Masami Aono

A transparent ultraviolet triggered amorphous selenium p-n junction

This paper will introduce a semitransparent amorphous selenium (a-Se) film exhibiting photovoltaic effects under ultraviolet light created through a simple and inexpensive method. We found that chlorine can be doped into a-Se through electrolysis of saturated salt water, and converts the weak p-type material into an n-type material. Furthermore, we found that a p-n diode fabricated through this process has shown an open circuit voltage of 0.35 V toward ultraviolet illumination. Our results suggest the possibility of doping control depending on the electric current during electrolysis and the possibility of developing a simple doping method for amorphous photoconductors.

Anneal-Induced Degradation of Amorphous Selenium Characterized by Photoconductivity Measurements

Photoconductivity measurements of amorphous selenium (a-Se) and arsenic selenide (a-As2Se3) were carried out. Samples where annealed from 333–358 K at 5 K step for 5 minutes and the photoconductivity was measured after each annealing step. It was found that the dark current increases permanently for a-Se and the photoconductivity ratio increased drastically after 338 K, but drops after further annealing. Amorphous-As2Se3 however, has no noticeable change in the dark current neither in the photoconductivity ratio. Arsenic is introduced into a-Se to prevent degradation, which causes loss of image quality, but the volume must be controlled in order to maintain high photoconductivity.

Amorphous carbon nitride deposition by nitrogen radical sputtering C+N+O+H

Abstract Amorphous carbon nitride films a-CNx were prepared by a nitrogen radical sputtering of a carbon target. A-CNx films were treated by atomic hydrogen or by an oxygen plasma, using a layer-by-layer method, forming LLa-CNx or LLa-CNxOy. These a-CNx films show large photosensitivity, high resistivity and low dielectric constants. Possible applications are also discussed briefly.

Nano Structural and Thermoelectric Properties of SiGeAu Thin Films

SiGeAu thin films exhibit large thermoelectric power. Recently, the formation of SiGe nano crystal has been confirmed after annealing. In this study, we investigated the relationship between the size of the nano crystal and thermoelectric power. SiGe nano crystals were fabricated by the crystallization of amorphous SiGeAu thin films. The grain size of SiGe was around 6 nm when Au composition was 2.2 at. %. The grain size decreased with increasing Au composition up to 2.2 at. % due to metal-induced crystallization, and increased depending on Au composition over 2.2 at. % due to liquid phase crystallization. When the grain size was around 6 nm, the thin films exhibited large thermoelectric power (4.8 mV K-1) and power factor (2.4×10-2 W m-1 K-2). Therefore, the grain size of SiGe around 6 nm is important for obtaining the large thermoelectric power and power factor. These results indicate that the quantum size effect enhances the thermoelectric power and power factor of SiGeAu thin films.

DC electrical conductivity study of amorphous carbon nitride films prepared by reactive RF magnetron sputtering

The effects of chemical bonding states on the electrical properties of hydrogen-free amorphous carbon nitride (a-CNx) films were reported. a-CNx films were prepared by reactive RF magnetron sputtering at various deposition temperatures. The electrical conductivity of the a-CNx films increased with increasing deposition temperature because of the predominant sp2C–C bonding sites. Their conductivity increased by almost one order of magnitude with a 25% decrease in the fraction of the N-sp3C bonding state. It was found that the fraction of the N-sp2C bonding state strongly contributed to the increase in the electrical conductivity. Nitrogen incorporation led to an increase in the sp3C–C bonding fraction in the films; as a result, the conductivity of the a-CNx films was found to be lower than that of the a-C films deposited under the same conditions.

Correlation of photothermal conversion on the photo-induced deformation of amorphous carbon nitride films prepared by reactive sputtering

The photo-induced deformation of hydrogen-free amorphous carbon nitride (a-CNx) films was investigated under visible-light illumination. The films gave rise to photothermal conversion by irradiation. In this study, we investigated the effects of thermal energy generated by irradiation on the deformation of a-CNx/ultrathin substrate bimorph specimens. The films were prepared on both ultrathin Si and SiO2 substrates by reactive radio-frequency magnetron sputtering from a graphite target in the presence of pure nitrogen gas. The temperature of the film on the SiO2 substrate increased as the optical band-gap of the a-CNx was decreased. For the film on Si, the temperature remained constant. The deformation degree of the films on Si and SiO2 substrates were approximately the same. Thus, the deformation of a-CNx films primarily induced by photon energy directly.

Influence of Chemical Bonding States on Electrical Properties of Amorphous Carbon Nitride Films

The electrical properties of amorphous carbon nitride (a-CNx) films have been investigated in terms of the nitrogen concentration (N/C) and chemical bonding states in the films. The films were deposited by the reactive rf magnetron sputtering method. Nitrogen concentration and chemical bonding states in the films were controlled by regulating the deposition temperature. C–C networks in the films changed to those having a graphite like structure with decreasing N/C, as deduced by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, the N–sp2C bonding state becomes more predominant. These results indicate the contributions of the N–sp2C component to the decrease in electrical resistivity and increase in photoconductivity.

Synthesis of mesostructured titanium dioxide films by surfactant-templated sol-gel method

Mesostructured titanium dioxide films have been synthesized by modifying sol-gel methods in the presence of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) block copolymer surfactants as a structure-directing agent. The synthesized films were analyzed using X-ray diffraction (XRD), field emission transmission electron microscopy (FE-TEM) and selected area electron diffraction (SAED). XRD investigation revealed that as-deposited and as-dried films showed a hexagonal arrangement. With increasing heat-treatment temperature, the mesostructure of the films degraded. From TEM and SAED studies, the degradation of the mesostructure can be explained by the grain growth of microcrystalline titanium dioxide.

Polyiodides in room-temperature ionic liquids

Polyiodides, Im-, were formed in room-temperature ionic liquids (RTILs). The RTILs were N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium iodide, [DEME][I], and 1-alkyl-3-methylimidazolium iodide, [Cnmim][I] (n = 1, 2, 3, 4, 6, and 12). In [Cnmim][Im], prepeak and intensity modulation of X-ray diffraction were caused by polyiodides. Above m = 4, polyiodide-mediated local structures were formed, although little alkyl side-chain length, n, dependence was seen in the X-ray diffraction patterns. The viscosity of the mixtures was measured as a function of m to obtain their dynamic properties. The m tendencies of the viscosity and density changed at around m = 4 when fixing n = 4. By 127I-NMR, the local environments of only polyiodides were extracted by changing n, m, and the temperature. Doublet/triplet peak splitting of pure [Cnmim][I] (n = 3, 4, and 6) occurred at low temperature. In [Cnmim][Im] (n = 2 and 4), the effect of polyiodides was clarified from the chemical shifts and peak splitting in the NMR spectra. The experimental results support a Grotthuss-type exchange mechanism of iodide between polyiodides.

Spectroscopic analysis of photo-induced deformation of amorphous carbon nitride films

Reversible photo-induced deformation of amorphous carbon nitride (a-CNx) films was investigated. The films were deposited at 400 and 600 °C on a rectangular shaped ultrathin SiO2 substrate by reactive radio frequency magnetron sputtering with graphite target and pure N2 gas. The amount of deformation change was estimated from bending curvature of a-CNx/SiO2 bimorph structure. For the film deposited at 400 °C, it bent toward the film side under illumination. Photo-induced deformation of the film estimated from the bending angle was observed in the excitation energy of 1.77 to 3.35 eV. The maximum deformation was observed under illumination of 2.6 eV which corresponds with π-π* bond of carbon nitride. In contrast, the deformation was hardly observed in the films deposited at 600 oC.The difference in these two films was probably due to difference in sp2 bonding and termination structures.