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Aluminum-germanium nanowires (NWs) thermal activated solid state reaction isa promising system as very sharp and well defined one dimensional contacts canbe created between a metal and a semiconductor, that can become a quantum dotif the size becomes sufficiently small. In the search for high performancedevices without variability, it is of high interest to allow deterministicfabrication of nanowire quantum dots, avoiding sample variability and obtainingatomic scale precision on the fabricated dot size. In this paper, we present areliable fabrication process to produce sub-10 nm Ge quantum dots (QDs), usinga 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 howthe heating electrode could be damaged due to the formation of Al crystals andvoids at the vicinity of the metal/NW contact, likely related withelectro-migration phenomena. We show that the contact quality can be preservedby including an additional ex-situ RTA step prior to the in-situ heating. Thein-situ observations also show in real-time how the exchange reaction initiatessimultaneously from several locations underneath the Al contact pad, and the Alcrystal grows gradually inside the initial Ge NW with the growth interfacealong a Ge(111) lattice plane. Once the reaction front moves out fromunderneath the contact metal, two factors jeopardize an atomically accuratecontrol of the Al/Ge reaction interface. We observed a local acceleration ofthe reaction interface due to the electron beam irradiation in the transmissionelectron microscope as well as the appearance of large jumps of the interfacein unpassivated Ge wires while a smooth advancement of the reaction interfacewas observed in wires with an Al2O3 protecting shell on the surface. Carefullycontrolling all aspects of the exchange reaction, we demonstrate a fabricationprocess combining ex-situ and in-situ heating techniques to precisely controland produce axial Al/Ge/Al NW heterostructures with an ultra-short Ge segmentdown to 8 nanometers. Practically, the scaling down of Ge segment length isonly limited by the microscope resolution.

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