Device-quality polycrystalline silicon films deposited at low process temperatures by hot-wire chemical vapor deposition

Device-grade undoped hydrogenated polycrystalline silicon thin films have been developed from a gas mixture of silane and hydrogen using a hot-wire chemical vapor deposition (HW-CVD) method, optimizing the deposition parameters. Proper design of the HW-CVD reactor helps to deposit a uniform quality of film over a large area (100 cm2) with a two filament configuration. Extensive studies have been made of the effects of hydrogen dilution (4-60), substrate temperature (180-400°C) and filament temperature (1500-1700°C) on the film growth. Atomic force micrographs give a quantitative estimate of roughness for these films. UV-visible ellipsometry analyses confirm their compactness and crystallinity while X-ray diffraction patterns allow for the determination of the crystallite sizes (up to 400 Å). Using a hydrogen dilution of 60, a substrate temperature of 300°C and a filament temperature of 1500°C, a dark conductivity of 2.5 x 10-5 S/cm and its activation energy of 0.45 eV have been obtained. For these films, the Hall mobility attains 10 cm2/V s. With these deposition parameters. the intrinsic layer of complete p-i-n HW-CVD solar cells has been realized. These cells, deposited on TCO coated Corning glass substrates, exhibit 1.8% conversion efficiency under 100 mW/cm2 irradiation.