Directed Positioning of Subsurface Single-Atom Dopants in Silicon for Quantum Computing*

The ability to controllably position single atoms inside a material is a prerequisite for the creation of next generation atomic-scale devices. Advances in aberration correction allow for precise control over the size and position of the electron probe in a scanning transmission electron microscope (STEM). Here we demonstrate for the first time the ability to controllably move and place subsurface bismuth dopants in a silicon crystal at room temperature using STEM. The controllable positioning of Bi dopants is indicated by two findings from our density functional theory calculations: (1) the strain induced by a substitutional Bi dopant allows for Si vacancies to be preferentially created adjacent to the Bi atoms, and (2) there is no significant energy barrier for the Bi atom to hop into these vacancies once they have formed. Using the electron beam, we exploit this vacancy-mediated motion to direct and place Bi dopants below the surface in specific columns within an oriented Si crystal, an important step towards creating a functional quantum device.