For release April 25, 1997
George Moffatt
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Carl Blesch
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Bell Labs Scientists Produce Pictures of Single Electrons in Semiconductors
MURRAY HILL, N.J. --A group of researchers at Bell Laboratories has developed a new instrument so sensitive it can take pictures of individual electric charges on and below the surface of semiconductor devices.
The instrument, known as the single-electron transistor scanning electrometer (SETSE), combines two major developments of recent times, the single-electron transistor (SET) and the scanning-probe microscope.
The SET, co-invented in the late 80's by Bell Labs researchers, is a cryogenic device of microscopic size whose electrical resistance is very sensitive to the electric fields emanating from nearby electric charges. This device is so sensitive it can easily detect not just single electrons but as little as one percent of an electron's electric field. The SET consists of two tunnel junctions, similar to diodes, interconnected by a minute metal "electron island" which senses the electric fields.
The second development, the scanning probe microscope, dating to the mid-80's and now in use in many forms, produces maps of physical properties of a surface on a scale comparable to that of electron-microscope images. The microscope uses a sharp tip as an ultra-small probe, which is brought gently into near contact with a material surface. A particular physical property, such as contact force, can be detected in the microscopic region below the tip. As the probe is scanned back and forth across the surface, images of this property can be captured by the microscope.
In creating the SETSE, the researchers have put the circuitry of the SET literally on the point of a needle (see Figure 1> in this press release at http://www.lucent.com/press/).
The probe consists of a sharpened glass fiber, similar in size and shape to a small sewing needle but with a much finer tip that tapers down to a tiny, nearly flat area at the end. This flat area is only 500 atoms (100 nanometers) wide. The SET is formed on this tiny area. The "electron island" covers most of the area and the two tunnel junctions are at either edge of the island. Electrical connections extend from the junctions up the side of the needle to the detectors. When brought close to a semiconductor or other surface, the SETSE probe picks up signals from all charges close to the island and measures the total charge to a small fraction of an electron. By scanning the SETSE probe back and forth, an "electric" picture emerges of the charge distribution on the surface. (see Figures 2 and 3 in this press release at http://www.lucent.com/press/).
The authors have published an article, entitled "Scanning Single-Electron Transistor Microscopy: Imaging Individual Charges," in the April 25, 1997 issue of Science. They are Michael J. Yoo, now at Philips Laboratories, and Theodore A. Fulton, Harald F. Hess, Robert L. Willett, LaRue N. Dunkleberger, Robert J. Chichester, Loren N. Pfeiffer, and Kenneth W. West, all of Bell Labs.
Hess noted that usually many electrical charges are sensed by the probe at once. "Typically there can be up to 300 electrons influencing the SET island at one time, which makes seeing individual electrons in the crowd a bit tricky," he said. "But, we were able to detect individual electrons by using light to move a very small fraction of them to different locations. By comparing the 'before' and 'after' picture, we can see individual spots where a new electron has arrived or
where an old one has disappeared."
Hess said, "The whole thrust of the semiconductor industry is to make transistors of ever smaller size and the day is not far away when only a handful of electrons will control a device's performance. The SETSE can help us locate where the electrons and charged dopant atoms are in a semiconductor material, such as gallium arsenide devices doped with silicon. The location and the amount of dopants and how they release electrons is critical to device performance. With the SETSE, we can image those charges with excellent sensitivity."
He cautioned that the SETSE is at present a research tool and commercial use is as yet far off. For example, the device currently operates only at cryogenic temperatures of minus 462=B0 F., requiring elaborate cooling techniques. The authors expect, however, that with time the device will be reduced in size, producing images with finer details, and will be made to operate at higher temperatures.
"In principle," said Fulton, "operation at room temperature with 100 times better detail is a reachable goal. The SETSE's proven ability to see single electrons is an incentive to work hard toward that goal."
Bell Labs is the research and development arm of Lucent Technologies (NYSE: LU), which designs, builds, and delivers a wide range of public and private networks, communications systems and software, consumer and business telephone systems, and microelectronics components. Lucent Technologies was formed as a result of AT&T's restructuring and became a fully independent company - separate from AT&T - on Sept 30, 1996.
For Figures 1-3, see the press release at Lucent Technologies website at http://www.lucent.com./press/
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