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31 DECEMBER 2002p2  2 Phase of Buckled Dimers of Si(100) Observed on n-Type Substrates below 40 Kby Scanning Tunneling Microscopy
Kenji Hata,* Shoji Yoshida, and Hidemi Shigekawa†
Institute of Applied Physics, 21st Century COE, University of Tsukuba, Tsukuba 305-8573, Japan(Received 25 October 2001; published 30 December 2002)
We have investigated the basic surface reconstruction of Si(100) on well defined surfaces fabricatedon various substrates at low temperatures ( 80 K) by scanning tunneling microscopy. Below 40 K, thesingle p2  2 phase, a phase never observed before, was observed exclusively on n-type substrates
doped in the range of 0.002 to 0:017  cm. We also exclude the possibility of the (2  1) symmetricdimer commonly observed at low temperature ( 10 K) being the basic surface reconstruction by
showing that a buckled dimer can be flip-flopped by the tunneling tip.
DOI: 10.1103/PhysRevLett.89.286104

PACS numbers: 68.35.Bs, 68.37.Ef

The surface reconstruction of Si(100) continues to be along lasting source of controversy [1–19]. In the history,
three possible phases have been considered as the groundstate. They are the (2  1) phase composed from symmetric dimers, and the c4  2 and p2  2 phases thatconsist of buckled dimers arranged in an antiferromagnetic and ferromagnetic ordering, respectively. Understanding regarding the ground state of Si(100) has beencultivated much by low temperature scanning tunnelingmicroscopy (LT-STM) experiments [3–12]. The first LTSTM observation of the Si(100) surface at 144 K unambiguously showed buckled dimers arranged in a c4  2phase, different from the (2  1) symmetric dimers observed at room temperature [3]. This observation led to
the general understandings of the surface accepted thesedays: (1) the c4  2 phase is the ground state and (2) the(2  1) symmetric dimers appear at room temperature asa result of a flip-flop motion of buckled dimers.
This picture is now challenged by recent STM experiments carried out at very low temperatures ( 10 K)[7–9]. At 6 K, Shigekawa et al. [7] have observed anemergence and a coexistence of the c4  2 and p2  2phases. On the other hand, two groups [8,9] have reported
a reemergence of the (2  1) symmetric dimers at
10 K and interpreted the result differently. Yokoyama
and Takayanagi [8] argued that the reemergence of the(2  1) symmetric dimers is due to an anomalous flip-flop
motion. In contrast, Kondo et al. [9] claimed that the(2  1) symmetric dimers at 10 K are static and posed aquestion regarding the ground state.
This work is motivated to solve the above conflict andto reconstruct a new general standard understanding of
the surface structure of Si(100). We investigated the surface reconstruction of a variety of Si(100) substrates
different in types (n and p, specifically), dopants, stepdensities, and conductivities by voltage dependent STM
imaging and tunneling spectroscopy at a wide range oftemperatures (9–80 K). In addition, we utilized the recent
progress to make a well defined surface with small defectdensities [10], a point that was crucial to rule out the in-fluence of defects and access to the intrinsic properties ofthe surface structure. Central to the results obtained is theobservation of a p2  2 phase, a surface reconstructionthat has never been observed before. The p2  2 phase
was observed specifically on n-type substrates doped in arange of (0.002 to 0:017  cm) and not on the p-type
substrates we investigated. This result suggests that theSi(100) surface might have a different ground state depending on whether the substrate is doped n or p type. Weshow that a buckled dimer can be easily flip-flopped by
the tunneling tip at low temperature, giving direct evidence that the (2  1) symmetric dimer phase observed at
low temperature is not an intrinsic surface phase.
Si(001) samples were ultrasonically cleaned for 10 min
in acetone and then loaded into the vacuum chamber andprebaked at 700  C for 5 h with a vacuum pressure
below 1:0  107 Pa. After prebaking, the sample wasonce flashed to 1200  C for 10 s to remove the oxidizedlayers. After the sample and sample holder are completely cooled, an additional flashing was carried out at
1200  C for a very short time ( 5 s) to reduce the defectdensity lower than 1% [10]. After flashing, the sample
was transferred to the STM and stable scanning at 9 K
could be carried out within 2 h after flashing. The surface
remained fairly clean for more than 24 h.
An STM image at 9 K of a wide and perfect p2  2phase formed in the middle of a defect-free domain
located far away from steps is shown in Fig. 1(a). Several important comments can be made about the observedp2  2 phase. First, and most importantly, the p2  2phase was observed only on n-type substrates. As listed inTable I, we observed the p2  2 phase on a wide rangeof n-type substrates different in dopant elements anddopant densities, though the p2  2 phase was never observed on any of the p-type substrates we studied. Second,
the p2  2 phase was observed only at temperatures
below 40 K. A phase transition occurred at around 40 K
[Fig. 1(b)] and the surface structure gradually converted
to the c4  2 phase as the temperature raised [Fig. 1(c)].
Third, the p2  2 phase is not caused by defect and286104-1

 2002 The American Physical Society

0031-9007=02=89(28)=286104(4)$20.00

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FIG. 1. Observation of the p2  2 phase. (a) An STM image(7  7 nm) of a p2  2 domain at 9 K (substrate 2 in Table I).
Imaging conditions: Vs  1:0 V; It  0:4 nA. (b),(c) STMimages (12  12 nm) at 38.5 and 40.4 K, showing the p2 2-c4  2 phase transition. Imaging conditions: Vs 1:0 V; It  0:4 nA. (d) Tunneling I-V curves. Dotted lines(i) and (ii): I-V curves from substrate 3 in Table I at 80 and 9 K,
respectively. Real line: I-V from substrate 2 in Table I at 9 K.

steps. In fact, dimers adjacent to steps and defects had astrong tendency to align in a c4  2 phase. Widep2  2 domains were observed only on surfaces withlow defect densities. In contrast, on surfaces with highdefect and step densities (substrate 1 in Table I), thesurface structure was a mixture of both the p2  2and c4  2 phases [7]. These experimental results suggest that the emergence of the p2  2 phase is an
intrinsic property of n-type substrates and is not caused
by defects or subsidiary effects such as decrease of theconductivity of the substrate at low temperatures.
The p2  2 phase was not observed previously because of two effects: (1) decrease of the conductivity ofthe substrate at low temperatures and (2) an intriguing
TABLE I. Substrates we studied the surface phase listed withthe resistivity, dopant, types, and the phase observed by STM.
No.

TypeResistivity ( cm)

DopantSurface phase

123
4
5
6
7
8nnnn
pp
p
p

0.002
0.004
0.012
0.014
0.001– 0.01
0.017
24AsAsPSbBB
BBp2  2p2  2p2  2p2  2

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31 DECEMBER 2002

dependence of the apparent surface structure observed bySTM on bias voltage. Figure 1(d) shows the temperaturedependence of the tunneling I-V spectra of an n-typeSi(100) substrate (substrate 3 in Table I) that has a dopantdensity (  0:012  cm) typically used for low temperature STM studies. At 80 K, the I-V spectrum showedsimilar conductivity at both negative (filled state) andpositive (empty state) surface biases. At 9 K, the conductivity at positive bias has decreased dramatically, andno tunneling current was measured at typical positive
biases used in STM. A similar tunneling rectifying behavior has been observed at room temperature on lowly
doped substrates (  10  cm) [13]. Thus, the rectifying behavior observed at low temperatures is caused by asuppression of ionization of dopants that leads to a thicker
depletion layer beneath the surface that hinders tunnelingof electrons ejected from the tip [14].
This rectifying behavior poses severe restrictions on
STM observations. Since no tunneling current flows at
positive biases, empty state imaging is not possible. Inaccessibility to the empty states at very low temperatures
explains why the p2  2 phase has not been observed
previously, because the p2  2 phase was observed by
STM only at the empty state as described in the following. We used degenerately doped substrates to image theempty states [note the same results were obtained on othern-type substrates if enough tunneling current could beobtained below 40 K (substrates 2 to 4 in Table I)].
Figure 1(d) shows a tunneling I-V spectrum (real line)
at 9 K of an n-type Si(100) substrate (substrate 2 in Table I)
that is heavily doped (  0:002  cm). A significant
tunneling current flows at positive biases enabling empty
state imaging. On these substrates, we found an interesting bias dependence of the surface structure by STM
observed only below 40 K.
An STM image [Fig. 2(a)] and a scanning tunnelingspectroscope (STS) spectrum [Fig. 2(b)] of the surface ofan n-type heavily doped substrate (substrate 2 in Table I)at 9 K are presented to show the bias dependence of STM
images and the corresponding imaged electronic states[Fig. 2(d)]. The STS spectrum was composed from threemain peaks (regions) and was identical to that observed
at higher temperatures [2]. Assignment of the three regions was given from recent spatial resolved tunnelingspectra studies [15] as (1) the peak in the filled state at1:0 V to the filled surface state, (2) the small peakin the empty state at 0:5 V to theempty surfacestate, and (3) tunneling from other states than these two
surface states compose the peak at 1:5 V. Interestingly,
the corresponding STM images of each region were different. First, and most importantly, buckled dimers in ap2  2 phase were observed when theempty state
was probed. Second, apparent (2  1) symmetric dimers
were observed at the filled state. Third, another (2  1)
symmetric feature that is out of phase from the dimerrows observed at the other two regions was observed inthe empty state high bias region. This (2  1) symmetric
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FIG. 2. Surface of n-type substrates. (a) An STM image(5:4  77 nm) of a surface of an n-type substrate at 9 K(substrate 2 in Table I) where the surface bias was changed
from 1:2, 1:0, and 1:5 V. It  0:4 nA. (b) Normalized
STS spectrum of the same surface at 9 K. Shaded regions
represent the surface band gap. (c) An STM image (7  4 nm)
at 9 K where the surface bias was changed from positive tonegative at locations indexed by arrows. Imaging conditions:Vs  1:0 V; It  0:4 nA. (d) A schematic showing the surface phase observed at different biases and the corresponding
states for n-type substrates (SSG: surface band gap; SS: surface
state).

feature was also observed in all p- and n-type substrates
we studied and was also observed at higher temperatures
up to room temperature [16,17]. It is established that STM
images at this bias region do not reflect the surface
structure because tunneling from other states (bulk stateand surface resonances) overwhelm tunneling from thesurface state [16,17]. If unintentionally this region isobserved, one might erroneously conclude that the surface structure is a (2  1) symmetric dimer. This might be
one reason of the conflict among researchers regarding
the surface phases observed at very low temperatures. The
most important issue that has to be clarified is whether
the p2  2 phase or the (2  1) symmetric dimer observed at the filled state reflects the true surface structure.
We found direct evidences that indicate the (2  1) symmetric dimers observed at low biases are caused by a flipflop motion induced by the STM tip. Figure 2(c) shows anempty state STM image of the p2  2 phase where thescanning was halted at locations indicated by arrows andthe bias was switched there from a positive to a negativevalue. A dramatic change in the surface configuration wasobserved when the bias was switched. This experimental
result is readily explained if a flip-flop motion of thebuckled dimers is induced by the tip at negative bias. A
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more direct evidence of tip induced flip-flop motion wasfound from experiments on p-type substrates.
STM images of the surface of p-type substrates (substrates 5 to 8 in Table I, doped in the range of 0.002 to0:017  cm) were very different from images of n-typesubstrates. First of all, the p2  2 phase was never observed on all of the investigated p-type substrates at any
tunneling conditions. This result is remarkably different
from n-type substates where essentially a p2  2 phase
was observed on all of the substrates studied, provided
enough tunneling current could be gained for imaging.
Instead, on many experimental conditions (on substrates 5to 8), we observed the (2  1) symmetric dimer. Exceptions were substrates 7 and 8 where a c4  2 phase, aphase we did not observe on p-type substrates, was observed at least at some bias voltages.
Substrate 8 in Table I showed a very interesting voltage
dependence [Fig. 3(a)]. Here, at the very upper edge oftheempty state (from 1:9 to 2.2 V), the c4  2phase was observed. When the voltage was lowered (below 1:9 V), even though we were imaging the same
empty state, the dimer converted into a (2  1) symmetric configuration. At higher voltages (above 2:2 V), the(2  1) symmetric features accompanied with the phaseshift were observed, defining the boundary of thesurface state. Importantly, at 1:9 V, a strong tunneling current dependence was observed: the buckled dimers
observed at low current [Fig. 3(b)] gradually transformed
into a (2  1) symmetric configuration [Fig. 3(c)] as thetunneling current was increased. Based on an extensive investigation, a phase diagram [Fig. 3(d)] of thephase observed by STM on various tunneling conditions
was constructed from which two important points weremade clear. First, the c4  2 phase was observed only ina very narrow window of tunneling conditions andthis must have precluded observation of the c4  2phase before. Second, the configuration of dimers became
symmetric at tunneling conditions where the tip-surface
interaction is strong (low bias and high tunneling current). Furthermore, at 80 K, we found that the buckled
dimer of the c4  2 phase could be forced into a symmetric configuration when the tip-surface interaction
is extremely strong (tunneling condition: surface bias
1:0 V; tunneling current 60 nA). These observations provide direct evidence that the observed (2  1)
symmetric dimers are induced by the tip. Indeed, theoretical calculations predict that a strong tip-dimer interaction would enforce a flip-flop motion giving a (2  1)
symmetric configuration in STM images [18].
The appearance of different phases on different substrates or the complicated voltage dependence of the STM
image of sample 8 are not readily explained. The experimental results strongly suggest that the conductivity ofthe substrate is an important factor for the former. We
think that the tip-surface interaction is affected by theconductivity of the substrate. Also, we would commentthat the surface bias necessary to observe the high bias
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However, observation of the p2  2 phase close to thevicinity of the step edges implies that a very small
perturbation or difference in environment might changethe stable phase. Possible causes we nominate are theexistence of the depletion layer beneath the surface andthe difference in the Fermi level position between n- andp-type substrates. We hope that our findings further invoke studies to elucidate this interesting issue.
In conclusion, we have implemented a thorough investigation of the surface structure of very clean and welldefined Si(100) surfaces at low temperatures below 40 K
by STM and STS. The p2  2 phase, a phase neverbefore observed, was observed specifically on n-type
substrates. Since the general belief that the c4  2 isthe ground state of this surface is based on an STM
observation of the c4  2 phase at higher temperatures(80 –200 K), our results might give rise to a controversy
to the long debated problem: whether the c4  2 orp2  2 phase is the ground state of this surface.

FIG. 3. Surface of p-type substrates. (a) An STM image(10  15 nm) of a surface of a p-type substrate at 9 K(substrate 8 in Table I) where the surface bias was changed
from 1:6, 1:9, and 2:5 V. (b),(c) STM images (7  7 nm)
at 9 K of the same location with different tunneling currents.
Vs  1:9 V. (d) Phase diagram of the existing probability ofthe c4  2 phase versus tunneling bias and current. (e) A
schematic showing the surface phase observed at different
biases and the corresponding states for p-type substrates.

empty state region is higher at sample 8 than in othersamples. This means that some voltage drop exists in thedepletion layer of sample 8. We speculate that this voltagedrop or boron segregation to the surface region [19] mightplay some role in the specialty of sample 8.
From STM studies alone, it is difficult to understand
why the n- and p-type substrates have different surface
structures at temperatures below 40 K. First principle calculations predict that the stabilities of the p2  2 andc4  2 reconstructions are essentially identical while
the (2  1) symmetric dimers are very unstable [20].
Indeed, by STM, at higher temperatures (80 –200 K),
occasionally a small domain of the p2  2 phase has
been locally observed close to step edges, though theirsize never spanned more than three or four dimer rows.
286104-4*Electronic address: khata@cmliris.harvard.edu†Electronic addresses: hidemi@ims.tsukuba.ac.jp; http://dora.ims.tsukuba.ac.jp[1] R. M. Tromp, R. J. Hamers, and J. E. Demuth, Phys. Rev.
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