High-rate deposition of high-quality hydrogenated amorphous silicon germanium using very high-frequency plasma-enhanced chemical vapor deposition with a high hydrogen dilution

Ways of improving the deposition rate of hydrogenated amorphous silicon germanium (a-SiGe:H) while maintaining its film quality, using the very high-frequency plasma-enhanced chemical vapor deposition (VHF-CVD) technique, were investigated. It was concluded that the VHF-CVD technique with a high hydrogen dilution of silane and germane is suitable for high-quality a-SiGe:H fabrication at a relatively high deposition rate. The reason why the high deposition rate is compatible with high quality is related to the high density of hydrogen radicals, which are proper to the VHF-CVD technique. The low plasma potential of VHF plasma contributes to high-quality a-SiGe:H film deposition by suppressing ion bombardment and excessive penetration of hydrogen atoms into the film. In device fabrication, the VHF-CVD technique is also useful because a high deposition rate with low ion bombardment may suppress negative effects to the underlying layers during deposition of the. a-SiGe:H photovoltaic layer. This investigation showed that almost the same initial and stabilized efficiency for an a-Si/a-SiGe tandem solar cell was obtained at a five times higher deposition rate for the a-SiGe:H photovoltaic layer than those in conventional RF-CVD methods using the same reaction chamber.