December 10, 2009 - Researchers at UCLA and IBM say they have successfully grown silicon germanium semiconducting nanowires that take a step closer to using nanowires in next-generation electronic devices.

Frames showing a ledge moving along the interface between a catalyst and a 21nm diameter Si wire during VSS growth at 490°C with 5×10-6 Torr Si2H6. (A-C) A ledge, arrowed, propagates along the interface; (D) the ledge reaches the end of the interface; (E) a new ledge appears. The images are labeled with the elapsed time since the appearance of the first ledge. The growth rate of the wire is 2.8nm/min. On average a new ledge forms every 12.7 seconds and crosses the interface at 1.5nm/sec; the next ledge forms after the previous one has crossed the interface. Note that ledge flow can not be clearly resolved in TEM if the ledge moves parallel to the viewing direction, so the incubation time for nucleation of a ledge could be shorter than the time between (D) and (E), 3.4 sec. (Source: Science)

Their work, published in the Nov. 27 issue of the journal Science, involved creating nanowires with layers of different materials (Si and Ge) that were defect-free and one-atom-thick sharpness at the junction, which they note are "critical requirements" for efficient transistors. The work also has application in thermoelectrics (converting heat to electricity); "the Jet Propulsion Laboratory uses bulk chunks of silicon-germanium to power their satellites, and now there is a lot of interest in using a similar technology in automobiles," according to Suneel Kodambaka, UCLA professor of materials science and engineering, in a statement.

In their work, they used tiny particles of a gold-aluminum alloy (instead of conventional liquid semiconductor-metal eutectic droplets), heated to >370°C and melted inside a vacuum chamber; a "silicon-containing gas" was then introduced to precipitate the silicon and form wires under the droplets. A germanium-containing gas was used to form the germanium wires. Cooling the liquid droplets into solid form enabled excess silicon to be removed from the alloy; then Ge wire segments could be grown on the silicon with introduction of germanium vapor, and sharp interfaces formed, explained Kodambaka.

From the journal paper abstract:

We demonstrated single interfaces that are defect-free and close to atomically abrupt, as well as quantum dots (i.e., Ge layers tens of atomic planes thick) embedded within Si wires. Real-time imaging of growth kinetics reveals that a low solubility of Si and Ge in the solid particle accounts for the interfacial abruptness. Solid catalysts that can form functional group IV nanowire-based structures may yield an extended range of electronic applications.

"This study is significant because it provides a solution to the problem of growing sharp interfaces in nanowires, thereby addressing an important limitation in the growth of nanowires," stated Frances Ross, manager of IBM's nanoscale materials analysis department and corresponding author of the study.

Next phase of work will involve scaling up area of the nanowire growth in a conventional growth reactor, instead of under a microscope, and properties will need to be improved to be comparable to conventional nanowires. At that point work will shift to explore new devices and different metal alloys for making devices, Ross said.