Electrothermal Local Annealing via Graphite Joule Heating on Two-Dimensional Layered Transistors

A simple but powerful device platform for electrothermal local annealing (ELA) via graphite Joule heating on the surface of transition-metal dichalcogenide, is suggested here to sustainably restore intrinsic electrical properties of atomically thin layered materials. Such two-dimensional materials are easily deteriorated by undesirable surface/interface adsorbates and are screened by a high metal-to-semiconductor contact resistance. The proposed ELA allows one to expect a better electrical performance such as an excess electron doping, an enhanced carrier mobility, and a reduced surface traps in a monolayer molybdenum disulfide (MoS₂)/graphite heterostructure. The thermal distribution of local heating measured by an infrared thermal microscope and estimated by a finite element calculation shows that the annealing temperature reaches up to >400 K at ambient condition and the high efficiency of site-specific annealing is demonstrated unlike the case of conventional global thermal annealing. This ELA platform can be further promoted as a practical gas sensor application. From an O₂ cycling test and a low-frequency noise spectroscopy, the graphite on top of the MoS₂ continuously recovers its initial condition from surface adsorbates. This ELA technique significantly improves the stability and reliability of its gas sensing capability, which can be expanded in various nanoscale device applications

Tunable Mobility in Double-Gated MoTe₂ Field-Effect Transistor: Effect of Coulomb Screening and Trap Sites

There is a general consensus that the carrier mobility in a field-effect transistor (FET) made of semiconducting transition-metal dichalcogenides (s-TMDs) is severely degraded by the trapping/detrapping and Coulomb scattering of carriers by ionic charges in the gate oxides. Using a double-gated (DG) MoTe₂ FET, we modulated and enhanced the carrier mobility by adjusting the top- and bottom-gate biases. The relevant mechanism for mobility tuning in this device was explored using static DC and low-frequency (LF) noise characterizations. In the investigations, LF-noise analysis revealed that for a strong back-gate bias the Coulomb scattering of carriers by ionized traps in the gate dielectrics is strongly screened by accumulation charges. This significantly reduces the electrostatic scattering of channel carriers by the interface trap sites, resulting in increased mobility. The reduction of the number of effective trap sites also depends on the gate bias, implying that owing to the gate bias, the carriers are shifted inside the channel. Thus, the number of active trap sites decreases as the carriers are repelled from the interface by the gate bias. The gate-controlled Coulomb-scattering parameter and the trap-site density provide new handles for improving the carrier mobility in TMDs, in a fundamentally different way from dielectric screening observed in previous studies

Large-Scale Graphene on Hexagonal-BN Hall Elements: Prediction of Sensor Performance without Magnetic Field

A graphene Hall element (GHE) is an optimal system for a magnetic sensor because of its perfect two-dimensional (2-D) structure, high carrier mobility, and widely tunable carrier concentration. Even though several proof-of-concept devices have been proposed, manufacturing them by mechanical exfoliation of 2-D material or electron-beam lithography is of limited feasibility. Here, we demonstrate a high quality GHE array having a graphene on hexagonal-BN (<i>h</i>-BN) heterostructure, fabricated by photolithography and large-area 2-D materials grown by chemical vapor deposition techniques. A superior performance of GHE was achieved with the help of a bottom <i>h</i>-BN layer, and showed a maximum current-normalized sensitivity of 1986 V/AT, a minimum magnetic resolution of 0.5 mG/Hz^0.5 at <i>f</i> = 300 Hz, and an effective dynamic range larger than 74 dB. Furthermore, on the basis of a thorough understanding of the shift of charge neutrality point depending on various parameters, an analytical model that predicts the magnetic sensor operation of a GHE from its transconductance data without magnetic field is proposed, simplifying the evaluation of each GHE design. These results demonstrate the feasibility of this highly performing graphene device using large-scale manufacturing-friendly fabrication methods

Suppression of Interfacial Current Fluctuation in MoTe₂ Transistors with Different Dielectrics

For transition metal dichalcogenides, the fluctuation of the channel current due to charged impurities is attributed to a large surface area and a thickness of a few nanometers. To investigate current variance at the interface of transistors, we obtain the low-frequency (LF) noise features of MoTe₂ multilayer field-effect transistors with different dielectric environments. The LF noise properties are analyzed using the combined carrier mobility and carrier number fluctuation model which is additionally parametrized with an interfacial Coulomb-scattering parameter (α) that varies as a function of the accumulated carrier density (<i>N</i>_acc) and the location of the active channel layer of MoTe₂. Our model shows good agreement with the current power spectral density (PSD) of MoTe₂ devices from a low to high current range and indicates that the parameter α exhibits a stronger dependence on <i>N</i>_acc with an exponent −γ of −1.18 to approximately −1.64 for MoTe₂ devices, compared with −0.5 for Si devices. The raised Coulomb scattering of the carriers, particularly for a low-current regime, is considered to be caused by the unique traits of layered semiconductors such as interlayer coupling and the charge distribution strongly affected by the device structure under a gate bias, which completely change the charge screening effect in MoTe₂ multilayer. Comprehensive static and LF noise analyses of MoTe₂ devices with our combined model reveal that a chemical-vapor deposited <i>h</i>-BN monolayer underneath MoTe₂ channel and the Al₂O₃ passivation layer have a dissimilar contribution to the reduction of current fluctuation. The three-fold enhanced carrier mobility due to the <i>h</i>-BN is from the weakened carrier scattering at the gate dielectric interface and the additional 30% increase in carrier mobility by Al₂O₃ passivation is due to the reduced interface traps

Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS₂/<i>h</i>‑BN Heterostructure

Layered hexagonal boron nitride (<i>h</i>-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and transition metal dichalcogenides. However, the <i>h</i>-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of <i>h</i>-BN thin film as a substrate for monolayer MoS₂ can induce ∼6.5 × 10¹¹ cm^–2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS₂ on <i>h</i>-BN was found to be ∼5 × 10¹³ cm^–2 at high temperature and was significantly reduced at low temperature. Further, the inserted <i>h</i>-BN enables us to reduce the Coulombic charge scattering in MoS₂/<i>h</i>-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal–insulator transition at a much lower charge density of ∼1.0 × 10¹² cm^–2 (<i>T</i> = 25 K). The reduced effective Schottky barrier height in MoS₂/<i>h</i>-BN is attributed to the decreased effective work function of MoS₂ arisen from <i>h</i>-BN induced <i>n</i>-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO₂

Understanding Coulomb Scattering Mechanism in Monolayer MoS₂ Channel in the Presence of <i>h</i>‑BN Buffer Layer

As the thickness becomes thinner, the importance of Coulomb scattering in two-dimensional layered materials increases because of the close proximity between channel and interfacial layer and the reduced screening effects. The Coulomb scattering in the channel is usually obscured mainly by the Schottky barrier at the contact in the noise measurements. Here, we report low-temperature (<i>T</i>) noise measurements to understand the Coulomb scattering mechanism in the MoS₂ channel in the presence of <i>h</i>-BN buffer layer on the silicon dioxide (SiO₂) insulating layer. One essential measure in the noise analysis is the Coulomb scattering parameter (α_SC) which is different for channel materials and electron excess doping concentrations. This was extracted exclusively from a 4-probe method by eliminating the Schottky contact effect. We found that the presence of <i>h</i>-BN on SiO₂ provides the suppression of α_SC twice, the reduction of interfacial traps density by 100 times, and the lowered Schottky barrier noise by 50 times compared to those on SiO₂ at <i>T</i> = 25 K. These improvements enable us to successfully identify the main noise source in the channel, which is the trapping–detrapping process at gate dielectrics rather than the charged impurities localized at the channel, as confirmed by fitting the noise features to the carrier number and correlated mobility fluctuation model. Further, the reduction in contact noise at low temperature in our system is attributed to inhomogeneous distributed Schottky barrier height distribution in the metal–MoS₂ contact region

Ultrastretchable Analog/Digital Signal Transmission Line with Carbon Nanotube Sheets

Stretchable conductors can be used in various applications depending on their own characteristics. Here, we demonstrate simple and robust elastomeric conductors that are optimized for stretchable electrical signal transmission line. They can withstand strains up to 600% without any substantial change in their resistance (≤10% as is and ≤1% with passivation), and exhibit suppressed charge fluctuations in the medium. The inherent elasticity of a polymeric rubber and the high conductivity of flexible, highly oriented carbon nanotube sheets were combined synergistically, without losing both properties. The nanoscopic strong adhesion between aligned carbon nanotube arrays and strained elastomeric polymers induces conductive wavy folds with microscopic bending of radii on the scale of a few micrometers. Such features enable practical applications such as in elastomeric length-changeable electrical digital and analog signal transmission lines at above MHz frequencies. In addition to reporting basic direct current, alternating current, and noise characterizations of the elastomeric conductors, various examples as a stretchable signal transmission line up to 600% strains are presented by confirming the capability of transmitting audio and video signals, as well as low-frequency medical signals without information distortion

Ultrastretchable Analog/Digital Signal Transmission Line with Carbon Nanotube Sheets

Stretchable conductors can be used in various applications depending on their own characteristics. Here, we demonstrate simple and robust elastomeric conductors that are optimized for stretchable electrical signal transmission line. They can withstand strains up to 600% without any substantial change in their resistance (≤10% as is and ≤1% with passivation), and exhibit suppressed charge fluctuations in the medium. The inherent elasticity of a polymeric rubber and the high conductivity of flexible, highly oriented carbon nanotube sheets were combined synergistically, without losing both properties. The nanoscopic strong adhesion between aligned carbon nanotube arrays and strained elastomeric polymers induces conductive wavy folds with microscopic bending of radii on the scale of a few micrometers. Such features enable practical applications such as in elastomeric length-changeable electrical digital and analog signal transmission lines at above MHz frequencies. In addition to reporting basic direct current, alternating current, and noise characterizations of the elastomeric conductors, various examples as a stretchable signal transmission line up to 600% strains are presented by confirming the capability of transmitting audio and video signals, as well as low-frequency medical signals without information distortion

Junction-Structure-Dependent Schottky Barrier Inhomogeneity and Device Ideality of Monolayer MoS₂ Field-Effect Transistors

Although monolayer transition metal dichalcogenides (TMDs) exhibit superior optical and electrical characteristics, their use in digital switching devices is limited by incomplete understanding of the metal contact. Comparative studies of Au top and edge contacts with monolayer MoS₂ reveal a temperature-dependent ideality factor and Schottky barrier height (SBH). The latter originates from inhomogeneities in MoS₂ caused by defects, charge puddles, and grain boundaries, which cause local variation in the work function at Au–MoS₂ junctions and thus different activation temperatures for thermionic emission. However, the effect of inhomogeneities due to impurities on the SBH varies with the junction structure. The weak Au–MoS₂ interaction in the top contact, which yields a higher SBH and ideality factor, is more affected by inhomogeneities than the strong interaction in the edge contact. Observed differences in the SBH and ideality factor in different junction structures clarify how the SBH and inhomogeneities can be controlled in devices containing TMD materials

Surface Modulation of Graphene Field Effect Transistors on Periodic Trench Structure

In this work, graphene field effect transistors (FETs) were fabricated on a trench structure made by carbonized poly­(methylmethacrylate) to modify the graphene surface. The trench-structured devices showed different characteristics depending on the channel orientation and the pitch size of the trenches as well as channel area in the FETs. Periodic corrugations and barriers of suspended graphene on the trench structure were measured by atomic force microscopy and electrostatic force microscopy. Regular barriers of 160 mV were observed for the trench structure with graphene. To confirm the transfer mechanism in the FETs depending on the channel orientation, the ratio of experimental mobility (3.6–3.74) was extracted from the current–voltage characteristics using equivalent circuit simulation. It is shown that the number of barriers increases as the pitch size decreases because the number of corrugations increases from different trench pitches. The noise for the 140 nm pitch trench is 1 order of magnitude higher than that for the 200 nm pitch trench