Microscopic adsorption process of CO on Si„ 100… c „ 4 Ã 2 … by means of low-temperature scanning
tunneling microscopy
Y. Yamashita,* M. Z. Hossain, K. Mukai, and J. Yoshinobu
The Institute for Solid State Physics, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
Received 2 December 2002; revised manuscript received 26 March 2003; published 31 July 2003
The microscopic adsorption process and bonding nature of CO on Si100 have been investigated by means
of low-temperature scanning tunneling microscopy and valence-band photoelectron spectroscopy, respectively.
CO molecules initially adsorb at the C-defect and then start to develop as an island. Thus, the C defect is an
inhomogeneous nucleation center for the CO island. Upon adsorption, 5 donation dominantly occurs from
CO to the silicon down dimer atom, forming a bonding state at 10.68 eV according to the valence-band
photoelectron spectra. Thus, CO preferentially interacts with the electron-deficient down dimer atom.
DOI: 10.1103/PhysRevB.68.033314 PACS numbers: 68.37.Ef, 68.35.Ja
The understanding of chemical reactivity of the Si100
surface is important, not only from the scientific perspective
but also from technological applications, because the Si100
surface has been used for the substrate of the most frequently
used semiconductor devices. The dimer of the Si100 sur-
face is asymmetric
1–8
where the up dimer atom is electron
rich, while the down dimer atom is electron deficient.
1–3,7,8
Thus, from the perspective of surface chemistry, the reactiv-
ity of up and down dimer atoms should be different towards
the polar molecules.
When CO, which is a typical polar and Lewis-base mol-
ecule, is dosed to the Si(100) c (4 2) surface, CO chemi-
sorbs molecularly on Si100 below 200 K Refs. 9–13
and adsorbs on the down dimer atom.
12
Despite these stud-
ies, the microscopic adsorption process and the detailed
bonding nature of CO on the Si(100) c (4 2) surface are
poorly understood. In the present study, we investigated the
microscopic adsorption process and the detailed bonding na-
ture of CO on the Si(100) c (4 2) surface using low-
temperature scanning tunneling microscopy STM and
valence-band photoelectron spectroscopy PES, respec-
tively.
The experiments were performed in ultrahigh vacuum
UHV chambers, where the base pressure was below 1
10
-8
Pa. Boron-doped p-type Si100 wafers were used in
the experiments. Clean Si(100) c (4 2) surfaces were ob-
tained after being outgassed at 900 K for 12 h, flashed up
at 1550 K several times, and cooled slowly from 1000 K
to 70 K. During flashing, the pressure was below 1.2
10
-8
Pa to prepare an almost defect free Si100 surface
( 0.3%).
14
The CO molecules were introduced into the
UHV chamber through a gas doser. The STM measurements
were performed with a JSPM 4500 microscope. In the STM
measurements, samples were cooled at 70 K using solid N
2
.
Valence PES measurements were performed at BL 16B in
KEK. The incident photon energy was 50 eV, and the pho-
toelectrons were detected at a normal emission condition.
The sample was cooled at 100 K and the overall instrumental
resolution of this PES system was 30 meV.
Figure 1 shows a series of occupied state STM images
before and after a small exposure of CO molecules to the
Si(100) c (4 2) surface. For the Si(100) c (4 2) clean sur-
face Fig. 1a, the buckled dimers with c (4 2) phase are
clearly observed. Bright zigzag lines are attributed to the up
dimer atoms of the silicon asymmetric dimer.
4
Note that
down dimer atoms of the silicon asymmetric dimer become
bright when the unoccupied states are probed.
15,16
For the
Si(100) c (4 2) clean surface Fig. 1a, few defects 0.2%
are observed. Type-A , type-B , and type-C defects were re-
ported on the Si100 surface.
17–19
In Fig. 1a, ‘‘Y’’-shaped
depressions are due to the A-defect
17,20
. The depressions in-
dicated by arrows in Fig. 1a are due to the C defect because
this defect becomes bright when the unoccupied states are
probed.
19
For the structure of the C defect, we have recently
reported that the C defect is caused by the dissociative ad-
sorption of H
2
O on the same side of two adjacent dimers;
there exist Si-OH and Si-H species and two reactive dangling
bonds in these two dimers.
20
It should be noted that the de-
fect density in this study is very low 0.2%, compared with
the previous STM images on the Si100 surface the defect
density is usually a few percent.
4,21
At the initial stage of adsorption, CO molecules are pref-
erentially adsorbed at the C defect sites Fig. 1b. Since the
depression area is increased as a function of exposure, these
depressions can be assigned to the effect of the adsorbed CO
molecules. Therefore, CO molecules are initially adsorbed at
the C defects and then start to develop as an island structure.
Since the C defect has very reactive dangling bonds,
20
it is
likely that CO molecules initially adsorb at the C defect site.
Note that bright protrusions are observed in the images Figs.
1b –1d. However, they have not been identified yet. They
may be precursor species.
According to the temperature programmed desorption
TPD study of CO on Si(100) c (4 2), two desorption
peaks were observed at 210 K and 235 K, when 0.1 ML of
CO was exposed to the Si(100) c (4 2) surface at 90 K,
13
where ML stands for monolayer. The intensity ratio became
1:0.07 at saturation 0.5 ML; one CO molecule per Si
dimer
10
.
13
Kubo et al. concluded that the former one was
attributed to CO adsorbed on the silicon dimer, whereas the
latter one was due to CO located in defect sites; the defect
site may be a more stable adsorption site than the defect free
surface. Therefore, we conclude that the initial adsorption
site of CO on the Si(100) c (4 2) surface is at the C defect.
PHYSICAL REVIEW B 68, 033314 2003
0163-1829/2003/683/0333144/$20.00 ©2003 The American Physical Society 68 033314-1