(Invited) Ultrashort Channel Length Black Phosphorus Field-Effect Transistors

Layered black phosphorus (BP) is an emerging two-dimensional (2D) semiconductor with a decent bandgap and great mobility that bridges the gap between graphene (extremely high mobility, no bandgap) and transition metal dichalcogenides (low mobility, large bandgap). As a result, 2D layered BP has attracted significant amount of research interest recently as a promising candidate for high-performance ultimately-scaled electronic and optoelectronic devices. To the best of our knowledge, the smallest BP field-effect transistor demonstrated so far has a channel length of approximately 100 nm fabricated by electron beam lithography (EBL). It gets significantly more challenging to further scale down the channel length to below 100 nm due to the limitations in ebeam resist and lift-off process used.

In this paper, we report a novel and facile process combining EBL and angle evaporation to fabricate high-performance top-gated BP transistors with channel length down to 20 nm (the smallest 2D material transistor to date). By controlling the evaporation angle, the channel length of the transistors can be effectively and reproducibly controlled to be anywhere between ~20 to ~70 nm. With the high quality few-layer BP obtained from mechanical exfoliation and ultrashort channel length, such devices exhibit respectable on-current and transconductance up to 174 μA/μm and 50 μS/μm, respectively, at a small drain-to-source voltage of 100 mV. Owing to the use of 2D BP as the channel material, the transistors exhibit no obvious short channel effects, preserving a decent on-off current ratio of 10² even at extremely small channel lengths. Additionally, unlike the unencapsulated BP devices, which are known to be chemically unstable in ambient conditions, the top-gated BP transistors passivated by the Al₂O₃ gate dielectric layer remain stable without noticeable degradation in device performance after being stored in ambient conditions for more than one week. This work demonstrates the great promise of atomically thin BP for applications in ultimately-scaled transistors.