Oxidation of tantalum disulfide (TaS2) films for gate dielectric and process design of two-dimensional field-effect device

Ta-based high-κdielectrics can be synthesized via the oxidation of TaS2films. In this study, we investigated the wet and dry oxidation of TaS2films via thermal annealing and plasma irradiation, respectively. The specific vibration observed via Raman spectroscopy, the bonding states observed via x-ray photoelectron spectroscopy, and capacitance measurements confirmed the oxidation of TaS2films with a dielectric constant of ∼14.9. Moreover, the electrical transport of the TaS2films along the in-plane direction indicated a change in conductivity before and after the oxidation. The thickness of the oxidized film was estimated. Accordingly, the layer-by-layer oxidation was limited to approximately 50 nm via plasma irradiation, whereas the TaS2films within 150 nm were fully oxidized via thermal annealing in ambient air. Therefore, a Ta-oxide/TaS2structure was fabricated as a stack material of insulator and metal when the thickness of the pristine film was greater than 50 nm. In addition, Ta-oxide films were integrated into bottom-gated two-dimensional (2D) field-effect transistors (FETs) using the dry transfer method. 2D FETs with multilayer MoTe2and MoS2films asp-type andn-type channels, respectively, were successfully fabricated. In particular, the Ta-oxide film synthesized via dry oxidation was used as a gate dielectric, and the device process could be simplified because the Ta-oxide/TaS2heterostructure can function as a stack material for gate insulators and gate electrodes. An anti-ambipolar transistor consisting of an MoTe2/MoS2heterojunction was also fabricated. For the transfer characteristics, a relatively sharp on-state bias range below 10 V and sufficiently high peak-to-valley ratio of 106atVDS = 3 V were obtained using the high-κ gate dielectric of Ta-oxide despite the presence of the multilayer channels (∼20 nm).