PERFORMANCE OF THIN-FILM TRANSISTORS ON POLYSILICON FILMS GROWN BY LOW-PRESSURE CHEMICAL VAPOR-DEPOSITION AT VARIOUS PRESSURES

Defect properties of undoped low-pressure chemical-vapor-deposited (LPCVD) polysilicon films have been investigated by capacitance techniques on a simple metal-oxide-semiconductor (MOS) capacitor structure. The results show that the effective density of bulk and interface trap states is almost independent of the deposition pressure. After reducing the polysilicon film thickness by etching, although the grain size decreases due to the columnar mode of growth at low pressures, the trap states density reduces significantly. This finding could be explained by suggesting that, during the growth of the material, impurities are segregated at the film surface by fast diffusion through the grain boundaries. The transport properties of 0.5-mu-m-thick polysilicon films deposited at a pressure ranging from 100 to 0.5 mtorr were evaluated from measurements on thin-film transistors (TFT's). The results demonstrate that at high pressures the grain boundaries and at low pressures the polysilicon-SiO2 interface roughness scattering are the main factors in determining the transistor performance. The 40-mtorr polysilicon is the best TFT material due to a compromise between surface roughness and grain size that optimizes device performance.