High Performance P-type Black Phosphorus Transistor with Scandium Contact.

A record high current density of 580 µA/µm is achieved for long-channel, few-layer black phosphorus transistors with the scandium contacts after 400 K vacuum annealing. The annealing effectively improves the on-state current and Ion/Ioff ratio by one order of magnitude and the subthreshold swing by ~2.5×, whereas Al2O3 capping significantly degrades transistor performances, resulting in 5× lower on-state current and 3× lower Ion/Ioff ratio. The influences of moisture on BP metal contacts are elucidated by analyzing the hysteresis of 3-20 nm thick black phosphorus transistors with scandium and gold contacts under different conditions - as-fabricated, after vacuum annealing and after Al2O3 capping. The optimal black phosphorus film thickness for transistors with scandium contacts is found to be ~10 nm. Moreover, p-type performance is shown in all transistors with scandium contacts, suggesting that the Fermi-level is pinned closer to the valence band regardless of the flake thickness.