Single-atom transistor ‘recipe’ simplifies atomic-scale fabrication

Technology News | May 11, 2020

By Rich Pell

Transistors consisting of only several-atom clusters or even single atoms, say the researchers, promise to become the building blocks of a new generation of computers with unparalleled memory and processing power, but are notoriously difficult to fabricate in quantity. Now, using the new instructions, the researchers have become only the second in the world to construct a single-atom transistor and the first to consistently fabricate a series of single electron transistors with atom-scale control over the devices’ geometry.

The scientists demonstrated that they could precisely adjust the rate at which individual electrons flow through a physical gap or electrical barrier in their transistor. That strictly quantum phenomenon – known as quantum tunneling – only becomes important when gaps are extremely tiny, such as in the miniature transistors. Precise control over quantum tunneling is key, say the researchers, because it enables the transistors to become “entangled” or interlinked in a way only possible through quantum mechanics and opens new possibilities for creating quantum bits (qubits) that could be used in quantum computing.

To fabricate single-atom and few-atom transistors, the researchers relied on a known technique in which a silicon chip is covered with a layer of hydrogen atoms, which readily bind to silicon. The fine tip of a scanning tunneling microscope then removed hydrogen atoms at selected sites. The remaining hydrogen acted as a barrier so that when the researchers directed phosphine gas (PH3) at the silicon surface, individual PH3 molecules attached only to the locations where the hydrogen had been removed (see video).

The researchers then heated the silicon surface, which ejected hydrogen atoms from the PH3 and caused the phosphorus atom that was left behind to embed itself in the surface. With additional processing, say the researchers, bound phosphorous atoms created the foundation of a series of highly stable single- or few-atom devices that have the potential to serve as qubits.

Two of the steps in the method – sealing the phosphorus atoms with protective layers of silicon and then making electrical contact with the embedded atoms – appear to have been essential to reliably fabricate many copies of atomically precise devices, say the researchers. Their method of applying the layers, they believe, provides more stable and precise atomic-scale devices.

The researchers also developed a novel technique for the crucial step of making electrical contact with the buried atoms so that they can operate as part of a circuit. The researchers gently heated a layer of palladium metal applied to specific regions on the silicon surface that resided directly above selected components of the silicon-embedded device.

The heated palladium reacted with the silicon to form an electrically conducting alloy called palladium silicide, which naturally penetrated through the silicon and made contact with the phosphorus atoms. Their contact method, say the researchers, has a nearly 100% success rate – a key achievement.

Fabricating single-atom transistors, say the researchers, “is a difficult and complicated process that maybe everyone has to cut their teeth on, but we’ve laid out the steps so that other teams don’t have to proceed by trial and error.”

For more, see “Atom-by-Atom Fabrication of Single and Few Dopant Quantum Devices.”