Hsun Jen Chuang

Improved carrier mobility in few-layer MoS2 field-effect transistors with ionic-liquid gating

We report the fabrication of ionic liquid (IL)-gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS2. Our transport measurements indicate that the electron mobility μ ≈ 60 cm(2) V(-1) s(-1) at 250 K in IL-gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from ≈ 100 cm(2) V(-1) s(-1) at 180 K to ≈ 220 cm(2) V(-1) s(-1) at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio >10(7) (10(4)) for electrons (holes), and a near ideal subthreshold swing of ≈ 50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin IL dielectric layer.

Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors.

We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ∼0.3 kΩ μm, high on/off ratios up to >10(9), and high drive currents exceeding 320 μA μm(-1). These favorable characteristics are combined with a two-terminal field-effect hole mobility μFE ≈ 2 × 10(2) cm(2) V(-1) s(-1) at room temperature, which increases to >2 × 10(3) cm(2) V(-1) s(-1) at cryogenic temperatures. We observe a similar performance also in MoS2 and MoSe2 FETs with 2D/2D drain and source contacts. The 2D/2D low-resistance ohmic contacts presented here represent a new device paradigm that overcomes a significant bottleneck in the performance of TMDs and a wide variety of other 2D materials as the channel materials in postsilicon electronics.

Mobility Improvement and Temperature Dependence in MoSe2 Field-Effect Transistors on Parylene-C Substrate

We report low-temperature scanning tunneling microscopy characterization of MoSe2 crystals and the fabrication and electrical characterization of MoSe2 field-effect transistors on both SiO2 and parylene-C substrates. We find that the multilayer MoSe2 devices on parylene-C show a room-temperature mobility close to the mobility of bulk MoSe2 (100-160 cm(2) V(-1) s(-1)), which is significantly higher than that on SiO2 substrates (≈50 cm(2) V(-1) s(-1)). The room-temperature mobility on both types of substrates are nearly thickness-independent. Our variable-temperature transport measurements reveal a metal-insulator transition at a characteristic conductivity of e(2)/h. The mobility of MoSe2 devices extracted from the metallic region on both SiO2 and parylene-C increases up to ≈500 cm(2) V(-1) s(-1) as the temperature decreases to ≈100 K, with the mobility of MoSe2 on SiO2 increasing more rapidly. In spite of the notable variation of charged impurities as indicated by the strongly sample-dependent low-temperature mobility, the mobility of all MoSe2 devices on SiO2 converges above 200 K, indicating that the high temperature (>200 K) mobility in these devices is nearly independent of the charged impurities. Our atomic force microscopy study of SiO2 and parylene-C substrates further rules out the surface roughness scattering as a major cause of the substrate-dependent mobility. We attribute the observed substrate dependence of MoSe2 mobility primarily to the surface polar optical phonon scattering originating from the SiO2 substrate, which is nearly absent in MoSe2 devices on parylene-C substrate.

Anisotropic photocurrent response at black phosphorus–MoS2 p–n heterojunctions

We investigate the photocurrent generation mechanisms at a vertical p-n heterojunction between black phosphorus (BP) and molybdenum disulfide (MoS2) flakes through polarization-, wavelength-, and gate-dependent scanning photocurrent measurements. When incident photon energy is above the direct band gap of MoS2, the photocurrent response demonstrates a competitive effect between MoS2 and BP in the junction region. In contrast, if the incident photon energy is below the band gap of MoS2 but above the band gap of BP, the photocurrent response at the p-n junction exhibits the same polarization dependence as that at the BP-metal junction, which is nearly parallel to the MoS2 channel. This result indicates that the photocurrent signals at the MoS2-BP junction primarily result from the direct band gap transition in BP. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in vertical 2D semiconductor heterojunctions, offering a new way of engineering future two-dimensional materials based optoelectronic devices.

Optimizing Charge Injection across Transition Metal Dichalcogenide Heterojunctions: Theory and Experiment

In search of an improved strategy to form low-resistance contacts to semiconducting transition metal dichalcogenides, we combine ab initio density functional electronic structure calculations for an NbSe2/WSe2 interface with quantum transport measurements of the corresponding heterojunction between a few-layer WSe2 semiconductor and a metallic NbSe2 layer. Our theoretical results suggest that, besides a rigid band shift associated with charge transfer, the presence of NbSe2 does not modify the electronic structure of WSe2. Since the two transition metal dichalcogenides are structurally similar and display only a small lattice mismatch, their heterojunction can efficiently transfer charge across the interface. These findings are supported by transport measurements for WSe2 field-effect transistors with NbSe2 contacts, which exhibit nearly ohmic behavior and phonon-limited mobility in the hole channel, indicating that the contacts to WSe2 are highly transparent.

Control and enhancement of graphene sensitivity by engineering edge defects

This paper presents a novel design to control defect sites and to enhance sensitivity in graphene-based pH and biological sensors. We showed that pristine graphene was not sensitive to the pH. We demonstrated that the pH sensitivity of graphene could be controlled by varying the patterns and lengths of edges that were created using simple processes of photolithography and dry etching.