At the center of modern technology lies the logic circuitry provided by semiconductor devices. Extending circuit logic to the realm of nanotechnology requires the construction of atomic-scale systems, which has proven challenging. Both the electric nature of individual atoms and the need to place them at specific points within a crystal lattice has kept scientists from creating atom-scale transistors until the present.
Now, a group of researchers has fabricated a single-atom transistor by introducing one phosphorous atom into a silicon lattice. Through the use of a scanning tunnelling microscope (STM) and hydrogen-resist lithography, Martin Fuechsle et al. placed the phosphorous atom precisely between very thin silicon leads, allowing them to measure its electrical behavior. The results show clearly that we can read both the quantum transitions within the phosphorous atom and its transistor behavior. No smaller solid-state devices are possible, so systems of this type reveal the limit of Moore's law—the prediction about the miniaturization of technology—while pointing toward solid-state quantum computing devices.
Transistors are devices that allow for precise control over electrical currents. They can amplify currents and control their direction through a circuit; in combination, they produce the very complex logic circuits that are the basis of modern computer technology. Most transistors in use today are built from complementary metal-oxide semiconductor (CMOS) components, and improvements in fabrication techniques have allowed engineers to shrink these devices.
The latest result approaches the logical limit to this miniaturization: a single-atom transistor is as small as solid-state technology can get. Don't expect single-atom transistors to appear on your desktop any time soon, however: the existing device requires the use of a liquid helium refrigerator to reach 20 millikelvin (20 mK) before it can operate. Nevertheless, this experiment has overcome some of the problems earlier attempts experienced.
Typical solid-state semiconductor devices exploit the bulk properties of multiple atoms in a lattice to operate. It's possible to tweak these bulk properties; the introduction of impurity atoms known as dopants can enhance the material's ability to conduct electricity. Scaling a semiconductor system down to its practical limit—a single dopant atom—is complicated by interactions between the lattice and the quantum behavior of the atom. Unless the dopant atom is placed very precisely, these interactions cause any transistor behavior to be lost.
Fuechsle and his collaborators finally managed to perform this precise placement. By using a scanning tunnelling microscope to manipulate individual atoms on a surface, they replaced a silicon atom with a phosphorous dopant atom, as shown in the electron micrograph above. The rest of the device is also fabricated from silicon, with leads built from single crystals of silicon deposited on a boron substrate.
The researchers then measured the electrical response across the dopant atom, showing that it behaves in the same way that bulk phosphorous-doped silicon does in larger CMOS devices. And, it does that without losing the quantum behavior expected of a single atom. These experiments make it clear that the transistor characteristic does arise from the dopant atom, and not from other possible bulk behaviors.
Future experiments on similar single-atom transistors may allow researchers to exploit properties of the dopant atom beyond its electrical characteristics, such as electron spin. Because the underlying device is a silicon semiconductor, it provides compatibility between essentially quantum systems from the atomic world and existing CMOS technology. Single-atom transistors are also the limiting point for Moore's law, where miniaturization can go no further. So fabricating logic circuits from these transistors will bridge the world of silicon-based semiconductors and quantum computing.
