Atom manipulation in silicon dances to the tune of an “atomic waltz”

In the race for quantum supremacy, scientists have been exploring different architectures, one of which is the solid-state qubit – the basic unit of quantum information processing. But how do you make a qubit? The industry’s technological achievements in silicon manufacturing to date, as well advantages based on the properties of silicon itself, have focused attention on the nuclear spins of positively charged donor atoms inside crystalline silicon. However, constructing such qubits is challenging, and a major obstacle is the precise positioning of these donor impurities, called dopants, that are added in tiny amounts to alter silicon’s properties.

An international research team led by the University of Vienna, Austria, studied the behaviour of group V dopants – phosphorus, arsenic, antimony and bismuth – in silicon under electron irradiation. With partial support from the EU-funded ATMEN project, the team has now discovered a non-destructive way to move dopant atoms in a silicon lattice with atomic precision. In this novel mechanism called an indirect exchange, two neighbouring silicon atoms are involved in what an article the university posted on ‘Phys.org’ refers to as “a coordinated atomic ‘waltz’.” Published in ‘The Journal of Physical Chemistry C’, the team’s findings are possibly a key to the manufacture of solid-state qubits.

To obtain these results, the researchers made use of scanning transmission electron microscopy (STEM), a technique that uses a focused electron beam to manipulate strongly bound materials with atomic precision. “The unique strength of this technique is its ability to access not only surface atoms but also impurities within thin bulk crystals,” observes senior author Assistant Prof. Toma Susi of the University of Vienna. “This is not only a theoretical possibility: the first proof-of-principle manipulation of bismuth dopants in silicon was recently demonstrated by our US collaborators.”

The team found that in the indirect exchange mechanism observed, there’s a knock-on effect in which the dopant atom takes over the lattice position that the impacted silicon atom originally occupied. However, unlike what happens with materials such as graphene, the silicon atom doesn’t end up as a neighbour of the donor impurity. Instead, it becomes the second-nearest neighbour, displacing another silicon atom.However, this process only works with the two heavier group V dopants, antimony and bismuth. With the lighter two, arsenic and phosphorus, simulations revealed no indirect exchange. “While this mechanism only works for the two heavier donor elements, bismuth and antimony, it was crucial to find that it is non-destructive, as no atoms need to be removed from the lattice,” notes lead author Dr Alexander Markevich, also from the University of Vienna.

With bismuth dopant manipulation having been proved, the team went on to demonstrate for the first time that antimony impurities can be successfully manipulated in a thin crystalline slab of silicon using STEM. So, what does this imply for the manufacture of qubits? Assistant Prof. Susi explains: “Very recently, antimony dopants in silicon were suggested as promising candidates for solid-state nuclear spin qubits, and our work may open a path for their deterministic fabrication.” The ATMEN (Atomic precision materials engineering) project ends in September 2022.

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ATMEN project