Researchers Discover Way To Grow Silicon Nanowires

A research group led by Raj Solanki, Ph.D., professor of electrical engineering professor in OHSU's OGI School of Science & Engineering, recently demonstrated it is possible to grow silicon nanowires exactly where you want them on an electrode using electrical fields. Solanki's team also can grow silicon-based nanowires in the exact direction necessary to fabricate electronic devices.

The researchers now are exploring electrical properties of the silicon nanowires. "Now that we know we can grow silicon nanowires in a precise location and in a specific direction, we want to know what happens to the nanowire when it contacts the metal on the electrode," said Solanki. "We also are studying how any kind of coating or contamination on the nanowire surface affects the passage of charges through it.

"These kinds of factors determine the performance of nanoelectronic devices, so we need to thoroughly understand and perfect this technological advancement before any devices with silicon and silicon-based nanowires can be mass produced," he said. "In addition, a better understanding of the effect of contamination on the nanowire can lead to development of very sensitive sensors for a wide range of applications, such as environmental pollution to bio-toxins."

Solanki's is the only lab in the Northwest studying silicon nanowires. His research is funded by Intel Corporation and supported by Sharp Laboratories and FEI Corporation.

Oregon has high hopes for nanotechnologies. In December 2003 President Bush signed into law the 21st Century Nanotechnology Research and Development Act. Sponsored by Oregon Sen. Ron Wyden, the act authorizes $3.7 billion over four years for nanotech research and development, beginning in 2005.

During the past 40 years, computer technology has undergone a revolution, driven by the miniaturization of the silicon transistor. An increase in the number of transistors (the fundamental component of most active electronic circuits) per chip has led to an increase in computer power and a decrease in manufacturing and retail costs.

The trend of doubling the number of transistors on a chip about every 18 months (Moore's Law) was predicted by Intel's Gordon Moore in 1965 and has held fairly true. But as device dimensions rapidly approach the nanometer (one billionth of a meter) scale, the traditional electrical engineering methods and materials are being pushed to their physical limits, and most experts now believe Moore's law cannot continue beyond the 2010 to 2015 time frame.

"A completely new approach needs to be developed to go beyond the current limit," noted Solanki. "One possible solution is to develop electronic devices that incorporate silicon nanowires or carbon nanotubes as active components operating under physics laws of quantum mechanics."

Silicon nanowires are typically between 5 and 20 nanometers in diameter (about 1,000 times smaller than a human hair) and can be up to several micrometers (one micrometer equals one thousandth of a millimeter or one millionth of a meter) long. On photos taken via electron microscope, the silicon nanowires resemble skinny needles.

Unlike semiconductor silicon nanowires, carbon nanotubes can be either semiconductor or metallic, and are difficult to dope -- the process of deliberately introducing impurities to change electrical behavior. For those reasons, the OGI team is focusing on silicon nanowires, which also would make it easier for the microelectronic industry to adopt this technology.

Research at other institutions involves growing nanowires or nanotubes in a chamber separate from the silicon integrated circuits, then forming a liquid suspension and flowing it over silicon wafers that have prefabricated electrodes. Some of the nanowires or nanotubes grown in this way will settle between desired electrodes, which are then fabricated into devices such as transistors. This method uses only a small fraction of the nanowires or nanotubes and is time-consuming and expensive for mass production, noted Solanki.

"Growing silicon nanowires in a specific location in whatever direction you desire, which we have done, is much more practical for gigascale integration -- putting a billion transistors on a chip -- in the long term," said Solanki.

Solanki grows his silicon nanowires in a quartz reactor using a technique developed decades ago by Bell Labs called vapor-liquid-solid deposition. "The addition of the electrical fields is what's new," said Solanki. "We have also grown nickel silicide conducting nanowires, which will be useful for contacting the silicon semiconductor nanowires."

Solanki has been on the OGI faculty since 1986. His current work on nanowires is an extension of OGI's research on atomic layer deposition. OGI was one of the first universities to investigate atomic layer deposition for growing extremely high-quality thin films, one atomic layer at a time. Such a technique is ideal for the growth of nanoscale films.

Besides growing ultra-thin films for fabricating nanowire devices, Solanki and his team at OGI (John Freeouf, Ph.D, and John Carruthers, Ph.D.) recently have demonstrated that atomic layer deposition can be used to grow semiconductor heterostructure nanowires consisting of very thin alternating layers of two materials, which has potential for optoelectronics applications.

Atomic layer deposition recently has been recognized by the microelectronic industry as the technology that will be required for fabrication of nanoscale electronic devices. The OGI research team also collaborates with Portland State University scientists Shankar Rananavare, Ph.D., Jun Jiao, Ph.D., and Rolf Konenkamp, Ph.D.

OGI and OHSU expect to play a major role in bringing the capabilities of nanostructured devices into the electronics and biomedical industries through a commitment to applied research and educational programs. For more information about silicon nanowires or electrical engineering at the OGI School of Science & Engineering, visit http://cse.ogi.edu/edu.

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The OGI School of Science & Engineering (formerly the Oregon Graduate Institute of Science & Technology) became one of four schools of Oregon Health & Science University in 2001.

Note: Photos of Solanki and silicon nanowires are available at http://www.ohsu.edu/news/2004/022404nano.html. To see a video clip, go to http://www.ogi.edu/about/videos/nanotechnology.wmv