REHOVOT, Israel - August 4, 1997 - Researchers at the Weizmann Institute of Science have set a new world record for purity.They have created the purest ever crystals of the semiconductor gallium arsenide - a feat that allows electrons to travel at a record speed.
Electrons zoom through this material at 14.4 million centimeters per second (equivalent to 518,400 kilometers or 324,000 miles an hour) - beating the previous world record of 11.7 million centimeters per second set by Bell Laboratories of the United States in 1989.
The achievement, reported in the August 4 issue of Applied Physics Letters, allows scientists to study the behavior of electrons in extremely pure materials and provides them with an important research tool for creating faster and more efficient electronic devices, such as semiconductor transistors for use in the telecommunications industry and other areas.
This research was conducted by Dr. Vladimir Umansky, Ph.D. student Rafael de-Picciotto and Prof. Mordehai Heiblum, all working at the Weizmann Institute's Joseph H. and Belle R. Braun Center for Submicron Research.
In the past few years, scientists at leading research laboratories around the world have invested great efforts in producing the purest possible semiconductors, the materials of which electronic processors, memory chips, signal transmitting circuits and other such devices are made. Purity is an important factor in increasing the speed with which electrons move through the semiconductor: the purer the material, the fewer the collisions that deflect the electrons and slow them down, a phenomenon called scattering.
"If you have fewer impurities, electrons will scatter less and move faster," says Heiblum. "This, in turn, makes the chip work faster and more efficiently. Moreover, electronic devices made of pure material behave in a predictable, uniform manner - a crucial factor for the electronics industry."
Weizmann's "champion" semiconductor consists of a gallium arsenide crystal. This material is gradually replacing silicon, the mainstay of the microelectronics industry, in an increasing range of applications because of its superior properties. For example, the main component of cellular phones and the laser element in CD players are made of gallium arsenide because, among other advantages, this semiconductor allows electrons to travel faster and emits and absorbs light very efficiently. Moreover, gallium arsenide stands up better than silicon to radiation in outer space, making it more suitable for satellites, which are expected to play a growing role in tomorrow's telecommunications.
The Weizmann semiconductor is so pure it contains only one foreign atom per five billion gallium arsenide atoms. This would be the equivalent of a single cube of sugar in a five-storey apartment block on a 300 sq. meter lot.
In order to grow such pure crystals, the scientists used a unique vacuum system that was installed and enhanced at the Braun Center for Submicron Research. It holds another world record (for such systems) - this time for emptiness - because it produces what appears to be a near-perfect vacuum, a pressure of 10 to the minus16th atmospheres.
An enormous temperature difference is maintained within the system: while its vacuum pumps operate at minus 268 degrees C, the crystals are grown from the raw materials, gallium and arsenic, at an oven temperature of 650 degrees C.
The researchers deduced the speed from the time it takes an electron to move through one centimeter of crystal in an electric field of 1 volt per centimeter. During the measurements, the crystals were maintained at a temperature of 0.1K (only one-tenth of one degree above absolute zero) to prevent the vibration of atoms that at higher temperatures interferes with the movement of electrons, causing scattering. Thus, apart from impurities, nothing slowed down the movement of electrons.
This research has important scientific and practical implications for the microelectronics of the future, particularly for the development of improved gallium arsenide semiconductors.
The project was supported in part by the Israel Academy of Sciences and Humanities, Austria's Ministry of Science, the Robert Bosch Foundation of Germany and the Israel Ministry of Defense.
Umansky, de-Picciotto and Heiblum are members of the Weizmann Institute's Condensed Matter Physics Department.
The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel. Its 2,400 scientists, students and support staff are engaged in more than 850 research projects across the spectrum of contemporary science.