High Mobility WSe2 p- and n-Type Field Effect Transistors Contacted by Highly Doped Graphene for Low-Resistance Contacts

We report the fabrication of both n-type and p-type WSe2 field effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e2/h, a high ON/OFF ratio of >107 at 170 K, and large electron and hole mobility of ~200 cm2V-1s-1 at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ~330 cm2V-1s-1 and that of holes to ~270 cm2V-1s-1. We attribute our ability to observe the intrinsic, phonon limited conduction in both the electron and hole channels to the drastic reduction of the Schottky barriers between the channel and the graphene contact electrodes using IL gating. We elucidate this process by studying a Schottky diode consisting of a single graphene/WSe2 Schottky junction. Our results indicate the possibility to utilize chemically or electrostatically highly doped graphene for versatile, flexible and transparent low-resistance Ohmic contacts to a wide range of quasi-2D semiconductors. KEYWORDS: MoS2, WSe2, field-effect transistors, graphene, Schottky barrier, ionic-liquid gate

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 cm2V-1s-1 - 160 cm2V-1s-1), which is significantly higher than that on SiO2 substrate (~50 cm2V-1s-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 e2/h. The mobility of MoSe2 devices extracted from the metallic region on both SiO2 and parylene-C increases up to ~ 500 cm2V-1s-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 rule 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.

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 about 60 cm2V-1s-1 at 250 K in ionic liquid gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from about 100 cm2V-1s-1 at 180 K to about 220 cm2V-1s-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 >107 (104) for electrons (holes), and a near ideal sub-threshold swing of about 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 ionic-liquid dielectric layer.

Mobility enhancement and highly efficient gating of monolayer MoS2 transistors with Polymer Electrolyte

We report electrical characterization of monolayer molybdenum disulfide (MoS2) devices using a thin layer of polymer electrolyte consisting of poly(ethylene oxide) (PEO) and lithium perchlorate (LiClO4) as both a contact-barrier reducer and channel mobility booster. We find that bare MoS2 devices (without polymer electrolyte) fabricated on Si/SiO2 have low channel mobility and large contact resistance, both of which severely limit the field-effect mobility of the devices. A thin layer of PEO/ LiClO4 deposited on top of the devices not only substantially reduces the contact resistance but also boost the channel mobility, leading up to three-orders-of-magnitude enhancement of the field-effect mobility of the device. When the polymer electrolyte is used as a gate medium, the MoS2 field-effect transistors exhibit excellent device characteristics such as a near ideal subthreshold swing and an on/off ratio of 106 as a result of the strong gate-channel coupling.

Electrical Transport Properties of Graphene Nanoribbons Produced from Sonicating Graphite in Solution

A simple one-stage solution-based method was developed to produce graphene nanoribbons by sonicating graphite powder in organic solutions with polymer surfactant. The graphene nanoribbons were deposited on silicon substrate, and characterized by Raman spectroscopy and atomic force microscopy. Single-layer and few-layer graphene nanoribbons with a width ranging from sub-10 nm to tens of nm and length ranging from hundreds of nm to 1 {mu}m were routinely observed. Electrical transport properties of individual graphene nanoribbons were measured in both the back-gate and polymer-electrolyte top-gate configurations. The mobility of the graphene nanoribbons was found to be over an order of magnitude higher when measured in the latter than in the former configuration (without the polymer electrolyte), which can be attributed to the screening of the charged impurities by the counter-ions in the polymer electrolyte. This finding suggests that the charge transport in these solution-produced graphene nanoribbons is largely limited by charged impurity scattering.

Approaching the Intrinsic Bandgap in Suspended High-Mobility Graphene Nanoribbons

We report electrical transport measurements on a suspended ultra-low-disorder graphene nanoribbon(GNR) with nearly atomically smooth edges that reveal a high mobility exceeding 3000 cm2 V-1 s-1 and an intrinsic band gap. The experimentally derived bandgap is in quantitative agreement with the results of our electronic-structure calculations on chiral GNRs with comparable width taking into account the electron-electron interactions, indicating that the origin of the bandgap in non-armchair GNRs is partially due to the magnetic zigzag edges.