Large-scale growth of high-quality hexagonal boron nitride has been a challenge in two-dimensional-material-based electronics. Herein, we present wafer-scale and wrinkle-free epitaxial growth of multilayer hexagonal boron nitride on a sapphire substrate by using high-temperature and low-pressure chemical vapor deposition. Microscopic and spectroscopic investigations and theoretical calculations reveal that synthesized hexagonal boron nitride has a single rotational orientation with AA' stacking order. A facile method for transferring hexagonal boron nitride onto other target substrates was developed, which provides the opportunity for using hexagonal boron nitride as a substrate in practical electronic circuits. A graphene field effect transistor fabricated on our hexagonal boron nitride sheets shows clear quantum oscillation and highly improved carrier mobility because the ultraflatness of the hexagonal boron nitride surface can reduce the substrate-induced degradation of the carrier mobility of two-dimensional materials.
Fast-growth of single crystal monolayer graphene by CVD using methane and hydrogen has been achieved on "homemade" single crystal Cu/Ni(111) alloy foils over large area. Full coverage was achieved in 5 min or less for a particular range of composition (1.3 at.% to 8.6 at.% Ni), as compared to 60 min for a pure Cu(111) foil under identical growth conditions. These are the bulk atomic percentages of Ni, as a superstructure at the surface of these foils with stoichiometry CuNi (for 1.3 to 7.8 bulk at.% Ni in the Cu/Ni(111) foil) was discovered by low energy electron diffraction (LEED). Complete large area monolayer graphene films are either single crystal or close to single crystal, and include folded regions that are essentially parallel and that were likely wrinkles that "fell over" to bind to the surface; these folds are separated by large, wrinkle-free regions. The folds occur due to the buildup of interfacial compressive stress (and its release) during cooling of the foils from 1075 °C to room temperature. The fold heights measured by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) prove them to all be 3 layers thick, and scanning electron microscopy (SEM) imaging shows them to be around 10 to 300 nm wide and separated by roughly 20 μm. These folds are always essentially perpendicular to the steps in this Cu/Ni(111) substrate. Joining of well-aligned graphene islands (in growths that were terminated prior to full film coverage) was investigated with high magnification SEM and aberration-corrected high-resolution transmission electron microscopy (TEM) as well as AFM, STM, and optical microscopy. These methods show that many of the "join regions" have folds, and these arise from interfacial adhesion mechanics (they are due to the buildup of compressive stress during cool-down, but these folds are different than for the continuous graphene films-they occur due to "weak links" in terms of the interface mechanics). Such Cu/Ni(111) alloy foils are promising substrates for the large-scale synthesis of single-crystal graphene film.
The decrease in signal processing speed due to increased resistance and capacitance delay resulting from aggressive miniaturisation of logic and memory devices is a major obstacle for continued down scaling of electronics. 1-3 In particular, minimizing the dimensions of interconnects -metal wires that connect different device components on the chip -is crucial for device scaling. The interconnects are isolated from each other by nonconducting or dielectric layers. Much of the recent research has focused on decreasing the resistance of scaled interconnects because integration of dielectrics using complementary metal oxide semiconductor (CMOS) compatible processes has proven to be exceptionally challenging. The key requirements for interconnect isolation materials are that they should possess low relative dielectric constants (referred to as values), serve as diffusion barriers against migration of interconnect metals such as cobalt into semiconductors and be thermally, chemically and mechanically stable. In 2005, the International Roadmap for Devices and Systems (IRDS) recommended dielectrics with -values of < 2.2 and the most recent report recommends dielectric values of ≤ 2 by 2028. 4 Despite this, state-of-the-art low- materials, such as silicon oxide derivatives, organic compounds, and aerogels exhibit values > 2 and possess poor thermomechanical properties. 5 Here, we report a dielectric thin film with ultra-low values of1.78 and 1.16 -close to that of air ( = 1) -at 100 kHz and 1 MHz, respectively, in amorphous boron nitride (a-BN) obtained using CMOS compatible low temperature process. We demonstrate that 3 nm thin a-BN is mechanically and electrically robust with breakdown strength of 7.3 MV/cm -exceeding requirements. Cross-sectional transmission electron microscopy reveals that a-BN is able to prevent diffusion of cobalt interconnect atoms into silicon under very harsh accelerated conditions -in contrast with
Digital logic circuits are based on complementary pairs of n-and p-type field effect transistors (FETs) via complementary metal oxide semiconductor (CMOS) technology. In three dimensional (3D or bulk) semiconductors, substitutional doping of acceptor or donor impurities is used to achieve p-and n-type FETs. However, the controllable p-type doping of low-dimensional semiconductors such as two-dimensional transition metal dichalcogenides (2D TMDs) has proved to be challenging. Although it is possible to achieve high quality, low resistance n-type van der Waals (vdW) contacts on 2D TMDs 1-5 , obtaining p-type devices from evaporating high work function metals onto 2D TMDs has not been realised so far.Here we report high-performance p-type devices on single and few-layered molybdenum
We report on an insulating two-dimensional material, hexagonal boron nitride (h-BN), which can be used as an effective wrapping layer for surface-enhanced Raman spectroscopy (SERS) substrates. This material exhibits outstanding characteristics such as its crystallinity, impermeability, and thermal conductance. Improved SERS sensitivity is confirmed for Au substrates wrapped with h-BN, the mechanism of which is investigated via h-BN thickness-dependent experiments combined with theoretical simulations. The investigations reveal that a stronger electromagnetic field can be generated at the narrowed gap of the h-BN surface, which results in higher Raman sensitivity. Moreover, the h-BN-wrapped Au substrate shows extraordinary stability against photothermal and oxidative damages. We also describe its capability to detect specific chemicals that are difficult to analyze using conventional SERS substrates. We believe that this concept of using an h-BN insulating layer to protect metallic or plasmonic materials will be widely used not only in the field of SERS but also in the broader study of plasmonic and optoelectronic devices.
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