Low-temperature hot-wire chemical vapor deposition of epitaxial films for large-grained polycrystalline photovoltaic devices

Large-grained polycrystalline silicon thin-films on low-cost substrates are an interesting area of research for photovoltaic devices. Such devices, with grain sizes larger than the thickness of the cell, have the potential to achieve multicrystalline-like efficiencies of 15%, but at a much lower cost by taking advantage of thin-film manufacturing techniques. In this thesis, low-temperature epitaxial growth, by hot-wire (or catalytic) chemical vapor deposition, is investigated for the epitaxial thickening of large-grained polycrystalline silicon templates formed by metal-induced crystallization on low-cost substrates. Low-temperature hot-wire chemical vapor deposition allows for the deposition of epitaxial silicon with polycrystalline breakdown and with open-circuit voltages close to that of monocrystalline silicon. This is possible due to the incorporation of hydrogen into the silicon lattice, at temperatures below 350°C, for internal surface and defect passivation. In addition with hot-wire chemical vapor deposition, the critical epitaxial thickness actually increases, with a decrease in the substrate temperature down to temperatures of 270°C. Epitaxial growth of 5.5 mum thick films at 300°C and twinned epitaxial silicon growth of 6.8 mum thick films at 230°C have been achieved, along with arbitrarily thick crystalline films at low temperatures. Since epitaxial and high-quality crystalline silicon can be deposited at such low deposition temperatures, low-cost substrates, such as ordinary soda lime glass and many polymers are possible. In order to work towards achieving an epitaxially-thickened large-grained polycrystalline device, this work studies the mechanisms that lead to epitaxial growth during hot-wire chemical vapor deposition on silicon (100) substrates under various growth regimes, examines the surface evolution of crystalline thin-films grown via hotwire chemical vapor deposition and their growth mechanisms (including the unusual rough epitaxial growth and arbitrarily thick crystalline films at low temperatures), and concludes by presenting the optical and electrical characteristics of these films and their resultant devices. This thesis demonstrates that low-temperature epitaxial silicon growth by hot-wire chemical vapor deposition is a promising material for low-cost thin-film silicon photovoltaic devices.