Quantum tunneling microscopy of an atomic scale device in silicon

Orateur : Benoit VOISIN (UNSW Sydney Australia)

Silicon is posed to take a major role in the rise of quantum technologies, with the ability to isolate and control electrons in nanoscale devices [1]. This opens the way to spin-based quantum computing schemes and the exploration of many-body physics using quantum simulators [2]. For donor-based approaches offering some of the longest coherence times in the solid-state, the small spatial extent of the localized wavefunctions makes interactions and device behavior unusually dependent on donor locations in the lattice. Yet to date, device behavior and dopant location are not both accessible in conventional measurements. Here we present a novel path to experimentally access and manipulate interacting donor wavefunctions in a functioning device, using scanning tunneling microscopy (STM). We have designed an atomically precise device using a hybrid of STM lithography [3] performed at low temperature and top-down implantation. The dot wavefunction is probed in real space using the same STM in the Coulomb blockade regime where the chemical potential and occupation number can be independently tuned. Moreover the STM tip used as a moveable electrode offers a unique way of tuning tunnel rates over a wide range, desirable for instance in readout schemes. A large degree of control of donor interactions is essential in silicon, where also valley degrees of freedom play a key role. Besides direct exchange interactions, hybrid systems made from donors and quantum dots have been proposed, notably to mediate exchange over large distances [4]. STM allows mapping with atomic resolution the exchange interaction between a donor and a quantum dot, revealing no lattice aperiodic oscillations. This evidences a valley filtering effect with the quantum dot containing only 2 out of the 6 valleys existing for the donor [5, 6], result supported by tight-binding and full-configuration interaction calculations. Strain also influences valleys : STM images of donors implanted in 0.7% strained silicon show a dominant contribution of the z-valleys. These results highlight how tuning valleys can assist to achieve uniform couplings in silicon. Combining atomic precision device fabrication with spatially resolved spectroscopy will benefit to a broad range of quantum systems, from qubit coupling to many-body physics of spin arrays.

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