Deposition of Amorphous Silicon

Abstract

In 1969 Chittick et al [1] discovered that hydrogenated amorphous silicon (a-Si:H) deposited from SiH₄ glow discharges is a semiconductor, and in 1975 Spear and LeComber [2] demonstrated that a-Si:H could be doped by adding B₂H₆, AsH₃ or PH₃ to SiH₄. This opened the possibility of making thin-film devices at low temperature (100–300°C) on glass substrates for various applications: photovoltaic solar cells, xerography, vidicon tubes, optical sensors, light emitting diodes and thin-film transistors (TFT) for scanners and active-matrix liquid-crystal displays (AM-LCD) [3]. A large effort of basic and applied research accompanying industrial developments has been spent to study deposition processes and properties of a-Si:H and related alloys (a-SiC_x:H and a-SiGe_x:H), and microcrystalline silicon (µc-Si:H). Simultaneously many techniques alternative to plasma-enhanced chemical vapour deposition (PECVD) were explored: photo-CVD using UV or IR photolysis of SiH₄, hot-wire-CVD using the pyrolysis of SiH₄ on a tungsten filament as a source of reactive species, sputtering or evaporation of Si in the presence of H₂, etc… Other molecules such as Si₂H₆ or SiF₄ were also used [4]. All these approaches provide materials with analogous if not identical properties. Nevertheless PECVD in radio-frequency (RF) parallel-plate discharges is still preferred for large area industrial deposition.