EFFECT OF PRESSURE ON THE GROWTH OF CRYSTALLITES OF LOW-PRESSURE CHEMICAL-VAPOR-DEPOSITED POLYCRYSTALLINE SILICON FILMS AND THE EFFECTIVE ELECTRON-MOBILITY UNDER HIGH NORMAL FIELD IN THIN-FILM TRANSISTORS

The morphology of polycrystalline films grown by low-pressure chemical -vapor deposition (LPCVD) is investigated by transmission electron mic roscopy (TEM) as a function of the film thickness, the deposition pres sure, and the level of contamination. An orientation filtering mechani sm, due to the growth-velocity competition in the early stage of growt h, is responsible for the preferred orientation of the films. The size of the crystallites, the surface roughness, and the type of the struc tural defects are investigated by combined cross-sectional and plane-v iew TEM analysis. In polycrystalline silicon thin-film transistors (TF Ts), the influence of surface roughness scattering on the mobility is investigated by measuring the effective electron mobility under high e ffective normal field at 295 and 77 K. Although the surface curvature is increased when the deposition pressure is decreased, the surface ro ughness scattering is constant in the deposition pressure range from 4 0 to 0.5 mTorr. By decreasing the deposition pressure from 40 to 10 mT orr, although the grain size increases, the TFT performance degrades d ue to the following factors: (a) the increase of the grain-boundary tr ap density which is related to the change of the mode of growth at 10 mTorr; and (b) the increase of impurity contamination in the environme nt of the LPCVD system with constant silane flow rate at all pressures . At a deposition pressure of 0.5 mTorr the TFT performance is improve d indicating that the grain size is the prevailing key factor.