Localized Heating and Switching in MoTe2-Based Resistive Memory Devices.

Two-dimensional (2D) materials have recently been incorporated into resistive memory devices because of their atomically thin nature, but their switching mechanism is not yet well understood. Here we study bipolar switching in MoTe2-based resistive memory of varying thickness and electrode area. Using scanning thermal microscopy (SThM), we map the surface temperature of the devices under bias, revealing clear evidence of localized heating at conductive "plugs" formed during switching. The SThM measurements are correlated to electro-thermal simulations, yielding a range of plug diameters (250 to 350 nm) and temperatures at constant bias and during switching. Transmission electron microscopy images reveal these plugs result from atomic migration between electrodes, which is a thermally-activated process. However, the initial forming may be caused by defect generation or Te migration within the MoTe2. This study provides the first thermal and localized switching insights into the operation of such resistive memory and demonstrates a thermal microscopy technique that can be applied to a wide variety of traditional and emerging memory devices.

Contact Engineering High-Performance n-Type MoTe2 Transistors.

Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 µA/µm at 80 K and >200 µA/µm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·µm from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

Unipolar n-Type Black Phosphorus Transistors with Low Work Function Contacts.

Black phosphorus (BP) is a promising two-dimensional (2D) material for nanoscale transistors, due to its expected higher mobility than other 2D semiconductors. While most studies have reported ambipolar BP with a stronger p-type transport, it is important to fabricate both unipolar p- and n-type transistors for low-power digital circuits. Here, we report unipolar n-type BP transistors with low work function Sc and Er contacts, demonstrating a record high n-type current of 200 µA/µm in 6.5 nm thick BP. Intriguingly, the electrical transport of the as-fabricated, capped devices changes from ambipolar to n-type unipolar behavior after a month at room temperature. Transmission electron microscopy analysis of the contact cross-section reveals an intermixing layer consisting of partly oxidized metal at the interface. This intermixing layer results in a low n-type Schottky barrier between Sc and BP, leading to the unipolar behavior of the BP transistor. This unipolar transport with a suppressed p-type current is favorable for digital logic circuits to ensure a lower off-power consumption.

Probing the Optical Properties and Strain-Tuning of Ultrathin Mo1- xW xTe2.

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo0.91W0.09Te2, a semiconducting alloy of MoTe2, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo0.91W0.09Te2 transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe2 counterpart. We apply tensile strain, for the first time, to monolayer MoTe2 and Mo0.91W0.09Te2 to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton-phonon intervalley scattering. Strained MoTe2 and Mo0.91W0.09Te2 extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.

HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides.

The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV), permitting low-voltage operation at reduced leakage current, and the existence of SiO2 as a high-quality "native" insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two-dimensional (2D) semiconductors HfSe2 and ZrSe2 have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable "high-κ" native dielectrics HfO2 and ZrO2, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air-stable transistors down to three-layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio > 106; on current, ~30 µA/µm), with native oxides reducing the effects of interfacial traps. These are the first 2D materials to demonstrate technologically relevant properties of silicon, in addition to unique compatibility with high-κ dielectrics, and scaling benefits from their atomically thin nature.

High Current Density and Low Thermal Conductivity of Atomically Thin Semimetallic WTe2.

Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomically thin limit. Here, we introduce a facile growth mechanism for semimetallic WTe2 crystals and then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 µm long devices allow us to separate intrinsic and contact resistance, revealing metallic response in the thinnest encapsulated and stable WTe2 devices studied to date (3-20 layers thick). High-field electrical measurements and electrothermal modeling demonstrate that ultrathin WTe2 can carry remarkably high current density (approaching 50 MA/cm(2), higher than most common interconnect metals) despite a very low thermal conductivity (of the order ∼3 Wm(-1) K(-1)). These results suggest several pathways for air-stable technological viability of this layered semimetal.

Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy.

Molybdenum disulfide (MoS2), with its active edge sites, is a proposed alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Recently, the inert basal plane of MoS2 was successfully activated and optimized with excellent intrinsic HER activity by creating and further straining sulfur (S) vacancies. Nevertheless, little is known about the HER kinetics of those S vacancies and the additional effects from elastic tensile strain. Herein, scanning electrochemical microscopy was used to determine the HER kinetic data for both unstrained S vacancies (formal potential Ev0 = −0.53 VAg/AgCl, electron-transfer coefficient αv = 0.4, electron-transfer rate constant kv0 = 2.3 × 10(­4) cm/s) and strained S vacancies (Esv0= −0.53 VAg/AgCl, αsv = 0.4, ksv0 = 1.0 × 10(­3) cm/s) on the basal plane of MoS2 monolayers, and the strained S vacancy has an electron-transfer rate 4 times higher than that of the unstrained S vacancy. This study provides a general platform for measuring the kinetics of two-dimensional material-based catalysts.