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FIM, RHEED STUDIES ON ATOMATIC
STRUCTURE OF Si(111) AND Si(111)-Ag SURFACES
T. Park, C. Chung, S. Jung, D. Jeon
To cite this version:
T. Park, C. Chung, S. Jung, D. Jeon. FIM, RHEED STUDIES ON ATOMATIC STRUCTURE OF
Si(111) AND Si(111)-Ag SURFACES. Journal de Physique Colloques, 1988, 49 (C6), pp.C6-275-C6-
280. �10.1051/jphyscol:1988648�. �jpa-00228145�
JOURNAL DE PHYSIQUE
Colloque C6, supplbment au nO1l, Tome 49, novembre 1988
FIM, RHEED STUDIES ON ATOMATIC STRUCTURE OF Si(ll1) AND. Si(ll1)-Ag SURFACES
T.S. PARK, C.I. CHUNG, S.M. JUNG and D.Z. JEON
Department of Physics, Kyungpook National University, Taegu 702-701, Korea
ABSTRACT
We have investigated the atomic structure of clean surfaces of Si(ll1) and Si(ll1)-Ag surfaces using both FIM and RHEED-TRAXS system. In FIM image of Si at 78K, the sur- face oxides layers were slowly desorbed as increasing the applied voltage, and then the ring structure of Si(ll1)surface appeared. From this stage silicon atoms evaporated violently.
Ag atoms were deposited on clean Si tip surface by thermal evaporation controlled accu- rately by a fast thickness monitor which was developed by authors. The desorption of Ag atoms were carried out by direct heating the Si tip. From our FIM and RHEED-TRAXS experimental results we could support the displacement model for the interpretation of Si(ll1) 7x7 superstructure and Lander-Morrison model for the atomic arrangement of S i ( l l 1 ) & X & - A ~ structure for 0=1.0 ML.
1-INTRODUCTION
Several eminent works on atomic structures of silicon(ll1) using FIM and Atom- Probe have been reported by Melmed/l/; Sakurai/2/; Tsong and Liu/3/; KeIIogg/4/;
schmidt/5/; et al. We have investigated the atomic structure of clean surfaces of Si(ll1) and Si(ll1)-Ag surfaces using both FIM and RHEED(Reflection High Energy Electron Diffraction)-TRAXS(Total Reflection Angle X-ray Spectroscopy) system. Si(ll1) has been known as it possesses various kinds of reconstructed surface structures and phases. the 7x7 superstructure of Si(ll1) is one of the most important and mysterious problems in surface sciences to be solved. When Si(ll1) substrate is heated in UHV at above 200°C the 7x7 structure is formed and remains up to 830°C. Beyond 830°C the 7x7 structure disappears, then 1x1 structure took over. As the sample is cooled down, the 7x7 structure reappears at 830°C, then continues to the room temperature. The trnsition process of this structure is reversible one. The 7x7 structure is energetically most stable in spite of its formation at rather high temperature. Fig.1 shows the RHEED Si(ll1) 7x7 superstructure pattern.
Takayanagi, Tanishiro et al/6/; proposed the DAS(Dimer, Adatom, Stacking-fault) model from their TED analysis for the interpretation of 7x7 structure. Currently the DAS model receives strong support. DAS model contains 19 dangling bonds: 12 from the adatoms, 6 from atoms which are in the stacking fault layer but not bonded to the adatoms and one from the atom below the vacancy. The vacancy can act as a unique adsorption site.
Binnig's adatom model/7/; based on his observation of STM image that shows a deep dip at the corner and 12 hills inside the unit cell of Si(ll1). Ino and authers suggested a new structure model from their RHEED-TRAXS analysis as shown in Fig.2. The structure
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988648
JOURNAL DE PHYSIQUE
consists of 12 raised atoms nearly perpendicular t o the surface plane and one vacancy at the coner in a unit cell. Twelve raised atoms can be classified into 4 groups, each of them has 3 equivalent atoms. Since a surface atom on Si(ll1) has a dangling bond, the remaining three bonds are tending to become type sp2 hybridization..
Sp2
bonds tend t o form planner trigonal bonds, although the second layer atoms tend to keep the bulk sp3 bonds. Then first layer atoms will be lowered into the crystal side, resulting in a large lateral strain with r wrnpnssian type, because shortening of the bond length by forming sp2 like configuration is estimated to be small. The heights of displacement of atoms normal to the surface, Z,, Z b , 2, and Zd hold an inequality relationship, 2,>
Z b>
Z,>
Zd. The lateral displacement ub of atom b toward atom A becomes larger than U , and U , of atoms a and c. The U , toward the vacancy of atom j is larger than u b . Then total displacement around the vacancy site is so far larger than that of atom A. So a vacancy at corner of unit mesh will be created. Atoms A and B are brighter than C and D. Ino's displacement model/B/; is more suitable to explain such phenomena like so called "a corner effect" from the view point of the FIM image.2 - EXPERIMENT AND RESULTS
The starting materials we used for both FIM tips and RHEED-TRAXS substrates were identical n-type silicon wafers(l50R-cm). For preparation of FIM tips we cut the silicon wafer in the shape of fine needles, then polished by chemical etching. We obtained good quality tips having radius of curvature ranging 50nm to 100nm. Silicon tip was inserted in platinum tube and mounted on a cold trap. The basic chamber pressure was ZXIO-' torr to 5X10-'~ torr. The hydrogen imaging gas of 4X10-5 torr was admitted into the chamber through Pd purifier. The enchancement of field evaporation of semiconductors, Si(ll1) and GaP(111) surfaces in the presence of hydrogen has been well studied/9/. In the case of Si(ll1) hydrogen gas absorbed on the tip surface forms Si-H covalent bond and reduces the Si-Si binding strength. The penetration depth increased to several
A.
At 78K, the surface oxides layers were slowly desorbed as increasing the applied voltages to 4KV, then the ring structure of Si(ll1) surface abruptly appeared. From this stage silicon atoms evaporate violently as fast as nearly two layers per second. Fig 3(a) and (b) show the typical FIM images of Si(ll1) surface before (a) and after (b) field evaporation. Before evaporation, the surface is covered with oxides and other contaminants layers. After field evaporation, good quality lattice layer ring structures of Si(ll1) is obtained. The clear appearance of brighter spots on the outer rings of FIM image was observed. This may be the corner effect which is consistent with our displacement model of Si(ll1) surface. The field eva~oratied clean Si(ll1) tips were annealed at 200°C to 60ooCfor several hours in UHV. We obtained fairly well stabilized He and Ne images,yet not atomatically resolved or ordered. As previously mentioned, the 7x7 structyre of Si(ll1) surface must be constructed when the plane sample of Si(ll1) is annealed at about 200°C. The reason why we cannot obtain the 7x7 super- structure for the annealed FIM tip is not evident. It is ,however, conceived that the number of atoms on the tip surface might be too small to form the 7x7 structure which is a long range order. Silver atoms were deposited by heat evaporation onto the field evaporated clean Si(ll1) tip. The deposition thickness was most accurately controlled by a fast thickness monitor which was developed by authors/lO/. This new monitor has a 0.1 Hz/O.lsec counting resolution and extremely faster than ordinary thickness monitors.The silver ad-atoms were also desorbed by direct heating of tip observing the change of i FIM image patterns. As the thickness of Ag atoms on the Si(ll1) tip surface became about 1 rnono-layer coverage, indivisual or clusters of Ag atoms start to form randomly three dimension-like islands. If the deposition thickness of Ag exceeds 2.02, the growth of the Stranski-Krastanov mode silver crystd is obrerved. Fig.4. shows the typical FIM images of Ag adsorbed-Si(ll1) surfaces. In Fig.qr) the thinly deposited silver atoms are seen, while in (b) the Stranski-Krastanov mode crystal growth is observed. At current stage of FIM experiment, we were unable to obtain the clear phase changes as the function of both Ag thickness and the substrate temperatures. The simultaneously deposited Ag films on clean Si(ll1) surface with FIM were analysed by RAEED and TRAXS. The detailed experimental procedures are described in Ref.10.. We obtained well defined two- dimensional state diagram of Ag adsorbed Si(ll1)-Ag system as shown in Fig.5.. For the lower coverage of ad-silver atoms at lower substrate temperature(from room temperature to 230°C) the surface structure is 6x1. If temperature is raised t o about 350°C the 6x1 structure suddenly changed into the &X& then ,7x7, finally 1x1 beyond 830°C. For the thicker deposion one can see quite different situation similar to that in the FIM images.
We could precisely determined that the the transion thickness between the 7x7 and the
&X& phases was 1.0 mono-layer(ML). From thier result, we strongly support the Lander-
Morrison model of Si(ll1)-Ag system/ll/.
We have concluded that, (1) it is extremely difficult to construct superstructure on not only Si(ll1) alone but also Si(ll1)-Ag system using solely with FIM, (2) it is also very little likehood to construct a surface structure model of Si(ll1) by FIM alone. This finding will not be merely for the Si(ll1) but for any other semiconductors and its metallic-adatom sys- tems as a whole. Nevertheless, it must be stressed that some ingeneous techniques/3/ for obtaining superlattice-like structures of semiconductors should be developed that enable to observe some-local-superstructure-like silicon FIM images, and other powerful technique like STM or RBtEED should be combined with FIM for further study of this interest- ing subject. The authors are greatly indebted to Professor S. Ino, Dr. H. Daimon of Tokyo University for their cooperation, and Professor S. Nishikawa of Tokyo Institute of Technology for his valuable advices. Present work has been supported by the Ministry of Education of Korea.
REFERENCES
/l/ A.J. Melmed and R.J. Stein, Surface Sci. 49 (1975)
/2/ T. Sakurai, T.T. Tsong and R.J. Culberston, J. Vac. Sci. Technol. 15 (1978) /3/ T.T. Tsong, H.M. Liu, et al, Jour. de Phys. C6, (1987) Proc. of 34th IFES /4/ G.L.Kellog, Phys. Rev. Lett. 55 (1985)
/5/ W.A. Schmidt, A.J. Melmed, M.F. Lovisa, M. Nashitzki and J.H. Bloch, C6 (1987) Proc. of 34th .IFES
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/6/ K. Takayanagi,
Y.
Tanishiro, et al, Surface Sci. (1985) 164/7/ G. Binnig, H. Rohrer, Ch Gerber and E. Weibel, Phys. Rev. Lett. 50 (1983) 120 /8/ S. Ino, J. Phys. Soc. Japan 53 No.6 (1984) 1911
/g/ T. sakata, J.H. Bloch, M. naschitzki and W.A. Schmidt, Jour. de Phys. C6 (1987) /10/
T.S.
Park, C f . Chung andS.M.
Jung, Jour.K- Phys.
Soci. 29,4 (1988) /ll/ G.V. hansson, R.S. Bauer, et al, Phys. Rev. Lett. 46 (1981) 1033Fig.1 Typical RHEED pattern of a Si(ll1) clean 7x7 structure. The electron beam is parallel to (211) orientation and its accelerating voltage is 20kV.
Fig.2 New structure model for the 7x7 structure. It contains twelve raised atoms and a vacancy in a unit mesh. The size of black circles A , B, C and D corresponds to the fieights of displacement
ZA, ZB, ZC
andZD
in which an inequalityZa ZB ZC ZD
holds consequently. Open circles are lowered atoms in first with sp2 like bonds. Small black circles and triangles are displaced atoms and fixed or nearly fixed atoms in the second layer, respectively.*
taken from ref./9/(a) (b)
Fig.3 Field ion micrographs of Si tip surface recorded in 2 ~ 1 0 - ' torr hydrogen at 78K (a) Before evaporation (b) After evaporation
C6-280 JOURNAL DE PHYSIQUE
( a ) ( b )
Fig.4 Field ion micrographs of Si(ll1) surface after Ag deposition. (a) 78K, 4.3kV (b) 78K, 7.4kV
I
1 I
I
(6x1) i Ag crystal
I
I IThickness of A ~ ( A )
Fig.5 Two-dimensional state diagram of Ag adsorbed Si(ll1)-Ag system/8/,/10/.
