Pitt Professor Reveals Fundamental Physical Behavior of Silicon

PITTSBURGH—Experiments by University of Pittsburgh Professor Hrvoje Petek recently revealed how electrons at the instant of their generation interact with the atoms in solid crystalline silicon to form quasiparticles. Petek and coauthors Muneaki Hase and Masahiro Kitajima of the National Institute of Materials Science reported their findings in the Nov. 6 issue of Nature.

Virtually all electronic devices rely on silicon. Complex electronic components such as microprocessors are built of basic silicon switching elements—transistors.

"We have made a significant step in understanding the fundamental properties of silicon that I hope, over the next few decades, will substantially impact the development of future electronic devices," said Petek, professor of physics and chemistry in the Department of Physics and Astronomy.

Using an ultrafast laser that generates flashes of light 10 femtoseconds—10 quadrillionths of a second—in duration, Petek and his coinvestigators could induce the formation of quasiparticles and quantify their properties from the changes in the optical properties of excited silicon.

Quasiparticles refer to charges that carry electrical current, or electrons, and vibrations of atoms, or phonons, that cause electrical resistance in semiconductor devices. According to the laws of quantum mechanics, a yin-yang relationship exists between electrons and phonons—the decay of one generates the other, and vice versa.

Understanding how quasiparticles form and decay reveals the quantum mechanical principles that underlie how electronic transistors operate. Once it is understood how silicon behaves in the quantum mechanical regime, it may be possible to devise semiconductor chips that are more than a thousand times faster than current chips.

Quasiparticle buildup has been observed in other semiconductors, but not in silicon, because it has unfavorable properties for optical experiments with what physicists call ultrafast lasers. The success of Petek's group was made possible by the unique laser and optical measurement system developed in his laboratory.

"We have been able to capture, for the first time, how electrons in silicon evolve under quantum mechanical principles," said Petek. "Our work shows how silicon transistors might operate in the future."

Quasiparticles are rarely expressed in present-day devices because, at their current size and speed, classical physics is adequate to explain how they function. However, in the next few decades, transistor size will shrink and switching time will speed up to the point where only quantum mechanical laws will describe electronic device operation. This transition may present a revolution in technology that surpasses even that of the invention of the semiconductor transistor.

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11/5/03/mgc