Analyzing the Carrier Mobility in Transition-Metal Dichalcogenide MoS2 Field-Effect Transistors
TL;DR: In this paper, a theoretical model that quantitatively captures the scaling of mobility with temperature, carrier density, and thickness of transition metal dichalcogenides (TMDCs) is introduced.
Abstract: Transition-metal dichalcogenides (TMDCs) are an important class of two-dimensional (2D) layered materials for electronic and optoelectronic applications, due to their ultimate body thickness, sizable and tunable bandgap, and decent theoretical room-temperature mobility. So far, however, all TMDCs show much lower mobility experimentally because of the collective effects by foreign impurities, which has become one of the most important limitations for their device applications. Here, taking MoS2 as an example, the key factors that bring down the mobility in TMDC transistors, including phonons, charged impurities, defects, and charge traps, are reviewed. A theoretical model that quantitatively captures the scaling of mobility with temperature, carrier density, and thickness is introduced. By fitting the available mobility data from literature over the past few years, one obtains the density of impurities and traps for a wide range of transistor structures. It shows that interface engineering can effectively reduce the impurities, leading to improved device performances. For few-layer TMDCs, the lopsided carrier distribution is analytically modeled to elucidate the experimental increase of mobility with the number of layers. From our analysis, it is clear that the charge transport in TMDC samples is a very complex problem that must be handled carefully.
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TL;DR: This review of the challenges in the CVD growth of 2D materials highlights recent advances in the controlled growth of single crystal 2Dmaterials, with an emphasis on semiconducting transition metal dichalcogenides.
Abstract: Two-dimensional (2D) materials have attracted increasing research interest because of the abundant choice of materials with diverse and tunable electronic, optical, and chemical properties. Moreover, 2D material based heterostructures combining several individual 2D materials provide unique platforms to create an almost infinite number of materials and show exotic physical phenomena as well as new properties and applications. To achieve these high expectations, methods for the scalable preparation of 2D materials and 2D heterostructures of high quality and low cost must be developed. Chemical vapor deposition (CVD) is a powerful method which may meet the above requirements, and has been extensively used to grow 2D materials and their heterostructures in recent years, despite several challenges remaining. In this review of the challenges in the CVD growth of 2D materials, we highlight recent advances in the controlled growth of single crystal 2D materials, with an emphasis on semiconducting transition meta...
893 citations
TL;DR: Two-dimensional transition metal dichalcogenides (TMDCs) have been considered as promising candidates for next generation nanoelectronics and their corresponding applications in electronic and optoelectronic devices.
Abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been considered as promising candidates for next generation nanoelectronics. Because of their atomically-thin structure and high surface to volume ratio, the interfaces involved in TMDC-based devices play a predominant role in determining the device performance, such as charge injection/collection at the metal/TMDC interface, and charge carrier trapping at the dielectric/TMDC interface. On the other hand, the crystalline structures of TMDCs are enriched by a variety of intrinsic defects, including vacancies, adatoms, grain boundaries, and substitutional impurities. Customized design and engineering of the interfaces and defects provides an effective way to modulate the properties of TMDCs and finally enhance the device performance. Herein, we summarize and highlight recent advances and state-of-the-art investigations on the interface and defect engineering of TMDCs and their corresponding applications in electronic and optoelectronic devices. Various interface engineering approaches for TMDCs are overviewed, including surface charge transfer doping, TMDC/metal contact engineering, and TMDC/dielectric interface engineering. Subsequently, different types of structural defects in TMDCs are introduced. Defect engineering strategies utilized to modulate the optical and electronic properties of TMDCs, as well as the developed high-performance and functional devices are summarized. Finally, we highlight the challenges and opportunities for interface and defect engineering in TMDC materials for electronics and optoelectronics.
541 citations
TL;DR: Ultraclean van der Waals bonds between gold-capped indium and a monolayer of the two-dimensional transition-metal dichalcogenide molybdenum disulfide show desirably low contact resistance at the interface, enabling high-performance field-effect transistors.
Abstract: As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal–semiconductor interface at the source and drain electrodes increases, dominating the performance of devices1–3. Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors4,5. However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide3,5–9. Recent studies have shown that van der Waals contacts formed by transferred graphene10,11 and metals12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS2. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS2. The contact resistance of the indium/gold electrodes is 3,000 ± 300 ohm micrometres for monolayer MoS2 and 800 ± 200 ohm micrometres for few-layered MoS2. These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS2, enabling high-performance field-effect transistors with a mobility of 167 ± 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 ± 50 ohm micrometres on ultrathin niobium disulfide (NbS2) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS2) and tungsten diselenide (WSe2) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors. Ultraclean van der Waals bonds between gold-capped indium and a monolayer of the two-dimensional transition-metal dichalcogenide molybdenum disulfide show desirably low contact resistance at the interface, enabling high-performance field-effect transistors.
460 citations
TL;DR: A new quasi-non-volatile 2D semi-floating gate memory with high speed and long refresh time with the potential to bridge the gap between volatile and non-Volatile memory technologies, enabling a high-speed and low-power random access memory.
Abstract: As conventional circuits based on field-effect transistors are approaching their physical limits due to quantum phenomena, semi-floating gate transistors have emerged as an alternative ultrafast and silicon-compatible technology. Here, we show a quasi-non-volatile memory featuring a semi-floating gate architecture with band-engineered van der Waals heterostructures. This two-dimensional semi-floating gate memory demonstrates 156 times longer refresh time with respect to that of dynamic random access memory and ultrahigh-speed writing operations on nanosecond timescales. The semi-floating gate architecture greatly enhances the writing operation performance and is approximately 106 times faster than other memories based on two-dimensional materials. The demonstrated characteristics suggest that the quasi-non-volatile memory has the potential to bridge the gap between volatile and non-volatile memory technologies and decrease the power consumption required for frequent refresh operations, enabling a high-speed and low-power random access memory.
299 citations
TL;DR: The overall recent progress made in developing MoS2 based flexible FETs, OLED displays, nonvolatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices is discussed.
Abstract: Flexible, stretchable, and bendable materials, including inorganic semiconductors, organic polymers, graphene, and transition metal dichalcogenides (TMDs), are attracting great attention in such areas as wearable electronics, biomedical technologies, foldable displays, and wearable point-of-care biosensors for healthcare. Among a broad range of layered TMDs, atomically thin layered molybdenum disulfide (MoS2) has been of particular interest, due to its exceptional electronic properties, including tunable bandgap and charge carrier mobility. MoS2 atomic layers can be used as a channel or a gate dielectric for fabricating atomically thin field-effect transistors (FETs) for electronic and optoelectronic devices. This review briefly introduces the processing and spectroscopic characterization of large-area MoS2 atomically thin layers. The review summarizes the different strategies in enhancing the charge carrier mobility and switching speed of MoS2 FETs by integrating high-κ dielectrics, encapsulating layers, and other 2D van der Waals layered materials into flexible MoS2 device structures. The photoluminescence (PL) of MoS2 atomic layers has, after chemical treatment, been dramatically improved to near-unity quantum yield. Ultraflexible and wearable active-matrix organic light-emitting diode (AM-OLED) displays and wafer-scale flexible resistive random-access memory (RRAM) arrays have been assembled using flexible MoS2 transistors. The review discusses the overall recent progress made in developing MoS2 based flexible FETs, OLED displays, nonvolatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices. Finally, it outlines the perspectives and tremendous opportunities offered by a large family of atomically thin-layered TMDs.
241 citations
References
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TL;DR: Monocrystalline graphitic films are found to be a two-dimensional semimetal with a tiny overlap between valence and conductance bands and they exhibit a strong ambipolar electric field effect.
Abstract: We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in concentrations up to 10 13 per square centimeter and with room-temperature mobilities of ∼10,000 square centimeters per volt-second can be induced by applying gate voltage.
55,532 citations
TL;DR: This work reviews the historical development of Transition metal dichalcogenides, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.
Abstract: Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.
13,348 citations
TL;DR: Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors, and could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
Abstract: Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
12,477 citations
TL;DR: By using micromechanical cleavage, a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides are prepared and studied.
Abstract: We report free-standing atomic crystals that are strictly 2D and can be viewed as individual atomic planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromechanical cleavage, we have prepared and studied a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2D molecules unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
10,586 citations
TL;DR: This Review describes how the tunable electronic structure of TMDs makes them attractive for a variety of applications, as well as electrically active materials in opto-electronics.
Abstract: Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.
7,903 citations