Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

18A/2004G192

ARPES study of low-dimensional structures on Tl/Ge(111)Shinichiro HATTA1, Chihiro KATO1, Shin TAKAHASHI1,Taichi OKUDA2, Ayumi HARASAWA2, Toyohiko KINOSHITA2,Tetsuya ARUGA*1
1
Department of Chemistry, Graduate School of Science, Kyoto Univ., Kyoto 606-8502, Japan
2
ISSP, Univ. Tokyo, Kashiwa, Chiba 277-8581, Japan

Experimental
The ARPES measurements were performed at beam line
18A. We used a vicinal Ge(111) substrate tilted from[111] by 1.5° toward the [ 1 1 2] direction for thepreparation of the single-domain (3×1) surfaces. The
substrate was cleaned by cycles of Ar-ion sputtering (700eV) and annealing (<950 K) by direct current heating. Tl
was deposited on the substrate from an alumina crucible.
When the (3×1) surface was prepared, following thedeposition, the surface was annealed to ~550 K for a few
minutes. In the case of the (1×1) surface, moderateannealing (<400 K) was adopted in order to obtain a wellordered surface and suppress the partial formation of the(3×1) structure. The sample surfaces were checked by
low-energy electron diffraction (LEED) and ARPES. All
the ARPES measurements were performed at roomtemperature.
Results and discussion
Figure 1 shows a set of spectra along the Γ – K line for
the Tl/Ge(111)-(1×1) surface. The data were taken athν = 22 eV. The Fermi level (EF) at an emission angle of~29˚ corresponds to the K point. Three surface bands,
which pass over gap regions of the projected bulk bands,
are observed. They are labeled S1, S2 and S3, as shown inFig. 1. S1 disperses to higher binding energies with aminimum energy of 1.5 eV at the K point. Although thedispersion of S2 is roughly parallel to that of S1 with thedifference of ~1.4 eV, its minimum is located at ~0.7 Γ K .
S3 disperses to lower binding energies with a maximum

energy of 5.7 eV at the K point. Beside these bands, anondispersive band lies at 0.9 eV. This is labeled as SE inFig. 1. This band is tentatively assigned to electronic
states localized at step edges intrinsic to the vicinal
substrate.
The S1 and S2 bands have a p-orbital character. The S1band with higher energies is assigned to a dangling bondband. Our measurements show that the S1 band is not
fully occupied in the surface Brillouin zone. That is, the(1×1) surface has a metallic character.
For the single-domain (3×1) surface, we obtained theARPES spectra both parallel ( Γ – C ) and perpendicular( Γ – A ) to the chains. A surface band observed on theΓ – C line showed the EF crossing near the Γ point. Thisis quite different from the results for the alkali-metal
induced (3×1) surfaces which are semiconducting. Inorder to understand the origin of the difference, further
analysis of the ARPES data is now in progress.
S3S2S1 S E

θe363330

Intensity (arb. units)

Introduction
Reconstructed semiconductor surfaces induced bymetal adsorption have gained much attention because they
have shown interesting physical properties particular to
low-dimensional systems. Adsorption of thallium onGe(111) induces two phases at room temperature [1]. The(3×1) surface at 1/3 ML has a one-dimensional chainstructure, as observed on the alkali-metal induced (3×1)
surfaces. On the other hand, the structure of the (1×1)
surface at 1 ML is a pseudomorphic (1×1) structure,indicating that this surface may be a well-confined twodimensional system in the surface region. Scanningtunneling spectroscopy (STS) measurements on thesesurfaces imply that these surfaces have a very narrow gap
or a metallic character. We investigated the electronic
structures of these surfaces by angle-resolved
photoelectron spectroscopy (ARPES).

27K242118151296308642Binding energy (eV)

ΓEFFig. 1. ARPES spectra along the Γ – K line for theTl/Ge(111)-(1×1) surface.
References[1] C. Castellarin-Cudia et al., Surf. Sci. 491, 29 (2001).
*aruga@kuchem.kyoto-u.ac.jp

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