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High vacuum produces ultrapure crystals.

For electrons zipping through a semiconductor, cleanliness is next to speediness: The fewer impurities in the material, the faster the electrons move.

A team of researchers has applied this rule of thumb to gallium arsenide, the semiconductor material used in the laser elements in compact disk players. By growing gallium arsenide crystals nearly 25 percent purer than those previously made, the group recorded a maximum electron speed of 14.4 million centimeters per second.

Although silicon is by far the most widely used semiconductor today, gallium arsenide has advantages in certain applications. It can emit light, hence its utility in lasers. It can also be used at high frequencies, making it ideal for radio-frequency electronic components in cellular telephones.

The improvement in purity "is more of a technological feat than a scientific breakthrough," says Mordehai Heiblum of the Weizmann Institute of Science in Rehovot, Israel. He and his colleagues improved the vacuum system used to grow their samples, thus significantly reducing the number of contaminants. The samples consisted of multiple alternating layers of gallium arsenide and aluminum gallium arsenide. A group at Bell Laboratories set the previous purity record in 1989.

Impurities act as roadblocks that scatter the moving electrons, thus reducing their speed. In order to see the effects of unwanted atoms most clearly, the researchers recorded the electron speeds at a very low temperature-just one-tenth of a degree above absolute zero.

At higher temperatures, the thermal vibrations of atoms have a greater effect than impurities. "If you want to avoid scattering of electrons from these thermal vibrations, you cool [the sample] down as much as you can," says Heiblum. The group reports its findings in the Aug. 4 Applied Physics Letters.

Real devices used today operate at room temperature, so impurities don't significantly limit electron flow. As electronic devices get smaller, however, the number of impurities becomes critical, he adds.

Ultrapure crystals are important for studying how electrons travel in a material. In their gallium arsenide crystals, Heiblum says, the electrons travel a very long distance--120 micrometers--before scattering. Over these path lengths, the electrons display wavelike properties. When they travel short distances before scattering, the electrons act more like particles.

"We scientists like to study the behavior of electrons at their quantum mechanical limit, because then they interfere and diffract and do all kinds of things that we never see at room temperature," Heiblum says.
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Title Annotation:speed of electrons in gallium arsenide enhanced with vacuum use
Author:Wu, Corinna
Publication:Science News
Article Type:Brief Article
Date:Aug 16, 1997
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