Bias dependence of instability mechanisms in amorphous silicon thin-film transistors

Abstract

We have measured the bias dependence of the threshold voltage shift in a series of amorphous silicon-silicon nitride thin-film transistors, where the composition of the nitride is varied. There are two distinct instability mechanisms: a slow increase in the density of metastable fast states and charge trapping in slow states. State creation dominates at low fields and charge trapping dominates at higher fields. The state creation is found to be independent of the nitride composition, whereas the charge trapping depends strongly on the nitride composition. This is taken as good evidence that state creation takes place in the hydrogenated amorphous silicon (a-Si:H) layer, whereas the charge trapping takes place in the a-SiN:H. The metastable states are suggested to be Si dangling bonds in the a-Si:H, and the state creation process similar to the Staebler–Wronski effect. The confirmation of state creation in a thin-film transistor means that states can be created simply by populating conduction-band states in the undoped material. The slow states are also thought to be Si dangling bonds, but located in the silicon nitride matrix.

Effect of channel length on the threshold voltage degradation of hydrogenated amorphous silicon TFTs due to the drain bias stress

“…a-Si:H TFTs exhibit a bias-induced metastability phenomenon that causes the threshold voltage shift over time in the presence of a gate bias [2]. a-Si:H TFTs under gate bias stress may degrade by two mechanisms.…”

Disorder and instability processes in amorphous conducting oxides

“…These can be due to charge trapping in the gate insulator, or changes in interface states. The role of charge trapping can be distinguished in oxide TFTs, as in a-Si:H TFTs, as occurring at higher applied fields and being rather temperature independent [57][58][59].…”

Persistent microscopic charge patterns in amorphous silicon thin films for guided assembly of colloids

“…A model explaining why a positive voltage induces negative charge in the sample was already presented and discussed elsewhere [14]. Briefly, for positive tip voltages the tip-sample junction is reverse biased, electrons are accumulated in the a-Si:H subsurface region and some of them are trapped in deep gap states (dangling bonds) there [19] or they can even generate additional states [20]. In response, the atomic structure re-arranges in the vicinity of the trapped charge due to the flexible amorphous network.…”