Boosting Carrier Mobility in Zinc Oxynitride Thin-Film Transistors via Tantalum Oxide Encapsulation

Novel TaOx encapsulation was presented to enhance the field-effect mobility (μFE) of ZnON thin-film transistors (TFTs) consisting of a metallic Ta film deposited onto the ZnON surface followed by a modest annealing process. The resulting TaOx/ZnON film stack exhibited a more uniform distribution of nanoscale ZnON crystallites with increased stoichiometric anion lattices compared to the control ZnON film. The control ZnON TFTs exhibited a reasonable μFE, subthreshold gate swing (SS), and ION/OFF ratio of 36.2 cm2/V·s, 0.28 V/decade, and 2.9 × 108, respectively. A significantly enhanced μFE value of 89.4 cm2/V·s was achieved for ZnON TFTs with a TaOx encapsulation layer, whereas the SS of 0.33 V/decade and ION/OFF ratio of 8.6 × 108 were comparable to those of the control device. This improvement could be explained by scavenging and passivation effects of the TaOx film on the ZnON channel layer. Density of states (DOS)-based modeling and simulation were performed to obtain greater insight with regard to increasing the performance of the ZnON TFTs with a TaOx encapsulation layer. A smaller number of subgap states near the conduction band (CB) minimum and a higher net carrier density for the TaOx-capped device increased the Fermi energy level toward the CB edge under thermal equilibrium conditions, leading to efficient band conduction and fast carrier transport under the on-state condition.