UNC-CH Scientists Observe For First Time How Bonds Behave At High Temperatures

January 23, 1998

By DAVID WILLIAMSON
UNC-CH News Services

CHAPEL HILL -- Using a sophisticated device known as a scanning tunneling microscope, chemists at the University of North Carolina at Chapel Hill for the first time have observed directly how hydrogen atoms behave and bond to surfaces at high temperatures.

What the scientists have seen is a form of atomic Ping-Pong in which hydrogen atoms unpair, hop back and forth on the surface of silicon and sometimes even exchange partners. Their work is important, they say, because it helps explain what happens during silicon growth and related processes central to microelectronics manufacturing and other industries.

One of the chemists, Dr. John J. Boland, an associate professor, likened the reactions to a high school romance. "These pairs are constantly splitting up and getting back together," Boland said. "Occasionally, a third party comes along, and a new pair is formed."

A report on the findings appears in the Jan. 23 issue of the journal Science. Besides Boland, authors of the paper, also in chemistry at UNC-CH, are postdoctoral fellow Dr. Marc McEllistrem and graduate student Matthew Allgeier.

The research involved bombarding with deuterium, or "heavy" hydrogen atoms, tiny samples of silicon in a vacuum chamber so that hydrogen atoms covered the silicon surface and filled in almost all reactive sites known as dangling bonds. Chemists then photographed every 15 seconds what happened on the silicon surface after they heated it to about 650 degrees Fahrenheit.

The surprise was that the remaining dangling bonds on the surface, which were initially paired, unpaired and efficiently re-paired multiple times, depending on the temperature.

"This unpairing and re-pairing behavior, which was unknown before, is good news because it shows favorable conditions for growing more silicon," McEllistrem said. "This could mean future computer chips that would work faster."

"What we have done doesn't fully explain what is going on at the atomic level, but it is an important part of the picture," Boland said. "There is no reason why it would not apply to many other materials besides silicon."

Reactions of the kind the UNC-CH scientists studied are important in a common growth process known as chemical vapor deposition, he said. Growing industrial diamonds and coating engine parts, for example, are achieved using this process, except with different atoms.

"The new work also helps explain why growth of solids may not occur even under conditions where dangling bones are available," Boland said.

"One of the especially fun things about this technique is that you can actually see the atoms and watch the chemistry in progress by seeing how the pictures evolve over time," McEllistrem said.

The UNC-CH research was supported by the National Science Foundation.

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Note: Boland and McEllistrem can be reached at (919) 962-5098 and 962-5466, respectively.
-end-


University of North Carolina at Chapel Hill

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