In Situ Transmission Electron Microscopy Analysis of Aluminum–Germanium Nanowire Solid-State Reaction

Abstract

To fully exploit the potential of semiconducting nanowires for devices, high quality electrical contacts are of paramount importance. This work presents a detailed in situ transmission electron microscopy (TEM) study of a very promising type of NW contact where aluminum metal enters the germanium semiconducting nanowire to form an extremely abrupt and clean axial metal–semiconductor interface. We study this solid-state reaction between the aluminum contact and germanium nanowire in situ in the TEM using two different local heating methods. Following the reaction interface of the intrusion of Al in the Ge nanowire shows that at temperatures between 250 and 330 °C the position of the interface as a function of time is well fitted by a square root function, indicating that the reaction rate is limited by a diffusion process. Combining both chemical analysis and electron diffraction we find that the Ge of the nanowire core is completely exchanged by the entering Al atoms that form a monocrystalline nanowire with the usual face-centered cubic structure of Al, where the nanowire dimensions are inherited from the initial Ge nanowire. Model-based chemical mapping by energy dispersive X-ray spectroscopy (EDX) characterization reveals the three-dimensional chemical cross-section of the transformed nanowire with an Al core, surrounded by a thin pure Ge (∼2 nm), Al2O3 (∼3 nm), and Ge containing Al2O3 (∼1 nm) layer, respectively. The presence of Ge containing shells around the Al core indicates that Ge diffuses back into the metal reservoir by surface diffusion, which was confirmed by the detection of Ge atoms in the Al metal line by EDX analysis. Fitting a diffusion equation to the kinetic data allows the extraction of the diffusion coefficient at two different temperatures, which shows a good agreement with diffusion coefficients from literature for self-diffusion of Al.

Hard Superconducting Gap and Diffusion-Induced Superconductors in Ge–Si Nanowires

“…This type of interface would fit our observation of a hard gap which requires a defect-free highly homogeneous heterointerface [13], combined with the tunneling behavior achieved close to depletion. We therefore conclude that the diffusion-induced formation of X1 is essential for the hard induced gap [35,36]. CONCLUSION We have shown that Ge-Si nanowire devices with Al contacts contain additional superconductors after annealing, caused by diffusion of Al into the nanowire channel.…”

“…As is the convention in EDX analysis, L and K denote the orbital to which an electron decays in a picture where K, L, and M are the outer atomic orbitals, while α and β indicate whether it decays from the first or second higher orbital. The Al Kα signal shows the opposite behavior, implying that Ge has been replaced by Al in Area 2 [35,36]. The counts for elements O, C and Si remain equal in both areas (see also Supplementary Information Fig.…”

“…A specific application could be the creation of Majorana fermions, either in this system [46] or in other materials where low g-factors would otherwise be limiting [47]. The challenge lies to both radially and longitudinally control Al-Ge inter-diffusion in the nanowire [36] to prevent metallizing the nanowire (see Fig. S2 for SEM images of several devices showing partially and fully metallized nanowire channels).…”

Coulomb blockade in monolithic and monocrystalline Al-Ge-Al nanowire heterostructures

“…This image has been obtained on a probe corrected Titan Themis working at 200 kV. For STEM observations, devices were fabricated on electron transparent Si 3 N 4 membranes, as described in ref [18].…”

Highly Transparent Contacts to the 1D Hole Gas in Ultrascaled Ge/Si Core/Shell Nanowires

“…To overcome these limitations, material combinations with no intermetallic phase formation, such as the Al-Ge system, enabling true metal-semiconductor heterostructures with abrupt metal-semiconductor interfaces received a considerable amount of attention. 12,13,14,15,16,17 Within this paper, we apply quasi 1D…”