Multifunctional Homogeneous Lateral Black Phosphorus Junction Devices

We demonstrate a controllable doping technique of few-layer black phosphorus (BP) via surface charge transfer using an ionic liquid mixture of EMIM(C 6 H 11 N 2 + ):TFSI(C 2 F 6 NO 4 S 2 – ) [EMIM:TFSI, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide]. A wide range of hole carrier densities, from 10 11 cm –2 (nondegenerate) to 10 13 cm –2 (degenerate), can be obtained by controlling the weight percentage of the ionic liquid mixture. The doping method we proposed in this paper can be applied to make a multifunctional homogeneous lateral p–n junction device. By doping a fraction of the BP sample and by applying a gate voltage to the other fraction of the BP, we obtain homogeneous lateral p + –p, p + –n, p + –n + junction diodes in a single BP channel. The homogeneous lateral BP p + –p and p + –n junctions display ideal rectifying behavior and a much stronger photoresponse due to the built-in potential. Furthermore, at high positive gate voltages, the interband tunneling enables the homogeneous lateral p + –n + junction transistors to provide both a negative differential resistance (NDR) and a negative transconductance (NTC) in the current–voltage characteristics at room temperature. On the basis of our results, it is possible to build novel devices utilizing the large NDR and NTC in BP such as amplifiers, oscillators, and multivalued logic systems.