Diverse diode characteristics were observed in two-dimensional (2D) black phosphorus (BP) and molybdenum disulfide (MoS) heterojunctions. The characteristics of a backward rectifying diode, a Zener diode, and a forward rectifying diode were obtained from the heterojunction through thickness modulation of the BP flake or back gate modulation. Moreover, a tunnel diode with a precursor to negative differential resistance can be realized by applying dual gating with a solid polymer electrolyte layer as a top gate dielectric material. Interestingly, a steep subthreshold swing of 55 mV/dec was achieved in a top-gated 2D BP-MoS junction. Our simple device architecture and chemical doping-free processing guaranteed the device quality. This work helps us understand the fundamentals of tunneling in 2D semiconductor heterostructures and shows great potential in future applications in integrated low-power circuits.
Formation
of an electric double layer (EDL) is a powerful approach
for exploring the electronic properties of two-dimensional (2D) materials
because of the ultrahigh capacitance and induced charge in the 2D
materials. In this work, epitaxial graphene Hall bar devices are gated
with an EDL using a 1 μm thick solid polymer electrolyte, poly(ethylene
oxide) and LiClO4. In addition to carrier density and mobility,
ion dynamics associated with the formation and dissipation of the
EDL are measured as a function of temperature over a gate bias range
of ±2 V. The room temperature EDL formation time (∼1–100
s) is longer than the dissipation time (∼10 ms). The EDL dissipation
is modeled by a stretched exponential decay, and the temperature-dependent
dissipation times are described by the Vogel–Fulcher–Tammann
equation, reflecting the coupling between polymer and ion mobility.
At low temperatures, approaching the glass transition temperature
of the electrolyte, the dissipation times of both cations and anions
exceed several hours, and both p- and n-type EDLs can persist in the
absence of a gate bias. The measured temperature-dependent relaxation
times qualitatively agree with COMSOL multiphysics simulations of
time-dependent ion transport in the presence of an applied field.
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