Shozo Hongo

Observation of the interface of Ba/Si(100) by MDS and TDS

Abstract The interaction of Ba overlayers with Si(100)2 x 1 surfaces and barium silicide formation are studied for Ba coverages up to θ = 10 ML by MDS (metastable deexcitation spectroscopy) and TDS (thermal desorption spectroscopy). TDS spectra of Ba from Ba/Si(100) are observed as a function of Ba coverage. The process of Ba dosing on Si(100)2 x 1 and the annealing process are observed by MDS as a function of Ba coverage and as a function of sample annealing temperature, respectively. The work function change is measured as a function of the annealing temperature simultaneously. It is found from the above experiments that no silicide formation takes place by heating up to 800°C in the system of submonolayer Ba on Si(100)2 x 1 and that silicide is formed very easily by heating up to 250°C in the system of 2 ML Ba/Si(100)2 x 1. The Si—Ba bond is so tight that the Ba atoms bonded to the substrate Si atoms cannot move easily to form three-dimensional barium silicide. Therefore more than a few Ba layers are necessary to form barium silicide on the Si(100) surface.

A study of the surface electronic properties of Na/D/Si(100) by metastable de-excitation spectroscopy

Abstract MDS was used to investigate how sodium adsorbs on deuterium precovered Si(100) surface. Upon depositing sodium atoms on the deuterated silicon, some of the pre-adsorbed deuterium atoms bound to the Si surface break the bonds to produce Na + -D − bonds with its D − end toward the vacuum. The produced system on the Si(100) is not independent layers of Na and D but a mixture of Na–D and metallic Na, whose ratio is dependent on the quantity of adsorbed Na and D. The surface of 5 ML Na adsorption on deuterated Si(100) might consist of islands of metallic Na and Na–D layers.

The difference of the singlet-triplet conversion efficiency between K/Si and K/Ta surfaces through oxidation process

Abstract The oxidation processes of potassium ultrathin films adsorbed on Si(100)2 × 1 and polycrystalline tantalum surfaces were studied by metastable de-excitation spectroscopy (MDS). At the surface of 1 ML K/Si(100)2 × 1, the singlet-triplet conversion efficiency increases with oxidation. On the other hand, in the case of 1 ML K/Ta, it decreases with oxidation. These experimental facts were explained as follows: the 1 ML potassium on Si(100)2 × 1 form a filled-band structure for the top band which is the same as that of semiconductors. Since this band structure makes it infeasible that a singlet 2s electron tunnels resonantly into the sample, the singlet-triplet conversion efficiency is small at 1 ML K/Si(100)2 × 1. This efficiency increases because of the flowing out of potassium electrons into oxygen atoms with oxidation.

Singlet-triplet conversion efficiency of metastable He atoms dependent upon structures of adsorbed alkali metals on Si(100) 2 × 1

Abstract The singlet-triplet conversion efficiency for metastable He atoms has been studied. It depends on the structure of adsorbed alkali metals on a surface. At a coverage of less than half a monolayer of K and Cs adsorbed on Si(100)2 × 1, the singlet-triplet conversion efficiency is high and it decreases upon reaching the saturation coverage at room temperature. On the other hand, it stays high up to the saturation coverage of Na. These facts suggest that the electronic bands of monolayer-covered Cs and K on Si(100)2 × 1 are semiconducting or insulating (band is fully filled), while that of Na is metallic (band is half-filled).

Cs adsorption on H-terminated CVD-diamond studied by thermal desorption spectroscopy

Cesium adsorption was investigated by means of thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and metastable de-excitation spectroscopy (MDS) in order to clarify why only a small amount of Cs is adsorbed on H-terminated diamond at room temperature. H-termination is not perfect and H-patches are formed on diamond surface. At room temperature this H-terminated surface can adsorb no Cs atoms and only bare diamond surface catches them. Therefore only a small amount of Cs is adsorbed on the H-terminated diamond surface at room temperature.

Direct Mapping of Reciprocal Lattice Rods of Metal-Induced Facet on Si(001) Surface

On Si(001) surfaces annealed at 700°C after the deposition of 1.5 monolayer (ML) of Ba at room temperature, facet spots were observed in addition to the 2×1 pattern of the Si(001) clean surface. These patterns were recorded using a TV camera and stored in a personal computer as an image. Line profiles were obtained on the line along which the facet spot moved. From these profiles the peak position of facet spots was extracted and superposed on a reciprocal lattice space to construct reciprocal lattice rods. From the angle of these rods to those of the Si(001) substrate, the facet is indicated to be a metal-induced Si(113) face.

Chemisorption of Ba on deuterium-terminated Si(100) surface

Abstract The system of Ba overlayers deposited on a deuterium-terminated Si(100) surface was investigated by means of MDS (metastable de-excitation spectroscopy) and TDS (thermal desorption spectroscopy). Deposition of Ba overlayers caused the reduction of SiD bond strength because of charge donation to Si substrate from Ba atoms. As a result, about half of preadsorbed D atoms was released from the sample surface at 1 ML Ba deposition. The other half reacted with adsorbed Ba atoms entirely to form BaD bonds. Therefore, all the SiD bonds were lost, which is quite different from the alkali/D/Si(100) system. More Ba deposition did not induce the desorption of D atoms. The formed BaD bonds are considered to stay between the first and the second layer.

Mechanisms of He*(21S) de-excitation on oxygen-adsorbed Si(100) surfaces

Abstract Si(100) surfaces exposed to O 2 were studied using metastable de-excitation spectroscopy. In the case of a low exposure to O 2 ( * (2 1 S) metastable electron is transferred from the excited atom into an empty resonant level of the Si surface, forming a He ion. Electron emission them occurs by subsequent Auger neutralization of the He ion at the surface. When it is exposed to much O 2 , some layers of the Si surface are oxidized, changing into insulator SiO 2 . Then the de-excitation of the metastable state takes place through the Penning ionization prosess. When even one layer of K is adsorbed on the Si surface, oxygen atoms are easily captured by this system, and the Si surface changes into SiO 2 easily.

Molecular arrangement of organic crystal N,N′-dimethylperylene-3,4,9, 10-bis(dicarboximide) studied with metastable de-excitation spectroscopy and atomic force microscopy

Abstract We have studied the molecular orbitals and arrangements of N,N′ -dimethylperylene-3,4,9,10-bis(dicarboximide) (Me-PTC) molecules deposited on clean Si(100)2 × 1 and the H-terminated Si(100) surfaces on the basis of metastable de-excitation spectroscopy (MDS) experiments. The charge density of the second layer Me-PTC molecular orbitals differs somewhat from that of the first layer. The desorption temperature of Me-PTC from H-terminated Si surface is lower than that from clean Si. This is because Me-PTC molecules combine weakly with the H-terminated Si substrate.

Adsorption and desorption of H2O on potassium precovered Si(100)2 × 1 surface

Abstract The adsorption and desorption of H2O on a potassium precovered Si(100)2 × 1 surface have been studied using MDS (metastable deexcitation spectroscopy) and TDS (thermal desorption spectroscopy). It is concluded that H2O is dissociatively attached to a 1 ML K/Si(100)2×1 surface as OH and H at a dosing less than 1 langmuir at room temperature, because no molecularly adsorbed H2O is observed either by MDS or by TDS. The thermal desorption spectrum of H2 (mass 2) consists of two peaks. The lower temperature peak at 140°C is attributed to the hydrogen of a OH species bound to a Si atom. The higher temperature peak at 510°C is caused by hydrogen bound to Si dangling bonds. The thermal desorption spectrum of SiO (mass 44) has two peaks. The SiO desorption peak at 280°C suggests that bonds weaker than the usual SiO2 bonds exist. A detailed description of the adsorption and desorption of H2O is given by comparing both spectra of MDS and TDS and a model is proposed.