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 silicon thin-film transistors (polysilicon TFTs) is investigated by field-effect conductance activation energy measurements. The bulk states consist of deep states with a characteristic wide peak near the midgap and shallow exponential tails near the conduction band. The interface states show an exponential distribution which cause a faster change of the gap states density near the conduction-band edge. For p≳40 mTorr, the bulk and interface states are controlled by the grain size and the degree of disorder of the material. For p<40 mTorr, the experimental results indicate that the films are contaminated by some impurities. The concentration of these impurities increases as p decreases and contributes mainly to the deep states generation.