Shoichi Sekiguchi

Nitrogen Ion Behavior on Polar Surfaces of ZnO Single Crystals

Nitrogen radicals were irradiated on the (0001) and (0001) surfaces of the ZnO single crystals, and the stability and the states of N ions on the surfaces were investigated by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). On the (0001) surface, many N ions replaced the O ions of ZnO during annealing in oxygen gas or vacuum after nitrogen treatment. However, few N ions replaced the O ions of ZnO on the (0001) surface. These results suggest that the nitrogen doping on the (0001) surface is more effective than that on the (0001) surface.

Atomic force microscopic observation of SrTiO3 polar surface

Abstract The structure change of SrTiO 3 (111) polar surface was studied as a function of atmosphere. Atomic force microscopy (AFM) and coaxitial impact-collision ion-scattering spectroscopy (CAICISS) were used to study the surface structure of the (111) polar surfaces of SrTiO 3 single crystals. From the result of AFM observation, the polar surfaces annealed in oxygen gas had a trench structure, and that annealed in argon gas formed large triangular terraces. The surface annealed in air, i.e. in intermediate oxygen partial pressure, consisted of complicated self-similar triangles with different sizes, forming a fractal shape called Sierpinskis gasket. The time of flight spectra of CAICISS suggested that a Ti 4+ plane was more stable than a SrO 3 4− plane on the substrate surface annealed in oxygen gas, and the Ti 4+ and SrO 3 4− planes almost equally existed on the substrate surface annealed in argon gas. Molecular dynamic calculations at constant temperature and pressure were applied for various SrTiO 3 (111) polar surfaces.

Electrical Characterization of SrTiO3 Thin Films Grown on Nb-Doped SrTiO3 Single Crystals

SrTiO3(100) films homoepitaxially grown on Nb-doped SrTiO3(100) substrates with various Nb concentrations and SrTiO3(111) films epitaxialy grown on the [111]-oriented Pt thin-film electrode formed on SiO2/Si substrates with Ta buffer layers were prepared by the helicon sputtering method. After the top Pt electrode formation, the obtained Pt/SrTiO3/SrTiO3:Nb metal-insulator-semiconductor (MIS) capacitors and Pt/SrTiO3/Pt/Ta/SiO2/Si metal-insulator-metal (MIM) capacitors were electrically evaluated by current versus bias voltage characteristic (I–V) and capacitance versus bias voltage characteristic (C–V) measurements to clarify the specific features of the SrTiO3/SrTiO3:Nb interface compared with the SrTiO3/Pt interface. The existence of a space charge layer at the SrTiO3/SrTiO3:Nb interface was clearly characterized.

Cu Multiply Twinned Particle Precipitationin Low-Temperature Fired Ni-Zn-Cu Ferrite

Cu metal precipitates and truncated Cu metal multiply twinned particles (MTPs) were observed in low-temperature fired ferrite with a small excess of Ni-Zn-Cu chemical composition. The Cu metal precipitates suggest the existence of Cu solid solution and accelerated grain growth during the sintering accompanied with the formation of defect structures, such as oxygen vacancies in the ferrite spinel structure. The defect structure compensation during the cooling process after sintering results in Cu metal precipitation at the multiple grain junction.

Artificial SrTiO3/SrO Superlattices by Pulsed Laser Deposition

Artificial SrTiO3/SrO superlattice thin films epitaxially grown on SrTiO3 substrates were fabricated by pulsed laser deposition. The artificial superlattices were designed and fabricated by introducing a Ruddlesden-Popper (RP) layer periodically. The intensity of reflection high-energy electron diffraction (RHEED) patterns during the growth of the superlattices increased at the deposition of SrTiO3 layers and decreased at the deposition of the SrO monolayer, alternately. The gradual intensity decrease of RHEED reflection during SrO layer deposition is attributable to the island growth of SrO. The different type of RP layers normal to the film surface, observed in a high-temperature deposited superlattice, may originate from the higher diffusion condition and island growth of SrO.

Structure Analysis of SrTiO3 (111) Polar Surfaces

The structure change of SrTiO3 (111) polar surfaces was studied as a function of atmosphere, by atomic force microscopy, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The SrTiO3 (111) surfaces annealed in oxygen gas had a trench structure, while the surfaces annealed in argon gas formed large triangular terraces. The surfaces annealed in air, i.e., in intermediate oxygen partial pressure, consisted of complicated self-similar triangles with different sizes, forming a fractal shape called Sierpinskis gasket. The XPS analyses indicated that surface oxygen states were different depending on the annealing atmosphere.

Surface Structure of ZnO Single Crystals Analysed by Ion Scattering Spectroscopy

A ZnO crystal of wurtzite-type structure has polar surfaces of (0001) and (0001), which are terminated by Zn and O ions, respectively. Observation conducted by atomic force microscopy showed that step/terrace structures were formed on the (0001) and (0001) surfaces, whose step height was predominantly half of lattice parameter, c0. The ionic arrangement of the surfaces was determined by coaxial impact-collision ion scattering spectroscopy (CAICISS); the CAICISS spectra had a periodicity of 60° for azimuth angle dependence. From the analyses of CAICISS spectra it was shown that the (0001) and (0001) surfaces had a single dangling bond, and no ionic distortion occurred.

Thermal Stability of Artificial SrTiO3/SrO Superlattice Epitaxially Grown on SrTiO3 Single Crystal

A post-annealed artificial (SrTiO3)5/SrO superlattice grown on a (100) SrTiO3 single crystal substrate was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), to investigate the thermal stability of inserted rock-salt layers of SrO, the so-called Ruddlesden-Popper (RP) layers, in the epitaxial thin film. The intensities of XRD satellite peaks stemming from the superstructure decreased with increasing annealing temperature and almost disappeared at 1000°C. The high-resolution cross-sectional TEM image of the superlattice after annealing at 1000°C showed that RP layers perpendicular to the film surface appeared, which was predominant over in-plane RP layers. The predominant formation of the vertical RP layers may be attributed to the strain release of RP layers at the film surface.