Atomically-precise positioning of a single atom transistor-VIDEO

A team led by Michelle Y. Simmons, who spoke on “Atomic-scale device fabrication in silicon” at the 2007 Productive Nanosystems: Launching the Technology Roadmap conference, which introduced the Technology Roadmap for Productive Nanosystems, has succeeded in the atomically precise placement of a transistor consisting of a single atom of phosphorous between source and drain electrodes and gate electrodes all made from phosphorous wires only a few atoms wide. A YouTube video illustrating this working transistor of a single atom of phosphorous placed with atomic precision on a silicon crystal includes an STM image that shows the single phosphorous atom placed several tens of rows of silicon atoms from source and drain electrodes of phosphorous that appear to be about 10 rows of atoms wide. To manufacture the phosphorous transistor and electrodes, a scanning tunneling microscope was used to remove precisely determined hydrogen atoms from the passivating layer covering a silicon crystal to form a mask that was then used to apply phosphorous atoms to the vacancies created. An overlay of silicon atoms then preserved these phosphorous nanostructures. The accomplishment is described in a NY Times article by John Markoff, which describes both the place of this work in the progression of Moore’s Law and its potential for a new generation of quantum computers: “Physicists Create a Working Transistor From a Single Atom“:

Australian and American physicists have built a working transistor from a single phosphorus atom embedded in a silicon crystal.

The group of physicists, based at the University of New South Wales and Purdue University, said they had laid the groundwork for a futuristic quantum computer that might one day function in a nanoscale world and would be orders of magnitude smaller and quicker than today’s silicon-based machines. …

“Their approach is extremely powerful,” said Andreas Heinrich, an I.B.M. physicist. “This is at least a 10-year effort to make very tiny electrical wires and combine them with the placement of a phosphorous atom exactly where they want them.”

He said the research was a significant step toward making a functioning quantum computing system. However, whether quantum computing will ever be harnessed for useful tasks remains uncertain, and the researchers also noted that their work demonstrated the fundamental limits that today’s computers would be able to shrink to.

“It shows that Moore’s Law can be scaled toward atomic scales in silicon,” said Gerhard Klimeck, professor of electrical and computer engineering at Purdue, referring to the rate at which computing gets faster and cheaper. “The technologies for classical computing can survive to the atomic scale.”

The results were published in Nature Nanotechnology [abstract]. At least for the moment (February 19, 2012), the full text is available without charge. Also available in the same issue is a commentary by Gabriel P. Lansbergen “Nanoelectronics: Transistors arrive at the atomic limit“, which gives additional background and details on this accomplishment.

… Single-atom transistors represent the ultimate limit in solid-state device miniaturization, but they are also interesting for another reason. Deterministically positioned single-dopant atoms in silicon, electrically addressable by metallic leads, are at the heart of a number of promising proposals for quantum-information-processing devices3. The long coherence and relaxation times associated with single dopants make them very attractive candidates for quantum-device architectures.

The atom-by-atom fabrication technique developed by Simmons and co-workers therefore fulfills a long-standing need for a method that is capable of atomic-scale device fabrication in silicon. And although the technique is not directly applicable on an industrial scale, it does bring the development of truly atomistic electronics — and the possibilities they offer — into the experimental realm.

This latest accomplishment from Prof. Simmons and her collaborators follows swiftly on their recent demonstration published just last month in Science [abstract], that Ohms law holds for nanowire only four phosphorous atoms wide. From the Purdue University news service “Down to the wire for silicon: Researchers create a wire 4 atoms wide, 1 atom tall“:

The smallest wires ever developed in silicon – just one atom tall and four atoms wide – have been shown by a team of researchers from the University of New South Wales, Melbourne University and Purdue University to have the same current-carrying capability as copper wires.

Experiments and atom-by-atom supercomputer models of the wires have found that the wires maintain a low capacity for resistance despite being more than 20 times thinner than conventional copper wires in microprocessors.

The discovery, which was published in this week’s journal Science, has several implications, including:

  • For engineers it could provide a roadmap to future nanoscale computational devices where atomic sizes are at the end of Moore’s law. The theory shows that a single dense row of phosphorus atoms embedded in silicon will be the ultimate limit of downscaling.
  • For computer scientists, it places donor-atom based silicon quantum computing closer to realization.
  • And for physicists, the results show that Ohm’s Law, which demonstrates the relationship between electrical current, resistance and voltage, continues to apply all the way down to an atomic-scale wire.

Although the path from this laboratory demonstration to a practical technology is not yet clear, as emphasized above by the researchers themselves and commentators, the progress at Zyvex Labs (and elsewhere) that we cited in Oct. 2010 in this basic technology of using an STM for atomically precise lithography holds hope that a convergence of manufacturing technology and demonstrated prototypes will not be too distant.
—James Lewis

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