INFLUENCE OF DEPOSITION PRESSURE ON THE BULK AND INTERFACE STATES IN LOW-PRESSURE CHEMICAL-VAPOR-DEPOSITED POLYCRYSTALLINE SILICON THIN-FILM TRANSISTORS

The effect of deposition pressure, p, on the bulk and interface states of undoped low pressure chemical vapor deposited polycrystalline sili con thin-film transistors (polysilicon TFTs) is investigated by field- effect conductance activation energy measurements. The bulk states con sist of deep states with a characteristic wide peak near the midgap an d shallow exponential tails near the conduction band. The interface st ates show an exponential distribution which cause a faster change of t he gap states density near the conduction-band edge. For p > 40 mTorr, the bulk and interface states are controlled by the grain size and th e degree of disorder of the material. For p < 40 mTorr, the experiment al results indicate that the films are contaminated by some impurities . The concentration of these impurities increases as p decreases and c ontributes mainly to the deep states generation.