Silicon-Based Spintronics

In a development that may pave the way for new types of microelectronic devices, researchers have demonstrated that electron spin can be transported and manipulated in silicon. The study may enable future spintronics (spin-based electronics) products to be fabricated from the most ubiquitous material in the microelectronics industry: silicon.

At the heart of the study is a test device that transports electron spin (designated as spin "up" or spin "down") across a silicon substrate and detects the spin at the other side. The device features a novel design that consists of an electron emitter and a silicon film that is sandwiched between a pair of ferromagnetic materials, Co₈₄Fe₁₆ and Ni₈₀Fe₂₀, which serve as spin filters that can be activated independently.

As electrons impinge upon the CoFe material, only one orientation of spin passes through to the silicon film. Electrons with the opposite spin are blocked. In a similar way, as the electrons reach the NiFe material, that filter transmits either all or none of the electrons depending on the orientation of their spins. By applying a magnetic field and measuring its influence on the current, Ian Appelbaum, an assistant professor of electrical and computer engineering at the University of Delaware, and graduate student Biqin Huang, along with Douwe J. Monsma of Cambridge Nanotech, in Cambridge, Mass., directly observe electron-spin transport through the silicon film. They report their findings in Nature (2007, 447, 295).

Nowadays, commercial semiconductor chips come packed with millions of transistors and other components. Yet for all the progress in manufacturing and miniaturization technology, nearly all of today's circuits, like those of earlier decades, rely solely on the flow of electrical charge to trigger switches or to carry out other functions.

Proponents of spintronics suggest that electron spin, a fundamental property closely associated with magnetism, could be exploited in tandem with electrical charge such that electron current could carry more information than it does in charge-only circuitry. As such, spintronic circuits could play a central role in a future generation of advanced computers and other microelectronic devices (C&EN, Aug. 28, 2006, page 30).

Commenting in the same issue of Nature, Igor uti and Jaroslav Fabian, physicists at the State University of New York, Buffalo, and the University of Regensberg, in Germany, respectively, note that in order for that type of advanced circuitry to materialize, "spintronics must conquer silicon," because it is the abundant, inexpensive, and entrenched material of choice for conventional semiconductor microelectronics.

Previous attempts by other researchers to transport electron spin through silicon have produced inconclusive results, uti and Fabian note. In contrast, the current study provides "convincing proof," they say.