Nanochains could yield single-electron transistors
By Paul Kallender
EE Times
(08/22/01 18:38 p.m. EST)
OSAKA, Japan — Researchers at Osaka University believe that cheaply-produced crystalline nanochains could lead to the development of single-electron transistors (SETs) and nanoscale photon devices such as field-effect transistors (FETs) within 10 years.
Professor Seiji Takeda and Hideo Kohno, both members of the university's department of physics, have bulk-produced carpets of micrometer long chains of alternating crystalline globules and silicon dioxide stems that they believe could vie with nanotubes to produce next-generation semiconductor circuits.
The chains consist of clusters of electricity-conducting crystal globules, averaging about 10 nanometers in diameter, and insulating silicon dioxide "joints" that average 7-to-8 nanometers in diameter. Takeda said he believes the globules can demonstrate quantum confinement, while the chains will be capable of quantum tunneling effects. At the moment, the distance between the crystal sphere nodes on the chain averages about 30 nm. While this length is too distant for electron tunneling, Takeda said he is confident his team can juggle the growth process to bring the clusters closer to the 2-nm distance needed to promote the tunneling effect.
"If we produce such nanochains, I am sure this technology has a strong potential for the development of SETs," Takeda told EE Times. "Also, the structure of the silicon crystalline spheres separated by these thin oxide links is similar to the construction of FETs," he said.
Using little more than a furnace, a vacuum chamber and a quartz container, Takeda and Kohno discovered that a standard vapor liquid solid (VLS) crystal growth technique with a lead-gold combination catalyst and a two-step anneal produced a "lawn" of the nanochains on silicon substrate.
The beauty of the discovery, said Takeda, was its simplicity and cost. Unlike the massive and highly publicized work by research labs around the globe into nanotube technology, Osaka University's team consists of Takeda, Kohno and an assistant. The group gets by on a budget of about $25,000 per year, and was kicked into motion by accident.
Peculiar discovery
Peering through a microscope back in 1997, Kohno and Takeda discovered a single peculiar silicon "wire" among a sludge of byproducts; it was a nanochain. After juggling the catalysts and annealing processes, the team has since grown "lawns" of the tubes over 40 times and has made great strides to perfect the process, Takeda said.
"What we get is very dense, it's like a carpet full of nanochains that covers the substrate," he said.
The chains are formed by a simple "eat and excrete" VLS process, he explained. A tiny piece of gold wire is evaporated in a vacuum to form small gold islands that act as catalysts on the substrate surface. These are then put into a silica tube. Then they are heated, cooled, re-encapsulated in new silicon, and reheated.
During heating, tiny droplets of gold and lead absorb the silicon gas until it becomes supersaturated. At that point, the saturated globule starts acting like a tiny slow motion Pac Man, "eating" silicon gas and "ejecting" the alternate silicon globules and silicon dioxide stems out of its rear caused by regular periodic instability within the gold-lead catalyst.
The VLS technique is a standard process, said Takeda, but the resulting chains are novel, he said.
"We think that during the crystal growth the diameter changes while, simultaneously, oxidation occurs to form the amorphous silicon dioxide necks. I am convinced that this is really a new finding," he said.
Moreover, "it's very easy to make," Takeda said. "The process is much simpler and much cheaper than the advanced techniques being developed for nanotubes. We think there is nothing wrong with nanotubes, which show great potential."
Takeda said his group can create "a big pile of nanochains" with a $10,000 vacuum pump, a $5,000 furnace and a $100 quartz test tube. "We don't need molecular beam expitaxy, we don't need CVD. People are very surprised when they come to our lab."
Takeda said he sees no reason why the process cannot be extended to two-inch or three-inch wafers, but his laboratory does not have a furnace or quartz containers big enough to house these as yet.
Industry players, including big-name Japanese semiconductor makers, have already expressed interest in the process, Takeda said. But reaction have generally been muted because scientists feel the technique is still far from manufacturability, he said.
"Some people have told us that we need to develop the process right up to the pre-application stage," Takeda said. "As a physicist, I am satisfied with the experimental data, but the private companies want us to develop a product precursor. I wonder if they are too busy for such basic research."
The project faces significant hurdles if its process is to show utility for semiconductor products, the team said. First, Takeda and Kohno said they must figure out a way to make arrays via regular dots of catalyst on future wafers. "I think this is pretty easy to do via lithographic techniques," said Kohno.
They must also straighten out a big kink in the process, which at the moment only produces windy chains. That will prove more problematic, Kohno said.
"To create straight wires could be very difficult," he said. "If we can do this we can make contacts at the ends of each chain much more easily."
Perhaps the technique's biggest obstacle at present is the lab's budget, Takeda said. The laboratory's first priority is to study the semiconductor properties of the chains before scaling up the process to bigger substrates. The lab needs new equipment, including new microscopes, and will require between $800,000 and $1.6 million.
"We are convinced we will obtain the funding," Takeda said. |
