Bell Labs scientists have discovered a method that promises a better understanding of how dopants influence a semiconductor's electrical properties. Using a scanning transmission electron microscope, researchers at the Lucent Technologies R&D facility in Murray Hill, NJ, have examined individual antimony dopant atoms within crystalline silicon. Federico Capasso, vice president of physical research, calls the breakthrough—the first time anyone has seen a single, undisturbed impurity atom within a crystal—"a new chapter in materials science."

"If you think of an eight-inch silicon wafer on which we grow our chips as the size of the United States, a single transistor is the size of a car, and a single atom is the size of a pin. We are able to locate the equivalent of a few pins, hidden in a few cars, somewhere in the United States," explains David Muller, a physicist leading the research team at Bell Labs.

The ability to examine a dopant atom in its crystalline habitat, instead of simply on the silicon surface, will give chipmakers a better understanding of "the chemical and physical environment within devices," asserts Elsa Reichmanis, director of the materials research department. The researchers note that atoms inside a crystal act differently from those on the surface. The technique can be used with virtually any material, and the researchers have already used it to characterize optoelectronics components. Additional members of the research team are Paul Voyles, John Grazul, and Paul Citrin of Bell Labs. Hans Gossmann of Agere Systems, a supplier of communications components, also participated.

1.) This simulated image depicts the electron probe of a scanning transmission electron (STE) microscope propagating through a silicon atomic column containing a single antimony atom. The "ball-and-stick" overlay represents a DP2 defect cluster. The probe grows more intense on the atomic column because the electrons are attracted to the positive charges of the atomic nuclei, says the Bell Labs team. The antimony atom causes electron streams to pour off the probe like a waterfall breaking over a rock.

2.) In these micrographs, a small region of an experimental image shows antimony-doped silicon. The image at left shows the even columns of silicon atoms in a crystal lattice. The brightest dots contain one or more antimony atoms. At right, the silicon lattice has been stripped by image processing to highlight the antimony columns.

3.) Bell Labs researchers say this six-part series of images supports the conclusion that the primary deactivating defect in silicon contains only two atoms instead of three or four, as previously believed. From the top, each column depicts a ball-and-stick model, a simulated micrograph, and an experimentalmicrograph for a two-atom antimony cluster and a three-atom antimony cluster (right column).