Allen Hsu

Integrated Circuits Based on Bilayer MoS2 Transistors

Two-dimensional (2D) materials, such as molybdenum disulfide (MoS(2)), have been shown to exhibit excellent electrical and optical properties. The semiconducting nature of MoS(2) allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of graphenes advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS(2) show promising performance as potential substitute of Si in conventional electronics and of organic and amorphous Si semiconductors in ubiquitous systems and display applications. An important next step is the fabrication of fully integrated multistage circuits and logic building blocks on MoS(2) to demonstrate its capability for complex digital logic and high-frequency ac applications. This paper demonstrates an inverter, a NAND gate, a static random access memory, and a five-stage ring oscillator based on a direct-coupled transistor logic technology. The circuits comprise between 2 to 12 transistors seamlessly integrated side-by-side on a single sheet of bilayer MoS(2). Both enhancement-mode and depletion-mode transistors were fabricated thanks to the use of gate metals with different work functions.

van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates

We present a method for synthesizing MoS(2)/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports, (1, 2) a much lower growth temperature of 400 °C is required for this procedure. The chemical vapor deposition of MoS(2) on the graphene surface gives rise to single crystalline hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temperature, which was then followed by post annealing. At an elevated temperature, the precursor self-assembles to form MoS(2) flakes epitaxially on the graphene surface via thermal decomposition. With higher amount of precursor delivered onto the graphene surface, a continuous MoS(2) film on graphene can be obtained. This simple chemical vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface functionalization of graphene. The synthesized two-dimensional MoS(2)/Graphene hybrids possess great potential toward the development of new optical and electronic devices as well as a wide variety of newly synthesizable compounds for catalysts.

Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition

In this contribution we demonstrate a method of synthesizing a hexagonal boron nitride (h-BN) thin film by ambient pressure chemical vapor deposition on polycrystalline Ni films. Depending on the growth conditions, the thickness of the obtained h-BN film is between ∼5 and 50 nm. The h-BN grows continuously on the entire Ni surface and the region with uniform thickness can be up to 20 μm in lateral size which is only limited by the size of the Ni single crystal grains. The hexagonal structure was confirmed by both electron and X-ray diffraction. X-ray photoelectron spectroscopy shows the B/N atomic ratio to be 1:1.12. A large optical band gap (5.92 eV) was obtained from the photoabsorption spectra which suggest the potential usage of this h-BN film in optoelectronic devices.

Synthesis of Monolayer Hexagonal Boron Nitride on Cu Foil Using Chemical Vapor Deposition

Hexagonal boron nitride (h-BN) is very attractive for many applications, particularly, as protective coating, dielectric layer/substrate, transparent membrane, or deep ultraviolet emitter. In this work, we carried out a detailed investigation of h-BN synthesis on Cu substrate using chemical vapor deposition (CVD) with two heating zones under low pressure (LP). Previous atmospheric pressure (AP) CVD syntheses were only able to obtain few layer h-BN without a good control on the number of layers. In contrast, under LPCVD growth, monolayer h-BN was synthesized and time-dependent growth was investigated. It was also observed that the morphology of the Cu surface affects the location and density of the h-BN nucleation. Ammonia borane is used as a BN precursor, which is easily accessible and more stable under ambient conditions than borazine. The h-BN films are characterized by atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy analyses. Our results suggest that the growth here occurs via surface-mediated growth, which is similar to graphene growth on Cu under low pressure. These atomically thin layers are particularly attractive for use as atomic membranes or dielectric layers/substrates for graphene devices.

Synthesis and characterization of hexagonal boron nitride film as a dielectric layer for graphene devices.

Hexagonal boron nitride (h-BN) is a promising material as a dielectric layer or substrate for two-dimensional electronic devices. In this work, we report the synthesis of large-area h-BN film using atmospheric pressure chemical vapor deposition on a copper foil, followed by Cu etching and transfer to a target substrate. The growth rate of h-BN film at a constant temperature is strongly affected by the concentration of borazine as a precursor and the ambient gas condition such as the ratio of hydrogen and nitrogen. h-BN films with different thicknesses can be achieved by controlling the growth time or tuning the growth conditions. Transmission electron microscope characterization reveals that these h-BN films are polycrystalline, and the c-axis of the crystallites points to different directions. The stoichiometry ratio of boron and nitrogen is close to 1:1, obtained by electron energy loss spectroscopy. The dielectric constant of h-BN film obtained by parallel capacitance measurements (25 μm(2) large areas) is 2-4. These CVD-grown h-BN films were integrated as a dielectric layer in top-gated CVD graphene devices, and the mobility of the CVD graphene device (in the few thousands cm(2)/(V·s) range) remains the same before and after device integration.

Dielectric screening of excitons and trions in single-layer MoS2.

Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics.

BN/Graphene/BN Transistors for RF Applications

In this letter, we demonstrate the first BN/graphene/BN field-effect transistor for RF applications. This device structure can preserve the high mobility and the high carrier velocity of graphene, even when it is sandwiched between a substrate and a gate dielectric, and is hence very promising to enable the next generation of high-frequency graphene RF electronics.

Applications of graphene devices in RF communications

Graphene, a one-atom-thick layer of carbon atoms arranged in a honeycomb lattice, has recently attracted great interest among physicists and engineers. The combination of the unique properties of graphene with new device concepts and nanotechnology can overcome some of the main limitations of traditional radio frequency electronics in terms of maximum frequency, linearity, and power dissipation. In this article we review the current status of research on graphene-based electronic devices for RF applications. The future challenges facing this rising technology and its feasibility for a new generation of applications in RF communications and circuits are also discussed.

Impact of Graphene Interface Quality on Contact Resistance and RF Device Performance

This letter demonstrates the importance of the graphene/metal interface on the ohmic contacts of high-frequency graphene transistors grown by chemical vapor deposition (CVD) on copper. Using an Al sacrificial layer during ohmic lithography, the graphene surface roughness underneath the ohmic contacts is reduced by fourfold, resulting in an improvement in the contact resistance from 2.0 to 0.2-0.5 kΩ·μm. Using this technology, top-gated CVD graphene transistors achieved direct-current transconductances of 200 mS/mm, maximum on current densities in excess of 1000 mA/mm, and hole mobilities ~ 1500-3000 cm²/(V·s) on silicon substrates. Radio-frequency device performance yielded an extrinsic current-gain cutoff frequency <i>f</i>_T of 12 GHz after pad capacitance de-embedding resulting in an <i>f</i>_T - <i>L</i>_G product of 24 GHz·μm.

Synthesis of large-area multilayer hexagonal boron nitride for high material performance

Although hexagonal boron nitride (h-BN) is a good candidate for gate-insulating materials by minimizing interaction from substrate, further applications to electronic devices with available two-dimensional semiconductors continue to be limited by flake size. While monolayer h-BN has been synthesized on Pt and Cu foil using chemical vapour deposition (CVD), multilayer h-BN is still absent. Here we use Fe foil and synthesize large-area multilayer h-BN film by CVD with a borazine precursor. These films reveal strong cathodoluminescence and high mechanical strength (Youngs modulus: 1.16±0.1 TPa), reminiscent of formation of high-quality h-BN. The CVD-grown graphene on multilayer h-BN film yields a high carrier mobility of ∼24,000 cm2 V−1 s−1 at room temperature, higher than that (∼13,000 2 V−1 s−1) with exfoliated h-BN. By placing additional h-BN on a SiO2/Si substrate for a MoS2 (WSe2) field-effect transistor, the doping effect from gate oxide is minimized and furthermore the mobility is improved by four (150) times.