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

Novel TaO _x encapsulation was presented to enhance the field-effect mobility (μ_FE) 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 TaO _x/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 μ_FE, subthreshold gate swing (SS), and I_ON/OFF ratio of 36.2 cm²/V·s, 0.28 V/decade, and 2.9 × 10⁸, respectively. A significantly enhanced μ_FE value of 89.4 cm²/V·s was achieved for ZnON TFTs with a TaO _x encapsulation layer, whereas the SS of 0.33 V/decade and I_ON/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 TaO _x 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 TaO _x encapsulation layer. A smaller number of subgap states near the conduction band (CB) minimum and a higher net carrier density for the TaO _x-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.