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Fishing for current with an STM rod.

Fishing for current with an STM rod

In the arcane and microscopic world of quantum mechanics, electrons routinely vanish from one side of an energy barrier and reappear on the other. Things like that don't happen in more familiar realms. Scientists first convincingly demonstrated the electronic tunnelling phenomenon in semiconductors in the late 1950s, and since that time it has become the basis of several electronic components--among them tunnel diodes, which are good for some amplification and high-speed switching applications. More recently, tunneling electrons have attracted attention because of their central role in scanning tunneling microscopes (STMs), which enable scientists to image surface and molecules with atomic-scale resolution.

Using an STM to probe how different microregions of a semiconductor's surface conduct electricity under a range of applied voltages, physicists working at Harvard University and the Rowland Institute for Science in Cambridge, Mass., report observing on an atomic scale the same odd electronic behavior that occurs in macroscopic tunnel diodes, which are millions of times larger. The findings could lead to faster electronic devices fitted with the tiny tunnel diodes, says Peter J. Bedrossian, one of the project's scientists. The researchers describe their work in the Nov. 16 NATURE.

The sine qua non of a tunnel diode is an unusual behavior called negative differential conductivity, which means that for a particular range of increasing voltages (when applied across a diode's two terminals) there is a reduction in the current of electrons that tunnel through the energy barrier between the terminals. Normally, the current through electronic components such as resistors increases as the voltage goes up.

In their experiment, the researchers positioned the tip of an STM, which served as one terminal of a diode, over specific sites on a silicon surface that either hosted a boron atom or did not. With the tip over the site, which served as the second terminal, the researchers varied the voltage between the tip and the surface while monitoring the changing tunneling current. The results showed that boron-free sites flanked by boron-occupied sites behaved as atomic-scale tunnel diodes.
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Title Annotation:scanning tunneling microscopes
Publication:Science News
Date:Nov 25, 1989
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