PHYSICAL REVIEW B 70, 165302 (2004)

Temperature effect as an essential factor in scanning tunneling microscopy studies demonstrated
by the analysis of an oxygen-adsorbed Si„111…-7 Ã 7 surface
Yasuaki Konishi, Shoji Yoshida, Yasuyuki Sainoo,* Osamu Takeuchi, and Hidemi Shigekawa†
Institute of Applied Physics, 21st century COE, NANO project, University of Tsukuba, Tsukuba 305-8573, Japan(Received 20 January 2004; revised manuscript received 15 April 2004; published 8 October 2004)
The very initial oxidation of a Si共111兲-7 ⫻ 7 surface was studied by scanning tunneling microscopy (STM).
The bias-dependent STM images of Si共111兲-7 ⫻ 7 surfaces with oxygen adsorbed at RT, 80 and 5 K werecompared by observing them at (RT and 80 K), (80 and 5 K) and 5 K, respectively. An essential influence ofthe measurement conditions on the STM images was clearly demonstrated. Through the experiment, the B-sitestate, which was defined as having a brighter site than that of normal adatoms, was found to have two different
structures at each adsorption site. The recently identified precursor state was shown to change into the welldistinguished B sites, and its thermal activation was also confirmed.
DOI: 10.1103/PhysRevB.70.165302

PACS number(s): 68.37.Ef, 68.43.Fg

The adsorption and chemical reaction of molecules onto
solid surfaces have been attracting considerable attention
from both fundamental and practical points of view. In particular, the understanding and control of the oxidation processes of silicon surfaces are the main subjects in the development of future nanostructural devices in utilizing
semiconductor techniques. Over the past decade, the interaction of oxygen molecules with silicon surfaces has been investigated by a large variety of experimental techniques andtheoretical methods.1–11 Of particular interest is the identityof the precursor of the stable oxidation products, i.e., thegeometry and electronic structures of the initial state of theO2 molecules adsorbed on the Si共111兲-7 ⫻ 7 surface. However, despite extensive research efforts, many aspects of themolecular-level mechanism of Si共111兲-7 ⫻ 7 oxidation remain controversial.
According to previous scanning tunneling microscopy(STM) studies at room temperature, the adsorption of oxygen
molecules produces two stable species at the adatom positions of Si共111兲-7 ⫻ 7. One appears brighter than normal Siadatoms (referred to hereafter as the B site) and is preferentially located on the corner adatoms in the faulted-half (FH)
unit cells. The other appears darker, similar to missing adatoms (referred to hereafter as the D site), and exhibits lesssite selectivity. At very low O2 exposure, most of the reaction
products are B sites, and the ratio of the number of D sites tothe total number of reacted sites increases with additional
oxygen exposure. Therefore, a widely accepted model is thatthe B site is formed by the oxidation of a Si adatom with asingle oxygen molecule, and the additional incoming oxygen
molecules react with B sites, resulting in the formation of Dsites. Existence of the precursor state of the B site was reported by analyzing the temperature dependence of the ultraviolet photoemission spectroscopy (UPS) spectra.2
Recently, using low-temperature STM and electron energy loss spectroscopy (EELS), a new adsorption species
was found.10 The new species is dark at bias voltages lower
than ⬃1 V, and changes into a bright site after the STM scan
at voltages higher than ⬃1 V. In comparison with the theoretical prediction that the metastable ad-ins and the stableins-ins structures appear in dark and bright images in STM,8

the new species were attributed to the precursor state of theB site. Here, ad and ins denote the O atom bonding on top of
the Si adatom and that inserted into a backbond of the Si
adatom, respectively. From the isotope-labeled vibrational
analysis, the precursor product was concluded to be formedby dissociated oxygen atoms instead of molecular oxygen.
All controversies seemed to have settled down.
However, in the EELS spectra as a function of exposure
time to O2, the second species appears after 0.03 L 共1 L = 1⫻ 10−6 Torr s兲 exposure following the first species that is
considered to be the precursor state, which was attributed toa different structure such as the ad-ins⫻ 3. Since the characteristic product appearing between the precursor state and theB site has not yet been observed by STM, there may be someother mechanisms for these phenomena.
In this paper, we present our results on the initial stage ofoxygen adsorption onto the Si共111兲-7 ⫻ 7 surface in consideration of the temperature effect in the STM images. In previous STM studies, oxygen adsorption species were studied
at only either negative or positive bias images. We examined
the oxygen adsorption species with both negative and positive bias voltages to understand their electronic structures inmore detail, and the difference in the four adsorption structures of the B site were clearly distinguished. Namely, eachof the adsorption species at corner 共Co兲 and center 共Ce兲 adatom sites both in faulted and unfaulted halves (FH and UH)of the Si共111兲-7 ⫻ 7 unit cell was found to have two types
with different temperature-bias voltage dependences.
The samples used were P-doped, n-type Si共111兲 surfaces,with a conductivity of 0.001– 0.005 ⍀ cm. In order to reduce
the effect of band bending in the study of bias dependence at
low temperature, highly doped samples were used in thisexperiment. The base pressure was 4.0⫻ 10−9 Pa. Oxygengas of ⬃99.97% purity was introduced through the variableleak valve (typically ⬃5 ⫻ 10−8 Pa). After confirmation of
the completed 7 ⫻ 7 structure, the STM tip was retracted
from the sample during oxygen exposure, and observation
was resumed over the same area with adsorbed oxygen. This
process ensured that all the observed reacted sites wereformed due to oxygen gas exposure. All STM images shownin this paper were taken in the constant-current mode by W

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KONISHI, YOSHIDA, SAINOO, TAKEUCHI, AND SHIGEKAWA

FIG. 2. (Color) The distribution of oxygen adsorbates on
Si共111兲-7 ⫻ 7 surface at adsorption and observation temperatures of(a), (b), and (i) (RT and RT), (c) and (g) (80 and 80 K), (d) and (h)(RT and 80 K). (e) (5 and 5 K), (f) (80 and 5 K兲, (j) (30, 30 K).
Yellow, blue, red, dark blue, gray, black, brown, and purple bars
indicate the B, DN, NB, BD, DD, D, NG, and DG sites, respectively. Data in (i) and (j) are from Ref. 11.

FIG. 1. (Color) STM images of the oxygen adsorption specieson Si共111兲-7 ⫻ 7 surface. (a) B site at 300 K 共Vs = ± 1.5 V ; It= 1.0 nA兲. (b) B and DN sites at 80 K 共Vs = ± 1.5 V ; It = 0.2 nA兲. (c)NB and BD sites at 80 K 共Vs = ± 1.5 V ; It = 0.2 nA兲. (d) DG and NGsites at 5 K 共Vs = ± 1.5 V ; It = 0.2 nA兲.

tip. Preparation of the STM tips was performed carefully,
and reproducibility was well confirmed to avoid tipdependent effect.
Figure 1 shows typical negative 共−1.5 V兲 and positive 共+1.5 V兲 sample bias voltage STM images of the B site obtained at RT. As has been observed in the previous STM
studies, the B sites are imaged bright both at negative andpositive bias voltages. The distribution of the B sites classified on the basis of four adsorption sites at RT is shown inFig. 2(a); adsorption species at corner 共Co兲 and center 共Ce兲
adatom sites both in faulted and unfaulted halves (FH andUH) of the Si共111兲-7 ⫻ 7 unit cell. Here, in order to analyze
the B site, formation of the D site was excluded by allowing
only very low oxygen coverage 共⬍0.03 ML兲 of the surface.

D sites appeared at higher coverages [Fig. 2(b)], which issimilar to the result observed by Mayne et al. as shown inFig. 2(i) (from Table I in Ref. 11).
On the other hand, when the oxygen adsorption and theobservation of the adsorbed surface were performed at lowtemperatures, the adsorption sites were imaged in completelydifferent forms. Namely, at 80 K, BD, NB, and DN sites
were observed in addition to the B site, as shown in Fig. 2(c).
And BD, DD, DG, and NG sites were observed at 5 K instead of the B sites [Fig. 2(e)]. Here, N, D, G, and B represent the brightnesses of the oxygen adsorbate STM images
compared to the brightness of the unreacted adatoms as isshown in Figs. 1(b)–1(d), namely, N: normal (unchanged),
D: dark, G: gray (less bright), and B: bright, as defined for
the B and D sites. For example, since the brightness wascompared for −1.5 and +1.5 V, DN site corresponds to sitewhose brightness is dark and normal at −1.5 and +1.5 V.
With this definition, although the B site should be represented as the BB site, we use the original definition to avoid
confusion. The DD site is different from the original D site.
As shown in Figs. 2(c) and 2(e), the observed species aredifferent, however, their relative proportions are similar if wereplace the B, NB, and DN sites at 80 K with the DG, DD,

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TEMPERATURE EFFECT AS AN ESSENTIAL FACTOR…

FIG. 3. (Color) STM images of the structuralchange of the precursors (P site) observed at 共Vs= + 0.7 V , It = 0.2 nA兲 before (a) and after (b)scanning at +1.5 V. The yellow, blue, and red
circles indicate the change of P → B sites, P→ DN sites and the B site thermally reacted fromthe beginning, respectively. STM images obtained at (c) 共Vs = −1.5 V , It = 0.2 nA兲 and (d)共Vs = + 1.5 V , It = 0.2 nA兲 are also shown to confirm the products. Distributions for the oxygen
absorbates obtained for the adsorption and observation temperatures of (e) (80 and 80 K) and (f)(RT and 80 K).

and NG sites, respectively. Their site dependence on the fouradatom sites (FH-Co, FH-Ce, UH-Co, UH-Ce) is close tothat obtained at RT [Fig. 2(a)]. In addition, D site appeared at
higher coverage as shown in Fig. 2(g), but the critical coverage seemed to be higher compared to the case at RT. Thisphenomenon is in good agreement with that observed byMayne et al., as shown in Figs. 2(i) and 2(j). Namely, appearance of D sites is suppressed at low temperature as waspointed out by them. Therefore, the most comprehensive explanation is to consider that the B sites originally have twodifferent structural types at each adatom site, which are imaged similarly at RT.
In order to confirm this point, we examined the temperature effect on the STM images. Namely, samples prepared athigher temperatures (300 and 80 K) were observed at lower
temperatures (80 and 5 K). For example, Fig. 2(d) shows theresult obtained for the samples on which oxygen was adsorbed at RT and which were observed at 80 K after quenching from RT. The new species BD, NB, and DN sites wereobserved similarly despite the samples being exposed to oxygen at RT. The distributions of these species observed at80 K [Fig. 2(d)] is almost the same as that obtained for thesamples prepared at 80 K [Fig. 2(b)]. In addition, D sites
appeared at higher coverage [Fig. 2(h)]. The result obtainedby quenching from 80 to 5 K is shown in Fig. 2(f), which
can be compared with that in Fig. 2(e).
According to our experimental result, the DN site wasimaged brighter at +2 V, and a slight difference in the effective bias voltage due to band bending is expected to occur at
low temperature depending on the difference in the dopant
density (doping of our sample is higher and the effect isless). If the NB and DN sites and BD site in Fig. 2(h) are
attributed to the B and D sites, respectively, the result in thecase of Mayne et al. [Fig. 2(j)], can be explained in the same
way systematically. The different appearance of the B site at
5 K may be caused by further charge transfer supposed tooccur at lower temperature.
In order to confirm the existence of the two types for theB site further, we examined the change of the precursor state

recently reported.10 The precursor state changes into the Bsite by scanning at a voltage higher than ⬃1 V. Therefore,
the surface was first observed at ⬃0.7 V to identify the precursor state sites [Figs. 3(a) and 3(b)]; the surface of what
following the scan at +1.5 V to transform the precursors into
B sites, was checked again at ±1.5 V to observe the distribution [Figs. 3(c) and 3(d)].
The precursor state was observed at the Co sites in theFH. When the oxygen adsorption and its observation wereperformed at 80 K [Fig. 3(e)], the amount of the precursor
state was 20% of the total oxygen adsorption species. By
comparing the positive and negative bias STM images ofoxygen adsorption species after the tip-bias-induced reaction,
the change of the precursor state into the B and DN sites wasclearly confirmed.
These results clearly indicate the extreme importance ofthe temperature effect on the STM images. Namely, the observed change in the STM images of the oxygen adsorption
species on the Si共111兲-7 ⫻ 7 surface is due not to the adsorption temperature but to the observation temperature. Recently, the temperature effect was pointed out for thehydrogen-adsorbed Ge surface.12 However, since structure isvery simple in that case, the oxygen-adsorbed Si共111兲-7 ⫻ 7
surface is an example of the detailed temperature effect on
the STM imaging of the local variety of the electronic structures. In fact, in addition to the adsorption site dependence,
the noteworthy point is that each adsorption site was foundto show two different species at low temperature.
The adsorption structures have been analyzed with comparison of the electronic structures between experimental andtheoretical results, assuming the oxygen atom forms bonds
with an adatom and one of the three backbonds with a threefold symmetry. However, the observed temperature dependence is more complicated as shown in Fig. 2. When thecomplex underlayers of the 7 ⫻ 7 unit cell are taken intoaccount, the atomic structure of the B site lacks the threefold
symmetry due to the asymmetry of the backbonds of theadatoms in the inside and outside directions of the unit cellas shown in Fig. 4 for the case of the corner adatom. There-165302-3

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fore, the asymmetry may result in an observable difference
between STM images and its temperature-dependent change
if a detailed theoretical analysis were performed. Chargetransfer at the adatom sites and its effect on the adsorption
processes has been studied extensively.13 However, in orderto understand the details of local electronic structures related
to chemical reactions, the temperature effect is very important, as has been observed, and must be included into thetheoretical analysis in addition to the real complicated structures.
On the basis of the obtained results, thermal reaction process of the precursor state was investigated by performing asimilar procedure for the analysis of the B site. Namely, oxygen was adsorbed at RT, and the sample was observed at
80 K after quick cooling to eliminate the thermal reaction at
RT [Fig. 3(f)]. As shown in the histogram, the amount ofprecursors was 5% of the total oxygen adsorption species,
which is much less than that in Fig. 3(e) which shows theresults of adsorption and observation performed at 80 K. Although difficulty in the bias-dependent discrimination of thespecies at 5 K enables us to compare the results at only two
different temperatures, which clearly shows that the precursors are thermally reacted. However, since the precursor
states were stable under observation with the voltage of0.7 V at 80 K, thermal energy of the adsorbed oxygen itself
must have played a role at least for the initial change of theminto the B and DN sites at the FH-Co site [Fig. 3(e)]. Since
the B site appears in the same form at the UH-Co site, chargetransfer may be less there, which may be related to the low
activity of the site.
Oxygen atoms were mostly adsorbed at the FH-Co sites,indicating that this site has the highest reaction probability.
Since the precursors remain only at the FH-Co adatom site,
the activation energy for the thermal reaction is the highest
there among all the adatom sites. Therefore, the reaction ofoxygen may be strongest at this site. Further experiments arenecessary to understand the site-dependent activation processes in more detail, which issues we leave for the future
study, together with the detailed analysis of the tip-inducedreaction.
In the EELS spectra, a new additional signal appeared
upon oxygen exposure following the first signal, which wasattributed to the structure between the precursor 共ad-ins兲 andthe B site 共ins-ins兲 states.10 However, since such a new structure was not observed in our STM study, and the B sites had
already formed even at 80 K, the structure may be related to
the newly identified different types of B sites. Since theprobe electron energy was 7 V for the EELS study, the measurement procedure may have changed the precursors to theB sites, in consideration of the tip-induced change at 1.5 V.
In such a case, the species analyzed by EELS would nolonger be the precursor state, and the conclusion of the dissociative interaction for the precursor state may need to bereconsidered.
In conclusion, we have demonstrated the importance ofthe temperature effect in the STM study by the analysis of
the very initial oxidation of the Si共111兲-7 ⫻ 7 surface. The B
site, which was originally defined at RT, was found to have
two different species at each adsorption site. The precursor
state, which was recently found, was confirmed to change
into the newly identified B sites, as expected. Since the adsorption species that was predicted by the EELS study toappear between the precursor and B site was not observed by
STM, and the B site was observed even at 80 K, reconsideration of the species attribution is necessary.

*Present address: Institute of Multidisciplinary Research for Ad-

7 C. Yan, J. A. Jensen, and C. Kummel, J. Chem. Phys.

FIG. 4. Schematic to show the asymmetry of ins structure depending on the adsorption site of oxygen atom.

vanced Materials, Tohoku University, Sendai 980-8577, Japan.
†URL: http://dora.ims.tsukuba.ac.jp
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