APPLIED PHYSICS LETTERSVOLUME 73, NUMBER 1

6 JULY 1998

Surface dynamics studied by perturbing the surface with the tipof a scanning tunneling microscope—Si„100… at 80 KK. Hata,a) M. Ishida, K. Miyake, and H. Shigekawa
Institute of Materials Science, TARA, and CREST (Japan Science and Technology Corporation (JST)),
University of Tsukuba, Tsukuba 305-8573, Japan~Received 19 December 1997; accepted for publication 6 May 1998!
We have utilized the tip of a scanning tunneling microscope to perturb a specific local structure ~theC defect! of Si~100! at 80 K, and observed the dynamical symmetric⇔buckled transition of thesurrounding dimers. The observed large-scale transition implies that the configuration of the dimers
is determined by a detailed balance among many elastic long-range forces generated by thesurrounding C defects. © 1998 American Institute of Physics. @S0003-6951~98!03727-9#

The ability of the scanning tunneling microscope ~STM!
is not only restricted to imaging the static atomic structure of
the surface. By manipulating the tip of the STM and applying a controlled electric field to a specified target, transfer ofsingle atoms and fabrication of nanoatomic structures havebeen demonstrated.1–3 Since the concept of atomic manipulation was introduced, this technique has been applied tomany surfaces, for example, to fabricate atomic structures ofdesired design,1 to confine electrons to artificial
nanostructures,2 and to extract Si adatoms.3 The technique of
atomic manipulation can also be used as a method to perturba specified local atomic structure and to excite a portion of
the surface into a nonequilibrium state. Another, and important, application of the STM is its ability to visualize realtime dynamical processes on surfaces.4 In situ observationsof crystal growth, adsorption, and atomic diffusion have revealed a wealth of information inaccessible by other methods.
Combining the technique of atomic manipulation anddynamic visualization STM would give a unique opportunity
to study the surface dynamics through observation of thelocal excited state and its relaxation to an equilibrium stage.
In this letter we report on an application of these techniques
to Si~100! at 80 K to study the dynamics of dimers. On
Si~100! at low temperatures ~80 K!, a special target ~C defect! was perturbed by positioning the STM tip above it andapplying a controlled electric field. The response of the surrounding dimers was dramatic and an avalanche type dynamical symmetric⇔buckled phase transition of the dimers
was observed. It can be understood by the fact that the surface is forced into an excited ~metastable! state by the perturbation. If an appropriate temperature is chosen, it is possible to observe, by subsequent scanning, both the excitedstate and the dynamical relaxation of the surface in responseto the localized perturbation until a new ground state is established. Direct observation of the dynamical transition reveals a wealth of important implications concerning surface
dynamics.
Si~100! samples were phosphorus doped with a conduca!

Electronic mail: hata@mat.ims.tsukuba.ac.jp or
hidemi@mat.ims.tsukuba.ac.jp
World Wide Web: http://www.ims.tsukuba.ac.jp/lab/shigekawa/index.html

tivity of 0.005 V cm. After the sample was prebaked at;700 °C for one night, it was flashed to 1200 °C for several
seconds and cooled down to 80 K.
An example of the perturbation and the accompanying
dynamical transition of the dimers is displayed in a set ofconsecutive STM images in Fig. 1. Figure 1~a! shows thesurface before perturbation. The white region at the lower
left represents a step edge and an upper terrace. The C defectin the center indicated by an arrow is the target. Every time,before perturbation, the surface was scanned with both negative and positive surface bias to confirm that the target has
the characteristics of the C defect mentioned afterward. Thetip of the STM was positioned above the target C defect, anda ramped bias ~typically 22–2 V, 7 ms! was applied severaltimes ~;16 rounds!. By this procedure we could destruct theFIG. 1. Four STM images showing the dynamical response of the surface toa perturbation. The red regions show the symmetric dimer domains inducedby the perturbation. Tunneling conditions: 0.6 V and 1 nA. The schematics
show the stick and ball atomic structure of the initial, excited, and finalstates.

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40© 1998 American Institute of Physics
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Appl. Phys. Lett., Vol. 73, No. 1, 6 July 1998

FIG. 2. ~a! Stick and ball model showing the down ~up! atoms of thebuckled dimers. Red and blue represent the c(432) and p(232) phases,respectively. The schematic structure of the C defect and P defects is represented by the black atoms. ~b! Schematic showing many P defects migrating along the dimer rows resulting in an apparent symmetric configurationof dimers observed by STM. ~c! The STM image on the right shows a wider
region of the STM image on the left ~region surrounded by a box!. Tunneling conditions: 0.6 V and 1 nA.

C defect routinely with a probability estimated roughly at50%. Never did the same perturbation induce a noticeable
transition when the tip was located above the dimers. It is
important to realize that STM images are not snap shots ofthe surface. Ordinary scanning was halted when the tip came
above the target, and the C defect was perturbed. After that,scanning was resumed. The x axes of the scanning are represented by the dotted lines in Figs. 2~b! and 2~c!, while theblack arrows show the y direction. An increase in the y coordinate not only means a change in location but also an
increase in time. Considering these facts, it is possible tounderstand that the excited state is composed of the lower
part of Fig. 2~b! below the dotted line and the upper part ofFig. 2~c! above the dotted line. At the moment of perturbation, the upper part of the perturbed dimer row and threebuckled dimer rows on the right turned into a symmetricconfiguration, seen in red. Dynamical relaxation from theexcited state was observed 3 min after perturbation as shownin Fig. 1~c!. In the course of the dynamical relaxation, thenewly born symmetric dimer rows returned into a buckled
configuration as shown in Fig. 1~c!. Figure 1~d! shows the
new ground state established.
Before giving a detailed analysis, we want to describe
the characteristics of the symmetric and buckled dimers observed at 80 K by the STM. The basic building block of theSi~100! reconstruction is a buckled dimer. Buckled dimersalign in an antiferromagnetic order making up a dimer row.

Hata et al.

41FIG. 3. Another example of perturbation. ~a! A large scale STM image
showing an overview of a large symmetric region ~red!. ~b! A closeup showing the boundary of the symmetric and buckled dimer domains. ~c! The C
defect located at the boundary indicated by the red arrows was perturbed. ~d!
Final configuration after perturbation. Tunneling conditions: 0.6 V and
1 nA.

An antiphase alignment of these buckled dimer rows gives asurface phase of c(432) @blue region in Fig. 2~a!#, while aninphase alignment gives a p(232) @red region in Fig. 2~a!#.
At 80 K, the STM images show surfaces covered with c(432) domains with a certain ratio of symmetric dimer
domains.5,6
By looking through a number of STM images at low
temperatures we reached the conclusion that the configuration of domains is mainly regulated by the defects on thesurface, particularly, by the C defects. When probing thefilled states, the C defects appear to be two adjacent Si atoms
missing along the dimer row direction as shown schematically in Fig. 2~a!, and these C defects are observed as brightprotrusions in the empty states ~this is our definition of the Cdefect!. The C defect is a phase shifter by nature. If one sideof the C defect is the c(432) phase, the other side must bethe p(232), as shown in Fig. 2. However, a wide p(232) domain is never observed at 80 K, and regions supposed to reconstruct to the p(232) phases are usually observed as symmetric dimer domains. A typical example isshown in Fig. 2~c!, where two antiphase c(432) domains~blue! are regulated by two C defects. The region sandwiched by the two c(432) domains, which is supposed toreconstruct to the p(232) phase, appears as a symmetricdimer domain ~red!.
It is proposed that numerous phasons nominated as Pdefects are migrating along the symmetric dimer rows far
faster than the scanning of the STM.7 In such a case, basically the dimers are buckled, although they appear symmetric in STM as shown schematically in Fig. 2~b!. The crystallographic structure of the P defects is exactly the same as theC defect, although the P defects are movable and do notappear as bright protrusions when probing the empty states.
When this P defect moves along a dimer row, the surface

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42

Hata et al.

Appl. Phys. Lett., Vol. 73, No. 1, 6 July 1998

reconstruction switches from the c(432) to the p(232)phase or vice versa. In this sense the P defect is a phason.
The existence of the P defect was first directly confirmed bySTM observations at 6 K.8 At 80 K, the symmetric dimer
regions can be regarded as a ‘‘sea’’ of these phasons imprisoned by the C defects.
At this stage, we return to the result of Fig. 1 and interpret the data in terms of the concept of the P defect in symmetric dimers. Structural models of the initial, the excited,
and the final state around the target C defect are presented inFigs. 2~e!, 2~f!, and 2~g!. By comparing the configuration ofdimers of the perturbed dimer row in Figs. 2~e! and 2~f!, wecan see that, by perturbation, the C defect has been converted
into an unidentified defect which is not a phase shifter. Thismeans that a P defect has been created by the perturbation,
which carries the phase shifter ingredient of the C defect. We
suppose that the newly born P defect has migrated upward,
and is the origin of the symmetric dimers observed there in
the excited state. In the course of the dynamical relaxation,
not only did the symmetric dimers convert into buckled
dimers, but the p(232) phase observed at the left side of theC defect also switched into a c(432) configuration. Indeed,
we assume that the existence of this p(232) phase is thedirect cause of the dynamical relaxation, because, at thistemperature, the c(432) phase is favored. The transition
can be explained in two ways: ~i! the P defect has migrated
downward into the step edge, and associated with this migration, the p(232) phase converted into a c(432) phase; ~ii!
a new P defect was produced at the step edge and migrated
upward, uniting with the P defect produced by perturbation.
In both cases, the perturbed dimer row is left with no P
defects, corresponding to the dissipation of the symmetricdimers in the final ground state.
When the weak interaction among the dimer rows isconsidered, it is surprising to find that the effect of perturbation extends to other dimer rows. In the excited state, symmetric dimers emerge not only in the perturbed dimer row
but also in adjacent rows. This suggests that phasons havecome into being even in the adjacent rows. At 80 K, it isreported that phasons in adjacent rows have a strong tendency to form pairs,9 which is explained, in first-order approximation, as a reduction of the total p(232) area by thispairing. Experimental results of Fig. 1 imply that the artificially created phason has induced another phason in the adjacent row to form a pair with it. A new single counterpart
single phason is created by this process because phasonsform in pairs. We assume that this newly born single phason
again induces another phason in the adjacent row. A chain
reaction of these processes would induce a large-scale dynamical transition as was observed.
It should be noted that the remainder of the C defect wasalways a defect which is not a phase shifter. This suggeststhat the C defect is an unfavorable factor in a set of buckled
dimers as was reported before.10 In other words, the C defecthighly stresses the surrounding buckled dimers by pinning

down the buckled dimer domain and forces the surface into a
metastable state. We believe that when the C defect isstrongly perturbed, the pinning effect is released, resulting in
the observed large-scale dynamical transition.
Since Figs. 1~a! and 1~d! are quite similar, one might
conclude that the initial and new ground states are quite
similar, but this is not correct. By comparing STM images ofwider regions before and after perturbation ~not shown!, it
became clear that a new symmetric dimer domain has
emerged in the upper region which was once a wide buckled
dimer domain. This means that the influence of the perturbation has extended a long distance. This example is not anexception. Frequently we observed a dynamical transition
induced by perturbing a single C defect, in which thousandsof dimers changed their configuration. An example is shownin Fig. 3. At the beginning, there exists a large symmetricdimer domain @colored red in Figs. 3~a! and 3~b!#. The C
defect located at the boundary of the buckled symmetricdimer domain indicated by the arrow was perturbed asshown in Fig. 3~c!. At the moment of destruction, the widesymmetric dimer domain converted into a buckled dimer domain. Numerous changes occurred and the final configuration became very different from the initial state as shown inFig. 3~d!, the large symmetric dimer domain has been replaced by a buckled dimer domain and a new symmetricdomain has emerged in the lower part of the terrace.
We can regard the surface morphologies before and after
perturbation as representing the equilibrium surface configuration with and without the C defect. Therefore comparing
the two configurations will give a unique opportunity to
study the influence of the C defects on the buckled dimers.
The drastic difference between the two configurations indicates that the elastic stress produced by the C defect extends
a long distance and that dimer configuration is determined bya detailed balance among many long-range forces producedby the surrounding defects.
In conclusion, we have combined the technique ofatomic manipulation and the dynamic visualization STM andapplied them to Si~100! at 80 K to study the dynamics ofdimers. By perturbing the C defect, an avalanche typesymmetric⇔buckled phase transition of dimers was observed.
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