Wafer-Scale Evaporated Metallic BiOx as Contact Electrodes of MoS2 Transistors with Enhanced Thermal Stability

Bismuth is one of the most widely used contact electrodes in MoS₂ transistors for its capability to minimize the Fermi-level pinning and form ultralow contact resistance. However, the low melting point of bismuth contact will undoubtedly make it incompatible to a high-temperature manufactory process and application scenarios. Here, we introduce that thermal evaporation of α-Bi₂O₃, a well-known insulating oxide, can form a highly metallic O-deficient BiO_x phase, showing a high Hall mobility (∼60 cm² V^–1 s^–1) and ultrahigh carrier density (1.5 × 10¹⁵ cm^–2). Detailed microstructural analysis reveals that the wafer-scale evaporated BiO_x film possesses a unique structure of polycrystalline Bi dispersed within an amorphous BiO_x matrix, preserving ultraflat surface even after heating above the melting temperature of bismuth. Furthermore, the evaporated BiO_x film is functionalized as contact electrodes of MoS₂ transistors, exhibiting a high on-state current and an ultrasmall contact resistance of 650 Ω μm. More importantly, thanks to the superior thermal stability, the BiO_x-contacted MoS₂ transistor undergoes very slight electrical decays after annealing at 300 °C for 6 h, while the Bi-contacted one becomes totally broken down. Our findings demonstrate thermally evaporated BiO_x thin films as an alternative promising contact for MoS₂ transistors.