Itaru Honma

Enhanced optical properties of metal‐coated nanoparticles

Metal‐coated, nanometer‐size particles are modeled with a realistic distribution of coating thicknesses. General expressions are given for the local‐field enhancement, absorption, and nonlinear optical response. In addition, heuristic arguments are used to determine the effects of a diffuse, rather than a sharp interface. The linear and nonlinear optical properties are discussed within the context of the effective medium theory for small volume fractions. An efficient method of solution is used with the flexibility to handle an arbitrary number of coatings.

Electrical conductivity of a pure C60 single crystal

The temperature dependence of the electrical conductivity of a C60 single crystal is presented in this letter. The single‐crystal samples free from solvent contamination were grown up to a size of millimeters order by sublimation of C60 powder with oscillation of the crystal temperature. The electrical conductivity of the single crystal was measured at temperatures between 250 and 295 K. The sharp decrease of the electrical conductivity with temperature around 256 K was observed in association with the phase transition of C60.

Synthesis and optical properties of coated nanoparticle composites

Abstract The synthesis of coated nanoparticles is a new direction in engineering, specifically in the study of physical properties of materials. We examine semiconductor coated semiconductor particles, CdS PbS , and metal coated particles, Au 2 S Au , and the theoretical basis for their unique properties. Coated self-assembled nanoparticles are also studied and recent progress is reported.

Synthesis of GaAs nanoparticles by digital radio frequency sputtering

Nanometer‐sized GaAs particles embedded in SiO2 were prepared by a digital rf‐sputtering method, where GaAs and SiO2 targets were alternately sputtered in an Ar atmosphere. The GaAs deposition time was kept shorter than the time required to form a continuous layer. Transmission electron microscopy observations showed that the sizes of the GaAs particles can be controlled from 2 to 8 nm by changing the sputtering cycle time of the GaAs target. In spite of their small size, the GaAs particles have crystallinity similar to the target material without substrate heating or postannealing. The optical absorption spectra of the GaAs particles show a blue shift as large as 1.6 eV, corresponding to strong quantum confinement of electrons and holes.

Properties of hydrogenated amorphous germanium nitrogen alloys prepared by reactive sputtering

Hydrogenated amorphous germanium‐nitrogen alloys (a‐GeNx:H) were synthesized as a new group of amorphous semiconductors by rf(13.56 MHz) reactive sputtering of a Ge target in a gas mixture of Ar+N2+H2 under a variety of deposition conditions such as gas ratio, rf‐discharge power, and substrate temperature. Structural, optical, and electrical properties of those a‐GeNx:H alloys were systematically measured and are discussed in relation to their preparation conditions. The optical band gap E04 of a‐GeNx:H alloys could be continuously controlled in the range from 1.1 eV to 3.3 eV primarily depending on the atomic N/Ge ratio in the film. The role of hydrogen and nitrogen in the optical and electrical properties of the material is also crucially demonstrated.

The optical absorption and photoluminescence spectra of C60 single crystals

A constant photocurrent method (CPM) was used to determine the relative optical absorption coefficient of the C60 single crystal in the weak-absorption region. The result suggested the existence of extrinsic states in the C60 single crystal. The photoluminescence (PL) peaks of the C60 single crystal were observed at 1.48 eV and 1.65 eV these positions are different from those of C60 thin films.

Thermally induced structural change of a‐Ge:H/a‐GeNx multilayer structures

Multilayer structures of a‐Ge:H/a‐GeNx and a‐Ge/a‐GeNx were prepared by a reactive‐sputtering technique and their structural stability was studied through thermal anneals. High‐resolution transmission‐electron‐microscope analyses show that crystallization takes place only in a‐Ge:H(a‐Ge) layers without disturbing atomically smooth and uniform a‐Ge:H(a‐Ge)/ a‐GeNx interfaces. The crystallization temperature of a‐Ge:H(a‐Ge) layers increases with either decreasing thickness of those layers or increasing thickness of a‐GeNx layers. The rise in crystallization temperature is most remarkable when the layer thickness becomes smaller than a couple of hundred angstroms. On the basis of the experimental results, the structural stability of multilayer films is discussed in the light of classical nucleation theory using the free‐energy change of the system. It is demonstrated that the crystallization temperature is strongly affected both by layer thickness and by the nature of a heterointerface which phenomenological...

Optical and electronic properties of reactively sputtered amorphous GeNx:H

Basic photoelectronic properties of hydrogenated amorphous germanium nitride (a‐GeNx:H) are presented for the first time. Amorphous‐GeNx:H films were prepared by an rf reactive sputtering of a Ge target using Ar+N2+H2 mixed gases and their optical band gaps were controlled continuously in the range from 1.0 to 3.3 eV by varying the compositional ratio of nitrogen to germanium. In contrast to H‐free a‐GeNx, the temperature dependence of the dark conductivity of a‐GeNx:H shows that thermally activated band conduction prevails in the carrier transport over a wide temperature range at least down to 180 K. The photo‐to‐dark conductivity ratio under AM1 light of unoptimized a‐GeNx:H amounts to 40.

Monolayer nitridation of silicon surfaces by a dry chemical process using dimethylhydrazine or ammonia

A hydrogen‐terminated silicon surface was successfully converted to a surface covered with a monolayer of nitrogen. Nitridation was carried out in a vacuum chamber using either dimethylhydrazine [H2N–N(CH3)2] or ammonia at a pressure of 1 mTorr and at temperatures ranging from 400 to 600u2009°C. In situ x‐ray photoelectron spectroscopy measurements revealed that the binding energy and the full width at half‐maximum in the nitrogen spectra are the same as those in bulk Si3N4. Nitrogen content at the surface increased as the nitridation time increased and, below 500u2009°C, saturated at a value that approximately corresponds to a monolayer thickness. These results show the effectiveness of dry chemical processes for preparing uniform Si surfaces terminated with specific atoms or molecules other than hydrogen.

The structural stability of reactively-sputtered amorphous multilayer films

Abstract It is reported for the first time on reactively-sputtered a-Ge:H(a-Ge)/a-GeNx multilayer structures that the structural stability changes systematically depending on the layer thickness and a nature of the layer-layer interface. This effect has been interpreted qualitatively on the basis of the thermodynamical discussion on the free-energy change in two-dimensional crystallization process of a thin-film multilayer structure.