Kunihiro Kamataki

Two-dimensional bispectral analysis of drift wave turbulence in a cylindrical plasma

Bispectral analysis and multichannel measurement are becoming attractive investigation tools in plasma fluctuation studies. In the Large Mirror Device-Upgrade, the measurement of fluctuations in the ion saturation-current with a 64-channel poloidal Langmuir probe array was performed. The two-dimensional (2D) (poloidal wave number and frequency) power spectrum showed a number of pronounced peaks and broadband fluctuations in the poloidal wave number-frequency space. We applied 2D bispectral analysis, which considers both the matching conditions of poloidal wave number and frequency, to the spatiotemporal waveform, and confirmed the nonlinear couplings between coherent-coherent, coherent-broadband, and broadband-broadband fluctuation components. More than ten peaks were revealed to have as their origins only three original parent modes generated in the plasma. Comparison between the theoretical estimate and experimental observation for the bicoherence showed good agreement.

Non-Gaussian properties of global momentum and particle fluxes in a cylindrical laboratory plasma

Non-Gaussian statistical properties of the azimuthally averaged momentum and particle fluxes driven by turbulence have been simultaneously observed in inhomogeneous magnetized plasmas for the first time. We identified the stretched Gaussian distribution of the both fluxes and the transition from the point-wise distribution to averaged ones was confirmed. The change of the particle flux precedes that of the momentum flux, demonstrating that the momentum flux is induced by the relaxation of density gradient.

Coexistence of Collisional Drift and Flute Wave Instabilities in Bounded Linear ECR Plasma

This paper reports the experimental and theoretical investigation on the coexistence of the collisional drift and flute wave instabilities in the bounded linear electron cyclotron resonance (ECR) plasma. The drift wave instability is excited by the steep density gradient and imposed axial boundary conditions in the device. The flute instability is excited by the bad curvature of the magnetic field. Emphasis is made on the effect of the ion–neutral (i–n) particle collisions. It is observed that the drift wave instability is excited when the i–n particle collision frequency ν in is low, but it is stabilized when ν in is high. Furthermore, the drift and flute modes are coexistent for the intermediate values of ν in . The Hasegawa–Wakatani model which describes the dispersion relation of the collisional drift-interchange mode is used for understanding the experimental observations. It is found that the experimentally observed drift frequency is consistent with the numerical calculation. The present result has...

Impacts of Amplitude Modulation of RF Discharge Voltage on the Growth of Nanoparticles in Reactive Plasmas

We have investigated the effects of plasma fluctuation on the growth of nanoparticles in capacitively-coupled rf discharges with amplitude modulation (AM). Nanoparticles grow more slowly for higher AM levels, which causes the density of nanoparticles to increase by 100% and their size to decrease by 23%. The increase in the number of radicals for nucleation and nanoparticles by AM is thought to cause a decrease in the radical flux for a nanoparticle because the rate of increase in the number of radicals is smaller than that of nanoparticles. Therefore, this causes the generation of a large amount of nanoparticles with small sizes.

Effect of Nitridation of Si Nanoparticles on the Performance of Quantum-Dot Sensitized Solar Cells

We developed Si quantum-dot (QD) sensitized solar cells using nitridated Si nanoparticle films. The Si/N content ratio of the Si nanoparticle films was combinatorially controlled in double multi-hollow discharge plasma chemical vapor deposition (CVD) process in a SiH4/H2 and N2 gas mixture. The short-circuit current density of Si QD sensitized solar cells increases by a factor of 1.3 with the nitridation of Si nanoparticles, and a high photon-to-current conversion efficiency of 40% was achieved at a short wavelength of 350 nm.

Improvement of Si Adhesion and Reduction of Electron Recombination for Si Quantum Dot-Sensitized Solar Cells

Quantum dots (QDs) based on multiple exciton generation have attracted much attention. They are capable of generating multiple electrons by single-photon absorption. Si is one of the good QD sources and its nontoxicity and abundance are advantageous for photovoltaics. In this work, Si QDs were fabricated by multihollow discharge plasma chemical vapor deposition, and they were applied to Si QD-sensitized solar cells. Their initial performance was poor because of the weak adhesion of Si and charge recombination. In this work, we solved these problems through the functionalization of Si QDs and a ZnO barrier. Functionalized Si QDs were more adsorbed on TiO2 with strengthened adhesion and the ZnO barrier prevented the contact between TiO2 and the redox electrolyte. Consequently, the improved adhesion and the reduced electron recombination led to the enhancement of overall photovoltaic characteristics.

Effects of Hydrogen Dilution on ZnO Thin Films Fabricated via Nitrogen-Mediated Crystallization

Hydrogenated ZnO thin films have been successfully deposited on glass substrates via a nitrogen mediated crystallization (NMC) method utilizing RF sputtering. Here we aim to study the crystallinity and electrical properties of hydrogenated NMC-ZnO films in correlation with substrate temperature and H2 flow rate. XRD measurements reveal that all the deposited films exhibit strongly preferred (001) orientation. The integral breadth of the (002) peak from the hydrogenated NMC-ZnO films is smaller than that of the conventional hydrogenated ZnO films fabricated without nitrogen. Furthermore, the crystallinity and surface morphology of the hydrogenated NMC-ZnO films are improved by increasing substrate temperature to 400 °C, where the smallest integral breadth of (002) 2θ–ω scans of 0.83° has been obtained. By utilizing the hydrogenated NMC-ZnO films as buffer layers, the crystallinity of ZnO:Al (AZO) films is also improved. The resistivity of AZO films on NMC-ZnO buffer layers decreases with increasing H2 flow rate during the sputter deposition of buffer layers from 0 to 5 sccm. At a H2 flow rate of 5 sccm, 20-nm-thick AZO films with low resistivity of 1.5×10-3 Ω cm have been obtained.

Epitaxial Growth of ZnInON Films with Tunable Band Gap from 1.7 to 3.3 eV on ZnO Templates

Epitaxial ZnInON (ZION) films with a tunable band gap have been successfully fabricated by RF magnetron sputtering on ZnO templates prepared via nitrogen mediated crystallization (NMC). X-ray diffraction (XRD) measurements show that the full widths at half maximum of the rocking curves from (002) and (101) planes are small at 0.10 and 0.08°, respectively, indicating a high crystallinity with good in-plane and out-of-plane alignments. Since the coherent growth of 35-nm-thick ZION films on NMC-ZnO templates is deduced from the reciprocal space mapping around the (105) diffraction, there is little lattice relaxation at the interface between the films and templates, which is significant in terms of the suppression of carrier recombination. The band gap of the ZION films has been tuned in a wide range of 1.7–3.3 eV by changing the Zn:In ratio. These results indicate that ZION is a potential absorption layer material of solar cells.

High Amount Cluster Incorporation in Initial Si Film Deposition by SiH4 Plasma Chemical Vapor Deposition

We have carried out in-situ measurements of Si cluster volume fraction in Si films during plasma chemical vapor deposition by using quartz crystal microbalances (QCMs) together with a cluster-eliminating filter. The cluster volume fraction in films is deduced from film deposition rates with and without Si clusters using QCMs. By employing this method we have revealed a depth profile of the Si cluster volume fraction. A high cluster volume fraction is observed in the initial phase of film deposition. This behavior is compared with time evolution of SiH*, Si* emission intensities and their intensity ratio.

Bifurcation of the plasma turbulence on LMD-U

Density fluctuation spectra are qualitatively modified depending on filling gas pressure and magnetic field strength in a linear magnetized plasma. Abrupt and spontaneous jumps between fluctuation states during a discharge are observed in the filling gas pressure–magnetic field plane. Those jumps are characterized using a spectrum pattern and have a bifurcation property on the plane.