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NANOELECTRONICS
"In the world of fabrication, it's not that hard to make really small structures" such as nanowires, says chemistry professor James R. Heath of California Institute of Technology. What's hard is making them "really close together." Heath and his coworkers, including postdoc Nicholas A. Melosh and Pierre M. Petroff, a professor in the materials department at the University of California, Santa Barbara, have now devised a general method for producing ultrahigh-density arrays of aligned nanowires and nanowire circuits [Science, published online March 13, http://www.sciencemag.org/cgi/content/abstract/1081940v1]. These arrays--some containing hundreds of aligned nanowires 2 to 3 mm long--are dramatically denser than previous arrays, Heath tells C&EN. The highest density pattern they report consists of 20 wires that are 8 nm across and 8 nm apart. "The results are indeed very impressive," comments chemist Peidong Yang of UC Berkeley. "It's highly innovative work." Heath's fabrication process can be carried out more than once on the same substrate to produce simple circuits of crossed nanowires. When a layer of organic molecules is inserted between the crossed arrays, each crossing point will function as a molecular electronic device, he says. The density of these crossbar arrays will allow the researchers to cram 100 billion or more devices--memories and logic circuits--into a single square centimeter. Lithographic techniques can't produce such a high density of wires and circuits, Heath points out. The method his team developed relies instead on using the edge of a commercially available superlattice (multilayered film) as a stamp for transferring nanowire arrays onto a substrate. The precisely controlled thickness of the superlattice's layers determines the widths and separations of the deposited nanowires. The nanowires can be made from a variety of metals and, if desired, are easily translated into semiconductor nanowires. One of the biggest challenges in this field has been finding a way to address every single nanowire in such a dense array. Heath's group has figured out how to do this, too, he says. "We can use big wires and tiny connections to address individual nanowires that are much more closely spaced than our lithographic capabilities allow us to get to." The team has demonstrated this concept, known as a binary tree multiplexer, by fabricating a 320-bit molecular memory. But none of that work is described in the latest Science paper. Heath is saving that for a subsequent paper, so stay tuned.
IT'S A SNAP A method for making ultrahigh-density nanowire arrays starts with a GaAs/AlGaAs superlattice. The AlGaAs layers are selectively etched so that they are recessed. Metal is deposited onto the protruding GaAs edges, forming nanowires. When the superlattice is used as a stamp, the nanowires are imprinted onto an adhesive layer coating a silicon wafer. The superlattice is then etched away. |
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