(100h) MoS2-Passivated Bilayer Phosphorene Phototransistors

Authors: 
Son, Y., Massachusetts Institute of Technology
Liu, A. T., Massachusetts Institute of Technology
Koman, V., Massachusetts Institute of Technology
Wang, Q. H., Arizona State University
Strano, M. S., Massachusetts Institute of Technology
Despite its unique and promising properties such as high carrier mobility and conspicuous in-plane anisotropy that generated a tremendous amount of excitement for the past few years, BP has been suffering from its one significant drawback that may dim its potential as an active material in the next-generation nanoelectronics: severe crystal deterioration upon exposure to oxygen and water. Field-effect transistors (FETs) based on a MoS2-passivated black phosphorus (BP) van der Waals (vdW) heterostructure could provide an effective solution to tackling this important challenge, while enhancing photoresponse of the device and retaining the conductivity of a BP channel almost intact. In this work, we fabricate 4 MoS2-passivated BP FETs with varying BP thicknesses, 1.5 (bilayer), 5, 13, and 20 nm, to explore the effect of the MoS2 passivation layer on the device stability and electrical characteristics both in the dark and under illumination (λ = 600 nm). When in contact with a trilayer MoS2, photoluminescence intensity of the bilayer BP crystal decreases by 25%, suggesting that a built-in electric field forms at the BP-MoS2 p-n interface that could help dissociate photo-generated electron-hole pairs, thereby reducing the probability of the recombination events. The effectiveness of a few-layer MoS2 as a vdW protection layer is tested by exposing BP-MoS2 vdW vertical heterostructures to the air for up to 3 weeks as well as by annealing at a high temperature (350°C) in an inert argon environment. Then, the heterostructure FETs are fabricated so that direct comparisons between exposed and MoS2-passivated BP regions on the same flake can be made. In general, the MoS2 passivation layer gives rise to only an insignificant decrease in the transport characteristics in the dark, with the trend where its impact decreases as the thickness of the underlying BP crystal increases. Upon illumination, the MoS2-passivated regions of the bilayer BP device shows enhanced photoresponse compared with the exposed regions by 75 % with no applied gate voltage. Also, a possible photogating effect is observed in the bilayer BP device while photoconductivity dominates the photoresponse process in the thicker devices. Thus, 2D MoS2 2D thin films may aid in the realization of the future transparent, flexible BP electronic and optoelectronic devices by acting not only as an atomically thin passivation layer but as a photoresponse enhancer.