Applied Surface Science 130–132 Ž1998. 78–83

Selective chemical reaction of HBO 2 molecules on theSi ž111 /-7 = 7 surface studied by scanning tunneling microscopyKoji Miyake a,) , Masahiko Ishida a , Hidemi Shigekawa a,b,1aInstitute of Materials Science, and Center for Tsukuba AdÕanced Research Alliance, UniÕersity of Tsukuba, Tsukuba 305, JapanbCREST, Japan Science and Technology Corporation (JST), Japan
Received 1 September 1997; accepted 19 December 1997

Abstract
In the chemical reaction of HBO 2 molecules with SiŽ111.-7 = 7 surface, change in the charge density on the adatoms ofthe 7 = 7 units during the process was found to play an exceedingly important role. The molecules preferentially reactedwith the center adatoms in the unfaulted half units first. In the filled state STM images, the center adatoms surrounding thereacted center adatoms became dark compared to those in the normal 7 = 7 units, which indicates existence of some chargetransfer caused by the reacted center adatoms. Since HBO 2 molecules tend to react with the adatoms in less charge density,
such charge redistribution is supposed to increase the chemical reactivity of the modified center adatoms. In fact, centeradatoms in the faulted half units, which were adjacent to the firstly reacted center adatoms, reacted subsequently with theHBO 2 molecules. According to this process, chain structures were formed by the reacted center adatoms, which may berelated to the B network structure previously observed. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: Adatoms; STM images; HBO 2 molecules; Charge redistribution; Center adatoms

1. Introduction
Group III atoms are known to form '3 = '3structures on the SiŽ111. surface. In the case of the Binduced '3 = '3 structure, the most stable site for
B atoms is the S5 site instead of the T4 site w1–5x,
differing from the structure formed by the othergroup III atoms ŽAl, Ga, and In. w6,7x. In addition,according to the recent results of scanning tunnelingmicroscopy ŽSTM. on the BrSiŽ111.- '3 = '3
structural formation process induced by HBO 2 irra)
C o rre s p o n d in g
a u th o rs .
ims.tsukuba.ac.jprlabrshigekawa
1Also corresponding author.

h ttp :r r w w w .

diation w8–10x, B atoms showed other characteristic
properties; B atoms reacted preferentially with thecenter adatoms in the unfaulted half units of theSiŽ111.-7 = 7 DAS structure. And after complete
destruction of the 7 = 7 structure, B atoms interactedwith each other to form a self-ordered network in theformation process of the '3 = '3 structure w10x.
The observed results are completely different fromwhat observed in the '3 = '3 structural formation
processes induced by other group III atoms; forexample, when metals such as Al atoms are deposited on a SiŽ111.-7 = 7 surface, 7 = 7 structural
domains coexist with the '3 = '3 structural domains until the end of the complete formation of the'3 = '3 structure w11x. Therefore, from both funda-0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 0 2 9 - 4

K. Miyake et al.r Applied Surface Science 130–132 (1998) 78–83

79

mental and practical standpoints of view, it is exceedingly interesting and important to study the detailed mechanism of the '3 = '3 structural formation process induced by the HBO 2 molecular irradiation.
In this paper, we present our recent results of theSTM study performed on the initial stage of the'3 = '3 formation process induced by HBO2 irradiation.

2. Experimental
Phosphorus-doped n-type SiŽ111. Ž r s 0.375 ;0.625 V cm. substrates were used in this study.
After chemical cleaning, samples were degassed at; 5008C and subjected to flash heating in ultrahighvacuum ŽUHV. condition. After confirming the formation of clean 7 = 7 structure, HBO 2 molecules
were irradiated on this surface with the substratetemperature being kept at ; 7508C. HBO 2 cell temperature during irradiation was kept at ; 8008C.
STM observation was performed at room temperature using electrochemically etched tungsten tip. Thebase pressure was ; 1 = 10y1 0 Torr and the pressure during HBO 2 irradiation was less than 2 = 10y8Torr.

Fig. 1. STM image obtained after HBO 2 irradiation of 3 minŽsample bias; Vs sy1.0 V, and tunneling current; It s1.0 nA..

the model, HBO 2 molecules react with Si surface
with a two-step process mechanism as follows: First,Si surface is oxidized by the oxygen atoms of theHBO 2 molecules as2HBO 2 q 2Si ™ 2B q 2SiO 2 q H 2or4HBO 2 q 3Si ™ 4B q 3SiO 2 q 2H 2 O.

3. Results and discussions
Fig. 1 shows an STM image of the SiŽ111.-7 = 7
surface obtained after the HBO 2 irradiation of 3 min.
As is shown in Fig. 1, some adatoms in the 7 = 7units have become dark during the HBO 2 irradiation,
so that these darker adatoms are attributed to thereacted sites with the HBO 2 molecules. In addition
to the preferential reaction of the HBO 2 moleculeswith the center adatoms in the unfaulted half units
w9x, the reacted adatoms seem to form chains rather
than the randomly scattered structure, indicating theexistence of some interaction between B atoms adsorbed on the surface. In the chemical reaction
mechanism of the HBO 2 molecules with the 7 = 7surface, these characteristic properties must be explained comprehensively.
A reaction mechanism of HBO 2 molecules withSi has been proposed by Lin et al. w12x. According toWhen the substrate temperature is high enough, thesilicon oxide ŽSiO 2 . formed in the first step is removed from the surface in the second step as
SiO 2 q Si ™ 2SiO.
In such a case, surface temperature is high enough toevaporate H 2 and H 2 O molecules too, so that theirinfluence on the process is trivial. In our experiment,
the substrate temperature was kept about 7508C during HBO 2 irradiation, which satisfies the condition
described here.
On the basis of the chemical reaction model
proposed by Lin et al., Si surface is oxidized by theoxygen ŽO. atoms of the HBO 2 molecules first.
Since it is well known that O atoms occupy the backbond of adatoms as an intermediate state at the initialstage of SiŽ111.-7 = 7 oxidization w13x, Si atoms inthe second layer are considered to desorb from thesurface during the second step. However, since anunderlayer Si atom has four covalent bonds, while an

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K. Miyake et al.r Applied Surface Science 130–132 (1998) 78–83

adatom has three covalent and one dangling bonds,
adatoms are more easily desorbed compared to theunderlayer Si atoms. In fact, defects formed after
HBO 2 irradiation have a structure similar to that of
the adatom vacancy, as shown in Fig. 1.
The observed structure seems to be explained wellby the chemical reaction model proposed by Lin etal. w12x. However, there are some observed criticaldifferences between the model and our experimentalresults. Fig. 2 shows a relation in the number of thereacted adatoms in the 7 = 7 units after 3 min HBO 2irradiation; horizontal axis indicates the number ofthe reacted adatoms in a 7 = 7 half unit, and thevertical axis does the relative ratio among the numbers of the 7 = 7 half units with the reacted adatoms
from 0 to 6. From the model proposed by Lin et al.,two Si adatoms must be desorbed from the surface inthe reaction of each HBO 2 molecule; however, according to the STM images, there are a lot of 7 = 7
half units in which odd number adatoms are reacted
and removed, as shown in Fig. 2. Therefore, somenew mechanism must be introduced in our case.
One possible mechanism for the comprehensive
explanation of the observed results is to take intoFig. 2. Number of 7=7 half units in which one to six adatoms arereacted with HBO 2 molecules. Result of a Monte Carlo simulation is shown together.

account the more detailed reaction processes. If thefragments such as HBO or BO molecules producedby the decomposition of HBO 2 molecules may play
an essential role in the reaction process, one HBO 2
molecule remains one adatom vacancy. As shown in
the previous reports w8,9x, HBO 2 molecules react
preferentially with the unfaulted half units of 7 = 7DAS structure, in spite of the fact that, in general,atoms andror molecules react preferentially with thefaulted half units. If the characteristic preference
observed in the HBO 2rSi chemical reaction process
is caused by the negative charge remaining on ions,e.g., HBO or BO, the anions prefer to react withpositively charged adatoms, i.e., center adatoms in
the unfaulted half units.
In order to understand the reaction mechanism
more in detail, we performed a Monte Carlo simulation to analyze the experimental results shown inFig. 2, in which Ž1. one Si adatom was assumed tobe removed in the reaction process of one HBO 2
molecule, and Ž2. all Si adatoms were assumed tohave the same reaction probability with the HBO 2
molecules during all through the process. The ratioof ŽHBO 2 molecules.rŽ7 = 7 half units. was chosen
as close to the experimentally obtained value, 1r2.
The distribution obtained by the simulation is shownin Fig. 2, which seems to be in good agreement withthe experimental result. However, there exist a slightdifference between the theoretical and experimentalresults. For the 7 = 7 half units in which the numberof reacted adatoms is less than 2, calculated values
are close to or larger than the measured values. On
the other hand, when the number of the reacted
adatoms in a half unit is more than 3, measured
values are larger than those of the calculated ones.
The obtained results indicate the characteristic thatthe half units in which some adatoms have beenreacted already with HBO 2 molecules have higher
reaction probability for the following HBO 2
molecules, compared to the half units which have notreacted; our assumption Ž2. described above looks
wrong.
Lin et al. based on the experimental results obtained for the B-doping process caused by the coadsorption of HBO 2 molecules with Si MBE growth,and, as can be seen in the chemical reaction equations, reaction sites are not considered at all. However, since HBO 2 molecules were irradiated onto theK. Miyake et al.r Applied Surface Science 130–132 (1998) 78–83

well ordered SiŽ111.-7 = 7 surface in our case, thereaction sites on an atomic scale must play an important role in our case; the reaction process must be
strongly affected by the change in the surface atomicstructures caused by the reaction process itself. Let
us see the detailed reaction processes next.
Fig. 3a shows the number of the half units, inwhich only one isolated Si adatom is reacted asshown in Fig. 3b and c. The reacted adatoms in theFig. 3. Ža. Number of 7=7 half units in which isolated center or
corner adatom is reacted with HBO 2 molecule as shown in Žb.
and Žc..

81

units are indicated by solid circles in the schematicsin Fig. 3b and c, and U and F indicate unfaulted andfaulted half units, respectively. According to theresults shown in Fig. 3a, center adatoms, especiallyin the faulted half units, seem to be less reactive with
HBO 2 molecules compared to the corner adatoms.
However, as we showed in our previous paper w9x,
the center adatoms were more reactive than thecorner adatoms. These results suggest the existenceof some more detailed processes in the reaction
mechanism as expected. The observed phenomena
may be due to the effect of the first reaction onto thesubsequent reaction processes.
Fig. 4a shows the number of the half units inwhich adatoms are reacted in the fashion of paring as
shown in Fig. 4b to d. As is shown in Fig. 4a, in thepairing form, most part is the center adatoms which
react with each other crossing the boundary between
the faulted and unfaulted half units; all the reacted
center adatoms in faulted half units are adjacent to
the reacted center adatoms in unfaulted half units. In
fact, the observed F–F pair shown in Fig. 4d had astructure consisting of two U–F units; U–F–U inFig. 4d. On the other hand, the number of the halfunits with one reacted center adatom is not so large
as shown in Fig. 3a, in spite of the preferential
reaction of them in the first stage. These results
support the mechanism that HBO 2 molecules preferentially react with the center adatoms in the unfaulted half units first, then the subsequent HBO 2
molecules tend to react with the center adatoms
adjacent to the firstly reacted center adatoms.
The mechanism described above was discussed
concentrating on the change in the electronic structure during the reaction, next. As is well known, theDAS structure has a characteristic electronic property; charge transfer from the top-layer adatoms tothe rest atoms in the under-layer w14x. Since theamount of charge transfer from adatoms to rest
atoms depends on the number of rest atoms whichsurround the adatoms, structures of the underlayers
can be discussed by considering the redistribution ofcharge density on adatoms in STM images at negative bias voltages w15,16x. In the case of our experiment, desorption of adatoms increases the number ofdangling bonds, which lower the charge density onthe surrounding adatoms. Such charge redistribution
is expected to result in the increase in the reactivity

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K. Miyake et al.r Applied Surface Science 130–132 (1998) 78–83

of the center adatoms surrounding the reacted centeradatoms. Experimentally, in the STM images obtained at negative bias voltages, center adatoms adjacent to the reacted center adatom were observed to
be darker compared to those in the normal 7 = 7structure, which agrees well with the proposed mechanism. In any case, the reaction process describedhere tends to form chains of the reacted sites insteadof the randomly reacted one, which may be related tothe network formation process observed previously
w10x.
From the analysis of the site-dependent adsorption
mechanism, initial stage of the HBO 2rSiŽ111.-7 = 7
surface chemical reaction process was explained
comprehensively. In order to clarify the reaction
mechanism on an atomic scale more in detail, furtherstudy, such as determination of the surface structure
including B atoms, must be performed.

4. Conclusions
We have investigated in detail the reaction process of HBO 2 molecules with the SiŽ111.-7 = 7
surface by using STM. HBO 2 molecules first reacted
preferentially with the center adatoms in the unfaulted half units. Charge redistribution on adatoms
in the 7 = 7 units was caused by the first reaction of
the center adatoms, which resulted in the modification of the reaction probability of the adatoms, andmade the center adatoms surrounding the firstly reacted center adatom more reactive for the following
HBO 2 molecules. As a result of the observed processes, a chain network consisting of the reacted
sites was formed on the surface.

Acknowledgements

Fig. 4. Ža. Number of half units in which adatoms are reacted inthe fashion of pairing forms as shown in Žb. to Žd..

We wish to thank Dr. T. Nagamura and Mr. T.
Ozawa of UNISOKU for their help in the STM
measurement. This work was supported by theShigekawa Project of TARA, University of Tsukuba.
Support from a Grant-in-Aid for Scientific Researchfrom the Ministry of Education, Science and Cultureof Japan is also acknowledged. One of the authors
ŽK.M.. was financially supported by the JapanK. Miyake et al.r Applied Surface Science 130–132 (1998) 78–83

Society for the Promotion of Science ŽJSPS. Fellowships for Japanese Junior Scientists.
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