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Surface Structure of Si(111)-√21×√21-(Ag, Cs) studied
by Reflection High-Energy Positron Diffraction
Y. Fukaya a) , I. Matsuda b) , A. Kawasuso a) and A. Ichimiya a)
a) Advanced Science Research Center, JAEA, b) The University of Tokyo
The Si(111)-√3 × √3-Ag surfaces have been extensively
investigated as a typical two-dimensional metal system 1) .
By the adsorption of small amounts of noble (Cu, Ag, and
Au) and alkali (Na, K, and Cs) metal atoms on the
Si(111)-√3 × √3-Ag surfaces, the √21 × √21 superstructures
are formed, accompanied with a drastic increase in the
surface electrical conductivity 1) . The atomic coordinates
of √21 × √21 superstructures have been studied experimentally
and theoretically. In the case of the adsorption of
noble metal atoms, we found that three noble atoms are
situated at the center of large Ag triangles, surrounding the
2)
Si trimer . According to the scanning tunneling
3)
microscopy observations , the alkali-metal-atom induced
√21 × √21 superstructure is considered to be different from
the noble-metal-induced ones. In this study, we measured
the rocking curve of reflection high-energy positron
diffraction (RHEPD) from the Cs-induced √21 × √21
superstructure and analyzed the atomic height of Cs atoms
by means of the dynamical diffraction theory.
The substrates (10 × 5 × 0.5 mm 3 ) were cut from a
mirror-polished n-type Si(111) wafer with a resistivity of
1-10 Ωcm. To prepare clean 7 × 7 surfaces, they were
heated at 670 K in several hours and flashed at 1470 K in a
few seconds several times in an ultra-high vacuum (UHV)
chamber with a base pressure less than 3 × 10 -8 Pa. Then,
1.0 ML Ag atoms were deposited on the Si(111)-7 × 7
surfaces held at 740 K to form the √3 ×√3-Ag structures (1
ML = 7.83 × 10 14 cm -2 ). Finally, 0.14 ML Cs atoms were
deposited on the Si(111)-√3 × √3-Ag surfaces at 230 K.
The formation of well-ordered Si(111)-√21 × √21-(Ag,Cs)
surfaces was confirmed by reflection high-energy electron
diffraction (RHEED).
The RHEPD measurements were carried out using a
highly parallel and well focused positron beam generated
from 22 Na positron source and electromagnetic lens. The
positron beam energy was set at 10 keV. The diffraction
patterns were magnified with a micro-channel plate with a
phosphor screen and recorded with a charge-coupled-device
camera. In the rocking curve measurements, the glancing
angle of the incident positron beam was changed from 0.3°
to 6.0° at a step of 0.1° by rotating the sample.
The open circles in Fig. 1 show the RHEPD rocking
curve measured from the Si(111)-√21×√21-(Ag,Cs)
superstructure at 170 K. The azimuth of the incident beam
corresponds to 7.5° away from the [ 11 2 ] direction. Under
this condition, the RHEPD intensity of specular spots is very
sensitive to the atomic positions normal to the surface. In
the total reflection region, two distinct dip structures are
clearly observed at around 1.2° and 2.2°.
JAEA-Review 2010-065
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Specular intensity (arb. units)
TR
(222)
(111) (333) (444) (555)
h ad (Å)
exp
0 1 2 3 4 5 6 7
Glancing angle (deg)
Fig. 1 RHEPD rocking curves from the Si(111)-
√21 × √21-(Ag,Cs) superstructure. The open circles
denote the experimental curve. The solid lines
indicate the calculated curves using various heights
(h ad) of Cs atoms from the underlying Ag layer. TR
stands for the total reflection region.
To determine the height of Cs atoms, we calculated the
rocking curves based on the dynamical diffraction theory.
The solid lines in Fig. 1 show the rocking curves calculated
using various heights of Cs atoms from the underlying Ag
layer. The shape of the curve drastically changes
depending on the height of Cs atoms. From the
comparison between the measured and calculated curves, the
optimum height of Cs atoms from the underlying Ag layer
was determined to be 3.04 ± 0.26 Å. The value is much
larger than those of noble metal adsorptions (0.53-0.59 Å) 2) .
The atomic radius of Cs (2.67 Å) is much larger than that of
noble metals (1.28-1.46 Å). It is considered that the large
atomic radius leads to the large adsorption height.
Consequently, we found that the adsorption height of Cs
atoms is much higher than those of noble metals.
References
1) S. Hasegawa et al., Prog. Surf. Sci. 60 (1999) 89.
2) Y. Fukaya et al., Surf. Sci. 600 (2006) 3141.
3) C. Liu et al., Jpn. J. Appl. Phys. 42 (2003) 1659.
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