Haruo Yokomichi

Photo-Induced ESR in Amorphous Si1-xNx:H Films

A large increase in the spin density for ESR signals originating from Si dangling bonds is observed by illumination at liquid nitrogen temperature for a-Si1-xNx:H films prepared by magnetron-sputtering and glow-discharge-decomposition. The photo-induced ESR can be explained as arising from the following three types of centers; (1) negative U centers which have unpaired spins metastable only under illumination, (2) positive U centers which are converted from negative U centers by illumination and (3) dangling bonds created by a bond-breaking by the intense illumination.

ESR Studies on the Light-Induced Effect in Si-Based Amorphous Semiconductors

Light-induced creation of defects and their decay by annealing are investigated by means of ESR in a-Si:H, a-Si1-xNx:H and a-Si1-xCx:H. The amount of increase in the ESR spin density by prolonged strong illumination is found to increase with an increase in the structural flexibility factor defined by the inverse of the average coordination number, regarding an H atom bonded to Si, C or N as an entity reducing the coordination number by one.

Electron Spin Resonance Centers and Light-Induced Effects in Porous Silicon

The nature of defects in porous silicon has been investigated by electron spin resonance measurements. The main defect has trigonal symmetry with a principal axis along the axis, and the principal values of the g-tensors, g// and g⊥, are 2.0022 and 2.0078, respectively. This defect is identified as a silicon dangling bond located at the surface or at the interface between c-Si and SiO2. Light-induced defect creation and accompanying photoluminescence fatigue have been observed. These changes are partly, but not completely, recovered by thermal annealing, unlike hydrogenated amorphous silicon. Consequently, it is suggested that the light-emitting region of porous silicon is mainly the single-crystal phase.

Light-Induced Effects and Their Annealing Behavior in a-Si:H

It is found from ESR measurements in a-Si:H films that dangling bonds created by short-time illumination can be removed by short-time annealing and that photo-created dangling bonds removed by short-time annealing can be recovered by short-time illumination. Combining these results with temperature dependence of illumination effects for two kinds of films prepared at 100°C and 250°C, a mechanism of the photo-creation of dangling bonds is proposed. The illumination effect on photoconductivity and photoluminescence is also compared with that on ESR.

Synthesis of Carbon Nanotubes by Arc Discharge in CF4 Gas Atmosphere

Carbon nanotubes were synthesized by arc discharge in a CF4 gas atmosphere involving fluorine atoms, which are able to terminate carbon bonding, while no fullerenes were synthesized in a CF4 gas atmosphere. The morphology of these nanotubes was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on these results, the synthesized conditions in CF4 gas were compared with those in other gases, i.e., in CH4, H2, He and Ar gases. In addition, electron spin resonance (ESR) measurements were performed in order to obtain information about the electronic properties of these nanotubes.

Photo-created defects in Si-based amorphous semiconductors

Abstract The density of the photo-created ESR center is found to increase with an increase in the structural flexibility defined by the inverse of the average coordination number. We propose a model for the photo-creation of the ESR center, in which the distance between the impurity atom and the Si atom changes according to charge state.

Electron Spin Resonance Study of Light-Induced Annealing of Dangling Bonds in Glow Discharge Hydrogenated Amorphous Silicon: Deconvolution of Electron Spin Resonance Spectra

We analyze the electron spin resonance (ESR) spectra of dangling bonds in a-Si:H samples before and after strong illumination which gives rise to the phenomenon of light-induced annealing of dangling bonds. The spectra are deconvoluted into two components, that due to normal dangling bonds and that due to H-related dangling bonds. Both before and after prolonged illumination, the normal dangling bonds constitute about 60% of the total spin density, while the H-related dangling bonds constitute about 40%. However, the photoannealed densities of the two types of dangling bonds are found to be different after strong illumination. In addition, we analyze the ESR spectra of dangling bonds observed after weak illumination.

Effects of Oxygen Gas Addition and Substrate Cooling on Preparation of Amorphous Carbon Nitride Films by Magnetron Sputtering.

Amorphous carbon nitride (a-C1-xNx) films were prepared by magnetron sputtering using nitrogen (N2) and oxygen (O2) gases in order to reduce the sp2 structural region and to reduce triple bonding between carbon and nitrogen atoms (C≡N) with oxygen radical. Furthermore, these films were prepared at liquid-nitrogen temperature in order to increase the nitrogen concentration. Electron spin resonance (ESR), infrared (IR) absorption, ultra-violet visual (UV-vis) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the effects of O2 gas addition and substrate cooling on the preparation of a-C1-xNx films by magnetron sputtering.

N2-plasma-nitridation effects on porous silicon

Abstract Both the mechanism and the effects of plasma nitridation of porous silicon (PS) were studied using X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) and photoluminescence (PL). The PS layers formed by anodization of (100) and (111) Si wafers were exposed to an rf N 2 plasma. The Si 2 p XPS spectra in the as-grown PS exhibited signals due to both Si−Si bonds and Si−O bonds. On the other hand, the signal due to Si−N bonds was observed after nitridation. This fact shows that some of the Si−O bonds were replaced with Si−N bonds after plasma nitridation. It was also found that P b -like ESR centers increase and the PL intensity decreases with plasma nitridation in the same manner as for light soaking of PS. Plasma nitridation processes for single-crystalline Si and for PS will also be compared.

Light‐induced creation of defects and related phenomena in silicon‐based amorphous semiconductors

Light‐induced creation of defects and their annealing process were studied by ESR, photoconductivity and photoluminescence measurements for a‐Si1−xNx:H and a‐Si1−xCx:H films besides a‐Si:H films with various spin densities and H contents. The results show that the degradation of the photoconductivity and the photoluminescence is mainly attributed to creation of dangling bonds due to bond breaking.