar structure was also observed on aquenched Si~111! surface. The results obtained indicate that corner holes play an important role in the formation mechanism of the 737 structure. In fact, structures in which a dimer-adatom-stacking fault structure wasconsidered to be formed from a corner hole were observed on a quenched Si~111! surface.
@S0163-1829~97!03507-8#

It is well established that the Si~111! surface forms theDAS ~dimer-adatom-stacking-fault! structure, and that the737 structure is the most stable of the DAS structures.1
Since the DAS structure consists of a few surface layers andis very complicated, the stability and formation mechanismof the structure has been attracting considerable attention.
According to recent scanning tunneling microscopy ~STM!
studies on quenched and high-temperature Si~111! surfaces,dimers in the second layer were confirmed to play an important role in stabilizing the DAS structure.2–6 In addition tothe characteristic, nucleation of faulted half units at theshared corner holes was observed in triangular domains of
131 area in a quenched Si~111! surface.6 Since corner holes
are related to the structure of the stacking fault formed in a737 unit, the observed results suggest that corner holes play
an important role in stabilizing the 737 structure. A model

FIG. 1. STM images of Si~111! surface after HBO2 irradiationfor 3 min at 750 °C. Tip bias voltage V t and tunneling current I t are
1.0 V and 1.0 nA, respectively. The number of the reacted adatoms
are shown in the inset.
0163-1829/97/55~8!/5360~4!/$10.00

55

in which oxygen atoms serve to form and stabilize cornerholes was proposed by Ohdomari.7 However, to our knowledge, the role of corner holes has not been clarified yet.
In order to clarify the mechanism for stabilizing the 737structure, it is necessary to study the breaking process of theDAS structure, in addition to the analysis of its formation
process. One possible way is to observe the structural change
caused by chemical reaction of the surface with atoms ormolecules. However, in general, atoms and/or molecules react preferentially with and disturb the faulted half units in-FIG. 2. ~a! STM image of Si~111! surface after HBO2 irradiation for 5 min at 750 °C, which includes 737 and disordered areas
introduced by HBO2 irradiation ~V t 521.0 V and I t 51.0 nA!. Corner adatoms forming corner holes of the 737 structure at theboundary are indicated by dots. ~b! Schematic structure of a cornerhole in the 737 structure. Ellipses represent dimers, and faulted andunfaulted half units are indicated by F and U, respectively.
5360© 1997 The American Physical Society

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ROLE OF CORNER HOLES IN Si~111!-737 . . .

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FIG. 3. ~a! STM image of quenched Si~111! surface ~V t 522.0 V and I t 50.3 nA!. Corner adatoms forming corner holes of 737 structure
at the boundary are indicated by dots. ~b! Magnification of the rectangular area in ~a!. ~c! Magnification of the 737 fragment indicated by
C in ~a!. Two schematic models for the structure shown in ~c!; without @~d!# and with @~e!# stacking faults.

cluding the stacking faults.8 Since, with STM, we have toanalyze the underlayer structure through observation of the
top-layer adatom structure, such a disturbance hampers detailed analysis of the structure of the faulted half units upon
breaking the 737 structure.
According to recent results obtained by STM,9 a uniform
A33 A3 structure was formed by irradiation of HBO2 molecules onto a Si~111!-737 surface at ;750 °C. In its initialstage, HBO2 molecules were found to react preferentiallywith the unfaulted half units of the Si~111!-737 structure.
Therefore, even after partial breakdown of the 737 structure,
it is expected to be possible to analyze the structure of stacking faults including the role of corner holes in the stabilization of the 737 structure.
In this work, we present the results of STM observation of
the initial structural change from the Si~111!-737 phase to
the ordering Si~111!- A33 A3 phase formed by HBO2 molecular irradiation. The results obtained on a quenched
Si~111! surface are also discussed.

Phosphorus-doped n-type Si~111! ~r50.4;1 V cm! substrates were chemically cleaned and degassed at ;500 °C
before being subjected to flash heating in order to form the737 structure. For HBO2 molecular irradiation, after confirming that a clean 737 structure had been obtained, HBO2
molecules were irradiated onto the surface with the substratetemperature kept at ;750 °C. The HBO2 cell temperatureduring irradiation was kept at ;800 °C. All STM observations were performed at room temperature using an electrochemically etched W tip. The base pressure was ;1310210
Torr and the pressure during HBO2 irradiation was;231028 Torr.
Figure 1~a! shows a STM image of a surface formed after
HBO2 irradiation for 3 min. The Si~111!-737 structure remained, but some half units of the 737 structure became
dark. The number of reacted adatoms on the terrace aroundthe imaged area is shown as a bar graph in the inset of Fig.
1~a! @Fig. 1~b!#. As shown in Fig. 1~b!, HBO2 molecules

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KOJI MIYAKE et al.

initially react preferentially with the center adatoms in theunfaulted half units, the details of which will be presented
elsewhere.10 Since HBO2 molecules react preferentially withthe center adatoms in the unfaulted half units, the stability of
the corner holes and the stacking fault structure is expected
to be analyzed after partial breakdown of the 737 structuralarea as mentioned above.
Figure 2~a! shows a STM image obtained after irradiationof HBO2 molecules for 5 min, which includes the remaining
737 and introduced disordered areas. The 737 structure wasbroken from the areas below step edges on the surface. Some
A33 A3 structural islands were formed in the disordered
area as indicated by arrows.
As indicated by dots in Fig. 2~a!, corner holes remainalong the boundary between the 737 and the disordered areas. Preferential reaction of HBO2 molecules with the centeradatoms in unfaulted half units made this observation possible. The observed result indicates that the structure of acorner hole is stable compared to other parts of the 737 unit.
Since a corner hole has a symmetric structure as shown inFig. 2~b!, the existence of a complete corner hole may balance the strain around it. Such stabilization mechanism may
be related to the dimer-adatom pairing mechanism proposed
by Payne,11 in which a combination of a dimer with twoadjacent adatoms reduces surface energy. In a previous STM
observation at ;600 °C, corner holes were not completed at
the boundary, which indicates that the structure of the isolated corner hole at the boundary is not stable enough at thetemperature.5
In order to compare the result obtained in the breakdown
process of the 737 structure with that in the 737 formation
process, a quenched Si~111! surface was observed by STM.
Figure 3~a! shows a STM image of a Si~111! surface obtained by quenching from 1200 °C. When the disordered
area is narrow, it is considered to be difficult to study thestability of isolated corner holes. Therefore, as shown in Fig.
3~a!, wide disordered area with 737 domains was searched
for and investigated. In addition to 737 domains, domains
consisting of c~238! and other structures also exist. However, no A33 A3 structural domains were formed on thequenched surface, as expected.
Figure 3~b! shows a magnification of the rectangular areain Fig. 3~a!. Similar to the surface formed by HBO2 irradiation shown in Fig. 2~b!, corner holes exist at the boundary
between the 737 and disordered areas as indicated by dots inFig. 3~b!. Since corner holes were observed at boundaries inboth the breakdown @Fig. 2~a!# and formation @Fig. 3~b!#processes of the 737 structure, they are considered to play
an important role in the stability and the formation mechanism of the 737 structure.
As indicated by arrows in Fig. 3~a!, three fragments ofDAS structural islands exist. A magnification of the central
one indicated by C in Fig. 3~a! is shown in Fig. 3~c!. Schematics of the two structural models of Fig. 3~c! are shown inFigs. 3~d! and 3~e!, in which the model in Fig. 3~d! does nothave stacking faults, but that in Fig. 3~e! does. Both models
can reproduce the observed structure well. However, in consideration of the number of dangling bonds in each model,
the model with stacking faults shown in Fig. 3~e! is morepossible. If the observed fragments of the 737 structure inFig. 3~c! are a quenched structure formed during the forma-55

FIG. 4. ~a! Magnification of the 737 fragment indicated by L in
Fig. 3~a!. ~b! Schematic of the structure of ~a!.

tion process as shown in Fig. 3~e!, frontier areas with excess
dangling bonds exist along the boundary between the normal
and the stacking fault areas as indicated by arrows in Fig.
3~e!; stacking faults are formed from a corner hole by recombining the Si-Si bonds one by one. With this assumption, theformation of a corner hole is considered to serve as a trigger
for 737 structural growth. As is observed in Fig. 2~a!, cornerholes were stably observed also in the breakdown process ofthe 737 structures, which supports the mechanism. Oxygen
atoms may play a role to form and stabilize a corner hole asproposed by Ohdomari.7
When formation of a corner hole serves as a trigger for
737 structural growth, fragment of DAS structural islandwith one corner hole is expected to exist. Figure 4~a!, a magnification of the area indicated by L in Fig. 3~a! shows anexample. Schematic of the structural model of Fig. 4~a! isshown in Fig. 4~b!, in which the structure of stacking faults
is assumed similar as that in Fig. 3~e!. As is shown in Fig. 4,
DAS structure is growing from a corner hole. Since adatoms
were always observed in the area with stacking faults asshown in Figs. 3 and 4, the dimer-adatom pairing mechanism
proposed by Payne11 is considered to play an important roleto stabilize the DAS structure from the beginning of its formation process.

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ROLE OF CORNER HOLES IN Si~111!-737 . . .

Fragment structure of the faulted halves is similar in both
Figs. 3~c! and 4~a! as the frontiers of the area with stacking
faults are indicated by arrows, which agrees well with theprocess that formation of a faulted half unit is triggered froma corner hole and ends at the other corner hole by recombining the Si-Si bonds one by one as mentioned above.
Stability of the faulted half units of the DAS structures
has been known.3,5,6 Here, as shown in Fig. 4, a 535 structure is formed from a corner hole. Similar formation processof a 939 structure exist at the fragment of DAS structure
indicated by R in Fig. 3~a!. These results may indicate that
corner holes play a similar role in all DAS structures.
In conclusion, corner holes were observed to remain at theboundaries between 737 and disordered areas on the Si~111!
*Author to whom correspondence should be addressed. Electronic

address: hidemi@mat.ims.tsukuba.ac.jp
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surfaces formed by HBO2 irradiation and quenching. Theobserved results indicate that corner holes play an importantrole in the stability and formation mechanism of the 737
structure. Structures of the DAS fragments formed byquenching were explained well with this model.
We wish to thank Dr. T. Nagamura and T. Ozawa ofUNISOKU Corp. for their help in the measurement for
HBO2 irradiation. This work was supported by theShigekawa Project of TARA, University of Tsukuba. Support from a Grant-in-Aid for the Scientific Research from theMinistry of Education, Science and Culture of Japan is alsoacknowledged. One of the authors ~K.M.! was financiallysupported by the Japan Society for the Promotion of Sciences ~JSPS!.
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6