10 research outputs found

    Scalable Graphite/Copper Bishell Composite for High-Performance Interconnects

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    We present the fabrication and characterizations of novel electrical interconnect test lines made of a Cu/graphite bishell composite with the graphite cap layer grown by electron cyclotron resonance chemical vapor deposition. Through this technique, conformal multilayer graphene can be formed on the predeposited Cu interconnects under CMOS-friendly conditions. The low-temperature (400 °C) deposition also renders the process unlimitedly scalable. The graphite layer can boost the current-carrying capacity of the composite structure to 108 A/cm2, more than an order of magnitude higher than that of bare metal lines, and reduces resistivity of fine test lines by ∼10%. Raman measurements reveal that physical breakdown occurs at ∼680–720 °C. Modeling the current vs voltage curves up to breakdown shows that the maximum current density of the composites is limited by self-heating of the graphite, suggesting the strong roles of phonon scattering at high fields and highlighting the significance of a metal counterpart for enhanced thermal dissipation

    Ultrafast and Low Temperature Synthesis of Highly Crystalline and Patternable Few-Layers Tungsten Diselenide by Laser Irradiation Assisted Selenization Process

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    Recently, a few attempts to synthesize monolayers of transition metal dichalcogenides (TMDs) using the chemical vapor deposition (CVD) process had been demonstrated. However, the development of alternative processes to synthesize TMDs is an important step because of the time-consuming, required transfer and low thermal efficiency of the CVD process. Here, we demonstrate a method to achieve few-layers WSe<sub>2</sub> on an insulator <i>via</i> laser irradiation assisted selenization (LIAS) process directly, for which the amorphous WO<sub>3</sub> film undergoes a reduction process in the presence of selenium gaseous vapors to form WSe<sub>2</sub>, utilizing laser annealing as a heating source. Detailed growth parameters such as laser power and laser irradiation time were investigated. In addition, microstructures, optical and electrical properties were investigated. Furthermore, a patternable WSe<sub>2</sub> concept was demonstrated by patterning the WO<sub>3</sub> film followed by the laser irradiation. By combining the patternable process, the transfer-free WSe<sub>2</sub> back gate field effect transistor (FET) devices are realized on 300 nm-thick SiO<sub>2</sub>/P<sup>+</sup>Si substrate with extracted field effect mobility of ∼0.2 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. Similarly, the reduction process by the laser irradiation can be also applied for the synthesis of other TMDs such as MoSe<sub>2</sub> from other metal oxides such as MO<sub>3</sub> film, suggesting that the process can be further extended to other TMDs. The method ensures one-step process to fabricate patternable TMDs, highlighting the uniqueness of the laser irradiation for the synthesis of different TMDs

    Low Temperature Growth of Graphene on Glass by Carbon-Enclosed Chemical Vapor Deposition Process and Its Application as Transparent Electrode

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    A novel carbon-enclosed chemical vapor deposition (CE-CVD) to grow high quality monolayer graphene on Cu substrate at a low temperature of 500 °C was demonstrated. The quality of the grown graphene was investigated by Raman spectra, and the detailed growth mechanism of high quality graphene by the CE-CVD process was investigated in detail. In addition to growth of high quality monolayer graphene, a transparent hybrid few-layer graphene/CuNi mesh electrode directly synthesized by the CE-CVD process on a conventional glass substrate at the temperature of 500 °C was demonstrated, showing excellent electrical properties (∼5 Ω/□ @ 93.5% transparency) and ready to be used for optical applications without further transfer process. The few-layer graphene/CuNi mesh electrode shows no electrical degradation even after 2 h annealing in pure oxygen at an elevated temperature of ∼300 °C. Furthermore, the few-layer graphene/CuNi mesh electrode delivers an excellent corrosion resistance in highly corrosive solutions such as electroplating process and achieves a good nucleation rate for the deposited film. Findings suggest that the low temperature few-layer graphene/CuNi mesh electrode synthesized by the CE-CVD process is an excellent candidate to replace indium tin oxide (ITO) as transparent conductive material (TCM) in the next generation

    Plasma-Assisted Synthesis of High-Mobility Atomically Layered Violet Phosphorus

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    Two-dimensional layered materials such as graphene, transition metal dichalcogenides, and black phosphorus have demonstrated outstanding properties due to electron confinement as the thickness is reduced to atomic scale. Among the phosphorus allotropes, black phosphorus, and violet phosphorus possess layer structure with the potential to be scaled down to atomically thin film. For the first time, the plasma-assisted synthesis of atomically layered violet phosphorus has been achieved. Material characterization supports the formation of violet phosphorus/InN over InP substrate where the layer structure of violet phosphorus is clearly observed. The identification of the crystal structure and lattice constant ratifies the formation of violet phosphorus indeed. The critical concept of this synthesis method is the selective reaction induced by different variations of Gibbs free energy (Δ<i>G</i>) of reactions. Besides, the Hall mobility of the violet phosphorus on the InP substrate greatly increases over the theoretical values of InP bulk material without much reduction in the carrier concentration, suggesting that the mobility enhancement results from the violet phosphorus layers. Furthermore, this study demonstrates a low-cost technique with high compatibility to synthesize the high-mobility atomically layered violet phosphorus and open the space for the study of the fundamental properties of this intriguing material as a new member of the fast growing family of 2D crystals

    Transfer-Free Growth of Atomically Thin Transition Metal Disulfides Using a Solution Precursor by a Laser Irradiation Process and Their Application in Low-Power Photodetectors

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    Although chemical vapor deposition is the most common method to synthesize transition metal dichalcogenides (TMDs), several obstacles, such as the high annealing temperature restricting the substrates used in the process and the required transfer causing the formation of wrinkles and defects, must be resolved. Here, we present a novel method to grow patternable two-dimensional (2D) transition metal disulfides (MS<sub>2</sub>) directly underneath a protective coating layer by spin-coating a liquid chalcogen precursor onto the transition metal oxide layer, followed by a laser irradiation annealing process. Two metal sulfides, molybdenum disulfide (MoS<sub>2</sub>) and tungsten disulfide (WS<sub>2</sub>), are investigated in this work. Material characterization reveals the diffusion of sulfur into the oxide layer prior to the formation of the MS<sub>2</sub>. By controlling the sulfur diffusion, we are able to synthesize continuous MS<sub>2</sub> layers beneath the top oxide layer, creating a protective coating layer for the newly formed TMD. Air-stable and low-power photosensing devices fabricated on the synthesized 2D WS<sub>2</sub> without the need for a further transfer process demonstrate the potential applicability of TMDs generated via a laser irradiation process

    Low-Temperature Chemical Synthesis of Three-Dimensional Hierarchical Ni(OH)<sub>2</sub>‑Coated Ni Microflowers for High-Performance Enzyme-Free Glucose Sensor

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    Since prevention methods of type-II diabetes and knowledge of prediabetes are lacking, the development of sensitive and accurate glucose sensors with an ultralow detection limit is imperative. In this work, the enzyme-free glucose sensor based on three-dimensional (3D) hierarchical Ni microflowers with a Ni­(OH)2 coating layer has been demonstrated in a simple one-step chemical reaction at a low temperature of 80 °C. The as-synthesized materials were characterized by several analytical and spectroscopic techniques. In addition, the thin Ni­(OH)2 layer formed at the surface of the Ni microflower was evidenced by Raman, HRTEM, and XPS, which is the key factor to achieve highly sensitive enzyme-free glucose sensors based on low-cost materials such as copper, nickel, and their oxide and hydroxide. Moreover, our modified electrode exhibits an outstanding detection limit as low as 2.4 nM with an ultrahigh sensitivity of 2392 μA mM–1 cm–2, which is attributed to not only the increased surface area due to the controlled formation of spikes but also the contribution of the Ni­(OH)2 coating layer

    Direct Synthesis of Graphene with Tunable Work Function on Insulators via In Situ Boron Doping by Nickel-Assisted Growth

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    Work function engineering, a precise tuning of the work function, is essential to achieve devices with the best performance. In this study, we demonstrate a simple technique to deposit graphene on insulators with in situ controlled boron doping to tune the work function. At a temperature higher than 1000 °C, the boron atoms substitute carbon sites in the graphene lattice with neighboring carbon atoms, leading to the graphene with a p-type doping behavior. Interestingly, the involvement of boron vapor into the system can effectively accelerate the reaction between nickel vapor and methane, achieving a fast graphene deposition. The changes in surface potential and work function at different doping levels were verified by Kelvin probe force microscopy, for which the work function at different doping levels was shifted between 20 and 180 meV. Finally, the transport mechanism followed by the Mott variable-range hopping model was found due to the strong disorder nature of the system with localized charge-carrier states

    Selection Role of Metal Oxides into Transition Metal Dichalcogenide Monolayers by a Direct Selenization Process

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    Direct reduction of metal oxides into a few transition metal dichalcogenide (TMDCs) monolayers has been recently explored as an alternative method for large area and uniform deposition. However, not many studies have addressed the characteristics and requirement of the metal oxides into TMDCs by the selenization/sulfurization processes, yielding a wide range of outstanding properties to poor electrical characteristics with nonuniform films. The large difference implies that the process is yet not fully understood. In particular, the selenization/sulfurization at low temperature leads to poor crystallinity films with poor electrical performance, hindering its practical development. A common approach to improve the quality of the selenized/sulfurized films is by further increasing the process temperature, thus requiring additional transfer in order to explore the electrical properties. Here, we show that by finely tuning the quality of the predeposited oxide the selenization/sulfurization temperature can be largely decreased, avoiding major substrate damage and allowing direct device fabrication. The direct relationship between the role of selecting different metal oxides prepared by e-beam evaporation and reactive sputtering and their oxygen deficiency/vacancy leading to quality influence of TMDCs was investigated in detail. Because of its outstanding physical properties, the formation of tungsten diselenide (WSe2) from the reduction of tungsten oxide (WOx) was chosen as a model for proof of concept. By optimizing the process parameters and the selection of metal oxides, layered WSe2 films with controlled atomic thickness can be demonstrated. Interestingly, the domain size and electrical properties of the layered WSe2 films are highly affected by the quality of the metal oxides, for which the layered WSe2 film with small domains exhibits a metallic behavior and the layered WSe2 films with larger domains provides clear semiconducting behavior. Finally, an 8′′ wafer scale-layered WSe2 film was demonstrated, giving a step forward in the development of 2D TMDC electronics in the industry

    Wafer-Scale Growth of WSe<sub>2</sub> Monolayers Toward Phase-Engineered Hybrid WO<sub><i>x</i></sub>/WSe<sub>2</sub> Films with Sub-ppb NO<sub><i>x</i></sub> Gas Sensing by a Low-Temperature Plasma-Assisted Selenization Process

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    An inductively coupled plasma (ICP) process was used to synthesize transition metal dichalcogenides (TMDs) through a plasma-assisted selenization process of metal oxide (MO<sub><i>x</i></sub>) at a temperature as low as 250 °C. In comparison with other CVD processes, the use of ICP facilitates the decomposition of the precursors at low temperatures. Therefore, the temperature required for the formation of TMDs can be drastically reduced. WSe<sub>2</sub> was chosen as a model material system due to its technological importance as a p-type inorganic semiconductor with an excellent hole mobility. Large-area synthesis of WSe<sub>2</sub> on polyimide (30 × 40 cm<sup>2</sup>) flexible substrates and 8 in. silicon wafers with good uniformity was demonstrated at the formation temperature of 250 °C confirmed by Raman and X-ray photoelectron (XPS) spectroscopy. Furthermore, by controlling different H<sub>2</sub>/N<sub>2</sub> ratios, hybrid WO<sub><i>x</i></sub>/WSe<sub>2</sub> films can be formed at the formation temperature of 250 °C confirmed by TEM and XPS. Remarkably, hybrid films composed of partially reduced WO<sub><i>x</i></sub> and small domains of WSe<sub>2</sub> with a thickness of ∼5 nm show a sensitivity of 20% at 25 ppb at room temperature, and an estimated detection limit of 0.3 ppb with a <i>S</i>/<i>N</i> > 10 for the potential development of a low-cost plastic/wearable sensor with high sensitivity
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