Physicists Uncover Solution To Resolve Atomic Structure

Their findings will be reported in an upcoming issue of Physical Review Letters.

University of Arkansas postdoctoral researcher Vincent LaBella , Paul Thibado, assistant professor of physics, researchers H. Yang and D. W. Bullock and colleagues Matthias Scheffler and Peter Kratzer at the Max Planck Institute in Berlin have resolved the atomic structure of Gallium Arsenide (GaAs), a material commonly used in making high-technology telephones, satellites, Global Positioning Systems and cellular phones. LaBella likened Gallium Arsenide's importance in telecommunications to that of silicon chips in personal computers.

Scientists have debated between four atomic structures for GaAs over the past 10 years, but until now no one has been able to single out one structure as the correct conformation. Without the correct structure, industry scientists and researchers have to use several models of the GaAs surface when building telecommunications equipment and experiment to see what works. Knowing the structure will save companies time and money, LaBella said.

"Knowing the structure is the starting point," LaBella said.

The researchers used Scanning Tunneling Microscopy (STM) at different voltages to examine the GaAs surface. The STM uses a small electron beam at a steady current to create a contour map of the surface. To keep the current stable, the STM tip moves up and down, depending upon the distance from the surface atoms, creating a contour map showing peaks and trenches containing atoms. At 3.0 volts, the researchers could see features on the peaks of the crystal, but the image of the structure in the trenches remained unclear.

At 2.1 volts, the trenches became well-defined, and fit the Beta-2 model, one of the four proposed models for GaAs.

After creating the image, the University of Arkansas group contacted Matthias Scheffler, a theoretical physicist at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin. Scheffler founded the institute specifically to model GaAs surfaces.

LaBella and Thibado asked Scheffler to mimic the circumstances they created in the lab using computer models, so they could see if theory agreed with, and gave an explanation for, their results. "STM alone can't give you the answer," LaBella said.

The models Scheffler's group generated not only agreed with the University of Arkansas results; they pointed to the reason why the structure can be seen at lower voltages. At higher voltages, the electron clouds that surround the atoms form a barrier that causes the STM tip to skip the trenches, leaving the researchers with no picture of what's in the trenches. But the electrons associated with the atoms cause an effect that changes the STM picture at different voltages.

"The electron clouds over the arsenic atoms retract at lower voltages, so the image becomes sharper," LaBella said.

LaBella also credited the group's well-sharpened tips, made of tungsten, with helping them resolve the structure. Dull tips may be so wide that they cannot fit in the trenches, making it difficult to get a clear image of the atomic structure within.

The University of Arkansas researchers spend days sharpening tips, and if they are not satisfied with the STM results, they go back to the drawing board and sharpen more tips, LaBella said. The theoretical models use an infinitely sharp tip, making the agreement between theory and data in this research even more significant, LaBella said.

The University of Arkansas research group has now turned to Indium Phosphide (InP), another technologically significant material which makes the highest-speed transistor in the world. About 10 different theoretical models exist for this surface, LaBella said.

Using the theory combined with the knowledge of the voltage-dependent resolution will help scientists better understand many of these technologically important surfaces, LaBella said.

"We wouldn't understand this mechanism without the theory. But without the experiment, the theorists could not prove they were right," he said.

EDITORS: Photographs are available at: http://pigtrail.uark.edu/news/aug99/thibado-UARK.jpg and http://pigtrail.uark.edu/news/aug99/thibado2-UARK.jpg