The resolution of a-Si thin film transistor instability mechanisms

Abstract

Bias stress measurements on amorphous silicon - silicon nitride ambipolar thim film transistors (TFTs) give clear evidence for the coexistence of two mechanisms responsible for the threshold voltage shift; the metastable creation of fast states and charge trapping in slow states. State creation dominates at low positive bias and charge trapping at higher bias. Measurements on a series of standard TFTs, where the composition of the nitride is varied, show the state creation to be independent of nitride composition, whereas the charge trapping depends strongly on the nitride composition. We conclude that the slow states are located in the nitride, whereasthe metastable states are confined to the a-Si:H layer. The metastable states are suggested to be Si dangling bonds in a-Si:H, created by populating the antibonding orbitals of weak Si-Si bonds. The slow states are also thought to be Si dangling bonds, but located in the nitride. We proposed that charge transfer into these latter states is by hopping.