BERKELEY -- Using an ultrastable scanning tunneling microscope (STM) that "walks" above surfaces, scientists at the Lawrence Berkeley Laboratory have imaged never-before-seen features in semiconductors, the electronic materials that are the basis for computer chips.
LBL researchers have turned conventional STM design upside down by developing a microscope that stands on three legs above a material sample and walks itself to areas of interest. Many conventional STMs hold a dime-sized sample atop finger-like prongs and examine the sample from below.
The microscope legs of the new device are made out of a piezo-ceramic material that bends slightly when electrified. By flexing its legs, the microscope can jerk itself forward a few nanometers at a time, or rotate in place.
LBL materials scientists Eicke Weber, Jun-Fei Zheng, and Miquel Salmeron have used the STM to produce the first atom-by-atom images of an indium gallium arsenide/gallium arsenide interface, one of the more common architectures found in the semiconductors of lasers. They have also produced the first images of silicon donor atoms, the electron-rich impurities that give semiconductor crystals their special electrical properties. Both sets of images have been published in the Physical Review Letters.
"We are entering an exciting new phase in materials science," Weber says. "Until now, people have had to use indirect methods such as spectroscopy to explore these atomic features. Now we can use a microscope to look at them directly. We'll be able to use the new information to improve the way semiconductors are created."
The ability to investigate semiconductors atom-by-atom is especially important as production begins of electronic devices on a nanometer scale. At such a small scale, the precise placement of individual atoms is critical since the size of the atoms themselves begin to set the limits of performance.
Since its invention in 1981, the STM has been the most important tool for studying the atomic surface structures of metals and semiconductors. The device images the arrangements of atoms by moving an electrified metal tip a few atoms away from a material's surface. At such a close range, electrons will spontaneously jump back and forth between the tip and the sample material, a phenomenon known as tunneling. By measuring the rate of electron tunneling as it scans the material, an STM can map the topography of the material's surface.
In the past, however, researchers were unable to bring an STM tip close enough to a semiconductor surface to image features such as donor atoms. This is because mechanical instabilities and slight temperature fluctuations -- which cause the microscope's different parts to expand and contract -- can smash a tip of a microscope into a sample material if it is too close.
In addition to being able to walk, the new STM can get its tip much closer to a surface because it is more compact, and therefore less susceptible to mechanical vibrations. It also has a symmetrical design, so that shape changes in its different parts offset one another. With such an ultrastable STM, the donor atoms that appeared as faint shadows in past images now emerge as glowing spheres.
LBL is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
NOTE: Images of the silicon donor atoms, indium gallium arsenide/gallium arsenide interface, and microscope are available. Please contact Mike Wooldridge, MAWooldridge@lbl.gov.