In-situ high resolution TEM observation of Aluminum diffusion in Germanium nanowires: fabricating sub-10 nm Ge quantum dots

: The thermal activated solid state reaction forming aluminum-germanium nanowire (NW) heterostructures is a promising system as very sharp and well-defined one-dimensional contacts can be created between a metal and a semiconductor, that can become a quantum dot if the size becomes sufficiently small. In the search for high performance devices without variability, it is of high interest to allow deterministic fabrication of nanowire quantum dots, avoiding sample variability and obtaining atomic scale precision on the fabricated dot size. In this paper, we present a proof of principle experiment to produce sub- 10 nm Ge quantum disks (QDs), using a combination of ex-situ thermal annealing via rapid thermal annealing (RTA) and in-situ Joule heating technique in a transmission electron microscope (TEM). First we present in-situ direct joule heating experiments showing how the heating electrode could be damaged due to the formation of Al crystals and voids at the vicinity of the metal/NW contact, likely related with electromigration phenomena. We show that the contact quality can be preserved by including an additional ex-situ RTA step prior to the in-situ heating. The in-situ observations also show in real-time how the exchange reaction initiates simultaneously from several locations underneath the Al contact pad, and the Al crystal grows gradually inside the initial Ge NW with the growth interface along a Ge<111> lattice plane. Once the reaction front moves out from underneath the contact metal, two factors jeopardize an atomically accurate control of the Al/Ge reaction interface. We observed a local acceleration of the reaction interface due to the electron beam irradiation in the transmission electron microscope as well as the appearance of large jumps of the interface in un-passivated Ge wires while a smooth advancement of the reaction interface was observed in wires with an Al 2 O 3 passivation shell on the surface. Carefully controlling all aspects of the exchange reaction, we demonstrate a proof of principle experiment combining ex-situ and in-situ heating techniques to precisely control and produce axial Al/Ge/Al NW heterostructures with an ultra-short Ge segment down to 7 nanometers. Practically, the scaling down of the Ge segment length is only limited by the microscope resolution. deterministically fabricate Ge QDs with atomic size control between perfectly sharp metal contacts. To this end, we carefully study this exchange reaction in-situ from the nucleation stage of the propagation which starts at the NW surface underneath the contact electrode, to the end of the exchange process with atomically accurate control of the reaction interface position. We will show how the direct Joule heating, where a current is passed through a

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