Scientists develop new homoepitaxial graphene tunnel barrier/transport channel spintronic device

Scientists at the U.S. Naval Research Laboratory (NRL) have created a new type of room-temperature tunnel device structure in which the tunnel barrier and transport channel are made of the same material, graphene. Such functionalized homoepitaxial structures provide an elegant approach for realization of graphene-based spintronic, or spin electronic, devices. The research results are reported in a paper published in the journal ("Hydrogenated Graphene as a Homoepitaxial Tunnel Barrier for Spin and Charge Transport in Graphene"). Low temperature and room temperature operation of the homoepitaxial graphene spin valve (left) and a schematic (right) of one of the homoepitaxial fluorinated graphene/graphene spin valve devices. Distinct steps in the resistance appear at the coercive fields of the ferromagnetic contacts, producing plateaus of higher resistance when the ferromagnetic contact magnetizations are antiparallel, as indicated by the black arrows. Only a 50% decrease in magnitude is observed from 10 K to room temperature. The top layers of graphene are used as a tunnel barrier. It is hydrogenated to decouple it from the bottom layer of graphene, which is the spin transport channel. Ferromagnetic permalloy (NiFe - red) contacts inject and detect the spin in the channel. The gold contacts are ohmic reference contacts (Ti/Au). (Image: U.S. Naval Research Laboratory) (click on image to enlarge) The NRL team shows that hydrogenated graphene, a hydrogen-terminated single atomic layer of carbon atoms arranged in a two-dimensional honeycomb array, acts as a tunnel barrier on another layer of graphene for charge and spin transport. They demonstrate spin-polarized tunnel injection through the hydrogenated graphene, and lateral transport, precession and electrical detection of pure spin current in the graphene channel. The team further reports higher spin polarization values than found using more common oxide tunnel barriers, and spin transport at room temperature. In spite of nearly a decade of research on spin transport in graphene, there has been little improvement in important metrics such as the spin lifetime and spin diffusion length, and reported values remain far below those predicted by theory based on graphene's low atomic number and spin-orbit coupling. Understanding the extrinsic limiting factors and achieving the theoretically predicted values of these metrics is key for enabling the type of advanced, low-power, high performance spintronic devices envisioned beyond Moore's law. Scattering caused by tunnel barriers, which are essential for solving the conductivity mismatch problem for electrical spin injection from a ferromagnetic metal into a semiconductor, is a topic that is just now attracting attention. Uniform, pinhole/defect free tunnel barriers on graphene are not easily attained with the conventional methods that use oxides. Hydrogenation of graphene offers an alternative method to achieve a homoepitaxial tunnel barrier on graphene. In contrast with fluorination and plasma treatments, the chemical hydrogenation process developed by team member Dr. Keith Whitener provides a rapid, gentler and more stable functionalization with much higher hydrogen coverage. Moreover, recent studies, also by NRL teams, show that hydrogenated graphene could be magnetic, which could be used to control spin relaxation in the graphene. Because of its extremely low spin-orbit coupling, such control has been difficult. "These new hydrogenated graphene homoepitaxial devices solve many of the issues plaguing graphene spintronics and, with the room temperature operation and possible control with magnetic moments, offer distinct advantages over previous structures for integration with modern electronics architectures," explains Dr. Adam Friedman, lead author of the study. The NRL scientists use chemical vapor deposition to grow and then sequentially deposit a four-layer (only 4 atoms thick) graphene stack. They then hydrogenate the top few layers so that they serve as a tunnel barrier for both charge and spin injection into the lower graphene channel. They deposit ohmic (gold) and ferromagnetic permalloy (red) contacts as shown in the figure, forming a non-local spin valve structure. When the scientists apply a bias current between the left two contacts, a spin-polarized charge current tunnels from the permalloy into the graphene transport channel, generating a pure spin current that diffuses to the right. This spin current is detected as a voltage on the right permalloy contact that is proportional to the degree of spin polarization and its orientation. The vectorial character of spin (compared to the scalar character of charge) provides additional mechanisms for the control and manipulation needed for advanced information processing. The NRL team demonstrated the higher spin injection efficiency (16.5%) than most previous graphene spin devices, determined spin lifetimes with the Hanle effect, and observed only a 50% loss in spin valve signal from 10 K to room temperature (left graph).

read more "Scientists develop new homoepitaxial graphene tunnel barrier/transport channel spintronic device"

New technique to synthesise nanostructured nanowires

A new approach to self-assemble and tailor complex structures at the nanoscale, developed by an international collaboration led by the University of Cambridge and IBM, opens opportunities to tailor properties and functionalities of materials for a wide range of semiconductor device applications. The researchers have developed a method for growing combinations of different materials in a needle-shaped crystal called a nanowire. Nanowires are small structures, only a few billionths of a metre in diameter. Semiconductors can be grown into nanowires, and the result is a useful building block for electrical, optical, and energy harvesting devices. The researchers have found out how to grow smaller crystals within the nanowire, forming a structure like a crystal rod with an embedded array of gems. Details of the new method are published in the journal ("Synthesis of nanostructures in nanowires using sequential catalyst reactions"). Images recorded in the electron microscope showing the formation of a nickel silicide (NiSi2) nanoparticle (colored yellow) in a silicon nanowire. (Image: Stephan Hofmann) “The key to building functional nanoscale devices is to control materials and their interfaces at the atomic level,” said Dr Stephan Hofmann of the Department of Engineering, one of the paper’s senior authors. “We’ve developed a method of engineering inclusions of different materials so that we can make complex structures in a very precise way.” Nanowires are often grown through a process called Vapour-Liquid-Solid (VLS) synthesis, where a tiny catalytic droplet is used to seed and feed the nanowire, so that it self-assembles one atomic layer at a time. VLS allows a high degree of control over the resulting nanowire: composition, diameter, growth direction, branching, kinking and crystal structure can be controlled by tuning the self-assembly conditions. As nanowires become better controlled, new applications become possible. The technique that Hofmann and his colleagues from Cambridge and IBM developed can be thought of as an expansion of the concept that underlies conventional VLS growth. The researchers use the catalytic droplet not only to grow the nanowire, but also to form new materials within it. These tiny crystals form in the liquid, but later attach to the nanowire and then become embedded as the nanowire is grown further. This catalyst mediated docking process can ‘self-optimise’ to create highly perfect interfaces for the embedded crystals. To unravel the complexities of this process, the research team used two customised electron microscopes, one at IBM’s TJ Watson Research Center and a second at Brookhaven National Laboratory. This allowed them to record high-speed movies of the nanowire growth as it happens atom-by-atom. The researchers found that using the catalyst as a ‘mixing bowl’, with the order and amount of each ingredient programmed into a desired recipe, resulted in complex structures consisting of nanowires with embedded nanoscale crystals, or quantum dots, of controlled size and position. “The technique allows two different materials to be incorporated into the same nanowire, even if the lattice structures of the two crystals don’t perfectly match,” said Hofmann. “It’s a flexible platform that can be used for different technologies.” Possible applications for this technique range from atomically perfect buried interconnects to single-electron transistors, high-density memories, light emission, semiconductor lasers, and tunnel diodes, along with the capability to engineer three-dimensional device structures. “This process has enabled us to understand the behaviour of nanoscale materials in unprecedented detail, and that knowledge can now be applied to other processes,” said Hofmann.

read more "New technique to synthesise nanostructured nanowires"

On the way to breaking the terahertz barrier for graphene nanoelectronics

A team of scientists at the Max Planck Institute for Polymer Research (MPI-P) discovered that electrical conduction in graphene on the picosecond timescale - a picosecond being one thousandth of one billionth of a second - is governed by the same basic laws that describe the thermal properties of gases. This much simpler thermodynamic approach to the electrical conduction in graphene will allow scientists and engineers not only to better understand but also to improve the performance of graphene-based nanoelectronic devices. Interaction of the terahertz field with graphene leads to efficient electron heating, which in turn strongly changes graphene conductivity. (Image: Zoltan Mics / MPIP) The researchers found that the energy of ultrafast electrical currents passing through graphene is very efficiently converted into electron heat, making graphene electrons behave just like a hot gas. "The heat is distributed evenly over all electrons. And the rise in electronic temperature, caused by the passing currents, in turn has a strong effect on the electrical conduction of graphene" explains Professor Mischa Bonn, Director at the MPI-P. The study, entitled "Thermodynamic picture of ultrafast charge transport in graphene", has recently been published in . Graphene - a single sheet of carbon atoms - is known to be a very good electrical conductor. As a result, graphene finds a multitude of applications in modern nanoelectronics. They range from highly efficient detectors for optical and wireless communications to transistors operating at very high speeds. A constantly increasing demand for telecommunication bandwidth requires an ever faster operation of electronic devices, pushing their response times to be as short as a picosecond. "The results of this study will help improve the performance of graphene-based nanoelectronic devices such as ultra-high speed transistors and photodetectors" says Professor Dmitry Turchinovich, who led the research at the MPI-P. In particular they show the way for breaking the terahertz operation speed barrier - i.e. one thousand billions of oscillations per second - for graphene transistors.

read more "On the way to breaking the terahertz barrier for graphene nanoelectronics"