Nanotechnologists dream of creating wild new electronic technologies by wiring individual atoms and molecules into complex circuits. That dream may be closer to reality now that a team of physicists has figured out how to replace individual silicon atoms with phosphorus atoms on the surface of a chip.
For more than a decade, physicists have known how to move individual atoms around on the surface of a crystal by dragging them with a tiny fingerlike device known as a scanning tunneling microscope (STM). But although this technique works well for metallic atoms on metallic surfaces, it falls down when the substances are semiconductors, the stuff of microchips. Semiconductors such as phosphorus and silicon bind so tightly that a phosphorus atom won't budge when researchers try to lug it across a silicon surface. Now, however, Steven Schofield, Michelle Simmons, and colleagues at the University of New South Wales in Sydney, Australia, have found another way to put phosphorus atoms precisely where they want them.
The researchers used an STM as a chisel instead of a hook. They began by exposing their silicon surface to hydrogen so that one hydrogen atom would bind to every silicon atom. They then used the STM tip to detach selected hydrogen atoms and exposed the etched surface to phosphorus trihydride, or "phosphine." The phosphine molecules bound to the exposed silicon, and when the researchers gently heated their sample to 350°C, the molecules disintegrated and the phosphorus atoms swapped places with the silicon atoms. The researchers crafted rectangular patches and lines of embedded phosphorus atoms, and they even replaced individual silicon atoms with phosphorus, as they report in a paper to be published in Physical Review Letters.
"This is really an important milestone that they've achieved," says Chris Hammel of Ohio State University, Columbus. The finding "brings to mind a vast array of exciting things you might do," Hammel says, such as fashioning a chip in which individual phosphorous nuclei serve as the "qubits" in a quantum computer. More immediately, it shows that it may be possible to control the addition of other substances to silicon atom by atom. Such control may become critical as the transistors in microchips sink toward atomic size. But, Hammel says, the researchers still face one tough practical challenge: laying down a protective capping layer of silicon without disturbing the carefully positioned phosphorus atoms.