Electric-double-layer p-i-n junctions in WSe2.

While p-n homojunctions in two-dimensional transition metal dichalcogenide materials have been widely reported, few show an ideality factor that is constant over more than a decade in current. In this paper, electric double layer p-i-n junctions in WSe2 are shown with substantially constant ideality factors (2-3) over more than 3 orders of magnitude in current. These lateral junctions use the solid polymer, polyethylene oxide: cesium perchlorate (PEO:CsClO4), to induce degenerate electron and hole carrier densities at the device contacts to form the junction. These high carrier densities aid in reducing the contact resistance and enable the exponential current dependence on voltage to be measured at higher currents than prior reports. Transport measurements of these WSe2 p-i-n homojunctions in combination with COMSOL multiphysics simulations are used to quantify the ion distributions, the semiconductor charge distributions, and the simulated band diagram of these junctions, to allow applications to be more clearly considered.

Enhanced carrier transport by transition metal doping in WS2 field effect transistors.

High contact resistance is one of the primary concerns for electronic device applications of two-dimensional (2D) layered semiconductors. Here, we explore the enhanced carrier transport through metal-semiconductor interfaces in WS2 field effect transistors (FETs) by introducing a typical transition metal, Cu, with two different doping strategies: (i) a "generalized" Cu doping by using randomly distributed Cu atoms along the channel and (ii) a "localized" Cu doping by adapting an ultrathin Cu layer at the metal-semiconductor interface. Compared to the pristine WS2 FETs, both the generalized Cu atomic dopant and localized Cu contact decoration can provide a Schottky-to-Ohmic contact transition owing to the reduced contact resistances by 1-3 orders of magnitude, and consequently elevate electron mobilities by 5-7 times. Our work demonstrates that the introduction of transition metal can be an efficient and reliable technique to enhance the carrier transport and device performance in 2D TMD FETs.

Atomic Layer Deposition of Al2O3 on WSe2 Functionalized by Titanyl Phthalocyanine.

To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistors (FETs) formed using this process have a leakage current of 0.046 pA/µm(2) at 1 V gate bias with 3.0 nm equivalent oxide thickness, which is a lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade.

Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment.

van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.

Reconfigurable Ion Gating of 2H-MoTe2 Field-Effect Transistors Using Poly(ethylene oxide)-CsClO4 Solid Polymer Electrolyte.

Transition metal dichalcogenides are relevant for electronic devices owing to their sizable band gaps and absence of dangling bonds on their surfaces. For device development, a controllable method for doping these materials is essential. In this paper, we demonstrate an electrostatic gating method using a solid polymer electrolyte, poly(ethylene oxide) and CsClO4, on exfoliated, multilayer 2H-MoTe2. The electrolyte enables the device to be efficiently reconfigured between n- and p-channel operation with ON/OFF ratios of approximately 5 decades. Sheet carrier densities as high as 1.6 × 10(13) cm(-2) can be achieved because of a large electric double layer capacitance (measured as 4 µF/cm(2)). Further, we show that an in-plane electric field can be used to establish a cation/anion transition region between source and drain, forming a p-n junction in the 2H-MoTe2 channel. This junction is locked in place by decreasing the temperature of the device below the glass transition temperature of the electrolyte. The ideality factor of the p-n junction is 2.3, suggesting that the junction is recombination dominated.