William Makoto Nakamura

Two-Dimensional Spatial Profile of Volume Fraction of Nanoparticles Incorporated Into a-Si:H Films Deposited by Plasma CVD

Using an optical-scanning method, we obtained 2-D spatial profile of deposition rate of hydrogenated amorphous silicon (a-Si:H) films deposited by a multihollow discharge plasma CVD with a high spatial resolution. From the profile, we deduced 2-D spatial profile of the volume fraction of nanoparticles incorporated into films, since nanoparticles affect optical and electronic properties of a-Si:H films.

Cluster incorporation control for a-Si:H film deposition

To study the effects of clusters on the light induced degradation and control their deposition into films, we have developed a multi-hollow plasma CVD method by which the incorporation of clusters is reduced in the upstream region using the gas flow that drives clusters formed in discharges toward the downstream region of the reactor. Thus, we can simultaneously deposit films in which the volume fraction of clusters incorporated into films varies by changing the position of the substrate in the reactor. A-Si:H films with a lower volume fraction of clusters tend to show better stability against light exposure.

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.

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.

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.

Control of Nanostructure of Plasma CVD Films for Third Generation Photovoltaics

One of the requirements for successful application of hydrogenated amorphous silicon (a-Si:H) to promising tandem cells, that aim at high efficiency and low production cost, is to overcome its light induced degradation, which reduces significantly the initial conversion efficiency with light exposure. Our previous studies indicate a relation between light induced degradation and the incorporation of amorphous silicon nanoparticles (clusters) into a-Si:H films. Here we report control of nanostructure of a-Si:H films using a multi-hollow plasma CVD reactor. Deposition with low or non-incorporation of clusters is realized in the upstream region far from discharges in the reactor, whereas in the downstream region the volume fraction of clusters in films increases with the distance from discharge region. Films with a lower volume fraction tend to show better stability against light exposure.