Characterization of trapping states in polycrystalline-silicon thin film transistors by deep level transient spectroscopy

Polycrystalline-silicon (poly-Si) thin film transistors (TFTs) have been assessed using deep level transient spectroscopy (DLTS) based on the measurement of current transients due to the thermal emission of carriers trapped at deep levels in the poly-Si. Measurements were made on fully hydrogenated TFTs and the DLTS signal was found to vary continuously between 77 K and room temperature, without showing the characteristic peaks normally associated with point defects in single crystal Si. This demonstrates the existence of a continuous distribution of states through the band gap rather than discrete monoenergetic states. Analysis of the DLTS signal suggests the density of states increases rapidly toward the conduction band edge indicating the presence of a relatively high density of tail states. After annealing the TFTs at 450 °C the current DLTS signal was found to increase due to an increase in trap state density, which occurs because Si–H bonds are thermally unstable at this temperature. The increase in signal was over the whole temperature range of the measurement showing that hydrogen passivates states up to at least 0.1 eV below the conduction band as well as states nearer mid-gap. The effects of varying the fill voltage on the measurement and the time dependence of the current transient were also investigated and added weight to arguments that the current transient was due to the thermal emission of carriers trapped at deep levels.