Researchers Fabricate Single-Atom Transistors Using a Step-By-Step Formula

Transistors having only a few atom clusters or even single atoms have now been proven to have the ability to become the building blocks of a new generation of computers with unmatched memory and processing power.

However, to employ the full potential of these small transistors, which are tiny electrical on-off switches, scientists have to find a way to make numerous copies of these components that are incredibly challenging to fabricate. Now, researchers from the National Institute of Standards and Technology (NIST) and fellow colleagues from the University of Maryland have created a step-by-step guide to develop the atomic-scale devices.

Recipe For Success

Using this formula, the NIST-led team has managed to develop a single-atom transistor, becoming the second group of researchers to achieve this, and the first team to fabricate a series of single-electron transistors with atom-scale control over the device’s geometry as well.

The researchers proved that they could accurately adjust the rate at which the standalone electrons travel through a physical gap or electrical barrier in the transistor, although fundamental physics would block the electrons from doing so because they don’t have sufficient energy.

To manufacture single-atom and several-atom transistors, the team used a known method in which a silicon chip is coated with a layer of hydrogen atoms, which easily bind to silicon. The tip of a scanning tunneling microscope then eliminated hydrogen atoms at certain sites, and the rest of the atoms acted as a barrier. When the researchers directed phosphine gas (PH3) at the surface, single PH3 molecules fixed themselves only to the places where the hydrogen had been removed.

The team then heated the chip and observed that the temperature expelled hydrogen atoms from the PH3 and made the remaining phosphorus atom to implant itself into the surface. With more processing, embedded phosphorus atoms created the foundation of a few highly stable individual- or few-atom devices that can be used as qubits.

[Image: S.Kelley/NIST/March, 2020]
Two of the steps in the technique created by the NIST teams, namely sealing the phosphorus atoms with protective sheets of silicon and making electrical contact with the bound atoms, seem to have been crucial to securely develop numerous copies of atomically accurate devices, NIST scientist Richard Silver said.

“We believe our method of applying the layers provides more stable and precise atomic-scale devices,” said Silver.

The team also created a new technique for the vital step of making electrical contact with the embedded atoms so that they can function as part of the circuit. The NIST researchers heated a layer of palladium metal placed in certain areas of the silicon ship that inhabited directly above the selected parts of the device.

The heated palladium reacted with the silicon to create an electrically-conducting mix known as palladium silicide, which usually seeped through the silicon and made contact with the phosphorus atoms.

In a recent paper published in the journal Advanced Functional Materials, Silver and his colleagues, who include Xiqiao Wang, Jonathan Wyrick, Michael Stewart Jr., and Curt Richter, highlighted the fact that their contact technique has an almost 100 percent success rate. According to Wyrick, that is a crucial achievement.

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