Appl. Phys. A 66, S1013–S1016 (1998)

Applied Physics AMaterials
Science & Processing
 Springer-Verlag 1998

Self-organizedstructure appearing in the√ network√B/Si(111)-( 3 × 3)R30◦ phase formation process studied
by scanning tunneling microscopy
K. Miyake, H. Shigekawa
Institute of Materials Science, and Center for TARA, Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba 305, Japan(Fax: +81-298/55-7440, E-mail: miyake@mat.ims.tsukuba.ac.jp, hidemi@mat.ims.tsukuba.ac.jp)
Received: 25 July 1997/Accepted: 1 October 1997√√
Abstract. In the initial stage of the Si(111)-( 3 × 3)R30◦
structural formation by HBO2 molecular irradiation ontoa Si(111)-7 × 7 surface at ∼ 750 ◦ C, B atoms interactedwith√ each√ other to form chains from the beginning, anda ( 3 × 3)R30◦ structure consisting of Si and B atoms
was formed.
√ The√ observed result is completely different
from the ( 3 × 3)R30◦ structural processes induced byother group III atoms; in general, when metals such as Al
areon a Si(111)-7 × 7 surface, large 7 × 7 and√√ deposited( 3 × 3)R30◦ structure domains exist separately. With anincrease in the amount of the B component to about half thenumber
B atoms formed a network with the same√√of adatoms,◦( 3 × √3)R30
resulting in the formation of an or√ phase,dered ( 3 × 3)R30◦ structure. Off-phase disordered areas
remaining on the surface were limited to the areas occupied
by Si atoms. The obtained results indicate that the interaction
caused by B atoms plays
role in the formation√ an essential√
process of the ordered ( 3 × 3)R30◦ structure of this surface.

With the recent development of scanning tunneling microscopy (STM) [1] and its application to the atom manipulation [2–4], nanometer-scale fabrication of semiconductor surfaces has been attracting considerable attention
these years. For example, selective adsorption of Ga atoms
onto the hydrogen-missing dangling bonds in the hydrogenterminated Si(100) surface was demonstrated by controllingthe structure of hydrogen-missing defects using STM tip [5].
Since characteristics of semiconductor surfaces are drastically changed by adsorption of foreign atoms, modificationof semiconductor surfaces terminated by adsorbates has very
high potential over the field of nano-scale fabrication. However, in general, the area modified by the manipulation using
STM tip is very limited spatially.
Recently, selective desorption of Ga atoms and formationof self-ordered Si(111)-7 × 7 structure near the step edge was√√
induced by STM tip on the Si(111)-( 3 × 3)R30◦ –Ga surface [6]. In this case, Si(111)-7 × 7 structural domain withwidth 7.0 nm and length more than 140 nm was formed on
the surface, which indicates the importance of self-ordering
processes in this field, as has been recognized in the structural control of organic materials. Thus, in order to realize thenano-scale science on semiconductor surfaces, it is essential
to understand and develop the self-organized structural formation process on semiconductor surfaces.
On the other hand, boron (B) is one of the major ptype dopants for Si crystal, and has been playing important roles in semiconductor technology. For example, since
the B atom is less segregable compared to other group IIIatoms (Al, Ga and In), it has attracted considerable attention
as a promising candidate for the formation of atomic-layerdoped (δ-doped) structure. Therefore it is very important tocontrol the B/Si surface structures on an atomic scale. However, the B-induced reconstructed surface has generally been
formed by annealing a heavily B-doped substrate [7–11].
Since the formation process requires a high treatment temperature of ∼ 1000 ◦ C in order to segregate the dopant fromthe surface, it was very difficult to control the B/Si √surface.
√Recently, a new process to form the B/Si(111)-(3 × 3)√√
R30◦ surface was found; the B-induced ( 3 × 3)R30◦
structure was found to be formed by the deposition of Bcompounds (B10 H14 , B2 O3 , HBO2 ) [12–17]. In general, theprocess temperatures is lower than the diffusion temperatureof B atoms. B atoms occupy the T4 sites on the surfaces
formed by the molecular deposition, and change their sites
from4 to S5 by annealing of the surface. Therefore, the√ T√( 3 × 3)R30◦ structure formed by molecular deposition
may be controllable√ sense of self-ordering. Especially,√ in thein the case of the ( 3 × 3)R30◦ structure formed by HBO2
molecular deposition, since B atoms selectively react with theSi(111)-7 × 7 center adatoms [18], formation of characteristic
structures is highly expected on this surface.
In this work, we present the results of STM√observationof√
the formation process of the ordered Si(111)-( 3 × 3)R30◦
structure formed by HBO2 irradiation in detail; a self-S1014

organized network formation process was found on this surface for the first time.
Phosphorus-doped n-type Si(111) (% = 1.5 Ω cm) substrates were chemically cleaned and degassed at ∼ 500 ◦ C
before being subjected to flash heating in order to form the7 × 7 structure. After it was confirmed that a clean 7 × 7 structure had been obtained, HBO2 molecules were irradiated onto
the surface with the substrate temperature kept at ∼ 750 ◦ C.
The HBO2 cell temperature during irradiation was kept at∼ 800 ◦ C. All STM observations were performed at room
temperature using an electrochemically etched W tip. Thebase pressure was ∼ 1 × 10−10 Torr and the pressure during
HBO2 irradiation was less than 2 × 10−8 Torr.
Figures 1a–d show STM images of surfaces formed by (a)3 min, (b) 15 min, (c) 20 min, and (d) 50 min of HBO2 irradiation. Since additional irradiation at ∼ 750 ◦ C was carried out
before each measurement, it was impossible to observe thesame area. However, the observed structures were checkedover a wide area for each condition. In the initial stage shownin Fig. 1a, the Si(111)-7 × 7 structure still remained, but someunfaulted half-units of the 7 × 7 structure became dark, whichindicates that HBO2 molecules initially react preferentiallywith the unfaulted half-units, as previously observed [18, 19].
A magnification of a portion of Fig. 1a and a schematic of thisstructure are shown in Figs. 2a, b, respectively. U and F indicate unfaulted and faulted halves, and reacted adatoms arerepresented by closed circles. In addition to this characteristic, the reacted adatoms seem to form chains rather than being

acrandomly scattered, as shown in Fig. 2, which suggests that
there exists some interaction with B atoms adsorbed on thesurface.
With further irradiation, the disordered area increased insize. After the 7 × 7 structure
was completely broken, the sur√√face was covered with ( 3 × 3)R30◦ domains consisting ofatoms exhibiting two different degrees of brightness (Fig. 1b).
According to the photoemission study by Cao et al. [13], B
atoms, which occupy the T4 site first, diffuse inwards uponheating at ∼ 900 ◦ C, to occupy S5 sites. In this case, since
the substrate temperature during HBO2 irradiation was lower
than the B diffusion temperature, B atoms are considered toadsorb on the surface. Since the ratio of dark to bright atoms
increased with further B deposition (Figs. 1c, d), the brightand dark atoms were inferred to be Si and B atoms, respectively.
In the stage of Fig. 1b, islands of B atoms, in a chainform,√ remain√ isolated and the surface consists of a numberof ( 3 × 3)R30◦ structural domains; the phase of the domains is not ordered to be single. A magnified image and its
schematic of the structure, indicated by a rectangle in Fig. 1b,is shown in Fig. 3. Open and solid circles√indicate√ Si and B
atoms, respectively, and boundaries of ( 3 × 3)R30◦ domains with mismatched phase to each other are indicated bysolid lines. The domains with three different phases are represented by white, light gray and dark gray areas in Fig. 3.
With an increase in the B amount of the component toabout half the number of adatoms, B atoms were found tobdFig. 1a–d. STM images of Si(111) surface after HBO2 irradiation at 750 ◦ C.
Irradiation time, tip bias voltage Vt ,tunneling current It and scan size are,respectively, a 3 min, 1.0 V, 1.0 nA,
70 nm×70 nm, b 15 min, –1.0 V, 1.0 nA,
35 nm×35 nm, c 20 min, –1.0 V, 0.5 nA,
35 nm×35 nm, and d 50 min, –1.5 V,
0.98 nA, 35 nm×35 nm

S1015

abFig. 2. a Magnified image of Fig. 1a, and b its schematic structure. In b, triangles indicate half-unit cells of 7 × 7 structure, and reacted adatoms arerepresented by closed circles√√form a network with the same ( 3 × 3)R30◦ phase, resulting√in the√formation of a large domain of a single phase◦structure on the surface. In Fig. 1c,of√ a ( √3 × 3)R30◦( 3 × 3)R30 phase formed by B atoms, darker atoms in
the figure, has the same√ phase√ all over the surface in the figure. Small off-phase ( 3 × 3)R30◦ domains remained onlyin the areas occupied by Si atoms, as indicated by arrows inFig. 1c. A magnified image and its schematic of the structure
indicated by a rectangle in Fig. 1c is shown in Fig. 4. Areas ofoff-phase Si atoms are enclosed by solid lines. The network ofB atoms shown in Fig. 1c and Fig. 4 form branches in threefold symmetric directions, as indicated by arrows in Fig. 4.
The remaining Si adatoms were replaced by B atoms withfurther √deposition√ of HBO2 molecules, and a completely ordered ( 3 × 3)R30◦ surface was formed in the stage shownin Fig. 1d, where small Si domains of 1–2 atomic widths areisolated from each other by the B network.
The formation process√observed√ here is completely different from that of the ( 3 × 3)R30◦ structures inducedby other group III atoms. For example, when Al atoms
areon a Si(111)-7 × 7 surface, large 7 × 7 and√√ deposited( 3 × 3)R30◦ domains coexist and their domain boundaries are stabilized by Al(Si) √atoms√[20]. On the other hand,in the case of the B-induced ( 3 × 3)R30◦ surface formedby HBO2 irradiation, B atoms seemed to interact with eachother√ in√a one-dimensional manner from the beginning, and a( 3 × 3)R30◦ structure consisting of mixed Si and B atoms
was formed. Boron atoms formed a network and induced
phase ordering between domains (Fig. 1a–c). The obtained

aabFig. 3. a Magnifiedof Fig. 1b, and b its schematic structure. In b,√ image√
boundaries of ( 3 × 3)R30◦ domains with different phases are indicatedby solid lines, and the domains with three different phases are represented
by white, light gray, and dark gray areas. Open and solid circles indicate Si
and B atoms, respectivelybFig. 4. a Magnified image of Fig. 1c, and b its schematic structure. In b,
areas of off-phase Si atoms are enclosed by solid lines. Open and solid
circles correspond to Si and B atoms, respectively

S1016

results indicate that the balance between Si−Si, Si−B andB−B adatom interactions plays√ an√essential role in the formation of the self-organized ( 3 × 3)R30◦ structure.
Further experiment and theoretical study is necessary fora more detailed understanding of the mechanism of the selfordering process described in this manuscript.
Acknowledgements. We greatly appreciate the fruitful discussions with Dr.
H. Hirayama of the Tokyo Institute and Technology, and Dr.Y. Kumagai ofTexas Instruments Tsukuba Research & Development Center Ltd. We also
thank Dr.T. Ozawa and Dr.T. Nagamura of Unisoku Co., Ltd. for their help
in the STM measurements. This work was supported by the Shigekawa
Project of TARA, University of Tsukuba. Support from a Grant-in-Aid for
the Scientific Research from the Ministry of Education, Science, Sports andCulture of Japan is also acknowledged. One of the authors (K.M.) wasfinancially supported by the Japan Society for the Promotion of Science(JSPS) Fellowships for Japanese Junior Scientists.

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