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2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS2 field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS2 FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS2 flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS2 interface, but also from the MoS2-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO2 substrate. Furthermore, we analyse MoS2 FETs of different channel thickness on three different substrates, SiO2, hBN and Al2O3, by correlating the p-branch ION/IOFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al2O3 which enables us to observe ambipolar transport in a bilayer MoS2 FET. These results highlight that MoS2 is indeed an ambipolar material, and the absence of ambipolar transport in MoS2 FETs is strongly correlated to its dielectric environment and processing conditions. [ABSTRACT FROM AUTHOR]

Defect characterization of 2D materials is a critical aspect for their successful integration in future electronic devices. Here, a simple characterization technique is proposed that opens a path for fast, non‐invasive, quality assessment of transition metal dichalcogenide layers, such as MoS2, and their interfaces. It relates to the correlation between substrate‐induced traps and the indirect‐to‐direct photoluminescence peak ratio. It is shown that the indirect peak is quenched when interfacial trap sites are present. A physical mechanism is proposed to explain the observations based on different recombination mechanisms. [ABSTRACT FROM AUTHOR]

Here, we review the most recent developments in the field of 2-D electronics. We focus first on the synthesis of 2-D materials, discussing the different growth techniques currently available and assessing their strengths and weaknesses. Moreover, we describe a possible roadmap to enable CMOS compatible integration of 2-D materials. We then shift our attention to 2-D devices and circuits and review the state of the art. Among the plethora of device concepts, we look closely at 2-D tunnel FETs (TFETs) and negative-capacitance FETs (NC-FETs) for low-power applications. We also put a particular emphasis on doping-free polarity-controllable systems that use electrostatic doping to eliminate the need for physical or chemical doping. We conclude with an analysis of simulations of scaled devices and discuss the possibilities enabled at circuit level by 2-D electronics. [ABSTRACT FROM AUTHOR]

2-D material FETs hold the promise of excellent gate control, but the impact of nonidealities on their performance remains poorly understood. This is because of the need, so far, to use computationally intensive nonequilibrium Green’s function (NEGF) simulations. Here, we therefore use a semiclassical model to investigate the role of nonidealities in the scaling of back-gated (BG) and top-gated (TG) monolayer MoS2 FETs. We verify the electrostatics and transport of the semiclassical model with density functional theory-based NEGF simulations and calibrate nonidealities, such as interface traps (${D}_{\textsf {it}}$) and Schottky contact barrier height ($\phi _{\textsf {SB}}$) to experimental monolayer and bilayer MoS2 FETs. We find that among the nonidealities, ${D}_{\textsf {it}}$ has the strongest subthreshold swing impact with 70 mV/dec obtainable in BG devices for a ${D}_{\textsf {it}}$ of $5\times {10}^{11}$ cm−2eV $^{-1}$ , an equivalent oxide thickness (EOT) of 1 nm, and a channel length (${L}_{\textsf {ch}}$) of 5 nm. For scaled EOT, $\phi _{\textsf {SB}}$ only strongly impacts ${I}_{ \mathrm{\scriptscriptstyle ON}}$ for the TG case, as the overlapping gate thins the Schottky barriers in the BG case. We show in TG devices that a spacer of only 5 nm results in a 1000-fold drop in ${I}_{ \mathrm{\scriptscriptstyle ON}}$ because of the nonidealities. We propose positive spacer oxide charge as a solution and show that a charge density of above 1013 cm−2 is required to fully recover the device performance. [ABSTRACT FROM AUTHOR]

Ultra-thin MoS2 film doping through surface functionalization with physically adsorbed species is of great interest due to its ability to dope the film without reduction in the carrier mobility. However, there is a need for understanding how the thickness of the MoS2 film is related to the induced surface doping for improved electrical performance. In this work, we report on the relation of MoS2 film thickness with the doping effect induced by the n-dopant adsorbate poly(vinyl-alcohol). Field effect transistors built using MoS2 films of different thicknesses were electrically characterized, and it was observed that the ION/OFF ratio after doping in thin films is more than four orders of magnitudes greater when compared with thick films. Additionally, a semi-classical model tuned with the experimental devices was used to understand the spatial distribution of charge in the channel and explain the observed behavior. From the simulation results, it was revealed that the two-dimensional carrier density induced by the adsorbate is distributed rather uniformly along the complete channel for thin films (<5.2 nm) contrary to what happens for thicker films. [ABSTRACT FROM AUTHOR]

The effects of oxidants both in the channel and contact regions of MoS2 transistors are discussed through a systematic experimental study. This letter highlights the issues of partial instability in metal-organic chemical vapor deposition MoS2 and proposes a procedure, which considerably improves the electrical characteristics of back-gated transistors. By avoiding ambient exposure and layer oxidation, contact resistance can be reduced and intrinsic mobility increased by 50%. [ABSTRACT FROM AUTHOR]

A novel method for fast and flexible fin field effect transistor (FinFET) prototyping using a Ga+ focused ion beam is presented. The fin width and height control is explored, aiming for the successful fabrication of prototypes. This method results in fins with negligible Ga incorporation, when compared to traditional focused ion beam milling techniques. Our method for multiple fin FinFET prototyping enables advanced device fabrication and great flexibility regarding both the number of fins and fin width. Working FinFET prototypes have been fabricated using the proposed fin definition method, and the electrical characterization is discussed. [ABSTRACT FROM AUTHOR]

The chemical patterning of graphene is being pursued tenaciously due to exciting possibilities in electronics, catalysis, sensing, and photonics. Despite the intense efforts, spatially controlled, multifunctional covalent patterning of graphene has not been achieved. The lack of control originates from the inherently poor reactivity of the basal plane of graphene, which necessitates the use of harsh chemistries. Here, we demonstrate spatially resolved multicomponent covalent chemical patterning of single layer graphene using a facile and efficient method. Three different functional groups could be covalently attached to the basal plane in dense, well-defined patterns using a combination of lithography and a self-limiting variant of diazonium chemistry requiring no need for graphene activation. The layer thickness of the covalent films could be controlled down to 1 nm. This work provides a solid foundation for the fabrication of chemically patterned multifunctional graphene interfaces for device applications.

2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS 2 field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS 2 FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS 2 flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS 2 interface, but also from the MoS 2 -substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO 2 substrate. Furthermore, we analyse MoS 2 FETs of different channel thickness on three different substrates, SiO 2 , hBN and Al 2 O 3 , by correlating the p-branch I ON /I OFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al 2 O 3 which enables us to observe ambipolar transport in a bilayer MoS 2 FET. These results highlight that MoS 2 is indeed an ambipolar material, and the absence of ambipolar transport in MoS 2 FETs is strongly correlated to its dielectric environment and processing conditions.

For the integration of two-dimensional (2D) transition metal dichalcogenides (TMDC) with high-performance electronic systems, one of the greatest challenges is the realization of doping and comprehension of its mechanisms. Low-temperature atomic layer deposition of aluminum oxide is found to n-dope MoS 2 and ReS 2 but not WS 2 . Based on electrical, optical, and chemical analyses, we propose and validate a hypothesis to explain the doping mechanism. Doping is ascribed to donor states in the band gap of Al x O y , which donate electrons or not, based on the alignment of the electronic bands of the 2D TMDC. Through systematic experimental characterization, incorporation of impurities (e.g., carbon) is identified as the likely cause of such states. By modulating the carbon concentration in the capping oxide, doping can be controlled. Through systematic and comprehensive experimental analysis, this study correlates, for the first time, 2D TMDC doping to the carbon incorporation on dielectric encapsulation layers. We highlight the possibility to engineer dopant layers to control the material selectivity and doping concentration in 2D TMDC.

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, 2D materials with a unique layered structure have attracted tremendous interest in recent years, mainly motivated by their ultra-thin body nature and unique optoelectronic and mechanical properties. The development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. Metal-organic chemical vapor deposition has recently been used for the synthesis of large area TMDs, however, an important milestone still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale to obtain an atomically flat, self-passivated surface. In this work, we explore various fundamental aspects involved in the chemical vapor deposition process and we provide important insights on the layer-dependence of epitaxial MoS 2 film's structural properties. Based on these observations, we propose an original method to achieve a layer-controlled epitaxy of wafer-scale TMDs.