kΩ Μm Research Articles

Ultra‐low power consumption flexible sensing electronics by dendritic bilayer MoS2

AbstractTwo‐dimensional transition metal dichalcogenides (2D TMDs) are promising as sensing materials for flexible electronics and wearable systems in artificial intelligence, tele‐medicine, and internet of things (IoT). Currently, the study of 2D TMDs‐based flexible strain sensors mainly focuses on improving the performance of sensitivity, response, detection resolution, cyclic stability, and so on. There are few reports on power consumption despite that it is of significant importance for wearable electronic systems. It is still challenging to effectively reduce the power consumption for prolonging the endurance of electronic systems. Herein, we propose a novel approach to realize ultra‐low power consumption strain sensors by reducing the contact resistance between metal electrodes and 2D MoS2. A dendritic bilayer MoS2 has been designed and synthesized by a modified CVD method. Large‐area edge contact has been introduced in the dendritic MoS2, resulting in decreased the contact resistance significantly. The contact resistance can be down to 5.4 kΩ μm, which is two orders of magnitude lower than the conventional MoS2 devices. We fabricate a flexible strain sensor, exhibiting superior sensitivity in detecting strains with high resolution (0.04%) and an ultra‐low power consumption (33.0 pW). This study paves the way for future wearable and flexible sensing electronics with high sensitivity and ultra‐low power consumption.image

Reducing the Metal-Graphene Contact Resistance through Laser-Induced Defects.

Graphene has been extensively studied for a variety of electronic and optoelectronic applications. The reported contact resistance between metal and graphene, or rather its specific contact resistance (R C), ranges from a few tens of Ω μm up to a few kΩ μm. Manufacturable solutions for defining ohmic contacts to graphene remain a subject of research. Here, we report a scalable method based on laser irradiation of graphene to reduce the R C in nickel-contacted devices. A laser with a wavelength of l = 532 nm is used to induce defects at the contact regions, which are monitored in situ using micro-Raman spectroscopy. Physical damage is observed using ex situ atomic force and scanning electron microscopy. The transfer length method (TLM) is used to extract R C from back-gated graphene devices with and without laser treatment under ambient and vacuum conditions. A significant reduction in R C is observed in devices where the contacts are laser irradiated, which scales with the laser power. The lowest R C of about 250 Ω μm is obtained for the devices irradiated with a laser power of 20 mW, compared to 900 Ω μm for the untreated devices. The reduction is attributed to an increase in defect density, which leads to the formation of crystallite edges and in-plane dangling bonds that enhance the injection of charge carriers from the metal into the graphene. Our work suggests laser irradiation as a scalable technology for R C reduction in graphene and potentially other two-dimensional materials.

Low Resistance Contact to P-Type Monolayer WSe2.

Advanced microelectronics in the future may require semiconducting channel materials beyond silicon. Two-dimensional (2D) semiconductors, with their atomically thin thickness, hold great promise for future electronic devices. One challenge to achieving high-performance 2D semiconductor field effect transistors (FET) is the high contact resistance at the metal-semiconductor interface. In this study, we develop a charge-transfer doping strategy with WSe2/α-RuCl3 heterostructures to achieve low-resistance ohmic contact for p-type monolayer WSe2 transistors. We show that hole doping as high as 3 × 1013 cm-2 can be achieved in the WSe2/α-RuCl3 heterostructure due to its type-III band alignment, resulting in an ohmic contact with resistance of 4 kΩ μm. Based on that, we demonstrate p-type WSe2 transistors with an on-current of 35 μA·μm-1 and an ION/IOFF ratio exceeding 109 at room temperature.

Phase-engineered synthesis of atomically thin te single crystals with high on-state currents

Multiple structural phases of tellurium (Te) have opened up various opportunities for the development of two-dimensional (2D) electronics and optoelectronics. However, the phase-engineered synthesis of 2D Te at the atomic level remains a substantial challenge. Herein, we design an atomic cluster density and interface-guided multiple control strategy for phase- and thickness-controlled synthesis of α-Te nanosheets and β-Te nanoribbons (from monolayer to tens of μm) on WS2 substrates. As the thickness decreases, the α-Te nanosheets exhibit a transition from metallic to n-type semiconducting properties. On the other hand, the β-Te nanoribbons remain p-type semiconductors with an ON-state current density (ION) up to ~ 1527 μA μm−1 and a mobility as high as ~ 690.7 cm2 V−1 s−1 at room temperature. Both Te phases exhibit good air stability after several months. Furthermore, short-channel (down to 46 nm) β-Te nanoribbon transistors exhibit remarkable electrical properties (ION = ~ 1270 μA μm−1 and ON-state resistance down to 0.63 kΩ μm) at Vds = 1 V.

Engineering TiOx interlayers in high vacuum for Al-contacted MoSe2 transistors

We present an enhanced performance of MoSe2 transistors via sequentially depositing Ti and Al in high vacuum to establish TiOx interlayers positioned between the MoSe2 channel and Ti/Al contacts. Transmission electron microscopy analysis revealed the presence of TiOx at the MoSe2/Ti interface. While MoSe2 transistors exhibited poor device performance in the absence of a TiOx interlayer, the introduction of a TiOx interlayer yielded a notable transistor performance, including an on/off ratio of ∼105, a field-effect mobility of ∼40 cm2 V−1 s−1, and a contact resistance of ∼100 kΩ μm. These enhancements were attributed to the beneficial effects of Fermi level unpinning and interfacial doping facilitated by TiOx interlayers. These results underscore the feasibility of incorporating TiOx interlayers to enable the use of conventional Al contacts in MoSe2 transistors, delivering significant implications for enhancing the performance of transition metal dichalcogenide transistors.

High-performance monolayer MoS2 nanosheet GAA transistor

In this article, a 0.7 nm thick monolayer MoS2 nanosheet gate-all-around field effect transistors (NS-GAAFETs) with conformal high-κ metal gate deposition are demonstrated. The device with 40 nm channel length exhibits a high on-state current density of ~410 μA μm−1 with a large on/off ratio of 6 × 108 at drain voltage = 1 V. The extracted contact resistance is 0.48 ± 0.1 kΩ μm in monolayer MoS2 NS-GAAFETs, thereby showing the channel-dominated performance with the channel length scaling from 80 to 40 nm. The successful demonstration of device performance in this work verifies the integration potential of transition metal dichalcogenides for future logic transistor applications.

Largely Reducing the Contact Resistance of Molybdenum Ditelluride by In Situ Potassium Modification

AbstractSemiconducting molybdenum ditelluride (MoTe2) is widely reported owing to its favorable electronic and optoelectronic properties. The effective modulation of its electrical characteristics has garnered growing attention in regard to building high‐performance MoTe2‐based complementary devices. However, the inherent Schottky barrier (SB) in MoTe2‐based devices severely inhibits the charge‐carrier injection efficiency, leading to a high contact resistance between MoTe2 and contact metals. Here, an efficient method is presented for reducing the SB height of field‐effect transistors (FETs) based on MoTe2 by in situ potassium modification. Interestingly, the electrons transported from K continuously change the electrical characteristics of MoTe2 FET from ambipolar to n‐type with an improvement of electron mobility of over one order of magnitude. Meanwhile, the contact resistance of MoTe2 FET is significantly decreased from 11.5 to 0.4 kΩ µm. By regulating the modification region spatially, it is possible to create a complementary inverter with a high gain of ≈32 at VDD = 3 V. This research demonstrates a relatively simple method for optimizing the contact for MoTe2‐based devices and tuning the electrical properties of MoTe2 for future high‐performance complementary electronics.

Achieving Ultrahigh Electron Mobility in PdSe2 Field‐Effect Transistors via Semimetal Antimony as Contacts

AbstractEven though atomically thin 2D semiconductors have shown great potential for next‐generation electronics, the low carrier mobility caused by poor metal–semiconductor contacts and the inherently high density of impurity scatterings remains a critical issue. Herein, high‐mobility field‐effect transistors (FETs) by introducing few‐layer PdSe2 flakes as channels is achieved, via directly depositing semimetal antimony (Sb) as drain–source electrodes. The formation of clean and defect‐free van der Waals (vdW) stackings at the Sb–PdSe2 heterointerfaces boosts the room temperature transport characteristics, including low contact resistance down to 0.55 kΩ µm, high on‐current density reaching 96 µA µm−1, and high electron mobility of 383 cm2 V−1 s−1. Furthermore, metal–insulator transition (MIT) is observed in the PdSe2 FETs with and without hexagonal boron nitride (h–BN) as buffer layers. However, the layered h–BN/PdSe2 vdW stacking eliminates the interference of interfacial disorders, and thus the corresponding device exhibits a lower MIT crossing point, larger mobility exponent of γ ∼ 1.73, significantly decreased hopping parameter of T0, and ultrahigh electron mobility of 2,184 cm2 V−1 s−1 at 10 K. These findings are expected to be significant for developing high mobility 2D‐based quantum devices.

Laser‐Induced Phase Transition and Patterning of hBN‐Encapsulated MoTe2 (Small 17/2023)

hBN-Encapsulated MoTe2 In article number 2205224, Gwan-Hyoung Lee and co-workers investigate phase transition in a laser-irradiated MoTe2 with various stacked geometries. Depending on the passivation and temperature of the laser-irradiated sample, 2H-MoTe2 transforms into various phases, such as Td, 1T′, Mo3Te4, and Te. In the hBN-encapsulated MoTe2, 2H-1T′ heterophase can be patterned by local laser-irradiation, leading to a low contact resistivity of 1.13 kΩ μm.

Aligned Carbon Nanotube Arrays with Germanium Protective Layers for Improving the Performance of Radio Frequency Transistors

Aligned carbon nanotube arrays with high semiconducting purity and high density have been regarded as an ideal channel material for constructing high-frequency field effect transistors and high-speed integrated circuits. However, contaminants (mainly resist residues) that leave the surface of carbon nanotubes during the fabrication process seriously deteriorate the interface contact quality, which will trigger large contact resistance and thus inhibit the ultimate performance of the carbon nanotube-based transistors. Here, we demonstrate a germanium protective layer technique that can keep a clean surface of carbon nanotube films from the contaminants during device fabrication and thus boost the quality of ohmic contact between carbon nanotubes (CNTs) and metal. The transfer length method (TLM) is used to evaluate the effect of the germanium protective layer technique on the ohmic contact resistance. The contact resistance from metal–CNTs extracted using the TLM was reduced by 93%, from 1.66 to 0.123 KΩ μm before and after the germanium process treatment. Owing to the great improvement of contact between metal and CNTs by introducing the germanium protective layer technique, the fabricated aligned CNT array-based radio-frequency field-effect transistors with a gate length of 110 nm on the quartz substrate present excellent DC performance, with an on-state current of 1.28 mA/μm and maximum transconductance of up to 1.05 mS/μm at a bias of −1.2 V. Meanwhile, the as-measured current gain cut-off frequency (fT,EXT) and power gain frequency (fMAX,EXT) increase from 12 to 110 GHz and 13 to 105 GHz, respectively. This approach provides a simple and reproducible means of improving the interface quality and reducing contact resistance in CNT-based devices to enhance performance.

Contact optimisation strategy for wafer-scale field-effect transistors based on two-dimensional semiconductors

• Wafer-scale multilayered MoS2 films were fabricated by vacuum transferring technique. • A novel electrical contact architecture was proposed to improve the contact capability between 3D metal and 2D semiconductors. • A novel edge-treatment method for the MoS2 side was developed by employing a mature magnetron sputtering system. • Approximately 16 times lower contact resistivity (22.8 kΩ μm) and 3 times lower Schottky barrier height (34.6 meV) than that of the conventional top contacted devices were achieved. • Simplified resistor network model and energy-band diagrams were elaborately illustrated to discuss the mechanisms. Two-dimensional (2D) semiconductors can be utilized to continually miniaturize nanoscale electronic devices. However, achieving a practical solution for satisfying electrical contact with 2D semiconductors remains challenging. In this study, we developed a novel contact structure with transferred multilayer (t-ML) MoS 2 by integrating both edge and top contact. After in-situ plasma treatment for the edge of the MoS 2 channel and successive metal deposition, we achieved 16 times lower contact resistivity (22.8 kΩ µm) than that of the top contacted devices. The thickness-dependent electrical measurement indicates that edge contact is highly effective with thick MoS 2 due to the alleviated current-crowding effect resulting from the small contact area. The temperature-dependent transport measurement further confirms the effective minimization of the influence from the Schottky barrier and tunnelling barrier. Finally, the simplified resistor network model and energy-band diagram were proposed to understand the carrier transport mechanism. Our work provides a practical strategy for achieving excellent electrical contact between bulk metals and 2D semiconductors, paving the way for future large-scale 2D electronic devices.

P-Type Ohmic Contact to Monolayer WSe2 Field-Effect Transistors Using High-Electron Affinity Amorphous MoO3

Monolayer tungsten diselenide (1L-WSe2) has been widely used for studying emergent physics due to the unique properties of its valence bands. However, electrical transport studies have been impeded by the lack of a reliable method to realize Ohmic hole-conducting contacts to 1L-WSe2 especially at low carrier densities and low temperatures. Here, we report low-temperature p-type Ohmic contact to 1L-WSe2 field-effect transistors at carrier densities (n) below n = 1 × 1012 cm–2 with negligible temperature dependence down to the lowest measured temperature (10 K). The non-rectifying barrier is achieved between 1L-WSe2 and molybdenum trioxide (MoO3), where 1L-WSe2 underneath MoO3 is heavily hole-doped through surface transfer doping. Electrical transport measurements reveal linear current–voltage relations at a temperature of 10 K and carrier densities from n = 7.7 × 1011 cm–2 to below the threshold. The finding is also supported by nearly temperature-independent output curves up to room temperature and a negligible contact barrier down to the subthreshold regime. Furthermore, the contact resistivity of MoO3-contacted 1L-WSe2 FET is 30.2–64.8 kΩ μm at n = 1.5 × 1012 cm–2, which is the lowest reported for 1L-WSe2 FETs at such low carrier density. Realizing robust p-type Ohmic contact to a 2D transition metal dichalcogenide semiconductor will enable direct electronic measurements of quantum transport in correlated phases in the valence bands of monolayer semiconductors.

P-type electrical contacts for 2D transition-metal dichalcogenides.

Digital logic circuits are based on complementary pairs of n- and p-type field effect transistors (FETs) via complementary metal oxide semiconductor technology. In three-dimensional (3D) or bulk semiconductors, substitutional doping of acceptor or donor impurities is used to achieve p- and n-type FETs. However, the controllable p-type doping of low-dimensional semiconductors such as two-dimensional (2D) transition-metal dichalcogenides (TMDs) has proved to be challenging. Although it is possible to achieve high-quality, low-resistance n-type van der Waals (vdW) contacts on 2D TMDs1-5, obtaining p-type devices by evaporating high-work-function metals onto 2D TMDs has not been realized so far. Here we report high-performance p-type devices on single- and few-layered molybdenum disulfide and tungsten diselenide based on industry-compatible electron beam evaporation of high-work-function metals such as palladium and platinum. Using atomic resolution imaging and spectroscopy, we demonstrate near-ideal vdW interfaces without chemical interactions between the 2D TMDs and 3D metals. Electronic transport measurements reveal that the Fermi level is unpinned and p-type FETs based on vdW contacts exhibit low contact resistance of 3.3 kΩ µm, high mobility values of approximately 190 cm2 V-1 s-1 at room temperature, saturation currents in excess of 10-5 A μm-1 and an on/off ratio of 107. We also demonstrate an ultra-thin photovoltaic cell based on n- and p-type vdW contacts with an open circuit voltage of 0.6 V and a power conversion efficiency of 0.82%.

Electrical contact properties between Yb and few-layer WS2

The charge injection mechanism from contact electrodes into two-dimensional (2D) dichalcogenides is an essential topic for exploiting electronics based on 2D channels, but remains not well understood. Here, low-work function metal ytterbium (Yb) was employed as contacts for tungsten disulfide (WS2) to understand the realistic injection mechanism. The contact properties in WS2 with variable temperature (T) and channel thickness (tch) were synergetically characterized. It is found that the Yb/WS2 interfaces exhibit a strong pinning effect between energy levels and a low contact resistance (RC) value down to 5 kΩ μm. Cryogenic electrical measurements reveal that RC exhibits weakly positive dependence on T until 77 K as well as a weakly negative correlation with tch. In contrast to the non-negligible RC values extracted, an unexpectedly low effective thermal injection barrier of 36 meV is estimated, indicating the presence of significant tunneling injection in the subthreshold regime and the inapplicability of the pure thermionic emission model to estimate the height of the injection barrier.

Soft-lock drawing of super-aligned carbon nanotube bundles for nanometre electrical contacts.

The assembly of single-walled carbon nanotubes (CNTs) into high-density horizontal arrays is strongly desired for practical applications, but challenges remain despite myriads of research efforts. Herein, we developed a non-destructive soft-lock drawing method to achieve ultraclean single-walled CNT arrays with a very high degree of alignment (angle standard deviation of ~0.03°). These arrays contained a large portion of nanometre-sized CNT bundles, yielding a high packing density (~400 µm-1) and high current carrying capacity (∼1.8 × 108 A cm-2). This alignment strategy can be generally extended to diverse substrates or sources of raw single-walled CNTs. Significantly, the assembled CNT bundles were used as nanometre electrical contacts of high-density monolayer molybdenum disulfide (MoS2) transistors, exhibiting high current density (~38 µA µm-1), low contact resistance (~1.6 kΩ µm), excellent device-to-device uniformity and highly reduced device areas (0.06 µm2 per device), demonstrating their potential for future electronic devices and advanced integration technologies.

Drop-on-demand printing of edge-enhanced and conductive graphene twin-lines by coalescence regulation and multi-layers overwriting

Fabrication of straight and highly conductive graphene lines, the cornerstones of high-performance graphene-based printed electronics, still faces considerable challenges. We have developed a convenient and effective way to print edge-enhanced highly conductive graphene twin-lines by coalescence regulation and multi-layers overwriting (CRMO), which enhances both outline accuracy and electrical conductivity. The overlapping traces and wavy edges were eliminated by droplets coalescence at the expense of introducing discrete footprints, which were transformed into continuous lines by multi-layers overwriting. We successfully fabricated the edge-enhanced graphene twin-line with an edge width of 72.33 ± 7.96 μm and a linear resistivity of 0.188 ± 0.160 kΩ μm−1, yielding the coinstantaneous enhancement of outline accuracy, printing efficiency, and electrical conductivity. Printed graphene twin-lines achieve one of the lowest relative linear resistivity reported so far and a conductivity of 359.88 S m−1. We attributed the highly concentrated and tightly interconnected graphene flakes at the edge to the synergetic effect of CRMO. Finally, we have demonstrated the feasibility of CRMO by printing graphene line resistors with excellent linearity and broad resistance ranges. Such findings establish relationships among the printing method, line morphologies, flakes distribution, and electrical conductivity. This work will be of great significance for the self-assembly of graphene-based functional materials and graphene-based printed electronics development.

Electron beam evaporated Au islands as a nanoscale etch mask on few-layer MoS2 and fabrication of top-edge hybrid contacts for field-effect transistors

Metal contacts to two-dimensional layered semiconductors are crucial to the performance of field-effect transistors (FETs) and other applications of layered materials in nanoelectronics and beyond. In this work, the wetting behavior of very thin Au films on exfoliated MoS2 flakes was studied and evaluated as a nanoscale, self-assembled dry etch mask. Etching nanoscale pits into MoS2 flakes prior to metallization from the top of the flake forms edge sites that contribute some fraction of edge contacts in addition to top contacts for additional carrier injection and lower contact resistance. The morphology and thickness of Au islands and MoS2 were studied with scanning electron microscopy and atomic force microscopy before and after etching with low-power plasmas. A Cl2 plasma etch of 10 s with a Au island mask of 6 nm (nominal) showed the best resulting morphology among the plasma conditions studied. Back-gated MoS2-based FETs on SiO2/p +-Si with Ti/Au contacts were fabricated using a Cl2 etch of only the contact regions, and they yielded devices with ON currents of 100s µA/µm, ON/OFF ratios ⩾106, and contact resistance <10 kΩ µm. The best set of devices had a very low contact resistance of ∼1 kΩ µm with almost no dependence of contact resistance on gating. Using nanoscale etch masks made from metal islands could be highly customizable and shows promise for engineering FETs with low contact resistance.

Large-scale flexible and transparent electronics based on monolayer molybdenum disulfide field-effect transistors

Atomically thin molybdenum disulfide (MoS2) is a promising semiconductor material for integrated flexible electronics due to its excellent mechanical, optical and electronic properties. However, the fabrication of large-scale MoS2-based flexible integrated circuits with high device density and performance remains a challenge. Here, we report the fabrication of transparent MoS2-based transistors and logic circuits on flexible substrates using four-inch wafer-scale MoS2 monolayers. Our approach uses a modified chemical vapour deposition process to grow wafer-scale monolayers with large grain sizes and gold/titanium/gold electrodes to create a contact resistance as low as 2.9 kΩ μm−1. The field-effect transistors are fabricated with a high device density (1,518 transistors per cm2) and yield (97%), and exhibit high on/off ratios (1010), current densities (~35 μA μm−1), mobilities (~55 cm2 V−1 s−1) and flexibility. We also use the approach to create various flexible integrated logic circuits: inverters, NOR gates, NAND gates, AND gates, static random access memories and five-stage ring oscillators. Wafer-scale monolayers of MoS2 can be used to create flexible transistors and circuits that exhibit on/off ratios of 1010, current densities of ~35 μA μm−1 and mobilities of ~55 cm2 V−1 s−1.

Quantum Transport in Two-Dimensional WS2 with High-Efficiency Carrier Injection through Indium Alloy Contacts.

Two-dimensional transition metal dichalcogenides (TMDCs) have properties attractive for optoelectronic and quantum applications. A crucial element for devices is the metal-semiconductor interface. However, high contact resistances have hindered progress. Quantum transport studies are scant as low-quality contacts are intractable at cryogenic temperatures. Here, temperature-dependent transfer length measurements are performed on chemical vapor deposition grown single-layer and bilayer WS2 devices with indium alloy contacts. The devices exhibit low contact resistances and Schottky barrier heights (∼10 kΩ μm at 3 K and 1.7 meV). Efficient carrier injection enables high carrier mobilities (∼190 cm2 V-1 s-1) and observation of resonant tunnelling. Density functional theory calculations provide insights into quantum transport and properties of the WS2-indium interface. Our results reveal significant advances toward high-performance WS2 devices using indium alloy contacts.

Self-Terminated Surface Monolayer Oxidation Induced Robust Degenerate Doping in MoTe2 for Low Contact Resistance.

We introduce an effective method to degenerately dope MoTe2 by oxidizing its surface into the p-dopant MoOx in oxygen plasma. As a self-terminated process, the oxidation is restricted only in the very top layer, therefore offering us an easy and efficient control. The degenerate p-doping with the hole concentration of 2.5 × 1013 cm-2 can be obtained by applying a ∼300 s O2 plasma treatment. Using the degenerately doped MoTe2, we demonstrate a record low contact resistance of 0.6 kΩ μm for MoTe2. Our measurement highlights an excellent stability for the plasma-doped MoTe2. The doped characteristics are robust with no significant degradation even after a one-year exposure to the air. The oxygen plasma doping technique is compatible with the conventional semiconductor processes, which can be utilized to realize high-performance MoTe2 field-effect transistors (FETs) or tunnel FETs in the future.