Achieving low contact resistance is one of the main challenges for black phosphorus (BP) transistors for both electronic and optoelectronic applications. Here we demonstrate a novel yet feasible lithium doping technique, which greatly reduces the contact resistance from 2 to 0.85 $text{k}Omega cdot mu text{m}$ and results in more than 2.5 times improvement in output current and ON/OFF ratio. This can be mainly attributed to the high hole doping density by lithium bis(trifluoromethylsulfonyl)-imide, which results in a narrower carrier injection barrier at the source end. The ON/OFF ratios of BP field-effect transistors with contact doping at 300 and 20 K are 432 and $7.2 times 10^{5}$ , respectively. A high drain current of $773~mu text{A}/mu text{m}$ with a $0.56~mu text{m}$ channel length at 20 K is also demonstrated. The doping technique provides a valid way to improve the overall performance of BP transistors.
Published in: "IEEE Electron Device Letters".