Scientists tone down silicon rockers

Silicon’s surface contains rows of paired atoms, each duo tilting slowly up and down like a seesaw. Theorists predict that these silicon pairs, or dimers, should be most reactive when other atoms or molecules catch them midway between a teeter and a totter, when the two silicon atoms are on an even plane. Subtle as this may seem, such details often become important for advancing semiconductor technology.

Now, researchers have for the first time created dimers that stay level while permitting scientists to study silicon-surface reactions in unprecedented detail. They’ve discovered that these level pairs are dramatically more reactive than rocking ones. Emily J. Buehler and John J. Boland of the University of North Carolina at Chapel Hill report the results in the Oct. 20 Science.

Scientists had known that silicon dimers lie flat if two other atoms bond to them. So, the researchers exposed a silicon surface to hydrogen atoms, which readily react with silicon. As expected, scanning tunneling microscopy and spectroscopy revealed level silicon pairs wherever two hydrogen atoms had stuck to a dimer. Yet the team also found that the process pinned some hydrogen-free dimers into horizontal positions.

“If you have a dimer that has two atoms on it, then the neighboring dimer which has no atoms on it actually gets constrained . . . which was quite surprising,” says Boland. Bare pairs forced into horizontal positions were more than a billion times more reactive than normal, rocking pairs, he and Buehler report.

“There are lots of reactions that occur on these surfaces,” says Boland. “Sometimes, they give you many products. If we can do these reactions on our prepared dimers that are never tilting, we may be able to control the product.”