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

Abstract

Semiconductor-superconductor hybrid systems have outstanding potential for emerging high performance nanoelectronics and quantum devices. However, critical to their successful application is the fabrication of high quality and reproducible semiconductor-superconductor interfaces. Here, we realize and measure axial Al-Ge-Al nanowire heterostructures with atomically precise interfaces, enwrapped by an ultra-thin epitaxial Si layer further denoted as Al-Ge/Si-Al nanowire heterostructures. The heterostructures were synthesized by a thermally induced exchange reaction of single-crystalline Ge/Si core/shell nanowires and lithographically defined Al contact pads. Applying this heterostructure formation scheme enables self-aligned quasi one-dimensional crystalline Al leads contacting ultra-scaled Ge/Si segments with contact transparencies greater than 98%. Integration into back-gated field-effect devices and continuous scaling beyond lithographic limitations allows us to exploit the full potential of the highly transparent contacts to the 1D hole gas at the Ge-Si interface. This leads to the observation of ballistic transport as well as quantum confinement effects up to temperatures of 150 K. Low temperature measurements reveal proximity-induced superconductivity in the Ge/Si core/shell nanowires. The realization of a Josephson field-effect transistor allows us to study the subharmonic energy-gap structure caused by multiple Andreev reflections. Most importantly, the absence of a quantum dot regime indicates a hard superconducting gap originating from the highly transparent contacts to the 1D hole-gas,

The germanium quantum information route

“…The observation of a tunable superconducting proximity effect in Ge/Si core/shell NWs was reproduced in more recent experiments using not only Al 74,183,217 but also NbTiN 185 superconducting contacts. The high transparency of the S-Sm contacts was further confirmed by tunnel spectroscopy measurements 218 , which revealed an induced superconducting gap containing only a small density of quasiparticle states.…”

“…In addition, radial multi-shell doping and even modulation doping core/shell NWs have been demonstrated 175,182 . Recently, an advanced radial heterostructure was synthesized, with a core made of short segments of crystalline Ge and aluminum surrounded by a Si shell (Ge-Al/Si core/shell NWs), providing avenues for the synthesis of epitaxial superconductor-semiconductor heterostructures in Ge NWs 183 .…”

“…atomic-layer deposition), the inner diffusion of aluminium is probably at the origin of the high contact transparencies observed in most of the experiments discussed above. Recently, Al-Ge-Al devices with an extremely short Ge channel could be obtained by a controlled annealing process 183 (Fig. 6b).…”

“…Panel a is reproduced with permission from ref 214 , Springer Nature Limited. Panel b is reproduced with permission from ref183 , American Chemical Society. Panel c and d are reproduced with permission from ref18,216 , Springer Nature Limited.…”

Al–Ge–Al Nanowire Heterostructure: From Single‐Hole Quantum Dot to Josephson Effect

“…∆* agrees with the extracted ∆ from MAR in Al–Ge/Si–Al core/shell NWs that had similar Al leads, [ 28 ] however, differs significantly from the ∆ = 220 ± μeV extracted in the CB regime. This difference is understood to be due to the reduction of the superconducting gap at the Al–Ge interface resulting from the exchange of charge between the normal conductor and superconductor.…”