A DEFECT-POOL MODEL FOR NEAR-INTERFACE STATES IN AMORPHOUS-SILICON THIN-FILM TRANSISTORS

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-states distribution more like an oxide transistor by suitable negative-bias annealing. These results show the improtance of the Fermi energy in controlling the near-interface state distribution and suggests that the defect pool model can account for most deep states in a-Si:H (near-interface as well as bulk states).