ABSTRACT:We report the fabrication of both n-type and p-type WSe 2 fieldeffect 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 e 2 /h, a high ON/OFF ratio of >10 7 at 170 K, and large electron and hole mobility of μ ≈ 200 cm 2 V −1 s −1 at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ∼330 cm 2 V −1 s −1 and that of holes to ∼270 cm 2 V −1 s −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/WSe 2 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: MoS 2 , WSe 2 , field-effect transistor, graphene, Schottky barrier, ionic-liquid gate L ayered transition metal dichalcogenides (TMDs) have recently emerged as promising materials for flexible electronics and optoelectronics applications. These systems have demonstrated many "graphene-like" properties including a relatively high carrier mobility, mechanical flexibility, chemical and thermal stability, and moreover offer the significant advantage of a substantial band gap. 1 Field-effect transistors (FETs) with atomically thin TMD channels are immune to short channel effects. 2 In addition, pristine surfaces of twodimensional (2D) TMDs are free of dangling bonds, which reduce surface roughness scattering and interface traps. Atomic layers of MoS 2 are probably the most extensively studied among the layered TMDs due to the availability of large natural molybdenite crystals from mining sources. 3 In addition to MoS 2 , several other semiconducting TMDs such as MoSe 2 , WS 2 , and WSe 2 with different band structures and charge neutrality levels may offer additional distinct properties. 1,4 However, the number of studies on TMDs other than MoS 2 is still small. 5−14 Among these studies, back-gated WSe 2 monolayer FETs with surface doping have already demonstrated a high field-effect mobility 8 reaching ∼140 cm 2 V −1 s −1 , which is substantially higher than most of the reported roomtemperature mobility values for MoS 2 . 15−19 A high intrinsic hole mobility of up to 500 cm 2 V −1 s −1 was also observed in bulk WSe 2 FETs. 11 Furthermore, WSe 2 is more resistant to oxidation in humid environments than MoS 2 . 10,20 A major challenge for developing WSe 2 -based electronic devices is that WSe 2 tends to form a substantial Schottky barrier (SB) with most metals commonly used for making electrical contacts. 8,13 This is a strong disadvantage, because lowre...

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.

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 WSe 2 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 μ FE ≈ 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 MoS 2 and MoSe 2 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 post-silicon electronics.

We investigate electrical transport and optoelectronic properties of field effect transistors (FETs) made from few-layer black phosphorus (BP) crystals down to a few nanometers. In particular, we explore the anisotropic nature and photocurrent generation mechanisms in BP FETs through spatial-, polarization-, gate-, and bias-dependent photocurrent measurements. Our results reveal that the photocurrent signals at BP-electrode junctions are mainly attributed to the photovoltaic effect in the off-state and photothermoelectric effect in the on-state, and their anisotropic feature primarily results from the directional-dependent absorption of BP crystals.