Takeaki Matsunaga

Deposition of cluster-free b-doped hydrogenated amorphous silicon films using sih 4+b10h1 4 multi-hollow discharge plasma chemical vapor deposition

We have deposited cluster-free B-doped hydrogenated amorphous silicon (a-Si:H) films using a SiH4+B10H14 multi-hollow discharge plasma chemical vapor deposition (CVD) method. We have studied the dependence of the deposition rate, band gap, and conductivity of the films on the gas flow rate ratio R=[B10H14]/[SiH4]. The deposition rate for SiH4+B10H14 plasmas is 2–3 times as high as that for pure SiH4 plasmas. Optical emission spectroscopy (OES) measurements indicate that SiH3 radical generation rate remains nearly constant regardless of R. These results suggest that BxHy radicals enhance the surface reaction probability and/or sticking probability of SiH3, being the predominant deposition precursor. Cluster-free B-doped a-Si:H films have a wide band-gap energy of 1.8–2.0 eV and a conductivity as high as 5.0×10-6 S/cm. These results demonstrate that cluster-free B-doped a-Si:H films deposited using SiH4+B10H14 multi-hollow discharge plasma CVD are promising as a p-layer of pin a-Si:H solar cells.

Combinatorial Deposition of Microcrystalline Silicon Films Using Multihollow Discharge Plasma Chemical Vapor Deposition

A high-pressure depletion method using plasma chemical vapor deposition (CVD) is often used to deposit hydrogenated microcrystalline silicon (µc-Si:H) films of a low defect density at a high deposition rate. To understand proper deposition conditions of µc-Si:H films for a high-pressure depletion method, Si films were deposited in a combinatorial way using a multihollow discharge plasma CVD method by which fluxes of H and SiH3 radicals and their flux ratio can be varied with the distance from the discharges. The higher gas pressure brings about the higher deposition rate, whereas the process window of device quality µc-Si:H films becomes quite narrower for the higher gas pressure.

Si quantum dot-sensitized solar cells using Si nanoparticles produced by plasma CVD

We have fabricated Si quantum dot-sensitized solar cells by three assembling methods; the conventional method and two novel methods in which Si nanoparticles/TiO2 blend paste is coated onto FTO glass, or TiO2 film. The highest current density is realized s solar cell for which Si nanoparticles/TiO2 blend paste is coated onto TiO2 films. The result indicates that excitons generated in Si nanoparticles are separated into electrons and holes and such carriers are extracted to the outer circuit. Photon-to-current conversion efficiency (PCE) of our Si quantum dot-sensitized solar cells is larger than that of the cell without Si nanoparticles in a wavelength range below 500nm. It suggests that carrier generation in the Si nanoparticles is realized in the wavelength range less than 500 nm in the solar cell. Light intensity dependence of photo-current density shows superliner one for photon energy larger than twice of the bandgap energy of the Si nanoparticles.

Optical Emission Spectroscopy of Low-Discharge-Power Magnetron Sputtering Plasmas Using Pure Tungsten Target

To study the mechanism of a tungsten oxide (WO3) thin film using an RF magnetron sputtering method, optical emission spectroscopic (OES) measurements for the RF plasma of a pure W target have been performed. We also examined the crystalline structure and atomic composition rate of the prepared thin film using X-ray photoelectron spectroscopy (XPS). Experimental results indicate that Ar I emission peak intensity slightly increased with increasing Ar gas mixture. On the other hand, W I emission peak intensity rapidly increased with increasing Ar gas mixture. The increase rates of these two emission peak intensities are different, which may be due to the difference in emission mechanism. O I emission peak intensity decreased with increasing Ar gas mixture, indicating that O I emission intensity increased with increasing O2 gas mixture. Electron density and deposition rate increased with increasing Ar gas mixture, and their dependences on Ar gas mixture were very similar to that of Ar I emission. XPS analyses indicate that the oxidation ratio of the prepared film was slightly decreased with decreasing Ar gas mixture. These results suggest that the W sources of the WO3 film on the substrate are W atoms and WOx molecules sputtered from the W target. The plasma phase reaction between W and O atoms and the intermediate-energy O atomic and/or O2 molecular reaction on the surface of the substrate are considered to be important for WO3 film production in low-energy magnetron sputtering deposition.

Effects of hydrogen dilution on electron density in multi-hollow discharges with magnetic field for A-Si:H film deposition

We have measured dependence of electron density n<inf>e</inf> on hydrogen dilution ratio R= [H<inf>2</inf>]/([SiH<inf>4</inf>]+[H<inf>2</inf>]) in the multi-hollow discharges with or without magnetic fields to obtain information about the deposition rate enhancement due to hydrogen dilution and applying the magnetic fields. The R dependence of the n<inf>e</inf> did not correlate with that of the deposition rate. The n<inf>e</inf> exponentially decreases with a distance from the discharges z. The n<inf>e</inf> decreases faster for higher R. These complicated behaviors of n<inf>e</inf> may be explained by electron attachment to the clusters generated in the SiH<inf>4</inf>+H<inf>2</inf> discharges. For R=0, the n<inf>e</inf> was almost same value regardless with or without magnetic fields. For R=1, n<inf>e</inf> with magnetic fields was 1/10 of that without magnetic fields. We also found that, for R=0, ne drastically decreased with increasing the z, while for R=1, ne showed a gradual decrease with z. The effects of applying magnetic fields on the deposition are unclear but applying the magnetic fields may affect the electron energy distribution.

Deposition of cluster-free P-doped A-Si:H films using a multi-hollow discharge plasma CVD method

We have deposited cluster-free P-doped a-Si:H films using SiH<inf>4</inf>+PH<inf>3</inf> multi-hollow discharge plasma CVD method. We have measured dependence of a deposition rate and SiH emission intensity on a gas flow rate ratio R=[PH<inf>3</inf>]/[SiH<inf>4</inf>]. The increase of deposition rate with R is much larger than that of SiH emission intensity. These results suggest PH<inf>x</inf> radicals enhance surface reaction probability of SiH<inf>3</inf> radicals. The P concentration in the films can be controlled by the gas flow rate ratio for R<5%. We have succeeded in depositing P-doped a-Si:H films of a low stabilized defect density of 2.9×10¹⁵ cm⁻³. The conductivity of the films is lower than that of the conventional films. Other optoelectronic properties such as conductivity, its activation energy, and bandgap energy are value ranges of the conventional films.

Deposition profiles of microcrystalline silicon films using multi-hollow discharge plasma CVD

We have studied deposition profiles of micro crystalline silicon (μc-Si) films using a multi-hollow discharge plasma CVD method, by which contribution of SiH3 and H to deposition varies with the distance between the substrate and discharge region. Under high pressure (6 Torr) depletion condition, crystalline films were deposited in a region near the discharges and the higher crystallinity was obtained at the closer to the discharges. Films of 0.6 in crystallinity ΦC were deposited in a very narrow region between 4 and 5 mm from the discharges. The process window of good quality μc-Si films is very narrow. These results indicate the multi-hollow discharge plasma CVD method allows us to optimize deposition conditions easier than the conventional deposition methods.

Photoluminescence of Si nanoparticles synthesized using multi-hollow discharge plasma CVD

We have measured photoluminescence spectra of crystalline and amorphous Si nanoparticles dispersed in methanol to demonstrate exciton generation under 244nm and 405nm laser light excitation. The photoluminescence spectra have broad peaks centered at 484–500nm, indicating exciton generation in crystalline and amorphous Si nanoparticles. In addition, the peaks corresponding to the recombination centers at their surface defects are observed at 324nm–380nm. This indicates the importance of termination of surface defects on nanoparticles for application to solar cells.

Cluster-free B-doped a-Si:H films deposited using SiH 4 + B 10 H 14 multi-hollow discharges

We have deposited cluster-free B-doped a-Si:H films using a SiH<inf>4</inf>+B<inf>10</inf>H<inf>14</inf> multi-hollow discharge plasma CVD method. We have studied gas flow rate ratio R=[B<inf>10</inf>H<inf>14</inf>]/[SiH<inf>4</inf>] dependence of deposition rate and absorbance of films together with plasma emission intensities. Deposition rate increases sharply from 0.8nm/s R=0.0 % to 2.2nm/s for R=0.53%, but SiH emission intensity is almost constant for R=0–2.0%. These results suggest B<inf>x</inf>H<inf>y</inf> radicals enhance surface reaction probability of SiH<inf>3</inf> radicals. The optical bandgap of films is around 1.9eV, being larger than that of conventional B-doped films.