Kaoru Ojima

Control of N Content of GaPN Grown by Molecular Beam Epitaxy and Growth of GaPN Lattice Matched to Si(100) Substrate

We investigate the N content of a GaPN epilayer grown on GaP(100) by radio-frequency molecular beam epitaxy under various growth conditions. It is found that the N content of GaPN increases with decreasing growth temperature and with increasing rf power of the nitrogen plasma source. The N content was controlled by means of the growth temperature and rf power, and the GaPN epilayer was grown on a Si(100) substrate with a thin GaP buffer layer. The epilayer is investigated by cross-sectional transmission electron microscopy and it is clear that misfit dislocations and threading dislocations are not generated in the GaPN epilayer. As a result, it is demonstrated that lattice-matched and defect-free GaPN epilayers can be grown on Si(100) with a thin GaP buffer layer. We also discuss the effect of N incorporation on the generation of dislocations.

Hardening Effect of GaP1-xNx and GaAs1-xNx Alloys by Adding Nitrogen Atoms

We investigated the strain relaxation process of GaP1-xNx/GaP and GaAs1-xNx/GaAs in order to clarify their mechanical characteristics by adding nitrogen atoms. It was observed by transmission electron microscopy (TEM) that the critical thicknesses were greater and the generation rates of the misfit dislocations were slower in the GaP1-xNx and GaAs1-xNx layers than those in the GaP layer with a similar lattice mismatch. The critical thickness of the GaAs1-xNx layer was greater than that of the GaP1-xNx layer for the same nitrogen composition of 2%. The direction of higher crack density was orthogonal to that of the higher misfit dislocation density. These results indicate that the propagation of dislocations is prevented in III–V–N alloys such as GaP1-xNx and GaAs1-xNx, so that these alloys are harder than III–V compounds that lack nitrogen atoms. This feature could be attributed to the dislocation pinning and alloy hardening effects due to nitrogen atoms.

Structural and Electronic Properties of Barium Silicide on Si(100)

Barium silicides formed on the Si(100) surface have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The structure of the silicide surface depends on the preparation procedure; three kinds of silicide phases coexist on the surface when 2 monolayers (MLs) of Ba are deposited on Si(100) at the specimen temperature of 850 K, while two of the three phases appear when 2 MLs of Ba are deposited on Si(100) at room temperature, followed by annealing at 850 K. The STS result indicates that all the phases show semiconducting behavior with a band gap of ~ 1.3 eV and that one of the three phases has local density of states (LDOS) at 2.5 eV below the Fermi energy, which coincides with the reported results of metastable de-excitation spectroscopy (MDS) and ultraviolet photoelectron spectroscopy (UPS).

Influence of sulfur on oxygen adsorption onto a polycrystalline Zr surface

Abstract Oxidation of a polycrystalline Zr surface is studied by a time-of-flight electron-stimulated desorption (TOF-ESD) method and Auger electron spectroscopy (AES). An S-segregated Zr surface is obtained after heat treatment at 1100 K for 15 min in an ultrahigh-vacuum (UHV) chamber. Several cycles of Ar+ bombardment at 925 K and heating at 1000 K remove sulfur layers, resulting in a nearly clean Zr surface. Although oxygen molecules in dissociative states adsorb onto both types of sample surfaces, the sticking coefficient of the oxygen atoms is considerably decreased on the S-segregated surface. Three different sites for the adsorption of oxygen atoms are noted, namely, the subsurface sites, the hollow sites, and the on-top sites. The S atoms segregate to the Zr surface to occupy the hollow or the on-top site, resulting in the observed decrease of the sticking coefficient of the O atoms.

Oxygen adsorption induced hydrogen segregation on a zirconium surface

Abstract The adsorption process of oxygen on a Zr surface was studied by a time-of-flight electron-stimulated desorption (TOF-ESD) method. Oxygen adsorption induced hydrogen segregation on the Zr surface. It was revealed that the increase in H+ yield following O2 exposure is not caused by adsorption from the gas phase but by the diffusion of H from the bulk of Zr. In addition, H atoms formed ZrH bonds rather than hydroxyl species. From the analysis of the experimental results, it is suggested that O atoms penetrating into the Zr sample substitute for H atoms at tetrahedral sites, causing the diffusion of H atoms.

Effect of Hydrogen Termination on Ba Reaction on the Si(100) Surface

The effect of hydrogen termination on Ba reaction on the Si(100) surface at the initial stage of Ba adsorption was investigated by means of scanning tunneling microscopy (STM). On the bare Si(100) surface, Ba atoms form chain-shaped structures of Ba coverage up to 1/3 monolayer (ML), which extend perpendicular to the Si dimer row. The chain-shaped structure can change its shape because of the movement of Ba atom along the dimer row. On the other hand, in the case of Ba adsorption on a monohydrogenated Si(100)2×1 surface, Ba atoms make clusters, particularly at defect sites. In addition, Ba clusters can move across the dimer row.