Flexible synthesized MoS2 transistors are advanced to perform at GHz speeds. An intrinsic cutoff frequency of 5.6 GHz is achieved and analog circuits are realized. Devices are mechanically robust for 10,000 bending cycles.

Adsorption of organic molecules passivates defect states on single-layer MoS2 via charge transfer.

We report on the gigahertz radio frequency (RF) performance of chemical vapor deposited (CVD) monolayer MoS2 field-effect transistors (FETs). Initial DC characterizations of fabricated MoS2 FETs yielded current densities exceeding 200 μA/μm and maximum transconductance of 38 μS/μm. A contact resistance corrected low-field mobility of 55 cm(2)/(V s) was achieved. Radio frequency FETs were fabricated in the ground-signal-ground (GSG) layout, and standard de-embedding techniques were applied. Operating at the peak transconductance, we obtain short-circuit current-gain intrinsic cutoff frequency, fT, of 6.7 GHz and maximum intrinsic oscillation frequency, fmax, of 5.3 GHz for a device with a gate length of 250 nm. The MoS2 device afforded an extrinsic voltage gain Av of 6 dB at 100 MHz with voltage amplification until 3 GHz. With the as-measured frequency performance of CVD MoS2, we provide the first demonstration of a common-source (CS) amplifier with voltage gain of 14 dB and an active frequency mixer with conversion gain of -15 dB. Our results of gigahertz frequency performance as well as analog circuit operation show that large area CVD MoS2 may be suitable for industrial-scale electronic applications.

We report the electrical characteristics of chemical vapor deposited (CVD) monolayer molybdenum disulfide (MoS2) top-gated field-effect transistors (FETs) on silicon nitride (Si3N4) substrates. We show that Si3N4 substrates offer comparable electrical performance to thermally grown SiO2 substrates for MoS2 FETs, offering an attractive passivating substrate for transition-metal dichalcogenides (TMD) with a smooth surface morphology. Single-crystal MoS2 grains are grown via vapor transport process using solid precursors directly on low pressure CVD Si3N4, eliminating the need for transfer processes which degrade electrical performance. Monolayer top-gated MoS2 FETs with Al2O3 gate dielectric on Si3N4 achieve a room temperature mobility of 24 cm2/V s with Ion/Ioff current ratios exceeding 107. Using HfO2 as a gate dielectric, monolayer top-gated CVD MoS2 FETs on Si3N4 achieve current densities of 55 μA/μm and a transconductance of 6.12 μS/μm at Vtg of −5 V and Vds of 2 V. We observe an increase in mobility at lower temperatures, indicating phonon scattering may dominate over charged impurity scattering in our devices. Our results show that Si3N4 is an attractive alternative to thermally grown SiO2 substrate for TMD FETs.

Recent studies have increased the cut off frequencies achievable by exfoliated MoS 2 by employing a combination of channel length scaling and geometry modification. However, for industrial scale applications, the mechanical cleavage process is not scalable but, thus far, the same device improvements have not been realized on chemical vapor deposited MoS 2 . Here we use a gate-first process flow with an embedded gate geometry to fabricate short channel chemical vapor deposited MoS 2 radio frequency transistors with a notable f T of 20 GHz and f max of 11.4 GHz, and the largest high-field saturation velocity, v sat = 1.88 × 10 6 cm/s, in MoS 2 reported so far. The gate-first approach, facilitated by cm-scale chemical vapor deposited MoS 2 , offers enhancement mode operation, I ON /I OFF ratio of 10 8 , and a transconductance (g m ) of 70 μS/μm. The intrinsic f T (f max ) obtained here is 3X (2X) greater than previously reported top-gated chemical vapor deposited MoS 2 radio frequency field-effect transistors. With as-measured S-parameters, we demonstrate the design of a GHz MoS 2 -based radio frequency amplifier. This amplifier has gain greater then 15 dB at 1.2 GHz, input return loss > 10 dB, bandwidth > 200 MHz, and DC power consumption of~10 mW.

We present an ultra-high vacuum scanning tunneling microscopy (STM) study of structural defects in molybdenum disulfide thin films grown on silicon substrates by chemical vapor deposition. A distinctive type of grain boundary periodically arranged inside an isolated triangular domain, along with other inter-domain grain boundaries of various types, is observed. These periodic defects, about 50 nm apart and a few nanometers in width, remain hidden in optical or low-resolution microscopy studies. We report a complex growth mechanism that produces 2D nucleation and spiral growth features that can explain the topography in our films.The many incredible properties of graphene including high carrier mobility (200,000 cm 2 V −1 s −1 ) 1 have made it a very special material both from fundamental science and an engineering point of view. However, the lack of a band-gap in graphene causes high leakage current which makes it unsuitable for many optoelectronic purposes and logic-based devices and circuits. In contrast, transition metal dichalcogenides (TMDs) with the general chemical formula MX2 (M = Mo, W; X = S, Se, Te) provide a large family of two-dimensional (2D) crystals that vary greatly in physical and chemical properties 2 , ranging from metallic to semiconducting to insulators. Of all the TMDs, molybdenum sulfide (MoS2), with its indirect-to-direct band gap transition as a function of layer thickness, has been of particular interest for digital and optoelectronic applications. MoS2 has already been used to fabricate functional electronic circuit elements 3-6 , as well as used for optoelectronics 7-9 , valleytronics, spintronics 10, 11 and coupled electro-mechanics 12 .Most of the MoS2 material characterization and device demonstrations so far have been on exfoliated samples which suffer from low yield, and cannot be scaled up for practical applications. In order to address these problems, significant work has been done to introduce different growth techniques. Processes including liquid exfoliation 13 and direct sulfurization of molybdenum thin films 14 have been achieved to synthesize large MoS2 monolayers. However, the overall simplicity and the high quality of films obtained using the sulfurization of MoO3 has made it one of the most widely used methods of synthesizing large area monolayer MoS2 15-17 .Just like different synthesis techniques, various analytical techniques have been introduced. In addition to the commonly used techniques like scanning electron microscopy (SEM) and atomic force microscopy (AFM), techniques like Raman and photoluminescence (PL) spectroscopy have become common to ascertain the number of layers of these 2D materials. However, because of the resolution limit, these techniques only reveal a partial picture. SEM and AFM together show us the topographical and structural information. The spectroscopic techniques ascertain the energy levels to a certain degree. Recent techniques like microwave impedance microscopy (MIM) 18 have been used to map the dielectric constant of these films. Howev...

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Many factors have been identified to influence the electrical transport characteristics of graphene field-effect transistors. In this report, we examine the influence of the exposure current level used during electron beam lithography (EBL) for active region patterning. In the presence of a self-assembled hydrophobic residual layer generated by oxygen plasma etching covering the top surface of the graphene channel, we show that the use of low EBL current level results in higher mobility, lower residual carrier density, and charge neutrality point closer to 0 V, with reduced device-to-device variations. We show that this correlation originates from the resist heating dependent release of radicals from the resist material, near its interface with graphene, and its subsequent trapping by the hydrophobic polymer layer. Using a general model for resist heating, we calculate the difference in resist heating for different EBL current levels. We further corroborate our argument through control experiments, where radicals are either intentionally added or removed by other processes. We also utilize this finding to obtain mobilities in excess of 18 000 cm2/V s on silicon dioxide substrates. We believe these results are applicable to other 2D materials such as transition metal dichalcogenides and nanoscale devices in general.