Intermediate structures appearing in the phase transition of Si„111…-737
to „A33A3…R30° induced by HBO2 molecular irradiation
K. Miyake,a) T. Kaikoh, and K. Hata
Institute of Materials Science, Center for TARA, CREST, Japan Science and Technology Corporation (JST),
University of Tsukuba, Tsukuba 305-8573, Japan

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

H. Shigekawab)
Institute of Materials Science, Center for TARA, CREST, Japan Science and Technology Corporation (JST),
University of Tsukuba, Tsukuba 305-8573, Japan, and Department of Chemistry and Biotechnology,
Graduate School of Engineering, The University of Tokyo, Hongo, Tokyo 113-8656, Japan~Received 26 October 1998; accepted 15 February 1999!
We have investigated the breakdown process of the 737 structure during its transition to the~A33A3!R30° phase induced by HBO2 irradiation. The 737 structure was destroyed over the wide
area before the nucleation of the ~A33A3!R30° phase. Even when the ~A33A3!R30° structure
nucleated in the disordered area, disordered structure existed between the domains of the 737 and~A33A3!R30° structural phases. The observed results are completely different from the ~A33A3!R30°
formation processes induced by other group III atoms, where ~A33A3!R30° phases are formed asadjacent to the 737 structural domains. The surface B atom concentration in the 737, disordered,and ~A33A3!R30° regions were estimated to be about one, three, and six per 737 unit cell. The
disordered structure remained on the surface even after annealing the sample at 750 and 900 °C.
And the B concentration in the disordered region did not change. These results indicate that thedisordered structure is an intrinsic nature of the B-induced reconstruction, and suggest the existenceof the relationship between the B atom concentration and the stability of the 737 structure.
© 1999 American Vacuum Society. @S0734-2101~99!10204-5#

I. INTRODUCTION
Structure of the dimer-adatom-stacking fault ~DAS!1 is
most widely accepted as the best model for the Si~111!-737
reconstructed surface because it has been compatible withthe theoretical2,3 and experimental4,5 results. However, mostof these studies were performed in the static form, and study
on the dynamic process of the 737 structure has not yet beenclarified. Therefore, study of the dynamics of phase transitions between the 737 and the other structures is still attracting considerable attention, and is important in order to understand the mechanism for the formation and stabilizing
processes of the DAS structure.
It is well known that the 737 structure changes into ~A3
3A3!R30° structures by deposition of the group III atoms
onto the surface. In general, there coexist osculating large
domains of the 737 and ~A33A3!R30° structures on the surface, and the ~A33A3!R30° structural domains are formed bybeing connected to the 737 domains during the phasetransitions.6–8 In the case of B atom, however, the ~A3
3A3!R30° domains are never adjacent to the 737 domains
and there always exists disordered structural region between
the 737 and ~A33A3!R30° domains. Therefore, it is interesta!

Electronic mail: miyake@ims.tsukuba.ac.jp
Electronic mail: hidemi@ims.tsukuba.ac.jp; world wide web: http://
dora.ims.tsukuba.ac.jp

b!

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J. Vac. Sci. Technol. A 17„4…, Jul/Aug 1999

ing and important to study the B-induced transition process
for the understanding of the stability of the 737 structure.
According to the previous reports,9,10 737 structure remained on the surface even when 1–2 B atoms were included in a 737 unit cell, but was destroyed with the concentration of several B atoms.10 It is intriguing to clarify thathow many B atoms break the 737 structure. However, the
B-induced reconstruction was introduced by annealing aheavily B-doped Si~111! substrate to segregate the dopant tothe surface.9,10 Therefore, in this case, B atoms, originally
included in the bulk, are considered to suppress the formation of the 737 structure instead of the breaking processdescribed above. Furthermore, the surface B concentration isdifficult to be controlled by this conventional heat treatment.
In addition, since a similar disordered structure was observedon the quenched Si~111! surface,11–13 the effect must be discriminated.
Recently, the B-induced ~A33A3!R30° structure wasfound to be formed by the deposition of B compounds(B10H14 B2O3, and HBO2). 14–21 In this case, control of thesurface B concentration is easier compared to the conventional high-temperature annealing processes.
In this article, we analyzed in detail the breakdown process of the 737 structure and the disordered structure formed
during the phase transition from the 737 to ~A33A3!R30°
structure induced by HBO2 molecular deposition.

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Miyake et al.: Intermediate structures appearing in the phase transition of Si„111…-737

FIG. 1. STM images of the surfaces formed by ~a! 3, ~b! 5, ~c! 6, and ~d! 15
min of HBO2 irradiation. Tip bias voltage V s , tunneling current I t , and scansize are ~a! 22.0 V, 0.4 nA, 35 nm335 nm, ~b! 2.6 V, 0.4 nA, 35 nm335
nm, ~c! 23.0 V, 0.33 nA, 35 nm335 nm, and ~d! 1.0 V, 1.0 nA, 35 nm335
nm, respectively.

II. EXPERIMENTS
Phosphorus-doped n-type Si~111! ~r51 V cm! substrates
were used in this study. The samples were chemicallycleaned by acetone before being loaded into the ultrahighvacuum ~UHV! chamber. The samples were outgassed at;500 °C for 12 h. Subsequently, the samples were flashed atabout 1150 °C in order to form the 737 structure. After confirming the formation of the clean 737 structure by scanningtunneling microscopy ~STM!, HBO2 molecules were irradiated onto the surface. HBO2 was irradiated from BN cellheated by a W wire coil. All irradiations were performed
with the substrate temperature being kept at ;750 °C in order to prevent oxygen incorporation. The HBO2 cell temperature during irradiation was kept at ;800 °C. All STM
observations were performed at room temperature using anelectrochemically etched W tip. The base pressure was ;1310210 Torr and the pressure during HBO2 irradiation wasless than 231028 Torr. When it was impossible to observe
the same area, for example due to additional irradiation or
annealing at high temperature, the observed structures werechecked over a wide area for each condition.
III. RESULTS AND DISCUSSION
A. Destruction of the 737 structure by HBO2
irradiation

Figures 1~a!–1~d! show STM images of the surfaces
formed by ~a! 3, ~b! 5, ~c! 6, and ~d! 15 min of HBO2 irradiation. In the initial stage, 737 reconstruction remained on
the surface as shown in Fig. 1~a!. However, some adatoms
became dark. Since these dark adatoms were observed after
HBO2 irradiation, they are considered to be resulted from the
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reaction with HBO2 molecules. In addition, they were observed preferably in the unfaulted half units, similarly as theprevious results.11,20,21
With increase in the HBO2 irradiation time, the disorderedstructure appeared on the surface through the destruction of
the 737 structure. However, the 737 structure still remainedon the surface at this stage, and the ~A33A3!R30° structure
was not formed @Fig. 1~b!#. With further irradiation, the 737
structure disappeared except around the step edge here, andthe disordered structure expanded over the surface @Fig.
1~b!–1~c!#. Recently, 737 domain was confirmed to havetemperature dependent critical size for the stable existenceon the surface.22 Since B atoms induce strain on the surface
here, similar critical size must exist. The observance of thesmall 737 domains at the step edges in Fig. 1~c! may be dueto the fact that 737 structure is stable at step edges. In fact,a larger 737 domain with the size of 12 nm312 nm size wasobserved in a different terrace.
In addition, the ~A33A3!R30° structure nucleated in thedisordered structure at this stage, but the ~A33A3!R30° structural area was not adjacent to the 737 structural domains.
There always existed the disordered structure between the
737 and ~A33A3!R30° domains. Finally, 737 structural area
disappeared and the surface was covered with the small domains of the ~A33A3!R30° structure @Fig. 1~d!#. The ~A3
3A3!R30° structural domain is grown and a large domain ofa single phase of a ~A33A3!R30° structure formed on thesurface after further irradiation.20 This formation process isquite different from the process induced by the other group
III atoms. In the next section, we consider and discuss thebreakdown process of the 737 structure in detail.
B. Selective reaction in the initial stage

When atoms or molecules are adsorbed on the Si~11!-737
surface, most of them generally react with the faulted halfunits of the 737 structure in the initial stage.23 However, B
atoms react preferentially with the unfaulted half units of the
737 structure as shown in Fig. 1~a!.11,20,21 It is well known
that the unfaulted half units of the Si~111!-DAS structure areobserved darker compared to the faulted half units at negative sample bias voltages by STM. This means that the unfaulted half unit of the 737 structure is more positive than
the faulted half unit. In addition, with comparison of thecenter and corner adatoms in each half unit cell, the amountof charge transfer from them to the rest atoms is larger forthe center adatoms than the corner adatoms.24,25 From thesefacts, center adatoms in unfaulted half units are most positive
among the adatoms in a 737 DAS unit. In consideration ofthis surface charge distribution with the fact that HBO2 molecules react preferentially with the center adatoms in the unfaulted half units, negatively charged molecules are considered to be included in the molecules deposited onto thesurface.
In order to clarify the reaction process on the surface, wemeasured the temperature dependence of the mass spectra of
the molecules from the HBO2 source. Figure 2 shows thechange of the mass signals with the HBO2 cell temperature.

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FIG. 2. Temperature dependence of mass signal from HBO2 source.

Two dominant peaks related to HBO2 molecule were observed; mass 28 and 44 signals corresponding to HBO andHBO2 molecules, respectively. Both components increase
with temperature, however, mass 28 peak becomes larger
from around 600 °C. For the HBO2 cell temperature of theexperimental condition, ;800 °C, mass 28 is two orders
larger than the other. This result indicates that the molecularflax onto the surface mostly consists of HBO molecules.
Since a HBO2 molecule has the structure of H–O–B5O,
HBO molecules is considered to be produced as the oxygen
molecules of the right end being removed ~H–O–B!. Therefore, HBO molecules are expected to become negatively
charged, and they may react with the positively charged adatoms; center adatoms in the unfaulted half units, which isconsistent with the STM results obtained.
C. Structures of the intermediate phases

In this section, we analyze the disordered structures observed during the phase transition from 737 to ~A33A3!R30°
structure @Figs. 1~a!–1~d!#, and its relation to the 737 structure in detail. The disordered structure existing between the
737 and ~A33A3!R30° appears only for the B/Si surface, but
not for the surface formed by the other group III atoms as
described above. Why does only a B atom induce the disordered structure?
According to the previous reports on the B-induced reconstruction formed by annealing a heavily B-doped Si~111!
substrate,9,10 737 structure remained on the surface evenwhen 1–2 B atoms were included in a 737 unit cell, but wasdestroyed at the B concentration of several atoms per thecell. Figure 3 shows a magnified STM image of the boundary
between 737 structure and the disordered structure. According to the previous reports,14,26 the bright and dark protrusions in the positive sample bias ~empty state image! correspond to the Si and B related adatoms, respectively. We
counted the concentrations of the B related adatoms in the737, disordered and ~A33A3!R30° structures. The average
concentrations were about one, three, and six per 737 unit
cell for the 737, disordered and ~A33A3!R30° structural regions, respectively. This is quite consistent with the previous
J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999

FIG. 3. ~a! Magnified STM image of the boundary between 737 and thedisordered structures (V s 52.1V, I t 50.45 nA, 35 nm335 nm!. ~b! The surface B atom concentration ~atoms per 737 unit cell size! in the 737, disordered, and ~A33A3!R30° regions.

results that the 737 reconstruction was destroyed by the existence of the several B atom concentration per 737 unit
cell.10
In a previous paper,21 we showed those adatoms surrounding the firstly reacted adatom have higher reaction
probability for the following HBO2 molecules compared to
the normal adatoms. Therefore, B atoms have a tendency toconcentrate from the beginning of the reaction. Then when
the number of concentrated B atoms reaches to the value of
three atoms per 737 unit cell, the 737 structure is considered to break. However, the 737 structure still remains in theregion with low B concentration. Consequently, there exist
the 737 and the disordered structure on the surface.
In the case of the ~A33A3!R30° structural formation byannealing heavily B-doped Si~111! substrate, B atoms areincluded in the bulk from the beginning and are consideredto suppress the formation of the 737 structure. On the other
hand, when the B atoms are deposited on the 737 surface,
they are considered to break the 737 structure at the critical
B concentration on the surface. In any case, existence ofalmost three B atoms is enough to make the 737 structure
unstable.
The 737 structure is stabilized by the energy balance between the reduction of the dangling bonds and introductionof the unfavorable strains concerning the dimer wall and the
local 232 reconstruction. In the case of the 737 structure,1599

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FIG. 5. STM image of the surface after annealing the sample of Fig. 1~b! at
750 °C for 5 min (V s 52.5 V, I t 50.3 nA, 35 nm335 nm!.

derived from covalent radii, is significantly less than theideal Si–Si bond length.31 Therefore, the shorter B–Si bondlength increases the tensile stress in the surface layers, resulting in the decrease of the relative stability of the 737 DAS
structure. Then the 232 and/or c(234) adatom arrangement
without dimer wall is considered to become more stable than
the 737 DAS structure at the critical surface B concentration.
On the other hand, about 16 B atoms per 737 unit arenecessary to form the ~A33A3!R30° structure completely.
Even in the actual, about six B atoms seem to be necessary tostart the transition as shown in Fig. 3. This value is stilllarger than the observed critical B concentration ~about three
B atoms per 737 unit cell!. Therefore, destruction of the
737 structure does not result in the ~A33A3!R30° structure,but changes into the other structures, such as 232 or c(234) first.

FIG. 4. STM images of ~a! the surface formed by 25 min of HBO2 irradiation ~ V s 52.0 V, I t 50.8 nA, 30 nm330 nm!, ~b! the surface after annealing~a! at 750 °C for 5 min (V s 52.0 V, I t 50.87 nA, 30 nm330 nm!, and ~c! thesurface after annealing ~b! at 900 °C for 8 s (V s 52.0 V, I t 51.0 nA, 30
nm330 nm!.

introduced strains are tensile strain.27 When the surface isalloyed with Ge or Sn, compressive strain is introduced,
which reduces the unfavorable strain in the surface and isconsidered to stabilize the 737 DAS structure.28–30 In thecase of B atoms, however, the ideal B–Si bond length, as
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FIG. 6. STM image of the surface after annealing the sample of Fig. 5 at
900 °C for 2 s (V s 53.0 V, I t 50.33 nA, 35 nm335 nm!.

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The disordered structure almost covers the surface insteadof the formation of the ~A33A3!R30°, which is completely
different from the ~A33A3!R30° structures formed by otherIII atoms. In the other cases, small ~A33A3!R30° structural
domains are formed adjacent to the 737 structural domains.
In order to clarify the relation between B atoms and theintermediate structures, we analyzed the disordered structures more in detail.
D. Structure of the annealed surface after HBO2
irradiation

First, we annealed the completed ~A33A3!R30° structure
in order to check the surface B concentration before and after
annealing. Figure 4 shows the STM images of ~a! the surface
formed by 25 min of HBO2 irradiation, ~b! the surface after
annealing the sample ~a! at 750 °C for 5 min, and ~c! thesurface after annealing the sample ~b! at 900 °C for 2 s.
As shown in Fig. 4~a!, the surface was covered with thecompleted ~A33A3!R30° structure. There exist bright anddark adatoms on the surface. These adatoms correspond to
the Si and B related adatoms, respectively.20 The surface
structure obtained after annealing the sample ~a! at 750 °C@Fig. 4~b!# is quite similar to that of ~a!, and there is littlechange of the ratio of the two kinds of adatoms. The surface
B concentration did not change with annealing at 750 °C.
This fact indicates that it is proper to keep the substrate temperature at 750 °C during HBO2 irradiation in order to prevent oxygen incorporation.
On the other hand, some defects are introduced after annealing the sample at 900 °C as shown in Fig. 4~c!. According to the x-ray photoelectron spectroscopy ~XPS!
measurement,15 the deposited B atoms remained on the surface after annealing at about 700 °C, but begun to diffuse
into the bulk at annealing temperature of 900 °C. Since theconcentration of B atoms is higher at the surface in this case,B atoms will not come back to the surface once they moveinto the bulk. Our STM results look consistent with the XPS
result.
On the basis of the obtained results, let us analyze thesurface after annealing the sample in the intermediate stageof the breakdown process. Figure 5 shows the STM image ofthe surface after annealing the sample of Fig. 1~b! at 750 °C
for 5 min. It clearly shows that there exists similar disorderedstructure on the surface even after annealing at 750 °C. As
we have checked above @Fig. 4~b!#, B atoms still remainedon the surface. In addition, the surface structure did notchange before and after annealing at 750 °C, and the surface
B concentration at the disordered structure is about threeatoms per 737 unit, which is equivalent to that before annealing. We did not observe ~A33A3!R30° structural domains
formed adjacently with the 737 structural domains like thecase of other group III atoms.
Next, the sample was heated up to 900 °C, where the B
atoms are considered to diffuse into the bulk. Figure 6 shows
an STM image after annealing the sample shown in Fig. 5 at
900 °C for 2 s. The disordered structure still remains on thesurface, even the size of the areas is smaller than that of Fig.
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5. This change must be due to the diffusion of B atoms into
the bulk, however, there still existed about three B atoms per
737 unit at the disordered structure, i.e., there is not change
in the B concentration at the disordered structure even after
annealing at 900 °C. The observed phenomena must be related to the balance among the diffusion of B and Si atoms,
formation of the 737 phase, and stability of the disorderedstructures.
The obtained results strongly suggest that the disorderedstructure observed here is an intrinsic nature of the
B-induced reconstruction, differing from the disorderedstructure formed by quenching. Tensile stress induced by B
atoms may play an important role in the mechanism. On theother hand, existence of almost three B atoms is enough todestroy the 737 structure, but about six B atoms are necessary to form the ~A33A3!R30° structural domains on the surface. This fact may suppress the formation of the ~A3
3A3!R30° structure in adjacent to the 737 structure.
In order to clarify the nature of the B/Si surface, it isnecessary to analyze the phase transition induced by othergroup III atoms from the view points discussed in this article.
IV. CONCLUSION
We analyzed the breakdown process of the Si~111!-737
DAS structure induced by HBO2 irradiation. HBO molecule
was a dominant species in the B source flax. Because HBO
was negatively charged, molecules preferentially reactedwith the center adatoms in the unfaulted half-units. Withfurther irradiation, the disordered area appeared on the surface before starting the nucleation of ~A33A3!R30°. The average surface B concentrations in the 737, disordered, and~A33A3!R30° regions were about one, three, and six atoms
per 737 unit cell, respectively. The disordered structure remained even after annealing at 750 and 900 °C and the surface B concentration in the disordered region did not change.
These results suggest that the disordered structure observed
here is induced by a stable intrinsic nature of the B/Si~111!
structure, and the critical value of the B concentration todestroy the 737 structure is about three per 737 unit cell.
ACKNOWLEDGMENTS
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports,
and Culture of Japan.
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