The effect of Nb doping on the performance and stability of TiOx devices

The effect of niobium (Nb) doping on the performance and stability of TiOx-based thin-film transistors (TFTs) was studied. While sputtered TiOx has an initial amorphous phase and begins to crystallize to anatase at an annealing temperature of 450 degrees C, Nb-doped TiOx preserves the amorphous structure up to annealing temperatures as high as 550 degrees C. TFT devices fabricated using Nb-doped TiOx as the active layer exhibit higher field-effect mobility and better stability upon negative and positive bias stress compared to pure TiOx devices. X-ray photoelectron spectroscopy analyses indicate that Nb doping induces higher levels of oxygen deficiency and a considerable amount of defect states near the valence band, which cannot account for the higher device stability. It is thus suggested that the grain boundaries in crystalline TiOx TiOx may act as the major charge traps, which induce larger shifts in threshold voltage (V-th) upon bias stress.