Wafer-Scale Evaporated Metallic BiO_x 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 BiOx 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 BiOx film possesses a unique structure of polycrystalline Bi dispersed within an amorphous BiOx matrix, preserving ultraflat surface even after heating above the melting temperature of bismuth. Furthermore, the evaporated BiOx 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 BiOx-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 BiOx thin films as an alternative promising contact for MoS2 transistors.