System Bits: May 27

A technique developed by MIT and University of Michigan researchers might enable advances in display screens and solar cells; engineers at UC Davis have recently demonstrated 3D nanowire transistors for use in integrating semiconductors, such as gallium nitride, on silicon substrates.

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Making sheets of grapheme more easily
Graphene’s promise as a material for new kinds of electronic devices, among other uses, has led researchers around the world to study the material in search of new applications but one of the biggest limitations to wider use of the strong, lightweight, highly conductive material has been the hurdle of fabrication on an industrial scale.

Initial work with the carbon material, which forms an atomic-scale mesh and is just a single atom thick, has relied on the use of tiny flakes, typically obtained by quickly removing a piece of sticky tape from a block of graphite — a low-tech system that does not lend itself to manufacturing. Since then, focus has shifted to making graphene films on metal foil, but researchers have faced difficulties in transferring the graphene from the foil to useful substrates.

To this end, researchers at MIT and the University of Michigan have developed a technique for producing graphene, in a process that lends itself to scaling up, by making graphene directly on materials such as large sheets of glass.

Currently, most of the methods of making graphene first grow the material on a film of metal, such as nickel or copper, and to make it useful, it has to be removed from the metal and put onto a substrate, such as a silicon wafer or a polymer sheet, or something larger like a sheet of glass. However, the process of transferring it has become much more frustrating than the process of growing the graphene itself, and can damage and contaminate the graphene.

The new development still uses a metal film as the template but instead of making graphene only on top of the metal film, it makes graphene on both the film’s top and bottom. The substrate in this case is silicon dioxide, a form of glass, with a film of nickel on top of it.

Work still needs to be done to improve the uniformity and the quality of the graphene to make it useful but the researchers believe the potential is great for use as large-format displays and touch screens, and for ‘smart’ windows that have integrated devices like heaters and sensors. It could also be used for small-scale applications, such as integrated circuits on silicon wafers, if graphene can be synthesized at lower temperatures than were used in the present study.

Illustrated here is a new process for making graphene directly on a nonmetal substrate. First, a nickel layer is applied to the material, in this case silicon dioxide (SiO2). Then carbon is deposited on the surface, where it forms layers of graphene above and beneath the SiO2. The top layer of graphene, attached to the nickel, easily peels away using tape (or, for industrial processes, a layer of adhesive material), leaving behind just the lower layer of graphene stuck to the substrate. (Source: MIT)

Illustrated here is a new process for making graphene directly on a nonmetal substrate. First, a nickel layer is applied to the material, in this case silicon dioxide (SiO2). Then carbon is deposited on the surface, where it forms layers of graphene above and beneath the SiO2. The top layer of graphene, attached to the nickel, easily peels away using tape (or, for industrial processes, a layer of adhesive material), leaving behind just the lower layer of graphene stuck to the substrate.
(Source: MIT)

Nanowire-bridging transistors may open way to next-gen electronics
Developed by engineers at UC Davis, a new approach to integrated circuits, combining atoms of semiconductor materials into nanowires and structures on top of silicon surfaces, shows promise for a new generation of fast, robust electronic and photonic devices. The researchers recently demonstrated 3D nanowire transistors using this approach that open opportunities for integrating semiconductors, such as gallium nitride, on silicon substrates.

Silicon is reaching its limits, the researchers pointed out, and circuits built on conventionally etched silicon have reached their lower size limit, which restricts operation speed and integration density. Additionally, conventional silicon circuits cannot function at temperatures above 250 degrees Celsius (about 480 degrees Fahrenheit), or handle high power or voltages, or optical applications.

The new technology could be used, for example, to build sensors that can operate under high temperatures, for example inside aircraft engines.

Devices that include both silicon and nonsilicon materials offer higher speeds and more robust performance, the UC Davis engineers pointed out. Conventional microcircuits are formed from etched layers of silicon and insulators, but it’s difficult to grow nonsilicon materials as layers over silicon because of incompatibilities in crystal structure (or “lattice mismatch”) and differences in thermal properties.

As such, the researchers created silicon wafers with “nanopillars” of materials such as gallium arsenide, gallium nitride or indium phosphide on them, and grown tiny nanowire “bridges” between nanopillars — and have been able to make these nanowires operate as transistors, and combine them into more complex circuits as well as devices that are responsive to light. They have developed techniques to control the number of nanowires, their physical characteristics and consistency.

The suspended structures have other advantages such as being easier to cool and handle thermal expansion better than planar structures — a relevant issue when mismatched materials are combined in a transistor.

The technology also leverages the well-established technology for manufacturing silicon integrated circuits, instead of having to create an entirely new route for manufacturing and distribution, they added.