Site preferences of oxygen and boron atoms during dissociative reactionof HBO2 molecules onto the Si„111…-7Ã7 surface
T. Kaikoh, K. Miyake, and Y. J. Li
Institute of Applied Physics, CREST, Japan Science and Technology Corporation (JST), Universityof 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. Shigekawaa)
Institute of Applied Physics, CREST, Japan Science and Technology Corporation (JST), Universityof Tsukuba, Tsukuba 305-8573, Japan and Department of Chemistry and Biotechnology, Graduate Schoolof Engineering, The University of Tokyo, Tokyo 113-8656, Japan

共Received 25 October 1999; accepted 20 March 2000兲
Using scanning tunneling microscopy 共STM兲, we succeeded in observing the initial process of the
HBO molecule on the Si共111兲-7⫻7 surface for the first time. Since HBO2 molecules are commonlyused as the B source in the molecular beam epitaxy system, and the dominant flux from the HBO2
source in the ordinary condition is the form of HBO, analysis of the HBO reaction process is highly
important. When the substrate temperature was kept at room temperature during the molecular
irradiation, the adsorbed HBO molecules remained on the surface without dissociation. With thesubstrate temperature being increased, the dissociation process was activated, and the oxygen- andboron-related adatoms were produced at the corner and center adatom sites in the 7⫻7 units,
respectively. The site dependence was well determined by the bias voltage dependent STM. The
selective reaction site of oxygen atoms was comprehensively explained by their electronegativity
and charge distribution in the Si共111兲-7⫻7 surface adatoms. On the other hand, regarding theselectivity for the boron atoms, introduction of the surface strain effect was found to be essential.
© 2000 American Vacuum Society. 关S0734-2101共00兲11604-5兴

I. INTRODUCTION
Boron 共B兲 atom shows characteristic properties on
Si共111兲-7⫻7 surface differing from other group III atoms
共Al, Ga, In兲. Despite group III atoms 共Al, Ga, In兲 occupying
the T 4 adatom site on the Si共111兲 surface, the B atom occupies the S 5 site in the second layer directly underneath the Si
T 4 adatom.1–3 In addition, B atoms are stable with respect tothe deposition of Si overlayers and the two-dimensional B
layer at the Si/Si共111兲 interface is electrically active. Therefore, boron is thought to be one of the most promising candidates for the atomic layer doped 共␦-doped兲 structure. Consequently, it is very important to clarify how B atoms behaveon the Si共111兲 surface in order to control the structure andproperty of atomic layer doped structure.
The B-induced reconstruction was conventionally introduced by annealing a heavily B-doped Si共111兲 substrate tosegregate the dopant to the surface.2–9 However, this conventional heat treatment makes it difficult to control the surface
B concentration. Recently, it was found that the B induced( 冑 3⫻ 冑 3)R30° structure was also formed by the depositionof B compounds (B10H14, B2O3, and HBO2). 1,10–14 In thiscase, control of the surface B concentration is easier compared to the conventional method. Especially, HBO2 molecules commonly used as a B source in the molecular beama兲Electronic mail: hidemi@ims.tsukuba.ac.jp; http://dora.ims.tsukuba.ac.jp

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epitaxy 共MBE兲 system showed the possibility of forming theatomic layer doped structure.12–14
Therefore, the introduction process of the B atoms on
Si共111兲-7⫻7 surface by the molecular irradiation, especiallyusing HBO2 source, is most important, and has been studied
extensively, for example, by scanning tunneling microscopy
共STM兲.15–18 However, its phase transition mechanism is still
unclear. The 7⫻7 structure is destroyed over the wide area
before the nucleation of the ( 冑 3⫻ 冑 3)R30° phase in the caseof B induced reconstruction process.18 Even when the ( 冑 3⫻ 冑 3)R30° structure nucleated in the disordered area, disordered structure existed between the domains of the 7⫻7 and( 冑 3⫻ 冑 3)R30° structural phase. Subsequently, a ( 冑 3⫻ 冑 3)R30° structure consisting of mixed Si and B atoms wasformed and B atoms formed a network and induced phaseordering between domains.17 These phenomena are completely different from the ( 冑 3⫻ 冑 3)R30° formation processes induced by other group III atoms, where ( 冑 3⫻ 冑 3)R30° phases are formed as adjacent to the 7⫻7 structural domains.
In order to understand the characteristic phase transition
processes, it is necessary to clarify the molecular reaction
process of HBO2 molecules on Si共111兲-7⫻7 surface. Recently, HBO2 molecules were found to preferentially reactwith unfaulted halves of the 7⫻7 unit in spite of the fact that
almost all atoms and molecules react with faulted halfunits.15,16 The selectivity of the chemical reaction process

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FIG. 1. STM images obtained after molecular irradiation at substrate temperatures of RT 关共a兲 and 共b兲兴, 300 °C 关共c兲 and 共d兲兴 and 600 °C 关共e兲 and 共f兲兴. Figures
共a兲, 共c兲, 共e兲, and 共b兲, 共d兲, 共f兲 are the filled 关tip bias (V t )⫽2.0 V and tunneling current (I t )⫽0.3 nA兴 and empty-state (V t ⫽⫺2.0 V and I t ⫽0.3 nA兲 images,
respectively. Arrows, open circles, and open triangles indicate bright protrusions, type 1 and type 2 adatoms, respectively.

was attributed to the charge distribution of the molecule.
However, detail reaction process still remains unsolved.
In this article, we present our recent result on the dissociative reaction process of HBO2 molecules on Si共111兲-7⫻7
surface studied by STM.

II. EXPERIMENT
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. After being outgassed at⬃500 °C for 12 h, the samples were flashed at about 1150 °C
in order to form the 7⫻7 structure. After confirming theformation of the clean 7⫻7 structure by STM, molecules
were irradiated on the surface by heating the HBO2 cell at⬃800 °C. With this condition, the HBO molecule was confirmed to be the dominant species in the flux from the B
source as reported previously.18 Irradiation time was fixed at
3 min. The substrate temperature during the treatment wasvaried from room temperature 共RT兲 to 600 °C. All STM observations were performed at RT using an electrochemicallyetched tungsten tip. The base pressure was ⬃1⫻10⫺10 Torr
and the pressure during the molecular adsorption process
was less than 2⫻10⫺8 Torr.
J. Vac. Sci. Technol. A, Vol. 18, No. 4, JulÕAug 2000

III. RESULTS AND DISCUSSION
Figure 1 shows STM images obtained after molecular irradiation at substrate temperatures of RT 关共a兲 and 共b兲兴,
300 °C 关共c兲 and 共d兲兴 and 600 °C 关共e兲 and 共f兲兴. Figures 1共a兲,
1共c兲, 1共e兲, and 1共b兲, 1共d兲, 1共f兲 are the filled- 关tip bias (V t
⫽2.0 V and tunneling current (I t )⫽0.3 nA兴 and empty-state(V t ⫽⫺2.0 V and I t ⫽0.3 nA兲 images, respectively. Whenthe substrate temperature during the molecular irradiation
was kept at RT, bright protrusions indicated by arrows in
Figs. 1共a兲 and 1共b兲 were observed at both bias polarities. On
the other hand, when the substrate temperature was kept at
300 °C, two characteristic-type adatoms that differ from normal 7⫻7 adatoms were observed on the surface in addition
to the bright protrusions. One is the adatoms, which werebright in filled-state images but were slightly dark in emptystate images 关type 1 adatoms; open circles in Figs. 1共c兲 and1共d兲兴. The other is the adatoms, which were darker than normal 7⫻7 adatoms in both filled- and empty-state images
关type 2 adatoms; open triangles in Figs. 1共c兲 and 1共d兲兴. Magnified images of the type 1 and type 2 adatoms are shown in
Fig. 2. When the substrate temperature during molecular irradiation was kept at 600 °C, the bright protrusions observedin Figs. 1共a兲–1共d兲 did not appear, and only type 1 and type 2
adatoms were observed on the surface.
Since the bright protrusions were observed at only lower

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FIG. 2. Magnified images of the type 1 and type 2 adatoms. Figures 共a兲, 共c兲,and 共b兲, 共d兲 are the filled- (V t ⫽2.0 V and I t ⫽0.3 nA兲 and empty-state (V t⫽⫺2.0 V and I t ⫽0.3 nA兲 images, respectively. Open circles and open
triangles indicate type 1 and type 2 adatoms, respectively.

temperatures, they are considered to be the nondissociated
HBO molecules. On the other hand, according to the previous reports about the oxygen molecule adsorption on Si共111兲
surface, bright adatoms were observed on the surface at
negative sample bias voltage 共filled-state image兲 after oxygen adsorption.19 And it is well known that boron-related
adatoms are observed darker than normal Si adatoms in bothbias polarities. Therefore, in consideration of the bias dependence of STM images in Fig. 1, and the fact that the ratio of
the type 1 and type 2 adatoms increased with the substratetemperature, the type 1 and type 2 adatoms are considered tocorrespond to the dissociated oxygen- and boron-related adatoms, respectively.
In order to confirm this reaction process, the sample
formed by molecular irradiation at 300 °C was annealed at
600 °C for 1 min. There existed the bright protrusions, type 1
and type 2 adatoms on the surface before annealing as shownin Figs. 1共c兲 and 1共d兲. After annealing at 600 °C, however,the bright protrusions disappeared and only type 1 and type 2
adatoms were observed on the surface. Namely, the HBO
molecules were dissociated, and oxygen- and boron-related
adatoms were produced during the annealing process at
600 °C. These results strongly support our assignment about
the oxygen- and boron-related adatoms on the basis of thebias dependence of STM images. Since the numbers of theincreased oxygen- and boron-related adatoms were comparable with the number of the original HBO molecules, desorption of the HBO molecules during annealing process was
negligible. In addition, taking these results into consideration
with the fact that H atoms are desorbed at 600 °C, the effectof the H atoms on the observed structures is also neglected.
Next, we consider the reaction process of HBO moleculeson Si共111兲 surface. As shown in Figs. 1共a兲 and 1共b兲, thebright protrusions were often observed at the adatom site. At
lower temperatures, HBO molecules remain on the surface
without dissociation. Since the electronegativity of the oxygen atom is larger than that of boron atom, HBO molecules
JVST A - Vacuum, Surfaces, and Films

FIG. 3. Site distribution of oxygen- and boron-related adatoms in the 7⫻7
unit cells obtained by counting their STM images.

are supposed to be polarized, i.e., oxygen and boron atoms
are expected to be charged as ⫺␦ and ⫹␦, respectively. On
the other hand, it is well known that the dimer-adatomstacking fault 共DAS兲 structure has a characteristic electronic
property; charge transfer from the top-layer adatoms to therest atoms in the underlayer.20,21 Consequently, the rest atom
and adatom may play as an electron donor and acceptor,
respectively. Taking the difference in the electronegativity
between the oxygen and boron atoms into consideration, theoxygen atoms in the molecules are expected to bond directly
to a Si adatom.
When the substrate temperature was higher, the dissociation process was activated. And oxygen and boron atoms
showed a tendency to occupy their preferential sites. Figure
3 shows the site distribution of oxygen- and boron-related
adatoms in the 7⫻7 unit cells obtained by counting their
STM images. With regard to the oxygen-related adatoms, thesite preference ratios were about 1.3:1 for the faulted half tothe unfaulted half, and 2.7:1 for the corner to the centeradatoms. According to the previous reports on the O2 molecular adsorption onto the Si共111兲-7⫻7 surface, the values
were 4:1 and 2:1, respectively.19 Therefore, the observed difference in the occupation between faulted and unfaulted sites
looks small compared to the previous case; however, theuncertainty was a few percent, and the site preference existed
all over the surface. Then, we concluded that the oxygen
adatoms preferentially occupy faulted corner sites. The difference between our results and previous results probably
comes from the processes used in the two experiments; oxygen molecule adsorption and HBO2 molecule irradiation. On
the other hand, the boron-related adatoms were observed at
the unfaulted center sites. In order to explain these site preferences, the difference in the electronegativity regarding theoxygen, boron, and Si atoms is considered.

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Kaikoh et al.: Site preferences of oxygen and boron atoms

Since the electronegativity of oxygen is larger than that ofsilicon, oxygen atoms play as an electron acceptor to Si atoms and are considered to react with the Si adatoms withhigher electron density. On the other hand, it is well known
that the faulted half units of the Si共111兲-DAS structure areobserved brighter compared to the unfaulted half units in
STM images at negative sample bias voltages. This meansthat the faulted half units in the 7⫻7 structure are more negatively charged compared to the unfaulted half units. In addition, regarding the center and corner adatoms in each halfunit cell, the amount of charge transfer from them to the rest
atoms is smaller for the corner adatoms than the centeradatoms.20,21 In consideration of this surface charge distribution together with the difference in the electronegativity between oxygen and silicon, it is probable that the oxygen atoms preferentially occupy the corner adatom sites in thefaulted half units.
On the other hand, the electronegativity of boron is almost
the same as that of silicon. Therefore, it is difficult to explain
the site preference of the boron-related adatoms by only thedifference in the electronegativity. One possible mechanism
is to consider the effect of the surface strain at the adatom
sites. From the 7⫻7 DAS model, the dimer wall and the
local 2⫻2 reconstruction introduce the tensile strain. Since
the center and corner adatoms are surrounded by one and two
dimers, respectively, the strain energy of the corner adatoms
is larger than that of the center adatoms. On the other hand,
the ideal B–Si bond length, as derived from covalent radii, issignificantly less than the ideal Si–Si bond length.22 Therefore, when Si atoms are replaced by B atoms, the shorter
B–Si bond length increases the tensile strain in the surface
layers. Consequently, boron atoms are considered to occupy
the center adatom sites.
As has been discussed here, surface strain is considered toplay an important role in the B introduction process. Suchphenomena have been observed in the case of B induced
Si共111兲 surface reconstruction as mentioned in the introduction of this article.17,18 For example, the 7⫻7 structure isdestroyed over the wide area before the nucleation of the( 冑 3⫻ 冑 30)R30° phase.18 Boron induced strain must be necessary to explain the mechanism. Compared to the previous
result,15 less difference of the B reaction site between FH
and UH shown in Fig. 3 is explained by the less mobility ofthe molecules/atoms in this case due to the lower substratetemperatures used for the experiment.
According to the previous x-ray photoelectron spectroscopy 共XPS兲10 and STM1 experiments, B atoms remain on thesurface up to ⬃700 °C occupying the T 4 adatom site, butdiffuse into the bulk side at ⬃900 °C and occupy the S 5
substitutional site. In fact, when the ( 冑 3⫻ 冑 3)R30° surface
formed by HBO2 irradiation at 750 °C was flashed at 900 °C,
two kinds of ( 冑 3⫻ 冑 3)R30° structures with different brightness were observed by STM. In the case of this article, thesubstrate temperature during HBO2 irradiation was kept at
300 and 600 °C. Therefore, B atoms are considered to remain on the surface adatom sites. However, in order to determine the precise position of boron atoms, further study
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such as scanning tunneling spectroscopy 共STS兲 measurement, together with the theoretical calculation, is necessaryfor each adatom site.

IV. CONCLUSION
We succeeded in observing the initial reaction process ofthe HBO molecule on the Si共111兲-7⫻7 surface for the firsttime. When the substrate temperature during molecular irradiation was kept at RT, the adsorbed HBO molecules remained on the surface without dissociation. On the other
hand, when the substrate temperature was kept at 600 °C,
HBO molecules were dissociated, and the oxygen- andboron-related adatoms were produced at the corner and center adatom sites in the 7⫻7 units, respectively. Intermediatetransition process was observed at 300 °C. The site preference of oxygen-related adatoms was explained by the difference in the electronegativity well. Regarding the selectivity
for the boron-related sites, importance of the surface strain
effect was pointed out.

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