VOLUME 86, NUMBER 6

PHYSICAL REVIEW LETTERS

5 FEBRUARY 2001

Electron Spin Resonance Observation of the Si(111)-共共7 3 7兲兲 Surface and Its Oxidation Process
Takahide Umeda,* Masayasu Nishizawa, Tetsuji Yasuda, Junichi Isoya, Satoshi Yamasaki, and Kazunobu Tanaka
Joint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research,
Angstrom Technology Partnership, Tsukuba, 305-8562, Japan(Received 5 July 2000)
Electron spin resonance (ESR) observation of dangling-bond states on the Si(111)-共7 3 7兲 surface
is demonstrated for the first time. The ESR spectra clearly show that a reaction of molecular oxygenwith the Si(111)-共7 3 7兲 surface is associated with the appearance of a new dangling-bond center at
unreacted Si adatoms. Most of the oxidized surface sites do not show ESR signals, but in a minor partof the surface another new type of surface defect is detected. The well known Pb center at the SiO2 兾Siinterface is found to evolve at an oxide thickness as thin as 0.3 nm.
DOI: 10.1103/PhysRevLett.86.1054

PACS numbers: 82.65.+r, 73.20.– r, 76.30. – v

Electron spin resonance (ESR) is widely used to study
defect structures and chemical reactions that involve
unpaired electrons. Although ESR has great potential
as a surface probe, its application in surface studies has
been limited to indefinable surfaces of powder or porous
samples [1,2]. To make it possible to study well-defined
single-crystal surfaces, an ultrahigh-vacuum scanningtunneling microscope (UHV-STM) was combined withradio-frequency (rf) ESR [3]. This successfully achieveda single-spin detection on the Si(111)-共7 3 7兲 surfaces.
However, the spectroscopic information on the rf signal
was rather limited as compared to the usual ESR spectroscopy, due to a lack of g-anisotropy data, a sensitivity
problem, and unknown factors for the spin-dependent
tunneling [3]. Alternatively, a conventional ESR technique
had been applied to cleaved Si surfaces; however, thedetection of surface signals was not successful [4,5].
In this Letter, we report ESR measurements on theSi(111)-共7 3 7兲 surface when it is chemisorbed by oxygen molecules. The combination of a state-of-the-art ESR
system and a carefully designed UHV sample cell enables us to detect and identify new ESR signals on theclean single-crystal surface. Since the Si(111)-共7 3 7兲surface [6–10] shows 19 surface dangling bonds (DBs)per unit cell, it is an ideal sample for demonstrating thecapability of the UHV-ESR technique. The spectroscopic
information establishes that those DBs can be revealed bya new ESR center. In addition, ESR spectra clearly showthat the oxygen chemisorption upon the Si(111)-共7 3 7兲surface [11–16] greatly changed the DB states and induced another new paramagnetic defect. The present results are useful in analyzing oxidation of Si layers which
is an important reaction in Si technology. Also defects in
SiO2 兾Si structures are of technological interest becausesuch defects are related to reliabilities of metal-oxidesemiconductor devices [17–19]. Our method is expectedly
applicable to surface studies on a variety of clean surfaces
and their reaction processes.
Our experimental setup consists of an X-band ESR
spectrometer and a UHV chamber to which a sample cell (asilica glass tube of 1 cm in diameter) is attached. The UHVchamber 共base pressure 苷 9 3 1029 Pa兲 is arranged sothat the sample cell is located within the microwave
cavity of the ESR system. A sample of Si single crystal
共8 3 0.3 3 0.030 cm3 兲 is supported on the tip of avacuum transfer rod. In the UHV chamber, we are able toprepare the sample surface by standard surface treatment
and analysis techniques. Subsequently, we perform ESR
measurements on the processed surface by inserting thesample into the UHV sample cell. The transfer rod isrotatable so that the ESR spectrum of the surface can bemeasured as a function of an angle, u, between the surface
normal and magnetic field B. When u 苷 0 and 90±, themagnetic field parallels 关111兴 and 关110兴 axes, respectively. Si(111) wafers used in our study had a resistivity
higher than 1000 V cm and were polished on both sides
in order to increase the signal-to-noise ratio in the ESR
measurements. A Si(111)-共7 3 7兲 surface was prepared in
the chamber by direct-current heating of the wafer above
1400 K. For oxidation experiments, molecular oxygen was
introduced in the chamber through a calibrated variableleak valve. After the surface cleaning and oxidation, we
confirmed the surface structure by reflection high energy
electron diffraction (RHEED). All measurements andsurface oxidations were carried out at room temperature.
Our Si sample originally involved the so-called Pb
center [17] that is formed at the native oxide-Si(111)interface. The oxide removal by the direct-current heatingcompletely annihilated the interfacial Pb signal [20]. As
a result, the ESR spectrum of the Si(111)-共7 3 7兲 surface[Fig. 1(a)] shows a weak but detectable resonance at ag factor of g 苷 2.006. A RHEED measurement [inset
of Fig. 1(a)] ensures well-ordered 共7 3 7兲 reconstruction
on our surface. The double integral intensity of the ESR
signal at g 苷 2.006 corresponds to a spin density on
the order of 1012 cm22 , which is much lower than thepredicted density of DBs on an ideal Si(111)-共7 3 7兲surface, 3.04 3 1014 cm22 . Thus, ESR is largely insensitive to the DBs on the Si(111)-共7 3 7兲 surface at room
temperature. A similar phenomenon has been observedfor powder systems that contain a high density of surface
paramagnetic ions [1]. In such systems the high degree1054© 2001 The American Physical Society

0031-9007兾01兾86(6)兾1054(4)$15.00

VOLUME 86, NUMBER 6

PHYSICAL REVIEW LETTERS

5 FEBRUARY 2001

FIG. 2. The g maps for the observed ESR centers. The solidline represents the known g map for the Pb center [17], anddashed lines are fitted to the experimental data.

FIG. 1. ESR spectra of the Si(111)-共7 3 7兲 surface. (a) Clean
surface. (b) After exposure of the clean surface to O2 for 1 L,(c) 43 L, and (d ) to 1-atm O2 for 1 h. The spectra shown bysolid and dashed lines were measured for u 苷 0 and 90±, respectively. Each spectrum was accumulated for 2 to 6 h witha microwave power of 1 to 200 mW and field modulation frequency and amplitude of 100 kHz and 0.4 to 0.8 mT, respectively. Deconvoluted Ps0 and Ps1 signals in (c) were obtainedby simulations of a combination of two Voigt peaks. A RHEED
image in (a) was taken for 关110兴-incidence configuration. Arrows above the image indicate elementary reflections.

of coupling between neighboring unpaired electrons
broadens the ESR line, making the signal unobservable[1]. The g 苷 2.006 resonance observed here is probably
due to a sort of defected surface sites or contaminants
rather than normal DB states of the Si(111)-共7 3 7兲surface [21], because the DB states can be revealed byanother resonance, as is seen in the following oxidation
experiments.
Chemisorption of oxygen on Si(111)-共7 3 7兲 eliminates
the DB states of the topmost Si atoms, the so-called Si
adatoms [6], as reported in previous STM studies [11–14].
We have found that a new ESR signal appears in conjunction with such a reduction in the surface DBs. The pairsof spectra measured for u 苷 0 and 90± are also shownin Fig. 1 for different oxygen exposures. As is shownin the figure, the spectra (b) taken at the 1-L exposure
共1 L 艐 1024 Pa sec兲 reveal a new resonance whose g factor is clearly anisotropic. We name this new paramagnetic
center Ps0 . The angular dependence of its g factor, theso-called g map (Fig. 2), shows that this center has an axial symmetry about the 关111兴 axis with principal g fac-

tors of gk 苷 g共u 苷 0±兲 苷 2.0015, which is close to thefactor for the free electron (2.0023), and g⬜ 苷 g共u 苷
90±兲 苷 2.0064. These spectroscopic features indicate that
the Ps0 center originates from the 关111兴-oriented DB statesof Si(111)-共7 3 7兲.
As the O2 exposure was increased, ESR spectra wereincreasingly dominated by another new resonance. Figure 1(c) obtained at a 43-L exposure shows this new resonance (solid thin lines) superposed on the decayed Ps0
signal (broken thin lines). The origin of this signal is notclear at present, and we tentatively call it Ps1 . The g map of
this center (Fig. 2) does not agree with that for any known
center in the SiO2 兾Si structure [17–19]. Further, it does
not agree with that for any of a variety of oxygen complexes and radicals in Si [22] and SiO2 [23]. We point out
that the Ps1 centers are a type of surface defect, because
they exist only transiently during oxidation of the adatom
and underlying “rest-atom” layers and thus disappear completely once oxidation proceeds to the next surface layers,
as is clear from the spectra (d) in Fig. 1.
The spectra (d) in Fig. 1 were measured after an oxide
layer of approximately 0.3 nm in thickness was formedon the surface by exposure of 1-atm O2 . The thickness
was estimated from an Auger electron measurement [24]of the sample. The 共7 3 7兲 RHEED pattern was no longer
detected from the surface. We attributed the observed ESR
signal to the Pb center at the oxide-Si(111) interface, since
the spectrum is identical to that reported for this center[17]. On the basis of the estimated oxide thickness of
0.3 nm, we estimate that the Pb center in our sample isformed in the third atomic layer from the surface. This
layer lies approximately 0.43 nm below the adatom layer[7]. The Si atoms in the third layer have 关111兴-oriented
bonding toward the second layer [7] and therefore canadopt 关111兴-oriented DBs.
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Figure 3 shows the ESR results as a function of the O2
dose up to 1000 L. The oxidation primarily involves theadatom and rest-atom layers, because the RHEED image at
1000-L exposure retained a faint 共7 3 7兲 pattern, in consistency with the previous result [15]. Also shown in the
figure are STM images of the surface measured at 0.4,
1, and 2 L. The STM observation was carried out in a
separate vacuum chamber under the identical condition asthat of the ESR experiments. First, we point out that thedensity of the Ps0 centers monotonically decreases withthe oxygen chemisorption [Fig. 3(c)], showing the same
behavior as unreacted adatom DBs [see Fig. 3(a)]. Reduction of the Ps0 density is completed by approximately
10-L exposure of O2 . The observed change is consistent
with previous observations by STM [11] and photoemission [15] that unreacted Si adatoms disappeared at 10 to
20 L. Therefore, it is reasonable to consider that the Ps0
centers correspond to the 关111兴-oriented DB states of unreacted Si adatoms. For the clean 共7 3 7兲 surface, the Ps0
centers are not detectable due to the extensive signal broad-

FIG. 3. Oxidation-induced changes of the Si(111)-共7 3 7兲 surface as a function of O2 dose. (a) Reduction in the density of
unreacted adatom DBs (bright spots) observed by STM. The
area shown here is approximately 20 nm by 20 nm and wasmeasured with a constant current of 0.1 nA and a sample voltage of 1 to 2 V. The sharpness of the RHEED pattern wasused as a common measure in order to guarantee that the STM
and ESR samples are similar in integrity in the original 共7 3 7兲reconstruction. (b) Change in linewidths of the Ps0 and Ps1 centers. These widths correspond to full-widths-at-half-maximumof absorption peaks of the ESR signals at u 苷 0±. (c) Change
in the spin densities.

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ening. However, the signal linewidth narrows by oxygen
chemisorption [Fig. 3(b)], resulting in the appearance of
the Ps0 signal. The Ps0 signal becomes detectable at 0.4-L
exposure [Fig. 3(b)], although a large signal broadening
and overlap of the g 苷 2.006 signal make it difficult toestimate the Ps0 density. As the spin density and linewidth
are reduced, the g⬜ factor of Ps0 increases and saturates to
2.008 (Fig. 2). The variable g⬜ factor and signal broadening for the Ps0 center reflect on the fact that the electronic
structures and environments are appreciably different between the Ps0 (surface Si DBs) and Pb (interface Si DBs).
The areal density of the Ps0 centers is approximately
3 3 1013 cm22 at the O2 exposure of 1 L [Fig. 3(c)].
However, the STM image for the same exposure [seeFig. 3(a)] gives an adatom DB density of 9 3 1013 cm22 .
We tentatively attribute this difference to the fact that ESR
selectively detects paramagnetic (i.e., neutral) Si adatoms.
It has been reported [8,9,12] that on the Si(111)-共7 3 7兲surface, electron transfer takes place from the adatom DBs(twelve atoms per unit cell) to rest-atom and corner-holeDBs (six and one per unit cell, respectively). This process
leaves five paramagnetic Si adatoms per unit cell, andall other DBs are stabilized in charged states. If we takethis effect into account, a reasonable agreement between
ESR and STM data is obtained: 5兾12 of the unreacted
Si adatoms observed by STM is 4 3 1013 cm22 , which
is close to the spin density measured by ESR. The same
relation is also found for the case of a 2-L exposure. In
the 2-L image of Fig. 3(a), the unreacted adatom DB
density is 6 3 1013 cm22 , and 5兾12 times that is about
3 3 1013 cm22 which is also comparable with ESR data.
We found that the broadened Ps0 signal remains unsaturated even at a microwave power exceeding 200 mW andthat its line shape is well fit by a Lorentzian distributionfunction. These facts indicate that the lifetime broadening of the signal is caused by a very short spin-lattice relaxation time 共T1 兲 of unpaired electrons. The origin ofsuch a short T1 may be attributed to metallic electronic
states of the Si(111)-共7 3 7兲 surface that originate froma high density of adatom DBs [7–9]. Specifically, if theunpaired electrons of the adatom DBs are delocalized at
the surface [9,10], T1 values as short as those for the conduction electrons in metals and doped semiconductors canbe obtained [25,26]. For instance, the 5-mT linewidth observed at 0.4-L exposure corresponds to 1028 sec of T1 .
The T1 value will be much shorter on the clean surface,
due to more delocalization of the DB’s electrons. In the
STM images shown in Fig. 3(a), most of the unreacted Si
adatoms neighbor upon each other. Such grouping of theunreacted adatoms can assist the delocalization of their unpaired electrons. The dipolar broadening of the Ps0 signal
may be compensated by the exchange or motional narrowing effects of the delocalized electrons.
Since the DB states of oxidized adatoms are removed
far from the Fermi level [12,15], the delocalization of thePs0 centers cannot extend to the oxidized adatoms. In addition, the oxidized adatoms are a barrier to the electron

VOLUME 86, NUMBER 6

PHYSICAL REVIEW LETTERS

conduction across them. Accordingly, the more oxygen
chemisorption on the Si(111)-共7 3 7兲 surface is considered to result in the less delocalization of the unpaired
electrons. Reduction of the signal broadening effect seenin Fig. 3(b) is supposedly a consequence that the localization of the unpaired electrons proceeds with the submonolayer chemisorption of O2 . If the electrons are localized,
their T1 will no longer be so short as to cause the lifetime
broadening of the Ps0 signal. The localization phenomenon is also suggested from the increase in g⬜ of the Ps0
center (Fig. 2), because the g⬜ value of the DB-like centers
becomes larger with the more localization [27]. In addition, we speculate that the local structure of the Ps0 centers is distorted with the oxidation-induced strain, which
also leads to the increase in g⬜ as observed in the Pb
system [28].
The formation of the Ps1 centers is induced by the O2
exposure with 2 to 4 L, as is shown in Fig. 3(c). They
are alive even after the whole surface area has been oxidized by the oxygen exposure much larger than 10 L.
These facts suggest that the Ps1 center is related tooxygen-chemisorbed sites. In contrast with the Ps0 center(苷 unreacted Si adatoms), the Ps1 center may involve
oxygen atoms in its structure. However, the areal densityof the Ps1 center is less than 1% of that of the oxidized
sites 共艐1014 cm22 兲. It means that a small number of spin
centers are introduced by the oxidation process of the top
surface, although it mainly annihilates spin centers on theSi(111)-共7 3 7兲 surface.
The result that the reaction of molecular oxygen withthe surface eliminates an electron spin from the oxidized
site would be valuable for examining the oxidation processes on Si surfaces. One possible process of the O2
chemisorption is that O2 attaches to a surface Si DB, form—Si-O-O兲 [16]. In the other models
ing the peroxy radical 共—[12,13,16], dissociated oxygens are assumed to generate—Si-O兲 or Si-O-Si backbonds ofthe nonbridging oxygen 共—
the Si DB. All of those atomic structures, however, havean unpaired electron [23], unless they are charged. Therefore, the formation of those sites should be associated withthe electron transfer between surface sites in order to annihilate the unpaired electron.
In summary, we have performed the ESR measurementson the chemisorption process of molecular oxygen on theSi(111)-共7 3 7兲 surface. It was shown that there exist
two kinds of new paramagnetic center on the surface: aDB center on unreacted Si adatoms 共Ps0 兲 and another new
type of surface spin center 共Ps1 兲. Most of the oxidized
sites were found to have no unpaired electrons. Our experiments have demonstrated that electron-spin states on
single-crystal surfaces can be resolved by the UHV-ESR
technique. The present technique will open up a new approach in surface science analysis.
This work, partly supported by NEDO, was performedin the JRCAT under the joint research agreement between
the NAIR and the ATP. One of us (T. U.) acknowledges the

5 FEBRUARY 2001

support of the Japan Society for the Promotion of Scienceand University of Tsukuba.

*Present address: System Devices and Fundamental Research, NEC Corporation, Tsukuba, 305-8501, Japan.
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