Studying diffusion and other low-energy surface processes with atomic resolution is relatively easy to do nowadays, thanks to scanning tunneling microscopy (STM) and related methods. But events marked by high-activation-energy barriers generally are not amenable to STM investigations because of competing low-energy events. So while hydrogen diffusion on silicon, for example, has been studied in detail with STM, hydrogen desorption has proven tougher. The limitation has left open questions about the mechanism of H₂ desorption, for example, during silicon film growth from SiH₄ decomposition--an important industrial process.

The problem, according to Tony F. Heinz, a professor of physics and electrical engineering at Columbia University, is that if experiments are conducted at temperatures low enough to follow the motions of individual hydrogen atoms with STM, then it's too cold for H₂ to desorb readily. And if the temperature is high enough to give molecules sufficient energy to desorb, then surface diffusion occurs so rapidly that STM has little chance of pinning down the positions of atoms and molecules, revealing little about reaction dynamics.

Now, by combining nanosecond laser heating with STM, Heinz, Albert Biedermann, and Zonghai Hu of Columbia, and Michael Dürr and Ulrich Höfer of the University of Marburg, in Germany, have flipped the relative rates of hydrogen diffusion and desorption, and probed the H-on-Si system with atomic resolution. Without interferences from diffusion, the group finds that hydrogen molecules are formed through reaction of H atoms on adjacent silicon surface dimers [Science, 296, 1838 (2002)]. The interdimer mechanism stands in contrast to a commonly accepted intradimer recombination mechanism.

"It's an exciting development in the study of reaction dynamics on surfaces," says John J. Boland, a chemistry professor at the University of North Carolina, Chapel Hill.

John H. Weaver, a professor of materials science and engineering at the University of Illinois, Urbana-Champaign, comments that the study "offers new insights" into the mechanism of H₂ desorption from silicon. But he notes that the work focuses strictly on thermal causes for desorption. In related studies, researchers have shown that electronic excitations can play a role. "It would be interesting to learn whether there is a coupling between thermal and electronic activation," he says.

ROUNDABOUT At low temperatures, H diffusion on Si (red line) outpaces H₂ desorption (blue) as shown here in a schematic Arrhenius plot. But with fast laser heating, researchers can cause desorption to outrun diffusion momentarily.

Chemical & Engineering News
Copyright © 2002 American Chemical Society