Toshio Urano

Surface structure of MgO(001) surface studied by LEED

MgO(001) surfaces prepared by three different heat treatments were studied by LEED. The first was the surface just after the vacuum cleavage; the second was that after the annealing of the first one at 300°C in UHV. The third was prepared by cleaving in air and heating in oxygen until the carbon Auger peak disappear. LEED I-V curves of the diffraction spots, (10), (11) and (20), were recorded and compared carefully. No remarkable difference corresponding to the preparing methods was found. Therefore, the atomic structures of the surfaces seem to be similar to each other. These experimental I-V curves were compared with the theoretical curves obtained by dynamical calculations for several grades of surface relaxation. The theoretical curves with no relaxation or with the smallest one in the calculation (2.5% expand) fitted best with the experimental curves. The experimental curves were also compared with the theoretical ones calculated for the rumpled surface. But no effect induced by the rumpling was found in the experimental curves.

Atomic and Electronic Structure of Ultrathin Iron Film on MgO(001)Surface

Iron film deposited on a MgO(001) surface grows layer by layer and forms an epitaxial film at room temperature. Low-energy electron diffraction (LEED) I-V curves are measured for films of various thicknesses. In comparison with the curves by theoretical analysis it is found that an iron film grows pseudomorphic and iron atoms sit just above oxygen ions at the stage of one mornolayer (1 ML) average thickness. The structure of an iron film is body-centered tetragonal (bct) in the early stage, but it begins to change to bcc structure at about 10 A average thickness and at the same time a widening of the valence-band electronic state is observed by Auger electron spectroscopy (AES).

Adsorption structure and silicide formation of Ba on the Si(001) surface

The behaviour of Ba atoms on the Si(001) surface was investigated by low-energy electron diffraction (LEED). An epitaxial silicide formation was observed for surfaces obtained both by deposition onto a substrate kept at 700 C and by annealing at 600 C after deposition at room temperature. These surfaces show ( 4 8 0 16 ) and ( 4 4 0 8 ) oblique patterns which have a unit cell close to six times and three times, respectively, as large as that of the BaSi 2 (111) surface. In addition to the (2x3 ), (2x1) and (2x4) structures at submonolayer coverage, a c(2x6) pattern was found. The two-fold periodicities of the (2x4) and (2x3) superstructures were determined to be parallel to the dimer direction using a single-domain Si(001)-(2 x 1) substrate.

Adsorption structure of Ba on an Si(001)-(2 × 1) surface

Abstract The behaviour of Ba atoms on a Si(001) surface was investigated by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), metastable-atom de-excitation spectroscopy (MDS) and thermal desorption spectroscopy (TDS). A 2D phase diagram of surface structures as a function of substrate temperature and Ba coverage was obtained. The existence of a saturation coverage of less than 1 ML with a corresponding LEED pattern, of (2 × 3), (2 × 1) and (2 × 4) dependent on a temperature higher than 800°C, was confirmed.

Structure of the K adsorbed Si(001)2 × 1 surface studied by LEED

Abstract The surface structures of K adsorbed Si(001))2 × 1 surfaces have been studied by the dynamical analysis of low-energy electron diffraction (LEED) beam intensity versus electron energy curves (the so-called I - V curves). A reliability factor ( R -factor) analysis has been carried out for three models of the site of the K atoms, that is, a pedestal site model (Levines model), a double layer model, and a cave site model. Although the difference of the numerical value of R is not so large among these models, the double layer model indicates the smallest value within a symmetric dimer model of the topmost Si atoms.

Structure analysis of an alkali metal adsorbed Si(001)(2×1) surface by tensor LEED

Abstract The structures of the alkali metal (K, Cs) adsorbed Si(001)(2 × 1) surface have been studied by tensor LEED. A reliability factor ( R factor) analysis has been carried out for models having different sites for the alkali atoms, that is, a pedestal site model (so called Levines model), a cave site model and a double-layer model. Although the difference between the numerical values of R is small for these models, the smallest R factor is obtained for the double-layer model with a symmetric dimer model for the topmost Si atoms.

Structures of iron films deposited on Si(111)7×7 surface studied by LEED

Abstract Iron films deposited on Si(111)7x7 surface grow layer-by-layer. The structure of an iron film of more than half monolayer (1/2ML) average thickness changes from 1x1 to 2x2 superstructure after annealing above 400°C. LEED I-V curves of several beams have been measured for various film thickness. For films less than 1 ML annealed at 350°C the energy positions of peaks are very similar to those of the clean Si(111) surface; the iron film is pseudomorphic and grows as if iron atoms are on the same sites as Si atoms. For films of more than 1/2 ML thickness annealed at 500°C, we see evidence that an Fe-Si compound has grown.

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 simple method of automatic LEED spot tracing

Abstract A simple system for the measurement of LEED spot intensity has been developed. This system operates by the following mode; the spot photometer is driven, then the electron energy is changed automatically so that the spot tracks the photometer. By this system an I(hk)−V curve can be obtained in a few minutes. Examples of I–V curves obtained during a thin film formation are shown.

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.