Researchers have developed a step-by-step recipe for making devices at the atomic scale.

With this instruction, the team led by NIST is the second in the world to build a single-atom transistor and the first to build a series of atomic-controlled electronic single-atom transistors in device geometry.

Scientists have shown that they can precisely regulate the speed at which individual electrons pass through physical gaps or electrical barriers in their transistors, even though classical physics will prohibit electrons because they lack sufficient energy.

This tight quantum phenomenon, known as quantum tunneling, only becomes important if the gap is very small, for example in miniature transistors.

Quantum tunneling precision control is key because transistors can be “tangled” or connected in ways that are only possible through quantum mechanics and open up new possibilities for making quantum bits (qubits) that can be used in quantum calculations.

The team relies on a well-known technique for producing mono-atom and low-atom transistors in which a silicon chip is coated with a layer of hydrogen atoms that binds easily to silicon.

The fine end of the scanning tunneling microscope then removes the hydrogen atom at the chosen location. The remaining hydrogen acts as a barrier. When the team applied phosphine gas (PH3) to the surface of silicon, individual PH3 molecules only bind to places where hydrogen was removed (see animation). The researchers then heated the surface of the silicon.

The heat removes hydrogen atoms from PH3 and causes the remaining phosphorus atoms to accumulate on the surface. In further processing, the bound phosphorus atoms form the basis for a series of mono devices or very stable atoms that can function as qubits.

According to NIST researchers, the two steps in the method developed by the NIST team – sealing the phosphorus atom with a silicon shield and then making electrical contact with the embedded atom – are crucial for making many reliable copies of atomic precision devices. Richard, “Silver said.

“We believe that our coating method offers a more stable and accurate device at the atomic scale,” Silver said. The release of only one atom can change the conductivity and other properties of the electrical components which are characterized by groups of single or small atoms.

The team also developed a new technique for critical steps of electrical contact with buried atoms so they can work as part of a chain.

The heated palladium reacts with silicon to form an electrically conductive alloy called palladium silicide, which naturally penetrates the silicon and comes in contact with the phosphorus atom.

This is an important achievement, said Warrick. “You can have the best single-atom transistor device in the world, but if you can’t reach it, it’s useless,” he said.

Producing mono-atom transistors “is a difficult and complicated process that might require everyone to cut their teeth, but we have planned the steps so that other teams don’t have to go through trial and error,” Richter said.

Current flow measurements show that by increasing or reducing the difference between transistor components by less than one nanometer (one millionth of a meter), the team can be predicted to control the flow of one electron through the transistor.

“Because quantum tunnels are very important for every quantum device, including the construction of qubits, the ability to control the flow of electrons at the same time is a significant achievement,” Weirick said. “”

“As engineers package more and more circuits on smaller computer chips and the gap between components becomes smaller, understanding and controlling the effects of quantum tunneling becomes increasingly critical,” Richter said.

Reference: X. Wang, J. Wyrick, R.V. Kashid, P. Namboodiri, S.W. Schmucker, A. Murphy, M.D. Stewart Jr., N. Zimmerman, and R.M. Silver. Atomic-scale Control of Tunnel Coupling. Communications Physics. Published May 11, 2020. DOI: 10.1038/s42005-020-0343-1