Room Temperature - Page 5

In his lab in one of the more venerable buildings at Berkeley Lab, Schenkel and his students have used a focused ion beam to implant single ions in devices mere millionths of a meter square. (An ion is an atom with net charge, typically lacking one or more electrons.)

"Single-atom effects have been observed before, but the yields are so low as to be impractical -- or the devices are randomly formed, with no control or predictability," Schenkel says. "Our approach to single-atom doping integrates ion beams with a modified scanning force microscope. We use the microscope's cantilever tip for both the nondestructive imaging of the target area and to position the ion beam."

The device has the advantage of using virtually any species of atoms, says Schenkel. "We can start with any source of neutral atoms, such as phosphorus or antimony -- manganese is fashionable right now -- and choose one of a number of different sources to ionize them."

The low-energy focused ion beam is sent through a hole in the microscope's cantilever. "The hole in the tip acts as a tiny aperture or mask," says Schenkel. "We've demonstrated holes with diameters as small as five nanometers" -- five billionths of a meter.

To confirm that an ion has been implanted in the silicon, the region is fitted with electrodes to form a transistor channel, placed under a bias voltage. Then the implantation of a single ion -- even in a target area as large as two micrometers on a side (two millionths of a meter) -- can be detected as a change in the resistance of the channel current. Schenkel compares the current to electrons sliding down a hill; the presence of an implanted ion, he says, "is a bump in the way -- it impedes the electron flow."

Says Schenkel, "The method is so sensitive that single-ion hits can be detected at room temperature in a device as large as four square microns. Inside that region we can form numerous single-atom devices, each with dimensions less than 100 nanometers." Before making a specific transistor or other device, single-ion implantation can be "practiced" with atoms of a noble gas that do not dope the substrate. "We wait until the transistor settles down, then switch to the species we want."

Graphene is a nanomaterial combining very simple atomic structure with intriguingly complex and largely unexplored physics. Since its first isolation about four years ago researchers suggested a large number of applications for this material in anticipation of future technological revolutions. In particular, graphene is considered as a potential candidate for replacing silicon in future electronic devices.

Theoretical physicists from the Swiss Federal Institute of Technology in Lausanne (EPFL) and Radboud University of Nijmegen (The Netherlands) performed a virtual crash-test of graphene as a material for future spintronic devices, possible components of future computers. The material successfully passed the test, although, with some reservations.

A major European initiative is underway to develop a new breed of faster, low-power computing devices based on the physical phenomenon of spintronics.Nanospin is a European Commission project bringing together eight academic and industrial collaborators to develop new types of spintronic nanoscale devices using ferromagnetic semiconductors. The University of Würzburg will co-ordinate the project.

The project will use gallium manganese arsenide, a ferromagnetic semiconductor that is well understood but only operates at extremely low temperatures, to prove the technology. The team hopes that the resulting technology will in the longer term work with room-temperature semiconductors.

Another collaborator in the project is Nottingham University, whose role is to supply and investigate suitable materials. Bryan Gallagher, professor of physics and a consultant for project industrial partner Hitachi Cambridge Laboratory, said: 'We grow perfect crystals monolayer-by-monolayer using molecular beam epitaxy. We deposit a set number of monolayers of one semiconductor, then more monolayers of another on top to make high-quality materials.'

The researchers believe the technology could take 10 years to come to market if a suitable room temperature semiconductor can be found, but they emphasis that spintronics is not the only technique being investigated.

Physicists in the US are the first to detect the spin Hall effect at room temperature, in what could be an important development in the quest for a practical source of spin-polarized electrons for spintronic devices.

David Awschalom and colleagues at the Center for Spintronics and Computation at the University of California, Santa Barbara observed the current-induced spin-polarization of electrons and the spin Hall effect in thin surface layers of ZnSe.

The 'spin Hall' is a spin current flowing in a transverse direction to the charge current in a non-magnetic material and in the absence of an applied magnetic field. The result is a measurable accumulation of “spin up” and “spin down” electrons at opposite edges of the conducting channel. 

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Researchers at MIT's Francis Bitter Magnet Lab have developed a novel magnetic semiconductor that may greatly increase the computing power and flexibility of future electronic devices while dramatically reducing their power consumption.

The new material is a significant step forward in the field of spin-based electronics -- or "spintronics" -- where the spin state of electrons is exploited to carry, manipulate and store information. Conventional electronic circuits use only the charge state (current on or off) of an electron, but these tiny particles also have a spin direction (up or down).

The magnetic semiconductor material created by Moodera's team is indium oxide with a small amount of chromium added. It sits on top of a conventional silicon semiconductor, where it injects electrons of a given spin orientation into the semiconductor. The spin-polarized electrons then travel through the semiconductor and are read by a spin detector at the other end of the circuit.

Although the new material is promising in itself, Moodera says the real breakthrough is their demonstration that the material's magnetic behavior depends on defects, or missing atoms (vacancies), in a periodic arrangement of atoms. This cause-and-effect relationship was uncertain before, but Moodera's team was able to tune the material's magnetic behavior over a wide range by controlling defects at the atomic level.

"This is what has been missing all along," he says. "The beauty of it is that our work not only shows this magnetic semiconductor is real, but also technologically very useful."

The new material's ability to inject spin at room temperature and its compatibility with silicon make it particularly useful. Its optical transparency means it also could find applications in solar cells and touch panel circuitry, according to Moodera.