Atresh Sanne

Radio Frequency Transistors and Circuits Based on CVD MoS2

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.

Large-Area Monolayer MoS2 for Flexible Low-Power RF Nanoelectronics in the GHz Regime.

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.

Top-gated chemical vapor deposited MoS2 field-effect transistors on Si3N4 substrates

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 a...

Defect passivation of transition metal dichalcogenides via a charge transfer van der Waals interface

Adsorption of organic molecules passivates defect states on single-layer MoS2 via charge transfer. Integration of transition metal dichalcogenides (TMDs) into next-generation semiconductor platforms has been limited due to a lack of effective passivation techniques for defects in TMDs. The formation of an organic-inorganic van der Waals interface between a monolayer (ML) of titanyl phthalocyanine (TiOPc) and a ML of MoS2 is investigated as a defect passivation method. A strong negative charge transfer from MoS2 to TiOPc molecules is observed in scanning tunneling microscopy. As a result of the formation of a van der Waals interface, the ION/IOFF in back-gated MoS2 transistors increases by more than two orders of magnitude, whereas the degradation in the photoluminescence signal is suppressed. Density functional theory modeling reveals a van der Waals interaction that allows sufficient charge transfer to remove defect states in MoS2. The present organic-TMD interface is a model system to control the surface/interface states in TMDs by using charge transfer to a van der Waals bonded complex.

Intra-domain periodic defects in monolayer MoS2

We present an ultra-high vacuum scanning tunneling microscopy 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.

Influence of electron-beam lithography exposure current level on the transport characteristics of graphene field effect transistors

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 ...

Embedded gate CVD MoS 2 microwave FETs

Recent studies have increased the cut off frequencies achievable by exfoliated MoS2 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 MoS2. Here we use a gate-first process flow with an embedded gate geometry to fabricate short channel chemical vapor deposited MoS2 radio frequency transistors with a notable fT of 20 GHz and fmax of 11.4 GHz, and the largest high-field saturation velocity, vsat = 1.88 × 106 cm/s, in MoS2 reported so far. The gate-first approach, facilitated by cm-scale chemical vapor deposited MoS2, offers enhancement mode operation, ION/IOFF ratio of 108, and a transconductance (gm) of 70 μS/μm. The intrinsic fT (fmax) obtained here is 3X (2X) greater than previously reported top-gated chemical vapor deposited MoS2 radio frequency field-effect transistors. With as-measured S-parameters, we demonstrate the design of a GHz MoS2-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.High-frequency electronics: embedded gates boost MoS 2 radio frequency transistors2D materials enable radio frequency transistors, yet the absence of a bandgap in graphene limits its maximum oscillation frequency. A team lead by Sanjay Kumar Banerjee at the University of Texas at Austin fabricated radio frequency field-effect transistors using monolayer MoS2 grown by chemical vapor deposition. The devices feature an embedded gate structure which ensures optimal gate control over the conducting channel and improves the channel-dielectric interface, whilst requiring a reduced number of fabrication steps. As a result, the device exhibits a maximum oscillation frequency as high as 11.4 GHz, an ION/IOFF current ratio of 108, and a remarkable transconductance of 70 μS/μm, among the highest achieved so far for MoS2 devices fabricated by means of chemical vapor deposition. These results advance the state-of-the-art performance of atomically thin radio frequency transistors.

Record fT, fmax, and GHz amplification in 2dimensional CVD MoS2 embedded gate fets

We report on chemical vapor deposited (CVD) M0S2 radio frequency (RF) transistors with a record fT of 20 GHz and fmax of 11.4 GHz, and the largest high-field saturation velocity, Vsat = 1.88 × 106 cm/s, in MoS2 reported so far. The gate-first approach, facilitated by cm-scale CVD MoS2, offers enhancement mode operation, Ion/Ioff ratio of 108, and the highest reported transconductance (gm) of 70 μS/μm. The intrinsic ft (fmax) obtained here is 3X (2X) greater than previously reported top-gated CVD MoS2 RF FETs. With as-measured S-parameters, we demonstrate the design of a GHz MoS2-based RF amplifier. This amplifier has gain greater than 15 dB at 1.2 GHz, input return loss > 10 dB, bandwidth > 200 MHz, and DC power consumption of ∼10 mW.

E-mode RF transistors and circuit model using CVD MoS 2

Molybdenum disulfide (MoS<inf>2</inf>), a member of the transition metal dichalcogenide (TMD) family, is a 2D semiconductor with a direct bandgap of ∼1.8 eV for single layers. Its bandgap allows for high I<inf>on</inf>/I<inf>off</inf> metal-oxide semiconducting field-effect transistors (FETs). More relevant for radio frequency (RF) wireless applications, theoretical studies predict MoS<inf>2</inf> to have saturation velocities, V<inf>sat</inf> > 3×10⁶ cm/s. Recent studies have increased exfoliated MoS<inf>2</inf> cutoff frequencies by employing a combination of scaling and geometry modification.¹ However, for industrial scale applications, the mechanical cleavage process is not scalable and, thus far, there have been few studies on chemical vapor deposited (CVD) MoS<inf>2</inf> RF FETs.^2,3 Here we take an embedded gate approach⁴ to yield record transconductance, cutoff frequencies, and current saturation in CVD MoS<inf>2</inf>. Using measured S-parameters and the MIT-MVS semi-empirical verilog-A model⁴ we demonstrate analog and mixed-signal circuit operation.

Towards wafer scale monolayer MoS 2 based flexible low-power RF electronics for IoT systems

There is a growing interest in the design of novel flexible electronics for future internet of things (IoT) systems [1]. IoT requires design of low power RF electronics operating at GHz frequency range. Molybdenum disulphide (MoS2) is the prototypical transitional metal dichalcogenide (TMD) affording a large semiconducting bandgap (1.8eV), high saturation velocity, good mechanical strength, high mobility (> 50cm2/Vs), high on/off ratio (> 106), good current saturation and GHz RF performance [2]. In this work, we demonstrate wafer scale monolayer MoS2 based flexible RF nanoelectronics that can be used for low power nanoelectronics and flexible IoT systems.