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-Si1-xGex) films at temperatures below 550-degrees-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 10(21) 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 X 10(20) cm-3 were achieved at a growth temperature of 500-degrees-C. Such a high activated carrier concentration resulted in a film resistivity less than 2 mOMEGA 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-degrees-C. These transistors with a maximum field effect mobility up to 28 cm2/V s and an on/off current ratio over 10(6) 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.