Symmetry-dependent phonon renormalization in monolayer MoS2transistor

Single-layer MoS2, with its ultimate atomic thickness, has shown promise to scale down transistors for modern integrated circuitry. On the way to implementing two-dimensional (2D) electronic devices, controlled wafer-scale synthesis of single-layer MoS2, single-layer MoS2 metal-oxide-semiconductor field-effect transistors, ohmic contact of single-layer MoS2 for low contact resistance, etc, have been extensively studied. However, the most commonly used two-terminal electronic component, a diode, which conducts current primarily in one direction, has rarely been reported based on single-layer MoS2. Here, a two-terminal high rectification ratio metal-insulator-semiconductor (MIS) tunnel diode was reported based on single-layer MoS2. The In/Au (10/70 nm) electrode via thermal evaporation was used to form a good ohmic contact with the single-layer MoS2. The Si3N4/Pd/Au (5/10/70 nm) electrode via electron beam evaporation was used to form an MIS tunneling structure with the MoS2, showing a current rectification ratio of up to 107 at room temperature. The high current rectification ratio is realized by controlling the quantum tunneling carrier density and the tunneling barrier width. The single-layer MoS2 MIS tunnel diode fabricated via the silicon technology compatible evaporation method has potential application as a fundamental electronic building block for future 2D electronics.

Layered semiconductors show promise as channel materials for field-effect transistors (FETs). Usually, such devices incorporate solid back or top gate dielectrics. Here, we explore deionized (DI) water as a solution top-gate for field-effect switching of layered semiconductors including SnS2, MoS2, and black phosphorus. The DI water gate is easily fabricated, can sustain rapid bias changes, and its efficient coupling to layered materials provides high on-off current ratios, near-ideal subthreshold swing, and enhanced short-channel behavior even for FETs with thick, bulk-like channels, where such control is difficult to realize with conventional back gating. Screening by the high-k solution gate eliminates hysteresis due to surface and interface trap states and substantially enhances the field-effect mobility. The onset of water electrolysis sets the ultimate limit to DI water gating at large negative gate bias. Measurements in this regime show promise for aqueous sensing, demonstrated here by the amperometric detection of glucose in aqueous solution. DI water gating of layered semiconductors can be harnessed in research on novel materials and devices, and it may with further development find broad applications in microelectronics and sensing.

Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5–20 cm2/V·s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 105) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm2/V·s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.

Strong electron-phonon interaction induced significant reduction in lattice thermal conductivities for single-layer MoS2 and PtSSe

The integration circuits are exposing to meet fundamental limits induced by short channel effect Single layer molybdenum disulfide (MoS 2 ) is a promising channel material for field effect transistors (FETs) because the short channel effect is largely reduced. In this paper, a common-source amplifier based on single layer MoS 2 is reported, indicating that single layer MoS 2 is capable to amplify signals. The carrier mobility of the as-made MoS 2 FETs was reached to ∼2.84 cm2/V·s. Furthermore, the output voltage gain of the amplifier is ∼1.3, which can be used as a unify gain amplifier.

III-V semiconductors have been intensively studied with the goal of realizing metal-oxide-semiconductor field-effect transistors (MOSFETs) with high mobility, a high on-off ratio, and low power consumption as next-generation transistors designed to replace current Si technology. Of these semiconductors, a narrow band-gap semiconductor InAs has strong Rashba spin-orbit interaction, thus making it advantageous in terms of both high field-effect transistor (FET) performance and efficient spin control. Here we report a high-performance InAs nanowire MOSFET with a gate-all-around (GAA) structure, where we simultaneously control the spin precession using the Rashba interaction. Our FET has a high on-off ratio (104~106) and a high field-effect mobility (1200 cm2/Vs) and both values are comparable to those of previously reported nanowire FETs. Simultaneously, GAA geometry combined with high- κ dielectric enables the creation of a large and uniform coaxial electric field (>107 V/m), thereby achieving highly controllable Rashba coupling (1 × 10−11 eVm within a gate-voltage swing of 1 V), i.e. an operation voltage one order of magnitude smaller than those of back-gated nanowire MOSFETs. Our demonstration of high FET performance and spin controllability offers a new way of realizing low-power consumption nanoscale spin MOSFETs.

Due to the extremely large surface to bulk ratio, the properties of single layer (SL) MoS2 are largely determined by its interaction with environment. One critical interaction process that has been intensively investigated is the oxidation process of MoS2. Despite numerous previous explorations, the detailed mechanism regarding how MoS2 reacts with oxygen is still not well understood. In this work, we systematically investigate the impact of intrinsic and pre-created defects on the oxidation process of SL MoS2. For pristine SL MoS2, the oxidation is found to initiate near point defects and grain boundaries, leading to the formation of triangle pits in the basal plane and cracks near the grain boundaries. The pre-created defects introduced by ion irradiation are found to serve as the oxidation center, resulting in a more uniform oxidation process. The oxidation is found to introduce p-type doping in the SL MoS2, leading to the blue shift of Raman and photoluminescence (PL) spectra. The shift is found to be more for the region near the grain boundary and for the samples with more pre-created defects. Our results suggest that the presence of defects can strongly promote the oxidation reaction of SL MoS2 in ambient condition, which significantly affects the stability and functionality of materials.

In this paper, the mechanical properties of single and bi layer molybdenum disulfide (MoS2) structures are investigated using uniaxial tensile molecular dynamics (MD) simulation. According to the results of MD simulations, these structures show superior mechanical properties (failure strain, ultimate tensile strength and Young’s modulus) for various applications of nanodevice. The mechanical properties of single and bi layer MoS2 structures are studied at four different temperatures between 300 K and 900 K and different strain rates from 107 s-1 to 109 s-1. As temperature increases up to 900 K, the mechanical properties of single and bi layer MoS2 structures gradually decrease, due to the high temprerature’s weakening effect. Also, changing of temperatures shows more effect on the bi layer MoS2 structure than single layer MoS2 structure. Furthermore, MD results show that the mechanical properties of single and bi layer MoS2 structures demonstrate increasing trend when the strain rate increases. Different strain rates indicate similar effects on the mechanical properties of single and bi layer MoS2 structures. On the other hand, the mechanical properties of these structures are adversely affected by structural defects. Accordingly, the influences of two different S atom types vacancy defect on the mechanical properties of single and bi layer MoS2 structures are examined. When the vacancy defect concentrations in MoS2 structures increase, the mechanical properties of these structures decrease significantly. In addition, S atom bi vacancy defects type exerts more effect on the mechanical properties of single and bi layer MoS2 structures than S atom single vacancy defect type do by increasing concentration. Additionally, vacancy defects indicate more influence on the bi layer MoS2 structure than single layer MoS2 structure. Finally, the results of this study make them excellent candidate for nano-mechanical systems.

Two-dimensional (2D) InSe-based field effect transistor (FET) has shown remarkable carrier mobility and high on-off ratio in experimental reports. Theoretical investigations also predicated the high performance can be well preserved at sub-10 nm nodes in the ballistic limit. However, both experimental experience and theoretical calculations pointed out achieving high-quality ohmic has become the main limiting factor for high-performance 2D FET. In this work, we proposed a new sandwiched ohmic contact with indium for InSe FET and comprehensively evaluated its performance from views of material and device based on ab initio methods. The material properties denote that all of fundamental issues of ohmic contact including tunneling barrier, the Schottky barrier, and effective doping are well concerned by introducing the sandwiched structure, and excellent contact resistance was achieved. At device performance level, devices with gate length of 7, 5, and 3 nm were investigated. All metrics of sandwiched contacted devices far exceed requirement of the International Technology Roadmap for Semiconductors (ITRS) and exhibit obvious promotion as compared to conventional structures. Maximum boost of current with 69.4%, 50%, and 49% are achieved for devices with 7, 5, and 3 nm gate length, respectively. Meanwhile, maximum reduction of the intrinsic delay with 20.4%, 16.7%, and 18.9% are attained. Moreover, a benchmark of energy-delay product (EDP) against other 2D FETs is presented. All InSe FETs with sandwiched ohmic contact surpass MoS2 FETs as well as requirement from ITRS 2024. The best result approaches the upper limit of ideal BP FET, denoting superior preponderance of sandwiched structures for InSe FETs in the next generation of complementary metal-oxide semiconductor (CMOS) technology.

A controlled and eco-friendly, scalable CVD method for the production of single and few layer MoS2 crystals is proposed. The MoS2 crystals are fabricated at atmospheric pressure through the reaction of pre-deposited sodium molybdate (Na2MoO4) in solution and elemental sulfur at 800 °C, offering the flexibility to achieve two growth regimes -either homogeneously distributed single layer MoS2 crystals or continuous MoS2 films- by varying the Na2MoO4 solution concentration. In particular, for low precursor concentrations, isolated single layer MoS2 crystals with controllable mean lateral size were produced. Higher concentrations resulted in continuous single layer films grown in tandem with highly oriented few layer epitaxial domains. The area of the monolayer relative to the few-layer domains can be adjusted. The significant impact on the optical properties of single layer MoS2 crystals due to the growth induced strain is also examined. The grown monolayer crystals are found to experience ~0.3% biaxial tensile strain relative to the exfoliated ones, while a strain relief of 0.6% is measured when these CVD crystals are transferred to another plastic substrate. Moreover, in their photoluminescence (PL) spectra, the neutral exciton and negative trion peaks are shifted linearly with biaxial strain. By correlating PL and Raman spectroscopies the deformation potential of the direct optical transition in single layer MoS2 can be determined.

Indium tin oxide (ITO) is generally used as an electrode material but has recently been demonstrated to be a competitive candidate for use in semiconductor layers in high-performance thin-film transistors (TFTs), due to its high mobility and strong resistance to wet-etching. Here, we demonstrate TFTs using solution-processed, ultra-thin ITO films with outstanding switching performance. These devices exhibit a mobility of up to 15 cm2 V−1 s−1 and a high on-off ratio of 108. Because the device exhibits significant instability under stress tests, moderate doping with Ga as a dopant is introduced to form Ga-doped ITO TFTs. The resulting device has much enhanced stability, near-zero turn-on voltage, and a high on-off current ratio of 108. Through further involvement of an AlOx dielectric layer, the Ga-doped ITO TFTs exhibit a high apparent mobility of more than 40 cm2 V−1 s−1 and operate at small gate voltages (3 V). Remarkably, the device maintains an on-off ratio of over 104 at drain voltages as small as 1 mV.

Top-gate engineering of field-effect transistors based on single layers of MoS2 and graphene

The miniaturization of electronic devices remains a primary focus in the semiconductor industry as it directly impacts both performance enhancement and cost reduction. However, achieving extreme scaling down often relies on high-resolution lithography techniques, which are limited by complexity and an intensive processing time. Two-dimensional transition metal dichalcogenides (2D TMDCs) have great potential for developing short-channel field effect transistors (FETs) due to their atomically thin nature and high Young's modulus. Here, the nanometer-scale channel length in a 2D TMDC-based FET is realized by constructing the sloped architecture without lithography techniques. Utilizing h-BN tunneling layers ensures the mitigated short channel effect (SCE), resulting in a high on-off ratio and low subthreshold swing (SS). This sloped architecture short channel FET (SSFET) exhibits an on-off ratio over 105 with an SS of 160 mV/dec and an on-current level of 3.70 μA. This new approach can provide an innovative pathway to realize the nanometer-scale FET without complicating fabrication processes.

A single-layer MoS2 achieves excellent gate controllability within the nanoscale channel length of a field-effect transistor (FET) owing to an ultra-short screening length. However, multilayer MoS2 (ML-MoS2) is more vulnerable to short channel effects (SCEs) owing to its thickness and long screening length. We eliminated the SCEs in an ML-MoS2 FET (thickness of 4-13 nm) at a channel length of sub-30 nm using a Schottky barrier (SB) variable graphene/ML-MoS2 heterojunction. Although the band modulation in the ML-MoS2 channel worsens with a decrease in the channel length, which is similar to the SCEs occurring in conventional FETs, the variable Fermi level (EF) of a graphene electrode along the gate voltage allows control of the SB at the graphene/MoS2 junction and backs up the current modulation through a variable SB. Electrical measurements and a theoretical band simulation demonstrate the efficient SB modulation of our graphene nanogap (GrNG) ML-MoS2 FET with three distinct carrier transports along Vgs: a thermionic emission at a low SB, Fowler-Nordheim tunneling at a moderate SB, and direct tunneling at a high SB. Our GrNG FET shows an extremely high on-off current ratio of ∼108, which is approximately three-orders of magnitude better than a previously reported metal nanogap (MeNG) FET and a self-aligned metal/graphene nanogap FET with a similar MoS2 thickness. Our GrNG FET also exhibits a 100,000-times higher on-off ratio, 100-times lower subthreshold swing, and 10-times lower drain induced barrier.

The dynamics of excited electrons and holes in single layer (SL) MoS2 have so far been difficult to disentangle from the excitons that dominate the optical response of this material. Here, we use time- and angle-resolved photoemission spectroscopy for a SL of MoS2 on a metallic substrate to directly measure the excited free carriers. This allows us to ascertain a direct quasiparticle band gap of 1.95 eV and determine an ultrafast (50 fs) extraction of excited free carriers via the metal in contact with the SL MoS2. This process is of key importance for optoelectronic applications that rely on separated free carriers rather than excitons.