Japanese Journal of Applied Physics
Vol. 43, No. 7B, 2004, pp. L 990–L 992#2004 The Japan Society of Applied Physics

Express Letter

Characteristic of the Si(100) Surface Low-Temperature Phasewith Two Competing Structures Investigated by Rare Gas Adsorption
Tomohiko K IMURA, Shoji Y OSHIDA, Osamu T AKEUCHI, Eiji M ATSUYAMA,Haruhiro O IGAWA and Hidemi S HIGEKAWA
Institute of Applied Physics, 21th Century COE, NANO project, University of Tsukuba, Tsukuba 305-8573, Japan(Received May 26, 2004; accepted June 7, 2004; published July 2, 2004)

The change of the Si(100) surface, exposed to Kr gas at 10 K with the scanning tunneling microscopy (STM) tip being
extracted, was observed by STM. For a n-type sample, a p(2  2)/c(4  2) coexisting structure was stably observed even at
low Kr coverage, and the amount of the c(4  2) area increased with increasing the coverage. These results clearly show theappearance of the p(2  2) phase at 10 K. The Kr growth process was observed by low-energy electron diﬀraction
measurement for both n- and p-type samples, suggesting the inﬂuence of the STM measurement on the c(4  2) structure
observed for the p-type sample at 10 K. [DOI: 10.1143/JJAP.43.L990]
KEYWORDS: STM, Si(100), rare gas adsorption, LEED, ground state, low temperature phase, dimer

The Si(100) surface is known to exhibit intriguingcharacteristics at low temperature. Si atoms in the top layer
form dimers. The energetically favored alternation of dimer
buckling along the dimer rows leads to two phases. One isthe c(4  2) reconstruction, in which alternating buckling
angles are also present between two neighboring dimer rows.
The other possible phase is the p(2  2) phase, in which thebuckling angle is the same between two neighboring dimerrows. The diﬀerence between the energies of the p(2  2)
and c(4  2) reconstructions obtained by theoretical calculations is only on the order of 1 meV per dimer.1) Thereappears a transition from the disordered (2  1) phase atroom temperature to the ordered c(4  2) one at 200 K,2–5)
however, below 40 K, the surface becomes unstable andthe p(2  2) phase was found to emerge again for n-typesamples. In addition, the 2  1 structure was also observed.
These results have caused the ground state issue to attract
much interest recent years.5–10)
Although all experimental conditions inevitably inﬂuence
the results, the Si(100) surface at low temperature has beenrecognized as being extremely sensitive to the measurement
conditions. Recently, scanning tunneling microscopy (STM)
parameters, such as bias voltage, tunneling current and tipsample distance, were found to strongly inﬂuence the surface
structure, due to the delicate balance between the c(4  2)
and p(2  2) phases below 40 K.11,12) The inﬂuence of thelow-energy electron diﬀraction (LEED) measurement wasalso pointed out.13) These results, suggesting the artifact of
the low temperature phase, gave rise to a further controversy
regarding the ground state of Si(100).
In order to consider the ground state issue, we studied therare gas adsorption on the Si(100) surface below 25 K, bymeans of STM and LEED. For the STM experiment, theSTM tip was extracted during the dose procedure in order toeliminate the inﬂuence of the STM tip on the surface
structure. On the basis of a consideration of the structuresfrozen by rare gases with suppressed dimer ﬂip-ﬂop motion,
the original substrate structure without any STM tip-sample
interaction was discussed.
In consideration of the dopant dependence in the previous
STM study, n- (As doped, 0.005 cm) and p-type (B doped,4 and 0.018 cm) Si(001) samples were used. Under STM
observation at 10 K, the p(2  2) phase becomes apparent
for the former sample, while the c(4  2) phase remainsapparent for the latter samples.11) After conﬁrming thestructure of the clean surface, the samples were cooled to10 K. The base pressure was 5  1011 Torr. Kr gasses weredosed with a variable leak valve. The sample temperature
was monitored using a Si diode or an Au+0.07%Fe-Cr
thermocouple located near the sample.
First, let us examine the structures of a n-type Si(100)surface. Figure 1 shows a typical STM image taken after theadsorption of a small amount of Kr atoms. The surface
structure with adsorbed Kr atoms was stably observed for
positive-sample-bias voltages below 1 V. Figure 1(b)shows a typical image after the further adsorption of Kr
atoms, and several characteristic adsorption structures areobserved. Since the STM tip was extracted during the dose
procedure, the area is diﬀerent from that in Fig. 1(a). Asindicated by arrows C to G, island structures with diﬀerent
heights exist. Magniﬁed images and their schematic models
are shown in Figs. 1(d) to 1(f), and Figs. 1(h) to 1(k). The
letters A to G in Figs. 1(c) to 1(k) correspond to those inFigs. 1(a) and 1(b), respectively.
At the initial stage, Kr atoms are adsorbed in both phases
as indicated by arrows A and B in Fig. 1(a), respectively. A
magniﬁed image of the square area in Fig. 1(a) and its
schematic are shown in Figs. 1(c) and 1(g). Kr atoms wereadsorbed at the bridge site for both cases. Since STM images
were taken at positive-sample-bias voltages, lower Si atoms
in the dimers are imaged in the ﬁgures, which is a point to be
kept in mind when the results are compared with theschematic models. The structures indicated by C and D inFigs. 1(b) and 1(d) are the ﬁrst Kr layers adsorbed on thec(4  2) and p(2  2) phases, respectively, and originate
from the structures shown in Figs. 1(c) and 1(g). In the caseof the adsorption on the p(2  2) phase, Kr atoms form azigzag structure due to the tight packing along the row.
According to the amount of Kr atoms adsorbed, thestructures are referred to as c(4  2)-1/4 ML and p(2  2)1/2 ML, respectively.
A noteworthy point is that although the STM tip wasextracted during the dose procedure, the p(2  2) andc(4  2) phases coexist (Fig. 1(a)). When the surface wasscanned by STM at higher voltage, Kr atoms on the p(2  2)
surface were removed ﬁrst. Then Kr atoms on the c(4  2)
area began to be removed. In the latter case, the amount of
the c(4  2) area decreased with decreasing the amount of

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Jpn. J. Appl. Phys., Vol. 43, No. 7B (2004)

(a)

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T. KIMURA et al.

(b)(c) AB(g) AB

c(4x2) p(2x2)(h)(d) C

D(e) EC(i)D(j)c(4x2)-1/4ML

p(2x2)-1/2ML: 1st layer: 2nd layerG(f)(k)Ep(2x2)-1MLG

c(4x2)-3/4M+ 3rd layer: 3rd layer

Fig. 1. (a) a typical STM image obtained for a Si(100) surface with a small amount of Kr adsorbed atoms (sample bias þ1 V, 0.25 nA).
(b) a typical STM image after further adsorption of the Kr gases (sample bias þ0:75 V, 0.1 nA). (c) to (f) and (g) to (k) are themagniﬁed STM images and schematics of the adsorption structures, respectively. A to G in (c) to (k) correspond to those in (a) and (b).

camera (30 frames/s), and sequences of 30 frames were
integrated to obtain images for the analysis of the change inthe LEED pattern. LEED measurement was performed at
25 K, because it was necessary to open the thermal shield.
Contrary to the STM experiment, a similar growth process
was observed for both n- and p-type samples. Figure 2shows a typical result for the change in the c(4  2) andp(2  2) LEED spot intensities observed for a p-type sample(4 cm). The 4  2 and 2  2 spots have almost the same
intensity before adsorption. When the adsorption wasstarted, the 4  2 spot intensity began to increase, while

Spot Intensity (a.u.)

the Kr atoms. After the additional adsorption of Kr atoms,
the c(4  2)-1/4 ML becomes dominant, and the p(2  2)1/2ML structural areas are conﬁned to the c(4  2)-1/4ML
domain boundaries.
The surface can be stably observed even with small
amount of the adsorbed Kr atoms, and the structural change
induced by the adsorption of Kr gas is from p(2  2) toc(4  2). These results clearly indicate that the p(2  2)phase exists from the beginning and is not due to any
measurement inﬂuence.
On the other hand, for the p-type Si(100) surface, no Kr
atoms were observed stably by means of STM and thec(4  2) structure remained. This may be partially due to thefact that the dimers of the p-type sample have a characteristic to ﬂip-ﬂop under STM observation.11)In order to examine the results of the STM measurements
further, the growth process was observed by means ofLEED. To enable comparison with the results obtained byLEED, let us consider the structure in detail. In the secondlayer, Kr atoms form similar zigzag structures in bothphases, which are named c(4  2)-3/4 ML (Fig. 1(k)) andp(2  2)-1ML (Fig. 1(j)), with reference to their total
amounts of coverage, respectively. In some areas, Kr atoms
in the third layer were adsorbed on the c(4  2)-3/4ML
structure before the completion of the second layer, as
shown in Fig. 1(f), indicating that the growth is characterized by the Stranski-Krastanov (SK) mode.
LEED patterns were recorded by a video-rate CCD

c(4x2)
c(4x2)p(2x2)p(2x2)

02

4Exposure (L)

68Fig. 2. LEED spot intensities for the (1=2, 1=2) and (3=4, 1=2)
spots as a function of the Kr gas adsorption (30 V, 25 K). The change at
1:6 L is just due to the measurement procedure.

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Jpn. J. Appl. Phys., Vol. 43, No. 7B (2004)

Express Letterthe 2  2 spot intensity decreased, which means that thep(2  2) phase began to change into the c(4  2) phase. Thismay be due to the instability of the p(2  2)-1/2ML
structure because of the stress caused by its tightly packed
structure. Since the SK-mode was observed by means ofSTM, the peak of the 4  2 spot intensity observed
corresponds to the formation of the c(4  2)-1/4ML
structure the entire surface. With further adsorption, the 4 
2 spot intensity decreases gradually, which is due to theisland structures formed randomly in the second layer. Theobserved change in the LEED measurement is in good
agreement with that observed by means of STM for then-type sample.
These results indicate that although the LEED measurement may inﬂuence the structure of the Si(100) surface,13) it
is not signiﬁcant when Kr gas is adsorbed. The balance
between c(4  2) and p(2  2) structures is originally similar
for both n- and p-type Si(100) samples below 25 K, and thec(4  2) structure stably observed for the p-type sample issuggested to be a result of the STM measurement inﬂuence.14)
Before summarizing the results, we brieﬂy mention theelectronic structure of the adsorbed Kr. As shown in
Fig. 1(g), it appears to be characteristic that the Kr atoms
interact preferentially with the downward Si atoms indimers, which is the side with a lower charge density.
Namely, Kr atoms are adsorbed between the downward Si
atoms of the two buckled dimers on the c(4  2) phase, andthe position of the Kr atoms shifts slightly toward thedownward Si atom of the two buckled dimers in the p(2  2)phase. In addition, Kr atoms on the p(2  2) surface show ahigher contrast in the STM image, although the heights of
the Kr atoms in the two phases are almost the same based on
the calculation using the rigid ball model. The observed

T. KIMURA et al.

chemical-bond like characteristics are similar to thoseobserved for the Kr and Xe adsorptions on the Si(111)-7 
7 surface.15)
In conclusion, on the basis of a consideration of thestructures frozen by Kr atoms with suppressed dimer ﬂipﬂop motion, the p(2  2) phase was conﬁrmed to appear at
10 K for a n-type sample. The STM measurement inﬂuenceis suggested for the c(4  2) structure for the p-type sample.
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