Deposition and device application of in situ boron-doped polycrystalline SiGe films grown at low temperatures

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Abstract
Deposition of in situ boron‐doped polycrystalline silicon‐germanium (poly‐Si1−xGex) films at temperatures below 550 °C was investigated using an ultrahigh‐vacuum chemical‐ vapor‐deposition system. These films with a fine grain structure were obtained for boron concentrations higher than 1021 cm−3. It is attributed to the enhanced nonequilibrium doping effect due to the addition of GeH4 gas during film deposition. Poly‐Si0.56Ge0.44 films with a carrier concentration of 8×1020 cm−3 were achieved at a growth temperature of 500 °C. Such a high activated carrier concentration resulted in a film resistivity less than 2 mΩ cm. Utilizing these characteristics, a novel approach was proposed and demonstrated to fabricate p‐channel polycrystalline silicon thin‐film transistors at process temperatures below 550 °C. These transistors with a maximum field effect mobility up to 28 cm2/V s and an on/off current ratio over 106 were achieved without employing any post‐treatment step, indicating the feasibility of this approach on the fabrication of polycrystalline silicon thin‐film transistors at low temperatures.