Electronic structure of Si(111)-737 phase boundary studied by scanningtunneling microscopy
Koji Miyake and Hidemi Shigekawaa)
Institute of Materials Science, University of Tsukuba, Tsukuba 305, Japan

Ryuzo Yoshizaki
Cryogenics Center, University of Tsukuba, Tsukuba 305, Japan~Received 3 January 1995; accepted for publication 2 April 1995!
Remarkably low electron density of Si adatoms at the Si~111!-737 phase boundary was found by
scanning tunneling microscopy. The observed charge transfer was apparent with sample bias
voltages down to ;20.8 eV, close to the value of the dangling bond state of the rest atoms in theSi~111! 737 surface. In consideration of the DAS ~dimer-adatom-stacking fault! model, theobserved charge transfer could be related to the structural change in the dimer layer caused by phasemismatching at the boundary. In fact, such charge transfer was not observed at the less disorderedboundaries formed by introducing 535 half unit cells. Similar large charge transfer was found tooccur in the quenched disordered 131 structure. These results agree with the similar chemical
reactivity observed in the two disordered structures. © 1995 American Institute of Physics.
In the chemical reaction processes occurring on material
surfaces, electronic structure of the substrate surfaces isknown to play essential roles. As Avouris et al. showed in
atom-resolved surface chemistry by scanning tunneling microscopy ~STM!, initial reactive sites and subsequent reactions of oxygen and NH3 molecules on Si surfaces were governed by the local electronic structure of the substrates.1,2 For
charge transfer between adsorbates and substrates, the importance of the energy level structure of the substrate surface
was shown by the analysis of the high-resolution photoemission spectroscopy spectrum for the alkali metal adsorption
onto Si surfaces.3 These properties are also very important inunderstanding, for example, the processes of atomic layer
epitaxy4 and initial growth of compound semiconductors.5
Therefore, in order to design and control the material structure on an atomic scale, it is very important to characterize
and understand the influential local electronic structures ofmaterial surfaces/interfaces.
Recently, in the molecular beam epitaxial growth of Siand Ge on Si~111! 737 substrate, the nucleation and growthmodes of Si or Ge islands were found to be strongly affectedby the existence of the phase boundary.6 – 8 Despite the important roles of the local electronic structure as mentioned
above, STM studies performed on the Si~111!-737 domainboundaries have clarified only the arrangement of the firstlayer adatoms.9–12 In addition, the formation and relaxation
mechanisms of the stacking fault are essential with regarding
to the Si~111!-737 surface reconstruction.8 Therefore, analysis of the phase boundary, including underlayers, is very important from both practical and fundamental points of view.
In this letter, with consideration of the bias-voltagedependent STM images, the structure of the Si~111!-737
phase boundary will be discussed in detail on the basis of theDAS ~dimer-adatom-stacking-fault! model.13
Phosphorous-doped ~1 V cm! Si~111! sample surfaces
were prepared by conventional heat treatment following a
one-day prebake. Base pressure was ;1.0310 28 Pa, anda!

Electronic mail: hidemi@mat.ims.tsukuba.ac.jp

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Appl. Phys. Lett. 66 (25), 19 June 1995

pressure during the heat treatment was kept below ;5310 28 Pa. STM was performed at room temperature using
an electrochemically etched tungsten tip, and all STM images shown in this letter were obtained in the constant current mode.
Figures 1~a! and 1~b! show empty-state ~sample bias:
V s 52.0 V), tunneling current: I t 5300 pA! and filledstate ~V s 522.0 V, I t 5300 pA) STM images of aSi~111!737 phase boundary, respectively. The phase boundary is indicated by A–A in the figures. Phase mismatching atthe boundary is apparent in the empty-state image, as shownin Fig. 1~a!; however, adatoms at the boundary are as brightas those in the regular 737 regions. On the other hand, STM
images of adatoms at the boundary are darker and vague inthe filled-state image, as shown in Fig. 1~b!, indicating lower
electron density of the adatoms at the boundary region. Since
adatoms with higher electron density were not observed

FIG. 1. STM images of domain boundary for ~a! empty state ~V s 52.0V, I t 5300 pA) and ~b! filled state ~V s 522.0 V, I t 5300
pA!.

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FIG. 3. Magnified image ~a! and its structural model ~b! of the squared areain Fig. 1.
FIG. 2. STM images of a domain boundary obtained at @~a! V s 5
20.5V and ~b! V s 521.0 V]. Tunneling current was 300 pA.

around the boundary, the observed charge transfer is considered to be occurring from the adatoms to the underlayers.
Among the adatoms in a regular 737 unit cell, centeradatoms are known to be less brighter than the corner adatoms, because center adatoms are surrounded by twofold rest
atoms; thereby, the amount of the charge transfer to the rest
atoms is larger for the center adatoms than the corner
adatoms.1,2 Therefore, a possible explanation for the observed bias-dependent STM images is that formation ofdimers in the DAS structure was interrupted by the phasemismatching at the boundary, and additional dangling bonds
were created along the boundary. When the number of dangling bonds surrounding adatoms is increased at the domainboundary compared to the regular 737 surface, larger chargetransfer from adatoms to the dangling bond states is expected
to make the adatoms in the boundary region darker in thefilled-state images.
In order to examine our model, we measured the biasdependent image of the phase boundary. Figures 2~a! and2~b! show two filled-state STM images of a boundary obtained at different negative sample bias voltages V s @~a! 20.5
V and ~b! 21.0 V#. As shown in Fig. 2~a!, adatoms at theboundary are dark at V s 520.5 V; however, they become less dark, as shown in Fig. 2~b! when the bias voltage
was increased to over 20.8 eV, the energy level of the rest
atom dangling bond state.14
Figures 3~a! and 3~b! show a magnified image and structural model of the squared are in Fig. 1~b!. Faulted and unfaulted sites are indicated by F and U in Fig. 3~b!, respectively. Since the 737 structure is known to be formed fromthe unfaulted half and the isolated faulted-half structure isunstable, the structure of the boundary region was assumed
to be unfaulted here. Dark adatoms at the phase boundary in
the filled-state image and the extra rest atoms formed aroundAppl. Phys. Lett., Vol. 66, No. 25, 19 June 1995

the boundary are indicated by large and small solid circles inFig. 3~b!, respectively.
According to the DAS model, dimers are formed between the faulted and the unfaulted halves of a 737 DAS
unit. For the left-hand side of the boundary, since the faultedhalf unit cells of the left 737 domain are in contact with theboundary which has unfaulted structure, dimers remain along
B–B, as shown in Fig. 3~b!. On the other hand, in the rightdomain, unfaulted halves of the 737 units are neighboring
the unfaulted structure of the boundary. Therefore, atoms on
the edge of the unfaulted-half unit cells along C–C in Fig. 2
cannot form dimers and remain as rest atoms, as indicated bysolid circles along C–C. As we expected, adatoms between
the two lines B–B and C–C are dark, indicating the existence of charge transfer. However, adatoms situated near thecorner adatoms of the 737 units in the right domain arebrighter, which corresponds to the fact that the number ofrest atoms surrounding the corner adatoms are relativelysmall. All the center adatoms in the right domain along theboundary indicated by arrows in Fig. 3 are influenced by theboundary and are darker than other center adatoms as shownin Fig. 3, as expected. In a larger phase boundary area indicated by E, there are many dangling bonds, as shown in Fig.
3. Therefore, we can expect more dark adatoms in this area
compared to the boundary between B–B and C–C, which infact was observed, as shown in Fig. 3~a!.
In consideration of the DAS model, it is possible to forma phase boundary with less disturbance of the Si-dimer layer.
One possible way is to introduce 535 half unit cells, as isschematically shown in Fig. 4~a!. Since dimers remain in thiscase, and the number of rest atoms surrounding the 535
adatoms is close to that of the corner adatoms in a regular
737 structure, charge transfer at the 535 half unit cell mustbe similar to that at the corner adatoms. In fact, such aboundary was observed by STM, as shown in Fig. 4~b!, andall adatoms in the 535 half unit cells were as bright as thecorner adatoms in the 737 unit cells even for the filled-state
Miyake, Shigekawa, and Yoshizaki

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FIG. 4. A structural model ~a! and an STM image ~b! of a domain boundary
formed by introducing 535 half unit cells, V s 522.0V,I t 5300 pA.

images, as expected. This rule was well verified for the structures formed by mixture of other 737 families appearing on
the quenched Si~111! surfaces.
Similar large bias-voltage dependence was observed forthe disordered triangular 131 phase,15 as is shown in Fig. 5.

The empty-state image of the adatoms in the disordered area
was as bright as that of other adatoms in the regular 737
area; however, their filled-state image was apparently dark
for bias voltages down to ;20.8 V. Recently, a quenched
Si~111! surface was imaged by atomic force microscopy~AFM! in air, and 737 domain boundaries and disordered
131 structures associated with the phase transition from
131 to 737 were similarly observed as protrusions 0.1–0.15
nm higher than the regular 737 region, indicating similar
surface reactivity of these structures.16 Extra atoms on thetop layer were bright for both bias polarities, indicating a
metallic electronic structure. The triangular 131 phase structure has not been clarified yet; however, similar charge transfer to that at the phase boundary may explain the similar
chemical reactivity observed by the AFM measurement forthese two disordered areas. According to our model, dimers
seem to be formed at the boundary.
In summary, lower charge density of the top-layer Si
atoms at the Si~111!-737 phase boundary was found by
studying the bias-voltage-dependent STM images. On thebasis of the DAS model at the phase boundary, the change inthe electronic structure could be attributed to the chargetransfer caused by the disorder in the dimer layer at the phaseboundary. Phase boundary with less disorder formed by introducing 535 half unit cells was found to exist, the STM
image of which was explained well by our model. Similarlarge charge transfer was found to occur at the disordered
131 triangular structure formed by quenching. These results
are consistent with the similar chemical reactivity observed
for these two disordered structures.
This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Scienceand Culture of Japan. Research Foundation for Materials Science is also acknowledged.

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2FIG. 5. STM images of a disordered 131 triangular structure, ~a! V s520.5 V, I t 5300 pA and ~b! V s 521.0 V, I t 5300 pA.
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