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Structure Analysis of K/Si(111)-√3 × √3-B Surface by
Reflection High-Energy Positron Diffraction
The K/Si(111)-√3 × √3-B surface has been extensively
investigated as a typical example of Mott-type insulating
1)
surfaces . Recently, it has been reported that the
K/Si(111)-√3 × √3-B surface undergoes the phase transition
2)
from the √3 × √3 to 2√3 × 2√3 surface at 270 K . The
electronic states of the Mott insulating surface have been
studied in detail. In the experimental point of view,
however, the atomic coordinates of the K/Si(111)- √3 × √3-B
surface still remain unresolved. In this study, to investigate
the atomic coordinates of the Mott insulating surface, we
measured the rocking curve from the K/Si(111)- √3 × √3-B
surface using a reflection high-energy positron diffraction
(RHEPD). By means of the intensity analysis based on the
dynamical diffraction theory, we determined the atomic
heights of the K on Si(111)- √3 × √3-B surface.
The substrates (10 × 5 × 0.625 mm 3 ) were cut from a
mirror-polished highly B-doped Si(111) wafer. They were
flashed at 1470 K a few times in an ultra-high vacuum
(UHV) chamber with a base pressure less than 2 × 10 -8 Pa.
After the annealing at 1170 K, well-ordered √3 × √3
structure was formed. Subsequently, 1/3 ML of K atoms
were deposited onto the Si(111)- √3 × √3-B surface at room
temperature (1 ML corresponds to the atomic density of
7.83 × 10 14 cm -2 ).
The experiments were carried out in the UHV chamber
equipped with a positron source of 22 Na and electromagnetic
lens system. The accelerating voltage of the incident
positron beam was set at 10 kV. The diffraction pattern
was enhanced using a micro-channel plate with a phosphor
screen and taken with a charge-coupled-device camera.
The glancing angle of the incident positron beam was varied
from 0.3° to 6.0° at 0.1° step by rotating the sample.
Figure 1 shows the RHEPD rocking curve of specular
spots from the K/Si(111)- √3 × √3-B surface at 60 K. The
incident azimuth corresponds to 7.5° away from the [ 11 2 ]
direction (one-beam condition). Under the one-beam
condition, the RHEPD intensity is very sensitive to the
atomic coordinates perpendicular to the surface. The
intense total reflection and 111 Bragg peaks are clearly
observable. In the total reflection region, the dip structure
at around 2.0° is also observed. Although in the previous
study a 2√3 × 2√3 structure is observed below 270 K 2) , the
pattern still displays √3× √3 periodicity at 60 K. It is
considered that the phase transition temperature is very
sensitive to the defects on the surface 2) .
We calculated the RHEPD intensity based on the
dynamical diffraction theory. We optimized the atomic
height of the K atoms so as to minimize the difference
between the measured and calculated curves. The solid
JAEA-Review 2010-065
Y. Fukaya, A. Kawasuso and A. Ichimiya
Advanced Science Research Center, JAEA
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Specular intensity (arb. units)
111
222
total reflection
333
555
444
: exp (60 K)
: cal
0 1 2 3 4 5 6
Glancing angle (deg)
Fig. 1 RHEPD rocking curve of specular spots from the
K/Si(111)- √3 × √3-B surface at 60 K. The open
circles and solid line indicate the measured and
calculated curves, respectively. The acceleration
voltage of the incident beam is 10 kV. The incident
azimuth corresponds to 7.5° away from the [ 11 2 ]
direction (one-beam condition). The critical angle of
the total reflection is estimated to be 2.0° via the
Snell's equation.
line indicates the calculated curve using the height of 1.99 ±
0.26 Å from the first Si layer. The calculated rocking
curve is in good agreement with the measured one.
3)
According to the first principles calculations , the
optimum heights of the K atoms at H3 (threefold
coordinated) and T’ 4 (ontop) sites correspond to the 2.03 Å
and 2.19 Å, respectively (see Fig. 2). The height of 1.99 Å
obtained in this study is close to the theoretical value at H3 site 3) . Therefore, it is considered that the K atoms are
situated at the H3 site. The adsorption of the K atoms at H3 3)
site is energetically favored theoretically .
References
1) H. H. Weitering et al., Phys. Rev. Lett. 78 (1997) 1331.
2) L. A. Cardenas et al., Phys. Rev. Lett. 103 (2009)
046804.
3) H. Q. Shi et al., Phys. Rev. B 70 (2004) 235325.