Cheiron School 2015 BL practice@BL13XU
Surface and Interface Structures

BL13XU: X-ray Diffraction Measurement of Reconstructed Surfacever. 2015/08/17
Hiroo TAJIRI
tajiri@spring8.or.jp
1. Introduction
The undulator beamline BL13XU[1,2] is dedicated to studies of surface/interface
structures using surface X-ray diffraction. Precise structure determination of a crystal
surface can be done by measuring crystal truncation rod (CTR) scatterings, which
originate from termination of the surface. In the beamline practical, we measure CTR
scatterings from a reconstructed surface in ultra-high vacuum (UHV) with a 1-ton UHVchamber mounted on a 2+2 type diffractometer (See Fig.1).

Figure 1. (a) Schematic layout of BL13XU. (b) Surface diffractometer combined withan UHV-MBE chamber.
2. Surface X-ray Diffraction
X-ray diffraction has contributed for decades, as well as electron diffraction, tostructure analysis of surfaces and interfaces since brilliant synchrotron sources became
available, which overcome a drawback of weak interactions of X-rays with matters andthen make X-ray diffraction surface sensitive. A simple kinematical interpretation of
X-ray scattering make it possible to determine constellations of surface atoms precisely.
Excellent guides of surface X-ray diffraction (SXRD) can be found in the literatures[3-6].
In SXRD, diffraction conditions are described as those for two-dimensional lattices
as shown in Fig.2, and satisfied where either CTRs or fractional-order rods (FORs)
intersect the Ewald sphere. CTR has contributions from both surface layers and the inner
crystal whose periodicity perpendicular to the surface is truncated. In the kinematicalexpression, by summation of contributions to X-diffraction over a half infinite latticeplane, we get CTR intensity ICTR as follows with neglecting a constant of proportion,

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Cheiron School 2015 BL practice@BL13XU
Surface and Interface Structures

Figure 2. Reflection geometry for (a) out-of-plane, (b) in-plane surface X-ray
diffraction, and (c) transmission geometry. Gray circles denote Ewald spheres.
Here, crystal structure factor F(K) is the Fourier transform of charge density ρ(r) in anunit cell. K, r represent respectively a reciprocal space vector with the indices hkl and aposition vector in the unit cell, and FhkB (l), FhkS (l) are structure factors of bulk crystal andsurface layers, respectively. Bragg conditions are satisfied when l is integer and the rightside of eq. (2) diverges. However, the CTR intensity enough away from the Bragg pointcompared to the Bragg width is well explained by the kinematical theory. This conditionis almost always satisfied in SXRD. On the other hand, FOR is originating from a surface
super-lattice with a lager unit cell in a different symmetry from that in the crystal by
surface reconstruction in general.

Figure 3. Calculated intensity of crystal truncation rod (CTR) scattering from a simple
cubic crystal with a surface.
2Cheiron School 2015 BL practice@BL13XU
Surface and Interface Structures
CTR is sensitive to a surface structure as shown in Fig.3 since contributions to CTRs
from an inner crystal and its surface become comparable according to leaving away fromthe Bragg point. The blue curve is a CTR profile from the ideal crystal that surface atoms
are in the same positions as they would be in the bulk. The orange curve shows a CTR
profile from a crystal in which the top layer is relaxed inward by 7%.
3. Beamline Practical
We make in-situ observations of CTR scatterings from the Si(111)-√3 × √3-B
reconstructed surface with the UHV-MBE chamber mounted on the S2+D2
diffractometer. Silicon surface is one of the most intensively studied surface as aprototypical semiconductor surface. This evidence comes from not only scientificinterests to understand behaviors of semiconductor surfaces but also requirements fromthe industrial side to need making technological progress for developing reliable and/ornovel-concept semiconductor electronic devices. According to device size small tonanometers, where surface states of electrons play a fundamental role in device
performance, scientific knowledge becomes indispensable. Therefore, to know atomic
constellations on surfaces is a fundamental issue in understanding physical and chemical
properties on the surfaces at all.
Our plan of the practical is as follows:
1) Introduction of the beamline including optics, surface x-ray diffraction, the control
software (SPEC) [7], and scientific activities at BL13XU.
2) Sample alignment: Alignment of the optical surface normal by observing laserreflection from a sample surface, and determination of a crystallographic sample
orientation by detecting at least two Bragg reflections from the substrate.
3) Sample cleaning by heating a high boron-doped substrate at 1050℃ several times4) Measurement of SXRD (CTR scatterings) at room temperature.
5) Data analysis: Deriving structure factors from the observed data by using correction
factors, and making a structure analysis using the least-square fitting code developed at
BL13XU.
References:[1] URL:http://www.spring8.or.jp/wkg/BL13XU/instrument/lang-en/INS-0000000394/
instrument_summary_view[2] URL:http:http://www.spring8.or.jp/wkg/BL13XU/solution/lang-en/[3] R. Feidenhans'l, Surface Science Reports, 10 (1989) 105-188.
[4] I.K. Robinson and D.J. Tweet, Reports on Progress in Physics, 55 (1992) 599-651.
[5] E. Vlieg, Surface Science, 500 (2002) 458-474.
[6] J. Als-Nielsen and D. McMorrow, "Elements of Modern X-ray Physics" (John Wileyand Sons, 2001).
[7] SPEC software, URL: http://www.certif.com/

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