HOUGHTON, MI - In high-end electronics, where success hinges on being lighter, faster, and smaller, size is the biggest barrier to progress. But when computer circuitry and other tiny devices get to be too small, Newton's comfortable laws yield to the quirky world of quantum physics, where nothing acts like it did before and nobody knows what's going to happen.
So last year, Mohan Krishnamurthy and his fellow researchers at Michigan Tech University had no particular expectations as they completed their latest experiment, in which they'd carefully applied a film of a tin-germanium alloy several atomic layers thick to a two-inch disk of germanium (a silicon-like element). Tin and germanium don't get along very well, metallurgically speaking, so the experiment's prospects were unusually cryptic.
"We knew something funny would happen," Krishnamurthy recalls. "But we never expected this."
What they discovered were little tin "worms" digging neat little ditches through the alloy down to the level of the pure germanium. "Like an earthworm, the globs of tin eat up the alloy, spit out the germanium, and keep the tin," he said. And this was no random tangle of trenches. Like soldiers on parade march, the worms had dug out a series of wobbly straight lines and right-angle turns. And, when they finally halted their excavations, they hadn't created the world's smallest circuitboard, exactly; instead, it looked more like an artist's fantasy of how such a circuitboard might look. How small was it? The trenches were 8 nanometers deep, each flanked by tiny mounds of germanium only 4 nanometers high. Plus, they were amazingly long by nano-standards, up to about 10 microns in length.
The researchers' work appeared in Physical Review Letters and later in the February 13 edition of Science, which published a short article on page 991 under the heading "Quantum Etch-A-Sketch."
Krishnamurthy, an assistant professor in the MTU Department of Metallurgical and Materials Engineering, is a leader in the new field of epitaxial self-assembly, in which a very thin film of one type of substance is applied on top of another. When conditions are right, the top film buckles in a very precise way, forming tiny mountains, islands, or other nanostructures on top of the substrate. The goal is to develop a pattern of these nanostructures that have applications in microelectronics, similar to the way wires conduct electricity in a circuit.
Since he coauthored a groundbreaking paper on the subject in 1993, Krishnamurthy has concentrated his efforts on films and substrates made of silicon, the element of choice for the electronics industry. However, he notes, at the nano-level--anything smaller than 100 nanometers, or one-tenth of a micron--quantum physics kicks in, which opens up all kinds of new possibilities. For instance, the properties of those quantum germanium mounds dug up by the tin earthworms could be far different from ridges of regular-sized germanium. As an example, nano-germanium might turn out to be a good light-absorber, and thus have potential as a laser.
"All kinds of new properties could come out of this," Krishnamurthy said, adding, "There's a lot of work that needs to be done."
Krishnamurthy credits Professor Stephen Hackney, an expert in the phase transformation of metals, for helping determine the underlying rationale for the tin worms' excavating behavior. PhD students Xurui "Sherry" Deng and Becky Yang also contributed to the research project.
The above story is based on materials provided by Michigan Technological University. Note: Materials may be edited for content and length.
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