Monolayer Field-effect Transistors Research Articles

Transport Study of Charge-Carrier Scattering in Monolayer WSe_{2}.

Employing flux-grown single crystal WSe_{2}, we report charge-carrier scattering behaviors measured in h-BN encapsulated monolayer field effect transistors. We observe a nonmonotonic change of transport mobility as a function of hole density in the degenerately doped sample, which can be explained by energy dependent scattering amplitude of strong defects calculated using the T-matrix approximation. Utilizing long mean-free path (>500 nm), we also demonstrate the high quality of our electronic devices by showing quantized conductance steps from an electrostatically defined quantum point contact, showing the potential for creating ultrahigh quality quantum optoelectronic devices based on atomically thin semiconductors.

Effect of Thickness and Thermal Treatment on the Electrical Performance of 2D MoS2 Monolayer and Multilayer Field-Effect Transistors

We deposited high-quality molybdenum disulfide (MoS2) monolayer and multilayer crystals on SiO2/Si substrates, by means of a chemical vapor deposition (CVD) process at atmospheric pressure. Notably, NaCl salt was used as component of the precursors to assist the growth of MoS2 crystals, which were intended for use as the active channel layer in the fabrication of field-effect transistors (FETs). The resulting MoS2 crystals from this CVD process were analyzed by optical, scanning electron, and atomic force microscopies, and by Raman and photoluminescence spectroscopies. The optical images and the micrographs obtained by SEM revealed the formation of dispersed MoS2 crystals with a triangular shape all over the SiO2 surface. The thickness of the MoS2 crystals, analyzed by atomic force microscopy, showed minimum values of around 0.7 nm, confirming the formation of monolayers. Additionally, multilayers with larger thickness were also identified. The Raman and photoluminescence spectra of the MoS2 crystals corroborated the formation of single and multiple layers. The fabrication of the FET back-SiO2 -gate configuration was made by depositing patterned source and drain Ti contacts on the dispersed MoS2 crystals to achieve the Ti/MoS2/SiO2/Si layer stacks. MoS2-based FETs with one and three layers were assembled and their electrical response analyzed by I–V output and transfer curves showing the typical characteristics of an n-type semiconductor channel operating in depletion mode. The electrical performance parameters of the devices, such as mobility and threshold voltage, were also determined from this analysis. Finally, to enhance their electrical response, the MoS2-based devices were thermally annealed at 200 °C for 30 min in Ar atmosphere. The increase in the mobility of the device was 176% compared to the device before the treatment.

Increased Mobility and Reduced Hysteresis of MoS2 Field-Effect Transistors via Direct Surface Precipitation of CsPbBr3-Nanoclusters for Charge Transfer Doping.

Transition-metal dichalcogenides (TMDs) and metal halide perovskites (MHPs) have been investigated for various applications, owing to their unique physical properties and excellent optoelectronic functionalities. TMD monolayers synthesized via chemical vapor deposition (CVD), which are advantageous for large-area synthesis, exhibit low mobility and prominent hysteresis in the electrical signals of field-effect transistors (FETs) because of their native defects. In this study, we demonstrate an increase in electrical mobility by ∼170 times and reduced hysteresis in the current-bias curves of MoS2 FETs hybridized with CsPbBr3 for charge transfer doping, which is implemented via solution-based CsPbBr3-nanocluster precipitation on CVD-grown MoS2 monolayer FETs. Electrons injected from CsPbBr3 into MoS2 induce heavy n-doping and heal point defects in the MoS2 channel layer, thus significantly increasing mobility and reducing hysteresis in the hybrid FETs. Our results provide a foundation for improving the reliability and performance of TMD-based FETs by hybridizing them with solution-based perovskites.

Sub‐5 nm 2D Semiconductor‐Based Monolayer Field‐Effect Transistor: Status and Prospects

A detailed review of sub‐5 nm 2D semiconductor monolayer (ML) field‐effect transistors (FETs) is presented. A brief introduction to the fundamentals of 2D materials and their properties is provided followed by a critical review to propose an appropriate channel material from the available 2D materials. Various device simulation techniques incorporating density functional theory (DFT) and nonequilibrium Green's function (NEGF) are discussed in order to comprehend the distinctive features of 2D materials and the ballistic performance of sub‐5 nm ML 2D FETs. Relevant, recent experimental progress is also discussed. The review suggests that sub‐5 nm monolayer FETs based on 2D semiconductors like black phosphorene, WSe2N4, silicene, tellurene, arsenene, WSe2, MoTe2, MoS2, InSe, α‐CS, MoSi2N4, WS2, Bi2O2Se, GeSe, SnSe2, SnS2, AsP, GeS, MoSi2N4, BiH, SnSe2 and MoTe2 show excellent device performance meeting the International Technology Roadmap for Semiconductor (ITRS) requirements (2013 Version) for both high‐performance (HP) and low‐power (LP) applications in terms of on‐state current, subthreshold swing (SS), delay time, and power delay product (PDP).

23.1: Controllable Growth of Conjugated Polymer Monolayer: from Field‐Effect Transistors to Integrated Circuits

The charge transport of planar organic field‐effect transistors (FETs) occurs in the nearest molecular layer to the dielectric layer. Therefore, organic monolayer FETs with two‐dimensional transport properties have been widely studied, and is considered to be promising for applications in bottom‐up integrated circuits. Based on the semiconducting polymer PffBT4T‐2DT, we fabricate the high‐performance monolayer FETs and unipolar inverters. Resultant polymer monolayer FETs exhibited excellent and uniform performance with a 107 on/off current ratio and ultra‐low leakage current with a magnitude of 10‐12 A. More importantly, the corresponding intrinsic gain of 1000 V/V is achieved, which surpass Si MOSFET. Zero‐VGS inverters based on these polymer monolayer FETs exhibit steep level transitions with a voltage gain of 70 V/V, which is among the record for inverters based on organic monolayer.

Bottom‐Up Growth of n‐Type Polymer Monolayers for High‐Performance Complementary Integrated Circuits

AbstractDownscaling the semiconductor into ultrathin film is of vital importance to high‐performance field–effect transistors (FETs), but the high‐mobility FETs based on conjugated polymer monolayers have been rarely realized. Especially, the lack of high‐performance n‐type polymer monolayer FETs hinders the development of complementary integrated circuits. Herein, by fine‐tuning the supramolecular assembly of two thiazole flanked naphthalene diimide‐based conjugated polymers, the ≈2.5 nm‐thick monolayers with well‐defined fibrillar morphology are grown in a controllable way, where the one‐dimensional solution‐state structures are inherited. The resultant monolayer FETs show the electron mobility up to 0.25 cm2 V−1 s−1, among the record for n‐type polymer monolayer FETs. More importantly, the first demonstration of polymer monolayer complementary integrated circuits is present, and a record‐high inverter gain of 113 is achieved, which is also identical to the best polymer thin‐film inverters.

Vapor-Liquid-Solid Growth of Morphology-Tailorable WS2 toward P-Type Monolayer Field-Effect Transistors.

Although substantial efforts have been made, controllable synthesis of p-type WS2 remains a challenge. In this work, we employ NaCl as a seeding promoter to realize vapor-liquid-solid (VLS) growth of p-type WS2. Morphological evolution, including a one-dimensional (1D) nanowire to two-dimensional (2D) planar domain and 2D shape transition of WS2 domains, can be well-controlled by the growth temperature and sulfur introduction time. A high growth temperature is required to enable planar growth of 2D WS2, and a sulfur-rich environment is found to facilitate the growth of high-quality WS2. Raman and photoluminescence (PL) mappings demonstrate uniform crystallinity and high quantum efficiency of VLS-grown WS2. Moreover, monolayer WS2-based field-effect transistors (FETs) are fabricated, showing p-type conducting behavior, which is different from previous reported n-type FETs from WS2 grown by other methods. First-principles calculations show that the p-type behavior originates from the substitution of Na at the W site, which will form an additional acceptor level above the valence band maximum (VBM). This facile VLS growth method opens the avenue to realize the p-n WS2 homojunctions and p/n-WS2-based heterojunctions for monolayer wearable electronic, photonic, optoelectronic, and biosensing devices and should also be a great benefit to the development of 2D complementary metal-oxide-semiconductor (CMOS) circuit applications.

High-performance monolayer or bilayer SiC short channel transistors with metallic 1T-phase MoS2 contact

In this paper, high-performance monolayer or bilayer SiC short channel transistors with metallic 1T-Phase MoS2 contact have been proposed and investigated by using ab initio simulations. The results show that monolayer or bilayer SiC Schottky-barrier field-effect transistors (SBFET) could conquer the short channel effect, even if the channel length is reduced to 4.1 nm. The p-type ON-currents of monolayer SiC-SBFET, bilayer AB-stacked SiC-SBFET, and bilayer AA-stacked SiC-SBFET are 1132 μA/μm, 980 μA/μm, and 1457 μA/μm, respectively. The all could satisfy the requirement of the high-performance transistor outlined by International Technology Roadmap for Semiconductors (ITRS, 2013 version) for production year 2028. However, the n-type ON-currents of above SiC-SBFETs are 35 μA/μm, 21 μA/μm, and 229 μA/μm, which are much lower than that of p-type ON-currents. Comparing spectral current and spatial local density of states are used to explore the great differences between the p-type and n-type ON-currents.

Oligothiophene Phosphonic Acids for Self-Assembled Monolayer Field-Effect Transistors.

Semiconducting self-assembled monolayers (SAMs) represent highly relevant components for the fabrication of organic thin-film electronics because they enable the precise formation of active π-conjugates in terms of orientation and layer thickness. In this work, we demonstrate self-assembled monolayer field-effect transistors (SAMFETs) composed of phosphonic acid oligomers of 3-hexylthiophene (oligothiophenes-OT) with systematic variations of thiophene repeating units (5, 10, and 20). The devices exhibit stable lateral charge transport with increased mobility as a function of thiophene unit counts. Importantly, our work reveals the packing and intermolecular order of varied-chain-length SAMs at the molecular scale via X-ray reflectivity (XRR) and quantitative X-ray photoelectron spectroscopy (XPS). Short oligomers (OT5-PA and OT10-PA) arrange almost perpendicular to the substrate, forming highly ordered SAMs, whereas the long-chain OT20-PA exhibits a folded structure. By tuning the molecular order in the monolayers via the SAM substitution reaction, the OT20-PA devices show a tripling in mobility.

Nondegenerate P‐Type In‐Doped SnS2 Monolayer Transistor

AbstractDoping can change the intrinsic properties of 2D materials by tuning their electronic structure. In this work, high‐quality 2D In‐doped SnS2 (In‐SnS2) monolayer is reported through chemical vapor transport method and following mechanical cleavage process. The In content of In‐SnS2 is ≈0.9%, and doping results in the downward shift of the Fermi level compared with the undoped one. Transmission electron microscopy images show that doping is uniform in the In‐SnS2 nanosheets with high quality. The In‐SnS2 monolayer field effect transistors (FETs) show p‐type feature which is different from the n‐type feature of undoped SnS2. The average hole produced by one In atom is estimated as 0.37 from FETs measurement. Density functional theory calculations show that the decreasing of hole concentration results from the hole killing mechanism induced by S vacancy. The results suggest that changing the polarity of 2D semiconductor by doping is successful, and In‐SnS2 monolayer has great potential in the applications of electronics and optoelectronics.

Carbon-nanoparticle-assisted growth of high quality bilayer WS2 by atmospheric pressure chemical vapor deposition

Two-dimensional (2D) WS2 offers great prospects for assembling next-generation optoelectronic and electronic devices due to its thickness-dependent optical and electronic properties. However, layer-number-controlled growth of WS2 is still a challenge up to now. This work presents controlled growth of bilayer WS2 triangular flakes by carbon-nanoparticle-assisted chemical vapor deposition (CVD) process. The growth mechanism is also proposed. In addition, the field effect transistors (FETs) based on monolayer and bilayer WS2 are also fabricated and investigated. The bilayer FET displays a mobility of 34 cm2·V-1·s-1, much higher than that of the monolayer FET. The high figures of merit make bilayer WS2 a promising candidate in high-performance electronics and optoelectronics.

Compact Model of Carrier Transport in Monolayer Transition Metal Dichalcogenide Transistors

Monolayer 2-D transition metal dichalcogenide (TMDC) field-effect transistors (FETs) have excellent scaling properties due to an extremely thin body and reduced quantum-mechanical tunneling. High-field and ballistic carrier transport can play an important role in aggressively scaled TMDC FETs. A compact model is developed to describe carrier transport in 2-D monolayer FETs based on a TMDC material. The model has the following features: 1) it treats high-field transport and velocity saturation; 2) it can be applied from ballistic to quasi-ballistic to diffusive transport regimes; and 3) it incorporates analytical expressions for carrier mobility values based on examining physical carrier scattering mechanisms in TMDC materials. Monte Carlo simulation, which solves the Boltzmann transport equation stochastically, is used to validate the model. The carrier transport model is further integrated with a virtual-source model to calculate the ${I}$ – ${V}$ characteristics of 2-D monolayer FETs. Based on the model, the effects of phonon scattering and ionized impurity scattering on the average carrier injection velocity and transistor ballisticity are discussed.

Bilayer h-BN barriers for tunneling contacts in fully-encapsulated monolayer MoSe2 field-effect transistors

The performance of electronic and spintronic devices based on two-dimensional semiconductors (2D SC) is largely dependent on the quality and resistance of the metal/SC electrical contacts, as well as preservation of the intrinsic properties of the SC channel. Direct metal/SC interaction results in highly resistive contacts due to formation of large Schottky barriers and considerably affects the properties of the 2D SC. In this work, we address these two important issues in monolayer field-effect transistors (FETs). We encapsulate the channel with hexagonal boron nitride (h-BN), using bilayer h-BN at the metal/SC interface. The bilayer h-BN eliminates the metal/ chemical interactions, preserves the electrical properties of and reduces the contact resistances by prevention of Fermi-level pinning. We investigate electrical transport in the monolayer FETs that yields close to intrinsic electron mobilities (≈ 26 cm2 V−1 s−1) even at room temperature. Moreover, we experimentally study the charge transport through metal/h-BN/ tunnel contacts and we explicitly show that the dielectric bilayer of h-BN provides highly efficient gating (tuning the Fermi energy) of the channel at the contact regions even with small biases. Also we provide a theoretical model that allows to understand and reproduce the experimental I– characteristics of the contacts. These observations give an insight into the electrical behavior of the metal/h-BN/2D SC heterostructure and introduce bilayer h-BN as a suitable choice for high quality tunneling contacts that allows for low energy charge and spin transport.

Development of highly soluble perylenetetracarboxylic diimide derivative for n-type monolayer field-effect-transistor

We have employed 1,7-dibromo-N,N’-ditridecyl-3,4,9,10-perylenetetra carboxylic diimide (DB-PTCDI-C13) for the development of novel n-type monolayer field-effect-transistor (FET). Functionalization of PTCDI core with long alkyl chain and bulky bromine atoms improved the solubility and made n-type monolayer FET performance possible via solution deposition processes. Annealing effect of spin coated films and drop casted films have been discussed. Liquid crystalline phase of DB-PTCDI-C13 turned out to be playing an important role in controlling the film quality and mobility. The highest mobility was 5.8 × 10−3 cm2 V−1 s−1 after annealing, of which device has been then tested for doping by potassium.

Integrated circuits based on conjugated polymer monolayer

It is still a great challenge to fabricate conjugated polymer monolayer field-effect transistors (PoM-FETs) due to intricate crystallization and film formation of conjugated polymers. Here we demonstrate PoM-FETs based on a single monolayer of a conjugated polymer. The resulting PoM-FETs are highly reproducible and exhibit charge carrier mobilities reaching 3 cm2 V−1 s−1. The high performance is attributed to the strong interactions of the polymer chains present already in solution leading to pronounced edge-on packing and well-defined microstructure in the monolayer. The high reproducibility enables the integration of discrete unipolar PoM-FETs into inverters and ring oscillators. Real logic functionality has been demonstrated by constructing a 15-bit code generator in which hundreds of self-assembled PoM-FETs are addressed simultaneously. Our results provide the state-of-the-art example of integrated circuits based on a conjugated polymer monolayer, opening prospective pathways for bottom-up organic electronics.

Effect of Structure and Disorder on the Charge Transport in Defined Self-Assembled Monolayers of Organic Semiconductors.

Self-assembled monolayer field-effect transistors (SAMFETs) are not only a promising type of organic electronic device but also allow detailed analyses of structure-property correlations. The influence of the morphology on the charge transport is particularly pronounced, due to the confined monolayer of 2D-π-stacked organic semiconductor molecules. The morphology, in turn, is governed by relatively weak van-der-Waals interactions and is thus prone to dynamic structural fluctuations. Accordingly, combining electronic and physical characterization and time-averaged X-ray analyses with the dynamic information available at atomic resolution from simulations allows us to characterize self-assembled monolayer (SAM) based devices in great detail. For this purpose, we have constructed transistors based on SAMs of two molecules that consist of the organic p-type semiconductor benzothieno[3,2-b][1]benzothiophene (BTBT), linked to a C11 or C12 alkylphosphonic acid. Both molecules form ordered SAMs; however, our experiments show that the size of the crystalline domains and the charge-transport properties vary considerably in the two systems. These findings were confirmed by molecular dynamics (MD) simulations and semiempirical molecular-orbital electronic-structure calculations, performed on snapshots from the MD simulations at different times, revealing, in atomistic detail, how the charge transport in organic semiconductors is influenced and limited by dynamic disorder.

Diketopyrrolopyrrole‐Based Semiconducting Polymer with Both Hydrophobic Alkyl and Hydrophilic Tetraethylene Glycol Chains for Monolayer Transistor and Sensing Application

AbstractFunctional field‐effect transistors (FETs) have received increasing attention in recent years. Herein, it is demonstrated that the incorporation of hydrophilic tetraethylene glycol (TEEG) chains into the diketopyrrolopyrrole (DPP)‐based semiconducting polymer (PDPP3T1) can facilitate the formation of polymeric monolayer at the air–water interface, and the resulting field‐effect transistors (FETs) show sensitive and selective response toward alcohol vapor. By using the Langmuir–Schaefer method, monolayer and ultrathin films from bilayer to five‐layer can be successfully prepared. Interestingly, nanopores with sizes of 50–100 nm are formed within the monolayer film, as observed by using atomic force microscopy. The monolayer FET exhibits relatively high hole mobilities up to 0.014 cm2 V−1 s−1, which is among the highest charge mobilities reported for ultrathin film FETs prepared with the Langmuir technique. Moreover, hole mobility increases with an increase in the thickness of the ultrathin film from monolayer to three‐layer, and it reaches 0.04 cm2 V−1 s−1 for three‐layer FETs. A variation of both on‐current and off‐current is detected for monolayer FET upon exposure to alcohol vapor. The monolayer FET displays ultrahigh sensitivity (down to 1 ppb) and good selectivity toward alcohol vapors, in particular ethanol vapor. This can be attributed to the synergetic effects of the hydrophilic TEEG chains in PDPP3T1 and the nanopores within the monolayers.

Self-assembled monolayer field-effect transistors based on oligo-9,9'-dioctylfluorene phosphonic acids.

The use of functional oligomers of π-conjugated oligofluorenes led to a region-selective assembly of amorphous monolayers which exhibit robust lateral charge transport pathways in self-assembled monolayer field-effect transistors over long distances and even in mixed monolayers of semiconducting and insulating molecules. This oligomer concept might stimulate a new molecular design of self-assembling semiconducting materials.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates**Supported by the National Natural Science Foundation of China under Grant No 61306006.

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors (GFETs) fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density IDS, improved transconductance gm, reduced sheet resistance Ron, and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasi-free-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

Subthreshold slope as a measure of interfacial trap density in pentacene films

Electrical properties in organic field effect transistors (FETs) are dominated by charge transport in the accumulation layer, few molecular layers close to the gate dielectric. Through comparison of the subthreshold slope between monolayer (ML) and thick pentacene FETs, formation of the second layer islands on top of the complete first layer is found to be crucial in determining the charge transport in ML pentacene FETs. It is demonstrated that a pentacene ML field effect transistor (FET) is an excellent probe that can detect electronic states of organic semiconductors interfacing with the gate dielectric at nanometer scale. Far higher sub-threshold slope in ML FETs, as a measure of interfacial charge trap density, than that in thick pentacene FETs is translated that the path of the induced carriers in ML FETs is limited into the molecular layer interfacing with the gate dielectric with a high density of charge traps, while carriers in thicker films have alternative pathways through more electrically conductive layer above the first layer with much less trap density.