John Richard Hughes

Time and temperature dependence of instability mechanisms in amorphous silicon thin‐film transistors

We have measured the time and temperature dependence of the two prominent instability mechanisms in amorphous silicon thin‐film transistors, namely, the creation of metastable states in the a‐Si:H and the charge trapping in the silicon nitride gate insulator. The state creation process shows a power law time dependence and is thermally activated. The charge trapping process shows a logarithmic time dependence and has a very small temperature dependence. The results for the state creation process are consistent with a model of Si dangling bond formation in the bulk a‐Si:H due to weak SiSi bond breaking stabilized by diffusive hydrogen motion. The logarithmic time dependence and weak temperature dependence for the charge trapping in the nitride suggest that the charge injection from the a‐Si:H to the nitride is the rate limiting step and not subsequent conduction in the nitride.

A defect-pool model for near-interface states in amorphous silicon thin-film transistors

Abstract We present evidence that the majority of deep states located near to the gate-insulator interface in amorphous silicon (a-Si) thin-film transistors are part of a defect pool of silicon dangling-bond states, whose density and energy position within the energy gap of the a-Si are determined by the Fermi energy during thermal equilibration. Transistors made with silicon nitride and silicon oxide gate insulators tend to have different densities-of-states distributions. We show it is possible to modify the entire energy distribution of states, by annealing the transistors with an applied gate bias. The density of states and their energy distribution re-equilibrates to the new Fermi energy position, causing the density of states to be increased or decreased in different parts of the bandgap. In particular, an oxide transistor can be made to have a density-of-states distribution similar to a nitride transistor by suitable positive-bias annealing, and a nitride transistor can be made to have a density-of...

Evidence for the defect pool concept for Si dangling bond states in a-Si:H from experiments with thin film transistors

Abstract We compare the characteristics of a-Si:H TFTs made using silicon nitride and silicon oxide gate insulators, and then determine the energy distribution of states in the amorphous silicon band -gap. We also subject the transistors to positive and negative bias-stress, which creates new states in the gap. We find the majority of intrinsic deep states occur at a higher energy for oxide transistors than for nitride transistors. We also find that the metastable states induced by positive bias-stress occur at a higher energy than those induced by negative bias-stress. The results are consistent with the defect pool model for the Si dangling bond states.

Thermal bias annealing evidence for the defect pool in amorphous silicon thin‐film transistors

Thin‐film transistors were thermally annealed while a bias voltage was applied to the gate electrode. The transfer characteristics were then measured, and the density of states distributions derived by field‐effect analysis. The results indicate that the equilibrium distribution and number of defects in the transistor channel region depend on the position of the Fermi energy during annealing. Thus the density of states can be increased or decreased in parts of the band gap. A high Fermi energy during annealing results in few states high in the gap and more states low in the gap. The reverse is true for annealing while the Fermi energy is low. This is consistent with the defect pool model for silicon dangling bond states and suggests that most deep states are part of the defect pool.

Correlation between Density of States Distributions and the Fermi Level Position in Interfacial Regions of Amorphous Silicon Thin Film Transistors

This paper investigates the variation of the integrated density of states with conduction activation energy in hydrogenated amorphous silicon thin film transistors. Results are given for two different gate insulator layers, PECVD silicon oxide and thermally grown silicon dioxide. The different gate insulators produce transistors with very different initial transfer characteristics, but the variation of integrated density of states with conduction activation energy is shown to be similar.