Faculty Profiles - OZAKI Nobuhiko

Published Papers - OZAKI Nobuhiko

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Topology optimised photonic crystal waveguide intersections with high-transmittance and low crosstalk

N. Ikeda, Y. Sugimoto, Y. Watanabe, N. Ozaki, A. Mizutani, Y. Takata, J. S. Jensen, O. Sigmund, P. I. Borel, M. Kristensen, K. Asakawa

ELECTRONICS LETTERS ( INSTITUTION ENGINEERING TECHNOLOGY-IET ) 42 ( 18 ) 1031 - 1033 2006.08 [Refereed]

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Significant enhancement of ferromagnetism in Zn1-xCrxTe doped with iodine as an n-type dopant

Nobuhiko Ozaki, Nozomi Nishizawa, Stephane Marcet, Shinji Kuroda, Osamu Eryu, Koki Takita （Part： Lead author )

PHYSICAL REVIEW LETTERS ( AMERICAN PHYSICAL SOC ) 97 ( 3 ) 2006.07 [Refereed]

The effect of additional doping of charge impurities was investigated in a ferromagnetic semiconductor Zn1-xCrxTe. It was found that the doping of iodine, which is expected to act as an n-type dopant in ZnTe, brought about a drastic enhancement of the ferromagnetism in Zn1-xCrxTe, while the grown films remained electrically insulating. In particular, at a fixed Cr composition of x=0.05, the ferromagnetic transition temperature T-C increased up to 300 K at maximum due to the iodine doping from T-C=30 K of the undoped counterpart, while the ferromagnetism disappeared due to the doping of nitrogen as a p-type dopant. The observed systematic correlation of ferromagnetism with the doping of charge impurities of both the p and n type, suggesting a key role of the position of Fermi level within the impurity d state, is discussed on the basis of the double-exchange interaction as a mechanism of ferromagnetism in this material.

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New design for wide/flat bandwidth in photonic crystal-based SMZ all-optical device (PC-SMZ)

K. Asakawa, Y. Sugimoto, Y. Watanabe, N. Ozaki, A. Mizutani, N. Ikeda, O. Sigmund, P. I. Borel, M. Kristensen

ICTON 2006: 8TH INTERNATIONAL CONFERENCE ON TRANSPARENT OPTICAL NETWORKS, VOL 2, PROCEEDINGS ( IEEE ) 146 - 149 2006

A two-dimensional photonic crystal (2DPC) slab waveguide-based symmetric Mach-Zehnder type all-optical switch (PC-SMZ) is promising for a future ultra-fast photonic network system. An ultra-fast (similar to ps) and ultra-low energy (similar to 100 fJ) switching has already been demonstrated as the 1(st) phase. The 2(nd) phase is to create a PC-SMZbased ultra-fast photonic logical switch with a latch function for an ultra-fast photonic digital processor. One of the steps towards the goal is to develop a new design method of waveguides with wide/flat bandwidth, high transmittance and low reflectivity for improving switching performances. Another one is to develop a selective area growth of quantum dots. This paper dominantly reviews these two recent topics as well as briefing the PC-SMZ performances.

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Topology optimization for photonic crystal waveguide intersection with wide and flat bandwidths in ultra-fast all-optical switch (pc-smz)

Y. Sugimoto, Y. Watanabe, N. Ikeda, N. Ozaki, A. Mizutani, Y. Takata, J. S. Jensen, O. Sigmund, P. I. Borel, M. Kristensen, H. Ishikawa, K. Asakawa

2006 European Conference on Optical Communications Proceedings, ECOC 2006 1 - 2 2006

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Planar focusing lens grating for vertical coupling on 2D photonic crystal slato waveguide

Akio Mizutani, Naoki Ikeda, Yoshinori Watanabe, Nobuhiko Ozaki, Yoshiaki Takata, Yoshinori Kitagawa, Frederik Laere, Roel Baets, Yoshimasa Sugimoto, and Kiyoshi Asakawa

2006 IEEE LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2 ( IEEE ) 843 - 844 2006

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Photonic crystals and quantum dots: Towards integrated optics for advanced ultra-fast all-optical signal processing

K. Asakawa, Y. Sugimoto, Y. Watanabe, N. Ozaki, A. Mizutani, Y. Takata, Y. Kitagawa, N. Ikeda, S. Ohkouchi, S. Nakamura, A. Watanabe, X. Wang, M. Kristensen, O. Sigmund, P. I. Borel, R. Baets

2006 European Conference on Optical Communications Proceedings, ECOC 2006 4801132 1-4 2006

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Magneto-optical study of ferromagnetic semiconductor (Zn,Cr)Te

N. Nishizawa, S. Marcet, N. Ozaki, S. Kuroda, K. Takita

PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 12 ( WILEY-V C H VERLAG GMBH ) 3 ( 12 ) 4102 - 4105 2006 [Refereed]

Magneto-optical properties of a ferromagnetic semiconductor (Zn,Cr)Te were investigated through magnetic circular dichroism (MCD) measurements in the visible spectral range. (Zn,Cr)Te films without and with an additional nitrogen (N) doping as an acceptor exhibited different MCD spectra around the F point; a broad MCD band extending over a few hundred meV was observed in the undoped (Zn,Cr)Te while a relatively sharp peak appeared in the heavily N-doped (Zn,Cr)Te. The different MCD spectra are considered to be correlated with the change in the Cr valence state due the acceptor doping.

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Suppression of ferromagnetism due to hole doping in Zn1-xCrxTe grown by molecular-beam epitaxy

N Ozaki, Okabayashi, I, T Kumekawa, N Nishizawa, S Marcet, S Kuroda, K Takita （Part： Lead author )

APPLIED PHYSICS LETTERS ( AMER INST PHYSICS ) 87 ( 19 ) 2005.11 [Refereed]

Electric and magnetic properties were investigated on p-type Zn1-xCrxTe doped with nitrogen (N) as an acceptor. Thin films of p-Zn1-xCrxTe (x <= 0.09) were grown by molecular-beam epitaxy with the supply of N-2 gas excited by rf plasma. With the increase of Cr composition x at an almost fixed N concentration of the order of 10(20) cm(-3), the temperature dependence of resistivity changed from metallic behavior to an insulating one, accompanied with a significant decrease of the hole concentration. The magnetization measurements revealed that ferromagnetic behaviors observed in undoped Zn1-xCrxTe were suppressed due to the nitrogen doping; with N concentrations of the order of 10(20) cm(-3), hysteresis loops in the magnetization curve disappeared, the magnitude of magnetization decreased, and the ferromagnetic transition were not observed down to 2 K according to the Arrott plot analysis. These experimental findings are discussed on the basis of the ferromagnetic double exchange interaction which is considered to work on the Cr 3d impurity level formed in the band gap of ZnTe. (C) 2005 American Institute of Physics.

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Growth and magnetic properties of novel ferromagnetic semiconductor (Zn, Cr)Te

S Kuroda, N Ozaki, N Nishizawa, T Kumekawa, S Marcet, K Takita

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS ( NATL INST MATERIALS SCIENCE ) 6 ( 6 ) 558 - 564 2005.09 [Refereed]

Magnetic properties of a novel ferromagnetic semiconductor (Zn, Cr)Te were investigated. Zn1-xCrxTe thin films, both without and with the additional hole doping by nitrogen, were grown by molecular beam epitaxy. In the magnetization measurement on Zn1-xCrxTe without carrier doping, the ferromagnetic behaviors such as a hysteresis loop in the magnetization vs. magnetic field curve were observed. Similar hysteretic behaviors in the field dependence were reproduced in the magnetic circular dichroism measurement. The ferromagnetic transition temperature TC deduced from Arrott plot increased almost linearly with Cr composition with the maximum T-C = 275 K at a Cr composition of x = 0. 17. The ferromagnetic behaviors observed in the undoped samples were found to be suppressed upon the p-type doping with nitrogen. These experimental findings are discussed based on the double exchange mechanism and the suppression of ferromagnetism by the hole doping is interpreted as due to the shift of the Fermi level in the Cr 3d level with the acceptor doping. (c) 2005 Elsevier Ltd. All fights reserved.

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Growth of silicon nanowires on H-terminated Si {111} surface templates studied by transmission electron microscopy

N Ozaki, Y Ohno, J Kikkawa, S Takeda （Part： Lead author )

JOURNAL OF ELECTRON MICROSCOPY ( OXFORD UNIV PRESS ) 54 I25 - I29 2005.09 [Refereed]

We have studied the growth of silicon nanowires (SiNWs) by means of transmission electron microscopy. SiNWs are grown from nanocatalysts via the Vapor-Liquid-Solid (VLS) mechanism using silane (SiH4) gas as a source gas. The nanocatalysts are prepared on a hydrogen (H)-terminated Si surface. We have examined the formation mechanism of nanocatalysts on H-terminated surface and have observed several structural variants of SiNWs. According to the study we have suggested that many structural variations of SiNWs are possible, which modify the structural properties of SiNWs to great extents.

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Magnetic behaviors of ferromagnetic semiconductor Zn1-xCrxTe grown by MBE

N Ozaki, N Nishizawa, S Marcet, S Kuroda, K Takita （Part： Lead author )

JOURNAL OF SUPERCONDUCTIVITY ( SPRINGER/PLENUM PUBLISHERS ) 18 ( 1 ) 29 - 32 2005.02 [Refereed]

The magnetic and structural properties of NIBE-grown films of Zn1-xCrxTe were investigated. The magnetization versus magnetic field (M-H) measurement of Zn1-xCrxTe (x = 0.01-0.17) showed clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T-C) estimated from the Arrott-plot analysis increased almost linearly with the Cr composition (x) up to 275 K at x = 0.17. However, in the magnetization versus temperature (M-T) measurement, the irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This is typically observed in the magnetic random system such as spin-glass or superparamagnetic phase. In the high resolution transmission microscopy (HRTEM) observations, structural defects such as stacking faults and polycrystalline-like structure were observed at high Cr compositions, whereas any apparent precipitates of different phases were not seen in all the range of Cr compositions examined. The correlation of the observed magnetic randomness with the local structural defects was discussed.

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Magnetic properties of undoped and N-doped Zn1-xCrxTe grown by MBE

N Ozaki, N Nishizawa, S Kuroda, K Takita （Part： Lead author )

Physics of Semiconductors, Pts A and B ( AMER INST PHYSICS ) 772 345 - 346 2005 [Refereed]

Nitrogen-doped Zn1-xCrxTe (x approximate to 0.02) thin films were gown by molecular beam epitaxy (MBE) with rf-plasma excited nitrogen source. p-type conductivity was confirmed through the Hall measurements with the highest hole concentration up to 1.8x10(18) cm(-3). The magnetization measurements of the hole-doped samples revealed the suppression of the ferromagnetism compared to the undoped one. The correlation of the magnetic properties with the position of Fermi level is discussed.

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Nucleation and growth processes of silicon nanowires

S Takeda, N Ozaki, K Ueda, H Kohno, J Kikkawa, Y Ohno

Group-IV Semiconductor Nanostructures ( MATERIALS RESEARCH SOCIETY ) 832 257 - 267 2005 [Refereed]

We have studied the nucleation and growth processes of silicon nanowires (SiNWs) by means of transmission electron microscopy and scanning tunneling microscopy. SiNWs are grown on hydrogen-terminated Si surface via the VLS (Vapor-Liquid-Solid) mechanism using silane (SiH4) as source gas. We have classified the growth process of SiNWs into three stages: the formation of nanocatalysts on a substrate, the nucleation of SiNWs in nanocatalysts, followed by the growth of SiNWs. We have shown that the structures of SiNWs are varied in several ways in each stage, and accordingly the structural properties of grown SiNWs can be modified to great extents. At the present moment, the phenomena at the each stage are not fully controlled, and this prevents us utilizing silicon nanowires more effectively.

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Magnetic and structural properties of MBE-grown Zn1-xCrxTe films

N Ozaki, N Nishizawa, S Kuroda, K Takita （Part： Lead author,　Corresponding author )

JOURNAL OF PHYSICS-CONDENSED MATTER ( IOP PUBLISHING LTD ) 16 ( 48 ) S5773 - S5776 2004.12 [Refereed]

The magnetic and structural properties of MBE-grown films of a novel diluted magnetic semiconductor (DMS), Zn1-xCrxTe, were investigated. The magnetization versus magnetic field (M-H) measurement exhibited a clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T-C) estimated from the Arrott-plot analysis increased almost linearly with the Cr concentration (x) up to 275 K at x = 0.17. In the magnetization versus temperature (M-T) measurement, irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This behaviour, which is typically observed in a magnetic random system such as the spin-glass or superparamagnetic phase, is considered to be related to the local structural inhomogeneity observed by high resolution transmission electron microscopy.

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Magnetotransport and magnetic properties of p-Zn1-xMnx Te : N - Carrier-induced ferromagnetism

KT Nam, S Kuroda, T Kumekawa, N Ozaki, K Takita

PHYSICA STATUS SOLIDI B-BASIC RESEARCH ( WILEY-V C H VERLAG GMBH ) 241 ( 3 ) 668 - 671 2004.03 [Refereed]

The magnetic properties of nitrogen-doped p-Zn1-chiMnchiTe grown by MBE were studied through the magnetotransport measurements. In a sample of p-Zn1-chiMnchiTe with chi = 0.034 and p = 4.7 x 10(19) cm(-3), a hysteresis loop was observed below 2.5 K in the Hall resistance and the magnetoresistivity, which indicates the ferromagnetic transition. From the plot of (R-Hall/R-sheet)(2) against B/(R-Hall/R-sheet) at several temperatures, which corresponds to the so-called Arrott plot, we can estimate the Curie temperature (T-c) of the sample as about 3K, suggesting that this sample has the higher T-c than ever reported. We also demonstrated, by means of light irradiation, that the strength of ferromagnetic interactions can be controlled by changing the hole concentration. An increase of the ferromagnetic behaviours such as a small increase of the coercive field was observed under light irradiation. This result strongly supports that the ferromagnetic transition in Zn1-chiMnchiTe is the carrier-induced one. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Magnetic properties of MBE-grown Zn1-xCrxTe

N Ozaki, N Nishizawa, KT Nam, S Kuroda, K Takita （Part： Lead author,　Corresponding author )

11TH INTERNATIONAL CONFERENCE ON II-VI COMPOUNDS (II-VI 2003), PROCEEDINGS ( WILEY-VCH, INC ) 1 ( 4 ) 957 - 960 2004 [Refereed]

We have grown Zn1-xCrxTe films by molecular-beam-epitaxy (MBE) and investigated their magnetic properties with variation of Cr concentrations (x) between 0.01-0.08. Systematic increase of the lattice constant was observed by XRD only up to around x = 0.02. Measurements of the magnetizations M-H reveal that ferromagnetic transition occurs in the films at low temperatures. The transition temperature (T-C) which was estimated from Arrott-plots increased with Cr concentration up to about 100 K for x = 0.08. However, the temperature dependence of the magnetization M-T shows different curves below T-C for field-cooled (FC) and zero-field-cooled (ZFC) processes. Moreover, their paramagnetic Curie temperatures (theta), estimated from the higher temperature M-T curves, are different from and much higher than T-C. These behaviours could be attributed to the superparamagnetism in the films. Indeed, many defects such as stacking faults are observed in the sample with x = 0.08 through HRTEM. It could give rise to the distribution of the Cr concentrations and/or the presence of precipitates including Cr. (C) 2004 WILEYNCH Verlag GinbH & Co. KGaA. Weinheim

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Formation mechanism of nanocatalysts for the growth of silicon nanowires on a hydrogen-terminated Si {111} surface template

S Takeda, K Ueda, N Ozaki, Y Ohno

APPLIED PHYSICS LETTERS ( AMER INST PHYSICS ) 82 ( 6 ) 979 - 981 2003.02 [Refereed]

We have observed the formation process of nanocatalysts that act for the growth of Si nanowires by means of UHV scanning tunneling microscopy. Gold-silicon nanocatalysts that we have examined were thought to form on a hydrogen (H)-terminated [111] silicon surface and to expel Si nanowires of extremely high aspect ratio via the vapor-liquid-solid mechanism. We have observed that a nanocatalyst, that is, a droplet of melted gold-silicon alloy of about 5 nm in diameter, is actually formed in a pit on a H-terminated surface in the narrow temperature range around 500 degreesC. We have concluded that, in this specific temperature range, nanocatalysts can be melted, remain mutually isolated, absorb silicon effectively, and expel Si nanowires. Based on the result, we have proposed a method of making a thin template, which facilitates to decide the nucleation sites and the sizes of nanocatalysts, resulting in the precise control of those of Si nanowires. (C) 2003 American Institute of Physics.

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Misleading fringes in TEM images and diffraction patterns of Si nanocrystallites

H Kohno, N Ozaki, H Yoshida, K Tanaka, S Takeda

CRYSTAL RESEARCH AND TECHNOLOGY ( WILEY-V C H VERLAG GMBH ) 38 ( 12 ) 1082 - 1086 2003 [Refereed]

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Novel amorphization process in silicon induced by electron irradiation

J Yamasaki, Y Ohno, H Kohno, N Ozaki, S Takeda

JOURNAL OF NON-CRYSTALLINE SOLIDS ( ELSEVIER SCIENCE BV ) 299 793 - 797 2002.04 [Refereed]

We recently found that amorphization is induced in Si by MeV electron irradiation. In order to account for the steady-state diagram under electron irradiation, we propose a phenomenological theory which takes into account the two competing mechanisms, namely amorphization by higher energy recoils and recrystallization by lower energy recoils as well as thermal process. We apply the new amorphization method to fabricating artificial arrangements of the columns of a-Si in a c-Si film, and discuss its use for photonic crystal based on photonic band calculation. (C) 2002 Elsevier Science B.V. All rights reserved.

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Fabrication of periodic nanohole multilayer structure on silicon surface toward photonic crystal

Y Ohno, N Ozaki, S Takeda

PHYSICA B-CONDENSED MATTER ( ELSEVIER SCIENCE BV ) 308 1222 - 1225 2001.12 [Refereed]

We have studied a porous structure on silicon surfaces that is introduced by electron irradiation. The structure consists of nanometer-sized holes arranged on silicon surfaces. Investigating the size and the distribution of surface nanoholes in a temperature range from about 4K to about 600K, we have estimated the porosity on a surface with nanoholes. We have fabricated a periodic dielectric multilayer structure on a silicon surface by introducing nanoholes periodically in one direction: alternating layers with different dielectric constants (the dielectric contrast of about 1.08), spaced by a distance of 100 nm. We can form periodic dielectric layers at arbitrary locations on surfaces by scanning an electron beam, changing the periodicity and the dielectric constant by varying irradiation condition. (C) 2001 Elsevier Science B.V. All rights reserved.

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Published Papers - OZAKI Nobuhiko