Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

The outstanding electrical, mechanical and chemical properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates. The films have sheet resistances as low as approximately 125 ohms square(-1) with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as approximately 30 ohms square(-1) at approximately 90% transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.

/pdf/roll-to-roll-production-of-30-inch-graphene-films-for-3syrg1ob8a.pdf

Roll-to-roll production of 30-inch graphene films for transparent electrodes

Graphene nanoribbons, narrow straight-edged strips of the single-atom-thick sheet form of carbon, are predicted to exhibit remarkable properties, making them suitable for future electronic applications. Before this potential can be realized, more chemically precise methods of production will be required. Cai et al. report a step towards that goal with the development of a bottom-up fabrication method that produces atomically precise graphene nanoribbons of different topologies and widths. The process involves the deposition of precursor monomers with structures that 'encode' the topology and width of the desired ribbon end-product onto a metal surface. Surface-assisted coupling of the precursors into linear polyphenylenes is then followed by cyclodehydrogenation. Given the method's versatility and precision, it could even provide a route to more unusual graphene nanoribbon structures with tuned chemical and electronic properties. Graphene nanoribbons (GNRs) have structure-dependent electronic properties that make them attractive for the fabrication of nanoscale electronic devices, but exploiting this potential has been hindered by the lack of precise production methods. Here the authors demonstrate how to reliably produce different GNRs, using precursor monomers that encode the structure of the targeted nanoribbon and are converted into GNRs by means of surface-assisted coupling. Graphene nanoribbons—narrow and straight-edged stripes of graphene, or single-layer graphite—are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices1,2,3. In particular, although the two-dimensional parent material graphene4,5 exhibits semimetallic behaviour, quantum confinement and edge effects2,6 should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical7,8,9, sonochemical10 and lithographic11,12 methods as well as through the unzipping of carbon nanotubes13,14,15,16, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling17,18 of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation19,20. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots21, superlattice structures22 and magnetic devices based on specific graphene nanoribbon edge states3.

Atomically precise bottom-up fabrication of graphene nanoribbons

Graphene nanoribbons—narrow and straight-edged stripes of graphene, or single-layer graphite—are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices. In particular, although the two-dimensional parent material graphene exhibits semimetallic behaviour, quantum confinement and edge effects should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical, sonochemical and lithographic methods as well as through the unzipping of carbon nanotubes, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots, superlattice structures and magnetic devices based on specific graphene nanoribbon edge states.

/pdf/atomically-precise-bottom-up-fabrication-of-graphene-4p6ixf0dyo.pdf

Atomically Precise Bottom-up Fabrication of Graphene Nanoribbons JINMING CAI, Empa, Swiss Federal Laboratories for Materials Science and Technology

The deposition of a metal onto a semiconductor surface to provide low contact resistance, high-reliability electrical contacts without adversely affecting the device during the metallization process is one of the most important challenges in device fabrication. Consequently, a fundamental understanding of how contacts work is essential for successful device manufacturing and commercialization. The physics of carrier transport across the metal-semiconductor junction renders metal contacts either rectifying (a.k.a. Schottky) or nonrectifying. A nonrectifying contact whose relationship between current and voltage has a low interfacial contact resistance Rc, and is preferably linear, is referred to as an Ohmic contact. Achieving low contact resistance Rc (Ω mm) or contact resistivity ρc (Ω cm2) has required a great amount of investigation for every relevant semiconductor material in the past. Typically, the successful formation of an Ohmic contact has relied on three constituent requirements: highly or degenerately doped semiconductor, choice of metallization, and thermal annealing. Recent advances in bulk growth of single-crystal β-Ga2O3 have changed how this material is regarded in the device community and have brought it into the spotlight as an emerging wide bandgap semiconductor with potential for radio frequency (rf) and power electronics applications. The academic study of the mechanism of forming Ohmic contacts to Ga2O3 has been a less pressing topic while fundamental issues in growth and devices have been addressed. A sufficient body of literature to date has established a preliminary understanding of how Ohmic contacts to Ga2O3 work. In this chapter, we review the fundamentals of Ohmic contact operation, review the existing body of literature on Ohmic contacts to Ga2O3, and recommend future directions of research.

Ohmic contacts to gallium oxide

High-yield wafer bonding with minimal bonding imperfections and high-aspect etch process of 4H-SiC/SiO2/4H-SiC stacks is presented The top 4H-SiC layer is thinned, patterned, etched, and is then ready to be released to form the MEMS device layer An anti-bosch process is used to etch high aspect ratio trenches within the 4H-SiC device layer in the presence of buried oxide; the oxide serves both as a sacrificial layer and as an etch stop The etch yields an aspect ratio of 105 (429 μm etch depth : 41 μm mask opening) within the 4H-SiC device layer The handle wafer is 4H-SiC, as opposed to Si, which allows for all SiC-based MEMS devices for high shock resistant and temperature applications

SiC Wafer Bonding and Deep Reactive Ion Etching Towards High-Aspect Ratio SiC MEMS Fabrication

Thermoreflectance Temperature Mapping of Ga2O3 Schottky Barrier Diodes

We considered electron trapping and detrapping processes in the AlGaN barrier of AlGaN/GaN high electron mobility transistors as that trapping is the process of electrons directly tunneling from the gate metal into the AlGaN barrier traps while detrapping is electrons emission from the traps into the AlGaN conduction band by phonon-assisted tunneling. By fully coupling the electron emission characteristic time with the device thermal and electrical behavior, simulations were performed to comprehensively analyze the contributions of device bias, accounting for self-heating and electric field dependent electron detrapping. Discussions on the current-transient method for traps characterization based on these simulations and impact of traps-dependent reverse gate current on drain current collapse are presented.

https://iopscience.iop.org/article/10.1088/0268-1242/30/1/015010/pdf

Simulation of temperature and electric field-dependent barrier traps effects in AlGaN/GaN HEMTs

Valence and conduction band offsets of atomic layer deposition (ALD) Al2O3 deposited on bulk AlN crystals were determined using x-ray photoelectron spectroscopy to be ΔEV = 0.75 eV and ΔEC = −1.45 eV, with a measured energy gap of the Al2O3 film of 6.9 eV. In addition, crystalline AlN deposited by atomic layer epitaxy on sapphire was evaluated, resulting in a valence band offset of ΔEV = −0.75 eV and a conduction band offset of ΔEC = 3.25 eV due to the wider bandgap of the crystalline Al2O3 substrate compared to amorphous ALD Al2O3. Both heterojunctions exhibited type-II behavior and similar valence band offsets.

Marko J. Tadjer

Papers

Ohmic contacts to gallium oxide

SiC Wafer Bonding and Deep Reactive Ion Etching Towards High-Aspect Ratio SiC MEMS Fabrication

Thermoreflectance Temperature Mapping of Ga2O3 Schottky Barrier Diodes

Simulation of temperature and electric field-dependent barrier traps effects in AlGaN/GaN HEMTs

Band offset determination for amorphous Al2O3 deposited on bulk AlN and atomic-layer epitaxial AlN on sapphire