an9b02564_si_005.mp4 (1.26 MB)
In-Situ Transmission Electron Microscopy Imaging of Aluminum Diffusion in Germanium Nanowires for the Fabrication of Sub-10 nm Ge Quantum Disks
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posted on 2020-01-16, 20:34 authored by M.A. Luong, E. Robin, N. Pauc, P. Gentile, M. Sistani, A. Lugstein, M. Spies, B. Fernandez, M. I. Den HertogThe 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 electro-migration
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 unpassivated
Ge wires, whereas a smooth advancement of the reaction interface was
observed in wires with an Al2O3 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 ultrashort Ge segment down
to 7 nm. Practically, the scaling down of the Ge segment length is
only limited by the microscope resolution.
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Keywords
in-situ Joule heating techniqueSub -10 nm Ge Quantum Disksreaction interfaceunpassivated Ge wiresIn-Situ Transmission Electron Microscopy ImagingAl contact padreaction front movesnanowire quantum dotselectron beam irradiationprinciple experimentTEMjoule heating experimentsGe segment lengthNWtransmission electron microscopeexchange reactionex-situ RTA stepin-situ heating techniquesAl 2 O 3 passivation shellultrashort Ge segmentQDsub -10 nm Ge quantum disks
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