Yusuke Ebata

High-Rate Deposition of Intrinsic Amorphous Silicon Layers for Solar Cells Using Very High Frequency Plasma at Atmospheric Pressure

We have investigated the electrical and optical properties of hydrogenated amorphous silicon (a-Si:H) films prepared with extremely high deposition rates using very high frequency (VHF) plasma of gas mixtures containing He, H2, and SiH4 at atmospheric pressure. VHF power is a very important deposition parameter governing the dissociation of SiH4 molecules and the structural relaxation of the film. When the deposition parameters are optimized, the optical gap of the film can be controlled by the H2/SiH4 ratio without appreciable deterioration in photoconductivity. On the basis of the results, the a-Si:H films have been applied to the intrinsic layers (i-layers) of p–i–n single-junction solar cells. An initial efficiency of 8.25% is obtained for the cell with an i-layer prepared at a very high deposition rate of 128.1 nm/s. However, the results suggest that unknown factors limiting cell performance still exist in the fabrication process of a-Si:H solar cells.

Influence of H2/SiH4 Ratio on the Deposition Rate and Morphology of Polycrystalline Silicon Films Deposited by Atmospheric Pressure Plasma Chemical Vapor Deposition

Using the atmospheric pressure plasma chemical vapor deposition (AP-PCVD) technique, polycrystalline silicon (poly-Si) films were deposited at high rates. The plasma was generated by 150 MHz very high frequency (VHF) power. Deposition rate, morphology, and structure of the poly-Si films were studied as a function of the H2/SiH4 ratio, using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Plasma length and optical emission were also investigated to examine the reaction process in the plasma. The maximum value of deposition rate was 7.4 nm/s at an SiH4 concentration of 0.1% (H2/SiH4=100). SEM and TEM observations revealed that the poly-Si films had a columnar structure, and that the crystallites in the films deposited at high H2/SiH4 ratios contained no visible defects except at the grain boundaries. It was also shown that the Si film prepared has a preferential orientation of (220) by XRD. In addition, it was found that the plasma length was sensitive to the H2 concentration, while it was not affected by the SiH4 concentration within the deposition conditions used. This fact suggested that almost all the VHF power was consumed in the decomposition of H2 molecules.