We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.

Emerging Device Applications for Semiconducting Two-Dimensional Transition Metal Dichalcogenides

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. Given that any passivated, dangling-bond-free surface will interact with another through vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through non-covalent interactions. We present a succinct and critical survey of emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices. By comparing and contrasting with all-2D vdW heterostructures as well as with competing conventional technologies, we highlight the challenges and opportunities for mixed-dimensional vdW heterostructures.

Mixed-dimensional van der Waals heterostructures

Thin-film electronics in its myriad forms has underpinned much of the technological innovation in the fields of displays, sensors, and energy conversion over the past four decades. This technology also forms the basis of flexible electronics. Here we review the current status of flexible electronics and attempt to predict the future promise of these pervading technologies in healthcare, environmental monitoring, displays and human-machine interactivity, energy conversion, management and storage, and communication and wireless networks.

Flexible Electronics: The Next Ubiquitous Platform

One important use of layered semiconductors such as molybdenum disulfide (MoS2) could be in making novel heterojunction devices leading to functionalities unachievable using conventional semiconductors. Here we demonstrate a metal-semiconductor-metal heterojunction photodetector, made of MoS2 and amorphous silicon (a-Si), with rise and fall times of about 0.3 ms. The transient response does not show persistent (residual) photoconductivity, unlike conventional a-Si devices where it may last 3–5 ms, thus making this heterojunction roughly 10X faster. A photoresponsivity of 210 mA/W is measured at green light, the wavelength used in commercial imaging systems, which is 2−4X larger than that of a-Si and best reported MoS2 devices. The device could find applications in large area electronics, such as biomedical imaging, where a fast response is critical.

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High Performance Molybdenum Disulfide Amorphous Silicon Heterojunction Photodetector

The authors discuss time and temperature dependences of the shift in threshold voltage (Delta V-T) of nanocrystalline silicon (nc-Si) thin film transistors (TFTs) stressed at constant drain currents. In contrast to the behavior of the hydrogenated amorphous silicon (a-Si:H) counterpart, a weak temperature dependence of Delta V-T was observed. The results follow the charge trapping model and the predicted stretched-exponential time dependence that saturates at prolonged stress times. In addition, Delta V-T does not fit into the thermalization energy concept that was developed based on the defect state creation model for a-Si:H TFTs. The results indicate absence of defect state creation in nc-Si TFTs.

Absence of defect state creation in nanocrystalline silicon thin film transistors deduced from constant current stress measurements

We report on the stability of nanocrystalline silicon (nc-Si) bottom-gate (BG) thin film transistors (TFTs) with various compositions of hydrogenated amorphous silicon nitride (a-SiNx:H) gate dielectric TFTs with nitrogen-rich nitride exhibit higher output transconductance, threshold voltage stability, and effective field effect mobility (μFE) than the devices with silicon-rich gate dielectric For example, μFE drops from 075to02cm2∕Vs when the gate dielectric composition [N]∕[Si] changes from 13 to 1 The corresponding threshold voltages (VT) are 4 and −2V Following 5h electrical stress tests, the shift in threshold voltage (ΔVT) is larger for dielectrics with lower [N]∕[Si] content, regardless of the operating regime Indeed, ΔVT in the saturation regime is considerably less and correlates with the charge concentration in the channel, ie, ΔVT in saturation is about 2∕3 of that in the linear regime Relaxation tests on the stressed TFTs show that the charge trapping is the instability mechanism in

Stability of nanocrystalline silicon bottom-gate thin film transistors with silicon nitride gate dielectric

We report performance characteristics of nanocrystalline silicon thin-film transistors (TFTs) fabricated at 280 degC by plasma-enhanced chemical vapor deposition. The TFTs exhibit field-effect mobility of 0.8 cm2V-1s-1, threshold voltage of 4 V, on/off current ratio about 108 with an off-current of 10-13 A, and subthreshold slope of 0.8 V/dec. Bias stress measurements show that the TFT is 3-5 times more stable than the hydrogenated amorphous silicon (a-Si:H) counterpart, with a shift in threshold voltage that is less than 5 % at a gate voltage of 15 V

High Stability, Low Leakage Nanocrystalline Silicon Bottom Gate Thin Film Transistors for AMOLED Displays

CMOS nanocrystalline silicon thin film transistors with high field effect mobility are reported. The transistors were directly deposited by radio-frequency plasma enhanced chemical vapor deposition at 150 degC. The transistors show maximum field effect mobility of 450 cm2/V-s for electrons and 100 cm2/V-s for holes at room temperature. We attribute the high mobilities to a reduction of the oxygen content, which acts as an accidental donor. Indeed, secondary ion mass spectrometry measurements show that the impurity concentration in the nanocrystalline Si layer is comparable to, or lower than, the defect density in the material, which is already low thanks to hydrogen passivation

Mohammad R. Esmaeili-Rad

Papers

High Performance Molybdenum Disulfide Amorphous Silicon Heterojunction Photodetector

Absence of defect state creation in nanocrystalline silicon thin film transistors deduced from constant current stress measurements

Stability of nanocrystalline silicon bottom-gate thin film transistors with silicon nitride gate dielectric

High Stability, Low Leakage Nanocrystalline Silicon Bottom Gate Thin Film Transistors for AMOLED Displays

How to Achieve High Mobility Thin Film Transistors by Direct Deposition of Silicon Using 13.56 MHz RF PECVD