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()トピックス

I. X-Ray Structural Study of Ge(001):Te 1×1 Performedat the Advanced Photon Source. II. Current Statusof the Surface-Interface Structure Beamline at SPring-8
Osami SAKATA
Japan Synchrotron Radiation Research Institute

Abstract
This article is composed of two parts. In the ˆrst half, we describe a study that we performed at 5ID–C of the DupontNorthwestern University-Dow (DND) CAT in the Advanced Photon Source, the Argonne National Laboratory for 1998
to 2000. A surface structure of Ge(001): Te 1×1 was determined by least-squares ˆts of x-ray scattered intensities with calculations based on some surface atomic structural models. The ˆtted structural model has a characteristic that a directionof a Ge–Ge dimer bond on the ˆrst Ge atomic layer is perpendicular to a Te missing row. It was distinct from those basedon ˆrst-principles total energy calculations. In the second half, we introduce up-to-the-minute status of BL13XU for surface-interface structural studies at SPring-8. Scientiˆc research goals we desire are mentioned as well.

1.

A structure of a Ge(100):Te 1×1 surfaceHeteroepitaxial crystal growth is classiˆed into threegrowth modes: Frank-Van der Merwe (FM; 2D) growth1),
Volmer-Weber (VW; 3D) growth2), and Stranski-Krastanov(SK; 2D followed by 3D) growth3). Whether a ˆlm undergoes
FM, VW, or SK growth depends on the balance between surface, interface, and ˆlm free energies. Copel et al.4) proposed
an elegant method to amend the kinetics at a growth surface
by using surface-active species (surfactants). They demonstrated that island formation and interdiŠusion were avoided
with a surfactant. Such surface mediated epitaxy can speciˆcally control the kinetics at the growth surface.
Te has proven to be an eŠective surfactant in forming asharp interface in Ge W
Si heteroepitaxial structures grown by
molecular beam epitaxy.5) To understand this growthmechanism on an atomic scale, we aimed at determining
structural parameters, such as adsorption sites and bondlengths for a Ge(100): Te 1×1 surface.
There were two research articles related to the surface
structures as far as we knew. One was an experimental study
using low energy electron diŠraction (LEED) and x-ray
standing waves.6) The researchers' ˆnding was that Te atoms
are located at a well deˆned position in the [004] direction
but has a spread in its surface plane. From these results, theresearchers expected Te lateral displacements from the bridge-sites in the 5×1 Te missing row model and dimerized Ge
surface atoms (Fig. 1). The other article was about a study
on the surface structure based on ˆrst-principles total energy
calculations.7) Two structural models were therein proposed
which are the 2×1 Te zigzag model (Fig. 2) and the 5×2 missing row model. The energy diŠerence between the twomodels was not shown due to di‹culties in comparing theenergies for the two models which used diŠerent size clusters.
It was noted that the orientation of Ge dimers in the 5×2
model diŠered by angle 90 degrees from those in the 5×1model.
Kouto 1–1–1, Mikazuki, Sayo, Hyogo 679–5198
Tel: 0791–58–2750 Fax: 0791–58–0830 E-mail:Figure 1.
surface.

5×1 Te missing-row model for the Ge(001): Te 1×1

Figure 2.

2×1 Te zigzag model for the Ge(001): Te 1×1 surface.

o-sakata＠spring8.or.jp――(C) 2002 The Japanese Society for Synchrotron Radiation Research

放射光Our question was which structural model between the 5×
1, the 5×2, and the zigzag one was most favorable from an
x-ray scattering point of view. To answer the question, we
measured x-ray scattered intensities of crystal truncation
rods (CTR's) and compared the intensities with those calculated from the above-mentioned structural models (, which
could be a sort of modiˆed bridge-site ones).
The experiments were carried out in an ultrahigh vacuum(UHV) surface chamber at the 5ID–C station on the DND
undulator beamline. Te was deposited at a substrate temperature of 540 K on a clean Ge(001) surface followed by annealing to 690 K. After Te deposition we observed a 1×1
LEED pattern with streaks between the 00 and 01 spots (Fig.
3). This indicates that the surface had 1×1 long-range orderbut was locally disordered.
We measured x-ray scattered intensities along the 1 0 L,
1 1 L, and 3 0 L CTR's. (The re‰ection indices are here expressed using a tetragonal unit cell with a base deˆned by the1×1 surface unit cell of Ge(001).) The results of analyses ofthe data were compared to simulations corresponding to thebridge, top, anti-bridge, and hollow site models. Te at thebridge site was in best agreement. More complex surface
structural models based on modiˆcations of Te at the bridge
site were then compared to the data with the missing-row
model being in better agreement than the zigzag model (Fig.
4). The missing-row model could result from an in-planestrain relief mechanism. Finally, the CTR data were used toreˆne the structural parameters of the 5×1 missing row
model for determining surface relaxation. In this structuralmodel, the displacement of the Te atomic layer along the Z
direction from the bulk Ge position is ＋0.01(nm), where＋
sign stands for atoms shifted outwards. The ˆrst throughfourth Ge atomic layers are shifted vertically by 0.002,－0.003, －0.004 and －0.001(nm), respectively, from thebulk Ge positions. For further information, you could refer
to our original article8).

2.

The latest information on BL13XU第巻第号()tem have been obtained. Let me ˆrst refresh your memory as
to the outline of BL13XU and second introduce up-to-theminute information. We are hoping that the description below would be helpful like a guided-tour brochure when you
plan to utilize the beamline facilities.
The light source is the standard SPring-8 in-vacuum undulator (ID)11) with a 32 mm period and its number of 140. The
gap of the ID is opened up to 50 mm and closed down to 9.6
mm. The fundamental energy range available is correspondingly from 18.9 to 5.5 keV. Figure 5 depicts the beamline lay-

The design of BL13XU was written by Goto et al.9); a
previous status of BL13XU was described a year ago.10)
Stages for the beamline monochromator have been upgradedsince then for making a direct beam intensity more stable. In
addition, preliminarily experimental results with a UHV sys-Figure 4. Crystal structure factors measured along the 1 0 L,
1 1 L, and 3 0 L for the Ge(001): Te 1×1 surface. 5×1 (—) and5×2 (------) stand for the 5×1 and 5×2 Te missing-row model,
respectively. 2×1 (･･) means the 2×1 zigzag model.

Figure 3. LEED pattern with streaks for the Ge(001): Te 1×1surface. E＝35 eV.

Figure 5. Layout of BL13XU. The inside of EH 3 is additionallyillustrated. q stands for an incidence angle.

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()out. The beamline double crystal monochromator with an Si
111 re‰ection is cooled down with a liquid nitrogen chiller12).
The two mirrors have two stripes of a rhodium (Rh) and aplatinum (Pt) ˆlm with a Cr binder. They are for rejecting
higher harmonics of incident photons and for focusing an xray beam in a horizontal scattering geometry in an optics
hutch. The beamline has three experimental hutches (EH's).
You can utilize three UHV chambers that can be independently mounted on an S2＋D2 (2 degrees of freedom (DOF)
on a sample and 2 fully independent DOF on an x-ray detector) diŠractometer (3.2×3.2×2.3 m in dimension) in EH3.
EH1 is furnished with a multi-axis diŠractometer andprecision-rotary tables.
It was observed that an intensity of x-rays diŠracted fromthe BL13XU monochromator quickly faded away.10) This
mainly arose from temperature variation of the stage for thesecond crystal. The stage was radiatively heated by Compton
x-rays scattered from the ˆrst crystal surface.13) We had
adopted a constant temperature unit by water ‰ow and sandwiched it in. The unit was designed for BL29XUL by
Tamasaku et al.14). (It is called a chiller but a de factro heateragainst the second crystal, which is funny.) The x-ray intensity from the monochromator has been more stable than it wasbefore. Fig. 6 shows a stability test at an ID gap of 10 mm. It
is noted that the intensity was considerably stable although
the ID almost gave the maximum heat load to the monochromator. The temperature of the second stage was successfully
controlled within ‰uctuations of 0.1 K (Fig. 7). It has beenfound that variations in the intensity were ascribed to thetemperature ‰uctuations of the ˆrst crystal stage (compareFig. 6 with Fig. 7).
We revised measured photon ‰ux densities as a function of
an incident photon energy with a silicon pin photo diode(Fig. 8). The data were followed by correction of absorptionof a 150 mm-thick Be window of the photo diode and taking
into consideration of detection e‹ciency of the diode in 300mm thickness of Si. The x-ray beam came in through a 0.3mm graphite ˆlter(, which was 0.1 mm in the previousarticle10)) and three Be windows in total thickness of 0.75
mm. In this revision, we removed harmonics contribution

carefully; the ‰ux densities for an energy range less than 9
keV look much weaker than those shown in the previousarticle10).
A UHV vacuum system includes three UHV chambers andthe S2＋D2 diŠractometer. The chambers are equipped withstandard surface-structure-analysis apparatus and samplegrowth tools. Chamber 1 (Fig. 9) is suitable for a structural

Figure 6. X-ray beam intensities and ring currents versus time.
After we opened the MBS, time was measured.

Figure 9. A UHV chamber for studying a metal surface mounted
on the S2＋D2 diŠractometer in EH 3.

Figure 7.

Temperature of monochromator stages versus time.

Figure 8. An absolute x-ray photon ‰ux measured. An FE slit
opening used was 1×0.8 mm2. The data were taken without themirrors. `1st' and `3rd' mean the fundamental light and third
harmonics generated from the undulator, respectively.

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study of a metal surface.15) Chamber 2 and 3 target determination of a semiconductor surface structure using x-ray scattering W
diŠraction and x-ray standing waves, respectively.
The optimal chamber between the three ones for a user's experiment can be mounted on the diŠractometer at will. We
performed preliminary experiments for commissioning purposes. One of the experiments was a structural study on an O
Pt(111) surface that Prof. M. Ito proposed. (The otherWUHV chambers have been being prepared.)
A main idea for cleaning a metal surface like Pt is as follows. In combination with sputtering and annealing, surface
segregation occurs; accordingly the concentration of impurities (such as Si, Ca, Al, S, C, and P) is enhanced. Oxygen exposure then produces a volatile compound of such an element and oxygen. Oxides like these will readily remove thesurface. A sample-surface cleaning series including sputtering, oxygen exposure, and annealing was typically repeatedthree times. The surface was cleaned by Ar＋ (1.5×10－3 Pa)
sputtering at room temperature and followed by oxygen exposure at a temperature of 770 K with an O2 pressure of 2×
10－5 Pa for 10 min to remove the volatile oxides. The successive ‰ash at 1270 K for 2 min made the ‰at and clean surface,which was ensured by a sharp LEED pattern (like a hexagon)

Scattered x-ray intensity along an integer (1 0 L) rod at3 1Figure 11. Scattered x-ray intensity along a fractional (－ 2 2 L)rod at 25 K.

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taken at room temperature. (Electron bombardment heatingwas used.) 2-Langmuir O2 was deposited at a substrate temperature of 100 K. The sample was annealed at 250 K for 2
min to dissociate to atomic oxygen; its surface structure
became ordered. After this process, LEED patterns showed
2×2 spots at 25 K. Closed-cycle cryogenic refrigerator wasutilized for the cooling. The high temperatures were monitored using an alumel-chromel thermocouple and the lowtemperatures were done using an Au＋0.07z Fe-chromel
one. The base vacuum pressure was 5×10－8 Pa. We recorded scattered intensities along an integer rod (Fig. 10) and afractional rod (Fig. 11) from the surface at 25 K in chamber
1. The both plots would have ample L ranges for atomicscale structure analysis of a crystal surface.
Capabilities of the instruments include x-ray scatteringstudies in grazing incidence, studies of CTR's, re‰ectivity
measurements, x-ray standing waves, and many others in
UHV and in air. While surface x-ray measurements would
take so long time, they have not reached an accurate structure analysis so far. We have two long-term goals of sciencethat look opposite. The keywords are high throughput (convenience or to very e‹ciently get research outputs) and accurate measurements. The latter goal is to determine an electron-charge density map of a crystal surface. (We hope wehave an opportunity to write the details after achieving thegoals.) Determining a structure of a locally disordered surface is, moreover, tempting as shown in the ˆrst part.

3.

Figure 10.
25 K.

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Acknowledgments

The scientiˆc research written in the ˆrst half of this article
was carried out by Prof. M. J. Bedzyk's Group at Northwestern University. We are grateful to friendly researchers
for working together, who were Drs. P. F. Lyman, B. P.
Tinkham, D. A. Walko, D. L. Marasco, T.-L. Lee, W. P.
Rodrigues, and Prof. M. J. Bedzyk. The commissioning job
related to the beamline in the second half was performed
with Drs. S. Goto, Y. Furukawa, T. Mochizuki, T. Uruga,K. Takeshita, T. Ohata, T. Matsushita, S. Takahashi, T.
Ishikawa, and H. Tajiri. The author discussed alteration ofthe stages for the beamline monochromator with Drs. K.
Tamasaku, M. Yabashi, and S. Goto. The UHV chamberfor studying the Pt surface was mainly prepared by Prof. M.
Ito's Group at Keio University and Prof. T. Takahashi'sGroup at the University of Tokyo. In particular, young gentlemen, Dr. M. Nakamura and Mr. K. Sumitani, performed
excellent work for sample-surface preparation and x-ray
measurements. Fig. 5 is modiˆed marginally—like a mirror
image—from a drawing (for the 5ID–C experimental setup)
being originally prepared by Dr. T.-L. Lee. The x-ray measurements were carried out under proposal numbers of
2001B0504–ND–np, 2002A0128–ND1–np, and 2002A0201–
ND1–np. Encouragement, suggestions, and supports havebeen provided by Drs. T. Ishikawa, T. Ueki, and H.
Suematsu.

References
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F. C. Frank and J. H. Van der Merwe: Proc. Roy. Soc. Lon-放射光

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()don A 198, 205 (1949).
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Marasco, T.-L. Lee, and M. J. Bedzyk: Phys. Rev. 61, 16692(2000).
S. Goto, K. Takeshita, and T. Ishikawa: SPring-8 Information
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O. Sakata, Y. Furukawa, S. Goto, T. Mochizuki, T. Uruga, Y.
Shimada, K. Takeshita, H. Ohashi, T. Ohata, T. Matsushita,
S. Takahashi, and T. Ishikawa: SPring-8 Information 6, 450(2001).
H. Kitamura: J. Synchrotron Rad. 7, 121 (2000).
The following article does not describe the chiller for BL13XU
but for a similar one. T. Mochizuki et al., Nucl. Inst. andMeth. A 467–468, 647 (2001).
K. Tamasaku, M. Yabashi, T. Mochizuki, and T. Ishikawa:
SPring-8 Information 5, 390 (2001).
K. Tamasaku, M. Yabashi, D. Miwa, T. Mochizuki, and T.
Ishikawa: Proc. SPIE 4782, to be published.
The chamber numbers here are diŠerent from those shown in
the previous article10).