H Seo

Synthesis and characterization of ZnInON semiconductor: a ZnO-based compound with tunable band gap

We have synthesized ZnInON (ZION) semiconductor, a ZnO-based compound with tunable band gap, by radio-frequency magnetron sputtering on quartz glass, a-SiO2/Si, and GaN/Al2O3 templates. From XRD analysis using wide-range reciprocal-space mapping, ZION is deduced to be a pseudo-binary system of wurtzite ZnO and wurtzite InN, the c-axis lattice parameter of which varies continuously from 0.53 to 0.58 nm with decreasing the chemical composition ratio [Zn]/([Zn] + [In]). From optical measurements, we found that ZION has tunability of the band gap over the entire visible spectrum, and the photo-to-dark conductivity ratio is high, of 2.2 × 103, demonstrating the high photosensitivity of ZION films. We have also succeeded in the epitaxial growth of ZION films with the composition ratio (InN)/(ZnO) of about 50 at.% by using GaN templates, where the FWHM of the ZION (002) rocking curve is small, of 350 arcsec. These results allow us to conclude that ZION will open up the field of group II–III–V–VI semiconductors that offer new opportunities for design of optoelectronic devices.

Effects of filter gap of cluster-eliminating filter on cluster eliminating efficiency

We have developed a cluster-eliminating filter which reduces amount of amorphous silicon nanoparticles (clusters) incorporated into a-Si:H films. We have measured film thickness dependence of a ratio of a hydrogen content associated with Si-H2 bonds (CSiH2) to that with Si-H bonds (CSiH) as a parameter of the filter gap (dgap). The cluster removal efficiency increases with decreasing dgap. CSiH2 and the ratio of CSiH2 to CSiH decrease with increasing the film thickness, suggesting the amount of incorporated clusters is high near the interface between the film and the substrate.

Emission spectroscopy of Ar + H2 + C7H8 plasmas: C7H8 flow rate dependence and pressure dependence

We have studied toluene flow rate dependence and pressure dependence of Ar emission intensities at 425.9 nm, 750.4 nm and 811.5 nm for Ar + H2 + C7H8 plasmas. Electron density and the electron impact excitation rate coefficients are nearly the same for Ar(100%) and Ar+H2(33%) plasmas. Toluene addition leads to a decrease in the high energy electron density, whereas the effective electron temperature and low energy electron density remain almost the same. The effective electron temperature, and Ar metastable density and/or low energy electron density decrease significantly with increasing the total pressure from 0.1 to 5 Torr.

Contribution of ionic precursors to deposition rate of a-Si:H films fabricated by plasma CVD

We have studied contribution of ionic precursors to deposition rate of a-Si:H films in the downstream region of a multi-hollow discharge plasma CVD reactor using a DC bias grid and QCMs. The deposition rate decreases from 1.1 to 0.93 by applying negative bias voltage to the bias grid. The ionic precursors contribute to 7% of the total deposition rate and the dominant ionic precursors are considered to be negatively charged clusters.

Preparation of Si nanoparticles by laser ablation in liquid and their application as photovoltaic material in quantum dot sensitized solar cell

Quantum dot sensitized solar cell was fabricated using silicon nanoparticles prepared by laser ablation in liquid. The obtained particles are spherical and have an average size of 6 nm. These particles were utilized as photosensitizing material in solar cell with polysulfide as an electrolyte. The conversion efficiency of solar cell with silicon nanoparticles was 5.3 times higher than the one with only TiO2 particles. Electrical impedance spectroscopy also revealed the decrease in resistance in the former case led to higher current density than the latter one.

ZnO-based semiconductors with tunable band gap for solar sell applications

In this study, we discuss the potential advantages of a new ZnO-based semiconductor, ZnInON (ZION), for application in multi quantum-well (MQW) photovoltaics. ZION is a pseudo-binary alloy of ZnO and InN, which has direct and tunable band gaps over the entire visible spectrum. It was found from simulation results that owing to the large piezoelectric constant, the spatial overlap of the electron and hole wave functions in the QWs is significantly small on the order of 10-2, where the strong piezoelectric field enhances the separation of photo generated carriers. As a result, ZION QWs have low carrier recombination rate of 1014–1018 cm-3s-1, which is much lower than that in conventional QWs such as InGaAs/GaAs QW (1019 cm-3s-1) and InGaN/GaN QW (1018–1018 cm-3s-1). The long carrier life time in ZION QWs (∼1μs) should enable the extraction of photo-generated carriers from well layers before the recombination, and thus increase Voc and Jsc. These simulation results are consistent with our experimental data showing that both Voc and Jsc of a p-i-n solar cell with strained ZION MQWs and thus the efficiency were increased by the superimposition of laser light with lower photon energy than the band gap energy of the QWs. Since the laser light contributed not to carrier generation but to the carrier extraction from the QWs, and no increase in Voc and Jsc was observed for relaxed ZION MQWs, the improvement in the efficiency was attributed to the long carrier lifetime in the strained ZION QWs.

Off-axis sputter deposition of ZnO films on c-sapphire substrates with buffer layers prepared via nitrogen-mediated crystallization

High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing off-axis sputtering configuration together with buffer layers prepared via nitrogen mediated crystallization (NMC). The role of NMC buffer layers is to provide high density of nucleation site and thus to reduce the strain energy caused by the large lattice mismatch (18%) between ZnO and sapphire. The NMC buffer layers allow two dimensional (2D) growth of subsequently grown ZnO films, being particularly enhanced by employing off-axis sputtering configuration, in which the substrate is positioned out of the high-energy particles such as negative oxygen ions originating from the targets. As a result, ZnO films with smooth surfaces (root-mean-square roughness: 0.76 nm) and high electron mobility of 88 cm2/V⋅sec are fabricated. Photoluminescence spectra of the ZnO films show strong near-band-edge emission, and the intensity of the orange-red defect emission significantly decreases with increasing the horizontal distance between the target and the substrate. From these results, we conclude that off-axis sputtering together with NMC buffer layers is a promising method for obtaining high quality epitaxial ZnO films.

Deposition of crystalline Ge nanoparticle films by high-pressure RF magnetron sputtering method

We report here deposition of crystalline Ge nanoparticle films using a radio frequency magnetron sputtering method in argon and hydrogen gas mixture under a high pressure condition. The size of Ge nanoparticles is deduced to be 6.3-6.4 nm from the peak frequency shift of Raman spectra. Raman and X-ray diffraction spectra show that the films are crystalline. The film crystallinity strongly depends on substrate temperature (Ts). Highly crystalline Ge nanoparticle films are successfully fabricated at Ts = 180?C.

Contribution of H2 plasma etching to radial profile of amount of dust particles in a divertor simulator

We have studied contribution of H2 plasma etching to radial profile of amount of dust particles generated due to interactions between H2 plasmas and graphite target in a divertor simulator. Dust fluxes of spherical particles and flakes are the maximum at the distance r = 100 mm and 120 mm from the centre axis of the plasma column, respectively. From ion density and dust flux, we have deduced etched volume of deposited dust particles due to H2 plasma irradiation. Sum of the etched volume and measured volume of spherical dust particles is almost constant for r 120 mm, whereas that of flakes is the maximum at r = 120 mm. H2 plasma etching significantly reduces size of dust particles for r smaller than 100 mm.

Crystallinity Control of Sputtered ZnO:Al Transparent Conducting Films by Utilizing Buffer Layers Fabricated via Nitrogen Mediated Crystallization

We have developed a fabrication method based on sputtering, “Nitrogen Mediated Crystallization (NMC)”, which enables control of crystal nucleation and growth of ZnO. By utilizing this NMC, ZnO:Al transparent conducting films with low resistivity of 2.8×10-3.2×10 Ω•cm in a film thickness range 20-100 nm have been fabricated. The resistivity is significantly lower than that of ZnO:Al films fabricated by conventional sputtering (6.3×10-1.5×10 Ω·cm). Thus, NMC method opens up a new pathway for development of oxide semiconductors.