Hiroshi Tochihara

Alloyed interface formation in the AuSi(111)2 × 1 system studied by photoemission spectroscopy

Abstract Room temperature deposition of gold (Au) on the Si(111)2×1 cleaved surface has been studied by photoelectron spectroscopy using synchrotron radiation. Binding energies of Si(2p) and Au(4f) and the splitting of the Au(5d) signal are changed with increasing coverage in a different manner below and above one monolayer coverage and reach to saturation values at 20–100 monolayers. The Au(4f) binding energy and the Au(5d) splitting at saturation are clearly different from those of pure Au metal. The origin of alloying in the Auue5f8Si(111)2×1 system is discussed from these results. A new model, “chemical bonding model” is proposed to explain the initial stage of the metallic overlayer formation in the Auue5f8Si(111) system.

Cs on Si(111)2×1: Si surface state and Cs valence state

Abstract Cesium atom adsorption on the Si(111)2×1 surface has been studied by angle-integrated photoemission spectroscopy. It was found that the intrinsic surface state of Si is always present throughout cesium monolayer formation and that the emission from Cs 6s electrons increases at high coverages. The character of cesium adsorption on Si(111)2×1 is discussed.

Observation of the Plasmon Excitation in a Cs Overlayer on Si(100) 2×1

Angle-integrated low-energy electron-energy-loss spectroscopy (EELS) was measured as a function of coverage for studying electronic excitations in Cs atoms deposited onto a clean Si(100) 2×1. surface and simultaneously AES, LEED, Δ φ, and UPS were observed. The energy loss peak (1 eV) in the EEL spectrum appears with large intensity at the saturation coverage of a half monolayer and is not observed at lower coverages. This energy loss is assigned to the plasmon excitation in Cs overlayer on silicon as observed in alkali atoms on noble and transition metals. It is suggested that the observed plasmon excitation in Cs atoms has a one-dimensional character from the standpoint of the geometrical model proposed by Levine [J. D. Levine: Surf. Sci. 34 (1973) 901], where Cs atoms form one-dimensional chains on the raised site at a half monolayer.

Lithium-atom adsorption on Si(001)2 × 1: Properties of lithium overlayer different from other alkali-metal atoms

Lithium-atom adsorption on Si(001)2 × 1 has been studied by electron energy loss spectroscopy (EELS). No overlayer plasmon (OLP) is observed even at high lithium-atom coverage. By contrast, sodium, potassium, rubidium and cesium monolayers show prominent peaks of OLP in EEL spectra. It is shown that the structure and electronic properties of lithium overlayers are also different from those of other alkali-metal atoms. This is deduced by low energy electron diffraction, Auger electron spectroscopy and work-function measurement.

Temperature dependence of potassium adsorption on Ag(001)

Abstract Low-energy electron diffraction on the K-adsorbed Ag(001) surface showed remarkably different pattern changes, when the K coverage dependence was observed in the different temperature ranges at 315–335 and 125–150 K. This different changes can be explained by the reconstruction of the substrate Ag(001) surface at higher temperatures and the K overlayer structure change at lower temperatures. Potassium coverage dependences of the work-function changes and electron energy loss spectra support the model of the missing-row type reconstruction at higher temperatures.

Mechanism of alloy formation due to noble metal deposition on silicon surfaces at room temperature: Chemical bonding model

Abstract Room temperature deposition of noble metals of Au, Ag and Cu on a clean Si(111)2×1 surface is studied through photoemission using synchrotron radiation in order to clarify the phenomenon of room temperature alloyed interface formation (RTAIF) at metal (M)-semiconductor(S) contacts. Ag atoms behave differently on a clean Si(111)2×1 surface from the point of view of the RTAIF, in spite of the fact that Ag belongs to the same group as Au and Cu in the periodic table. As for the phenomenon of the RTAIF, experimental results are examined in the light of the “chemical bonding model” proposed by some of the authors and others.

Core-level photoemission spectra from Cs monolayers on Si(1 1 1)2 × 1

Abstract Band shape in the Cs 4 d core level of cesium atom monolayers adsorbed on Si(1 1 1)2 × 1 has been studied by photoemission spectroscopy using synchrotron radiation. Asymmetry appeared on the low kinetic energy side at high coverages of cesium atoms. This is caused by the energy loss due to the overlayer plasmon in the cesium atom monolayer of photoemitted Cs 4 d electrons.

Photoemission of Cs Valence Electrons from a Cs Monolayer on Si(111) 2×1

The adsorption of Cs atoms on the Si(111) 2×1 surface has been studied by photoelectron spectroscopy using synchrotron radiation. A photoemission peak due to Cs 6s valence electrons is clearly observed at full monolayer coverage by tuning photon energy at around 50 eV over an available energy range from 30 to 130 eV. The peak appears at almost the same initial energy (-1 eV) as that of the surface state of the clean Si(111) 2×1 surface. It is shown that the insulator-metal transition in the Cs monolayer takes place with increasing coverage.

DOMAIN GROWTH OF THE DAS STRUCTURE ON A QUENCHED Si(111) SURFACE STUDIED BY STM

We continuously observed the growth of the dimer–adatom–stacking-fault (DAS) domain, in unreconstructed regions remaining after quenching a Si(111) surface to 370–380°C, by using scanning tunneling microscopy. It was observed that a single faulted (F) half of the 9×9 unit cell of the DAS structure grows to a small 9×9 DAS domain. Continuous measurements showed that new F-halves are created sharing corner holes with existing F-halves. The creation of new isolated F-halves was very seldom at 370–380°C, and the region of the DAS structure was grown by expanding the area of the existing DAS domains.

Alkali-Metal Overlayers on Silicon Surfaces

Alkali-metal overlayers on the Si(001)-(2x1) surface at saturation coverage form a one-dimensional metal which is verified with observing dispersion relation of an overlayer plasmon for Si(001)-(2x1)-K. The metallization is considered to take place in the K chains as the results of photoelectron-spectroscopic measurements. Then, we tried to find the Peierls transition for Si(001)-(2x1)-K and Cs but have not found it. The Li atom adsorption on Si(001)-(2x1) is very different from the other alkali-metal atoms. The Li overlayer has no metallic character and seems to form lithium silicide. It is also emphasized that the surface preparation is important.