Nanowires are, as the name suggests, cylinders just a few nanometers in diameter but much longer in length. They attract a great deal of attention because they are the perfect way to connect tiny electronic devices. Much of the research in that area has focused on alloys made from elements in either groups II and VI, or III and V, of the periodic table. Guo-Ping Guo, Wei Lu and colleagues from the University of Science and Technology of China and the University of Michigan in the USA1 have now demonstrated that group IV-based nanowires possess a number of intriguing properties that could be particularly useful for the rapidly growing field of spintronics.

Electrons possess not only charge, which is used as the basis for electronics, but also spin — a property often pictured as the direction the electron is spinning on its own axis and which can be exploited in ‘spintronics’. In materials such as indium arsenide, the length of time that information can be stored in the spin of an electron, called the coherence time, is limited because of interactions between the spins of the electron and atomic nuclei. Silicon does not have this problem because the nucleus is ‘spin-zero’, and so long coherence times are anticipated in silicon-based systems. “Also, because of the importance of silicon-based materials in mainstream electronics,” explains Guo, “it should be possible to combine storage, detection, logic and communication capabilities on a single semiconductor nanowire chip.”

Fig. 1: Schematic illustration showing how nanowires (red) can be connected to electrical contacts using modern microfabrication techniques. Their small size means that many such devices can be formed as an array on a single chip. The spin on the electrons in the nanowires are controlled by applying an electric field — a property that is useful for spintronics.© 2010 G.-P. Guo

Guo and his team grew nanowires with a 10 nm germanium core and a 2 nm-thick silicon shell onto a silicon substrate (Fig. 1). They measured how much current flowed through the wire as a function of the voltage applied across it. The results pointed to the crucial role played by ‘spin–orbit interactions’, or the interplay between an electron’s spin and its orbital motion around the nucleus. Applying a voltage to the substrate enabled the researchers to tune the strength of this interaction. “Such electrical manipulation of electron spin is essential for spin-based information processing such as quantum computing,” says Guo. “In the future, we hope to realize full coherent control of spin using an oscillating electric field.”