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(C₆H₁₁N₂⁺):TFSI­(C₂F₆NO₄S₂^–) [EMIM:TFSI, 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl) imide]. A wide range of hole carrier densities, from 10¹¹ cm^–2 (nondegenerate) to 10¹³ 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

Plasma-Treated Thickness-Controlled Two-Dimensional Black Phosphorus and Its Electronic Transport Properties

We report the preparation of thickness-controlled few-layer black phosphorus (BP) films through the modulated plasma treatment of BP flakes. Not only does the plasma treatment control the thickness of the BP film, it also removes the chemical degradation of the exposed oxidized BP surface, which results in enhanced field-effect transistor (FET) performance. Our fabricated BP FETs were passivated with poly(methyl methacrylate) (PMMA) immediately after the plasma etching process. With these techniques, a high field-effect mobility was achieved, 1150 cm²/(V s), with an <i>I</i>_on/<i>I</i>_off ratio of ∼10⁵ at room temperature. Furthermore, a fabricated FET with plasma-treated few-layer BP that was passivated with PMMA was found to retain its <i>I</i>–<i>V</i> characteristics and thus to exhibit excellent environmental stability over several weeks