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

Deposition of in situ boron-doped polycrystalline silicon-germanium (poly-Si_1-xGe_x) 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-Si_0.56Ge_0.44 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.