Development of a Nb-based semiconductor-superconductor hybrid platform

TL;DR

This work tackles the difficulty of forming transparent Nb-based semiconductor–superconductor interfaces by inserting a thin in-situ Al interlayer between InAs 2DEG and Nb/NbTi films, enabling metal-to-metal epitaxy and a highly transparent interface. The optimized stack achieves an induced superconducting gap of about $1 ext{ meV}$ in NbTi-based devices, substantially larger than typical Al-based hybrids, and supports operation at high magnetic fields and temperatures (Tc up to several kelvin and Bc2 in the tesla range). MAR spectroscopy on Josephson junctions validates high interface transparency and a pronounced induced gap, with gating enabling supercurrent control and Fraunhofer-like patterns under perpendicular fields. The approach demonstrates generality across Nb and NbTi, suggesting pathways to even larger-gap materials (e.g., NbTiN) for robust hybrid quantum devices with broad operating windows.

Abstract

Semiconductor-superconductor hybrid materials are used as a platform to realise Andreev bound states, which hold great promise for quantum applications. These states require transparent interfaces between the semiconductor and superconductor, which are typically realised by in-situ deposition of an Al superconducting layer. Here we present a hybrid material based on an InAs two-dimensional electron gas (2DEG) combined with in-situ deposited Nb and NbTi superconductors, which offer a larger operating range in temperature and magnetic field due to their larger superconducting gap. We overcome the inherent difficulty associated with the formation of an amorphous interface between III-V semiconductors and Nb-based superconductors by introducing a 7 nm Al interlayer. The Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs 2DEG. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor-semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms.

Figures (9)

• Figure 1: STEM images showing the development of epitaxial interfaces between the semiconductor heterostructure and the Al and Nb superconducting layers.a HAADF STEM image of the interface in case of Nb deposition directly the semiconductor surface. An amorphous layer at the interface is indicated by dashed lines. b Angular Dark Field STEM image showing InAs heterostructure, the Al and the Nb superconducting layer together with the fast Fourier transform (FFT) of the Nb, which highlights its single crystalline structure. c Overview STEM image of the upper part of the material stack, showing a homogeneous and defect free Al interlayer. d High-resultion HAADF STEM images showing the details of the semiconductor/Al and the Al/Nb interfaces, for a grain where the [110] projection of the Nb matches the [110] projection of the substrate. To identify the observed grain orientations the obtained STEM data is overlaid with crystallographic models. e Bragg-filtered portion of the interface in d, highlighting the presence of misfit dislocations that occur at the Nb/Al interface. f Details of the semiconductor/Al and the Al/Nb interfaces, for a grain where the [100] projection of the Nb matches the [110] projection of the substrate. g Bragg-filtered portion of the interface in f, highlighting the presence of misfit dislocations that occur at the Nb/Al interface.
• Figure 2: The effect of Al thickness on the material interfaces. a HAADF STEM image of the interfaces for a 3 nm thick Al layer that is not completely crystalline. b, c and d show the exact same material stack but with an Al thickness of 5 nm, 7 nm and 20 nm respectively.
• Figure 3: GaAs capping of the semiconductor structure.a BF STEM images of the material stack with 7 nm Al and without GaAs capping. The dark triangular region within the Al layer corresponds to In migration into the Al interlayer and formation of a triangular inclusion. b Identical structures with 2 monolayers, c 4 monolayers and d 6 monolayers of GaAs deposited between the semiconductor and Al film. The addition of GaAs capping layers prevents In migration, but has a roughening effect on both the InAs/Al and Al/Nb interfaces.
• Figure 4: Material stack with NbTi as the top superconductor. a BF STEM images showing the interface of NbTi deposited directly on the semiconductor heterostructure, where a fully amorphous region is highlighted by the dashed lines. b BF STEM images of the high-quality interfaces formed due to insertion of the epitaxial Al interlayer. c Critical magnetic field as a function of temperature for Nb, Nb with Al, NbTi and NbTi with Al obtained from DC transport measurements. The substrate for all four films is the optimised semiconductor heterostructure with the InAs 2DEG and 2ML of GaAs capping. The grey arrows indicate the critical transition temperatures.
• Figure 5: Transport measurements of a Josephson Junction fabricated from this material.a An optical image of the characterized device, where the source-drain electrodes, top-gate and the AC voltage probes are labeled. b The dependence of the resistance on the bias current and gate voltage. c The dependence of the resistance $R$ on the bias current and applied magnetic field. d The resistance of the JJ as a function of temperature at zero DC bias current. In e the resistance of the JJ as a function of DC bias voltage for increasing temperature is plotted. Indicated are the peaks that we assign to MAR. Traces are offset for clarity. Figure f shows the resistance at 600 mK as a function of $V_{\rm SD}$ with in the figure inset the first five MAR peaks plotted as a function of 1/$N_{\rm MAR}$. The peak locations are fitted with an induced gap of 0.99 meV. In g the temperature related shift of the $N=2$ MAR peak is plotted and fitted with equation \ref{['BCSstar']}.