Optimized Transport Properties in Lithium Doped Black Phosphorus Transistors

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.