In the ongoing drive to make circuitry ever smaller and more powerful, researchers in Canada have taken things to a very small extreme. They devised a method for turning individual silicon atoms into electronic bits and have used the method to make atom-sized logic gates (Nat. Electron. 2018, DOI: 10.1038/s41928-018-0180-3). Logic gates are the basic building blocks of digital circuits.
Molecular electronic devices were proposed about 40 years ago. Since that time, scientists have been searching for ways to make circuits that are controlled by a single molecule—usually organic—or perhaps a few of them. If molecular electronics could be made reliably and in large numbers, they would offer the ultimate in miniaturization and dense data storage.
One such approach uses quantum dots as the basic elements. Because these nanosized specks are made of conventional semiconductors like silicon, not organic molecules, experts predict they’ll lead to faster, less-power-hungry electronics, and they’ll be more easily integrated into the vast array of semiconductor circuitry. But their electrical properties and some hurdles in synthesizing them have made it tricky to turn quantum dots into bits. Due to their large number of closely spaced energy levels, quantum dots need to be cooled to cryogenic temperatures to control their electronic properties, which is impractical. In addition, synthesis methods tend to yield a distribution of quantum dot sizes, which leads to variations in electronic properties.
To get around these problems, Taleana Huff, Robert A. Wolkow, and coworkers at the University of Alberta have made their electronic elements even smaller. They came up with a way to use individual silicon atoms, which are inherently identical, as stable electronic building blocks. But the atoms don’t need to be individually placed: The group worked out a way to carve out atomic circuit elements using the silicon atoms on the surface of a wafer.
The team patterns a silicon surface with dangling bonds, or unsatisfied valencies, and uses them as atom-sized bins to securely hold an electron. Dangling bonds are well-studied and can be generated easily and reproducibly on a hydrogen-capped silicon surface. By applying a brief voltage pulse with a scanning probe tip, the team removes a hydrogen atom and exposes a dangling bond on one silicon atom. They repeat the procedure, forming a pair of closely spaced dangling bonds. Then they deposit a single electron into the pair and control its position by using another electron to repel it to one dangling bond or the other to represent the 1 and 0 of digital logic.
The team’s earlier studies show that these systems are stable at room temperature. That’s because, unlike the case for quantum dots, the spacing between energy levels in dangling bonds is much greater than room temperature thermal energy.
Having worked out a procedure for making atom-sized binary bits, the team went on to make patterns of these bits, forming binary wires and logic circuit elements known as OR gates and studied them at cryogenic temperatures.
“This is elegant and very impressive work,” says University of Nottingham nanoscientist Philip Moriarty. The key advance, he explains, is that the researchers devised a system in which the silicon substrate does not short-circuit the dangling bond structures they create. Other teams have had to use insulating films to isolate their circuit building blocks from the underlying substrate, complicating the arrangement and adding fabrication steps.
These tiny binary bits are unlikely to make it out of the lab and into manufacturing plants in the short term, Moriarty says. Although the Alberta researchers showed previously that the dangling bond configurations are stable at room temperature, they have not yet demonstrated that dangling-bond logic circuits work properly at those temperatures.
Even so, there is potential for this approach to form the basis of new devices, Moriarty asserts. Researchers also need to figure out how to encapsulate the silicon surface to protect the binary bits and how to connect them to external wiring. Both are within the realm of possibility, he says.