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Register a new userHigh Performance Single Layered WSe2 p-FETs with Chemically Doped Contacts
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We report high performance p-type field-effect transistors based on single layered (thickness, ~0.7 nm) WSe2 as the active channel with chemically doped source/drain contacts and high-{kappa} gate dielectrics. The top-gated monolayer transistors exhibit a high effective hole mobility of ~250 cm2/Vs, perfect subthreshold swing of ~60 mV/dec, and ION/IOFF of >10^6 at room temperature. Special attention is given to lowering the contact resistance for hole injection by using high work function Pd contacts along with degenerate surface doping of the contacts by patterned NO2 chemisorption on WSe2. The results here present a promising material system and device architecture for p-type monolayer transistors with excellent characteristics.
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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 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-1 s-1 at room temperature, which increases to >2x103 cm2 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 post-silicon electronics.
Recently, two-dimensional materials and in particular transition metal dichalcogenides (TMDs) were extensively studied because of their strong light-matter interaction and the remarkable optoelectronic response of their field-effect transistors (FETs). Here, we report a photoconductivity study from FETs built from few-layers of p-WSe2 measured in a multi-terminal configuration under illumination by a 532 nm laser source. The photogenerated current was measured as a function of the incident optical power, of the drain-to-source bias and of the gate voltage. We observe a considerably larger photoconductivity when the phototransistors were measured via a four-terminal configuration when compared to a two-terminal one. For an incident laser power of 248 nW, we extract 18 A/W and ~4000% for the two-terminal responsivity (R) and the concomitant external quantum efficiency (EQE) respectively, when a bias voltage Vds = 1 V and a gate voltage Vbg = 10 V are applied to the sample. R and EQE are observed to increase by 370% to ~85 A/W and ~20000% respectively, when using a four-terminal configuration. Thus, we conclude that previous reports have severely underestimated the optoelectronic response of transition metal dichalcogenides, which in fact reveals a remarkable potential for photosensing applications.
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
This letter reports the impact of surface morphology on the carrier transport and RF performance of graphene FETs formed on epitaxial graphene films synthesized on SiC substrates. Such graphene exhibits long terrace structures with widths between 3-5 {mu}m and steps of 10pm2 nm in height. While a carrier mobility above 3000 cm2/Vs at a carrier density of 1e12 cm-2 is obtained in a single graphene terrace domain at room temperature, the step edges can result in a vicinal step resistance of ~21 k{Omega}.{mu}m. By orienting the transistor layout so that the entire channel lies within a single graphene terrace, and reducing the access resistance associated with the ungated part of the channel, a cut-off frequency above 200 GHz is achieved for graphene FETs with channel lengths of 210 nm, which is the highest value reported on epitaxial graphene thus far.
We report on p-WSe2/n-MoS2 heterojunction diodes fabricated both on glass and SiO2/p+-Si substrates. The electrostatic performance and stability of our diode were successfully improved toward ideal current-voltage (I-V) behavior by adopting the fluoropolymer CYTOP encapsulation layer on top of our diode; reduction of reverse-bias leakage current and enhancement of forward-bias on current were achieved along with good aging stability in air ambient. Such performance improvement is attributed to the intrinsic properties of CYTOP materials with C-F bonds whose strong dipole moment causes hole accumulation, while the strong hydrophobicity of CYTOP would prevent ambient molecule adsorption on 2D semiconductor surface. Moreover, fabricated on glass, our p-n diode displayed good dynamic rectification at over 100 Hz, without displacement current-induced signal overshoot/undershoot which was shown in the other diode on SiO2/p+-Si. Little I-V hysteresis in our diode is another benefit of glass substrate. We conclude that our CYTOP-encapsulated WSe2/MoS2 p-n diode on glass is a high performance and ambient stable 2D nanodevice toward future advanced electronics.
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