A study of hydrogen passivation of grain boundaries in polysilicon thin-film transistors

A series of different annealing temperatures and gases leads to steadily improving performance, i.e. lower leakage current, for five wafers containing both n-channel and p-channel transistors. Initially the n-channel and p-channel leakage currents improve proportionally for the same passivation treatment. For the best wafer, however, the p-channel leakage current improves by a factor of forty more than the n-channel leakage current. Measurement of leakage current activation energy shows that for most wafers, the activation energy is close to one half the silicon bandgap, but for the optimum wafer the activation energy for p-channel leakage current is higher by 0.1 eV. An interpretation is given in terms of the density of defect states in polysilicon. Initially passivation reduces the number of defect states near midgap, but for the optimum passivation the grain-boundary Fermi level shifts by 0.1 eV toward the conduction band, leading to a much lower leakage current for p-channel versus n-channel transistors