Superconductor-semiconductor Interface Research Articles

Supercurrent, Multiple Andreev Reflections and Shapiro Steps in InAs Nanosheet Josephson Junctions.

We report an experimental study of proximity induced superconductivity in planar Josephson junction devices made from free-standing InAs nanosheets. The nanosheets are grown by molecular beam epitaxy, and the Josephson junction devices are fabricated by directly contacting the nanosheets with superconductor Al electrodes. The fabricated devices are explored by low-temperature carrier transport measurements. The measurements show that the devices exhibit a gate-tunable supercurrent, multiple Andreev reflections, and a good quality superconductor-semiconductor interface. The superconducting characteristics of the Josephson junctions are investigated at different magnetic fields and temperatures and are analyzed based on the Bardeen-Cooper-Schrieffer (BCS) theory. The measurements of the ac Josephson effect are also conducted under microwave radiations with different radiation powers and frequencies, and integer Shapiro steps are observed. Our work demonstrates that InAs nanosheet based hybrid devices are desired systems for investigating the forefront of physics, such as two-dimensional topological superconductivity.

Momentum-resolved electronic structure and band offsets in an epitaxial NbN/GaN superconductor/semiconductor heterojunction.

The electronic structure of heterointerfaces is a pivotal factor for their device functionality. We use soft x-ray angle-resolved photoelectron spectroscopy to directly measure the momentum-resolved electronic band structures on both sides of the Schottky heterointerface formed by epitaxial films of the superconducting NbN on semiconducting GaN, and determine their momentum-dependent interfacial band offset as well as the band-bending profile. We find, in particular, that the Fermi states in NbN are well separated in energy and momentum from the states in GaN, excluding any notable electronic cross-talk of the superconducting states in NbN to GaN. We support the experimental findings with first-principles calculations for bulk NbN and GaN. The Schottky barrier height obtained from photoemission is corroborated by electronic transport and optical measurements. The momentum-resolved understanding of electronic properties of interfaces elucidated in our work opens up new frontiers for the quantum materials where interfacial states play a defining role.

InSbAs Two-Dimensional Electron Gases as a Platform for Topological Superconductivity.

Topological superconductivity can be engineered in semiconductors with strong spin-orbit interaction coupled to a superconductor. Experimental advances in this field have often been triggered by the development of new hybrid material systems. Among these, two-dimensional electron gases (2DEGs) are of particular interest due to their inherent design flexibility and scalability. Here, we discuss results on a 2D platform based on a ternary 2DEG (InSbAs) coupled to in situ grown aluminum. The spin-orbit coupling in these 2DEGs can be tuned with the As concentration, reaching values up to 400 meV Å, thus exceeding typical values measured in its binary constituents. In addition to a large Landé g-factor of ∼55 (comparable to that of InSb), we show that the clean superconductor-semiconductor interface leads to a hard induced superconducting gap. Using this new platform, we demonstrate the basic operation of phase-controllable Josephson junctions, superconducting islands, and quasi-1D systems, prototypical device geometries used to study Majorana zero modes.

Hard-Gap Spectroscopy in a Self-Defined MesoscopicInAs/AlNanowire Josephson Junction

Superconductor/semiconductor-nanowire hybrid structures can serve as versatile building blocks to realize Majorana circuits or superconducting qubits based on quantized levels such as Andreev qubits. For all these applications it is essential that the superconductor-semiconductor interface is as clean as possible. Furthermore, the shape and dimensions of the superconducting electrodes needs to be precisely controlled. We fabricated self-defined InAs/Al core/shell nanowire junctions by a fully in-situ approach, which meet all these criteria. Transmission electron microscopy measurements confirm the sharp and clean interface between the nanowire and the in-situ deposited Al electrodes which were formed by means of shadow evaporation. Furthermore, we report on tunnel spectroscopy, gate and magnetic field-dependent transport measurements. The achievable short junction lengths,the observed hard-gap and the magnetic field robustness make this new hybrid structure very attractive for applications which rely on a precise control of the number of sub-gap states, like Andreev qubits or topological systems.

Enhanced two-photon amplification in superconductor-semiconductor plasmonic waveguides.

We theoretically demonstrate significant enhancement of two-photon amplification by using a superconductor for both a Cooper-pair source and surface plasmon-polariton mode guiding. Cooper-pair-based gain active region restriction to the superconductor-semiconductor interface limits its potentially highly efficient two-photon gain process. Using the superconductor layer for a plasmonic waveguide structure allows strong photon confinement while reducing design and fabrication constraints. This results in three orders of magnitude enhancement of the superconducting two-photon gain (TPG) compared to superconductor-based dielectric waveguides. Moreover, a superconducting TPG produced by a plasmonic waveguide increases with carrier concentration, meeting practical device requirements. Our results pave the way for efficient two-photon amplification realization in nanoscale devices.

Toward durable Al-InSb hybrid heterostructures via epitaxy of 2ML interfacial InAs screening layers

The large Land\'{e} g-factor, high spin-orbit coupling, and low effective mass of the two-dimensional electron gas in InSb quantum wells combined with proximal superconductivity may realize a scalable platform for topological quantum computation. Aluminum thin films directly deposited on top of InSb planar structures result in the formation of a reactive AlInSb layer at the interface. This interlayer progressively consumes the whole Al film, resulting in a disordered AlInSb layer after few months at room temperature. We report on a heterostructure design that results in a significant increase of the durability of these hybrid Al-InSb heterostructures with the preservation of a pure Al film and sharp superconductor-semiconductor interface for more than one year. Two monolayers of epitaxial InAs at the superconductor-semiconductor interface prevent interfacial reactivity as evidenced by X-ray reflectivity and energy dispersive spectroscopy measurements. Structural characterizations of the Al films by transmission electron microscopy reveal the presence of tens of nanometers wide grains predominantly oriented with Al(110) parallel to InSb(001).

Andreev Reflection in a Superconducting Light-Emitting Diode.

We experimentally demonstrate Cooper-pair injection into a superconducting light-emitting diode by observing Andreev reflection at the superconductor-semiconductor interface, overcoming the contradicting requirements of an electrically transparent interface and radiative recombination efficiency. The device exhibits electroluminescence enhancement at the quasi-Fermi energy at temperatures below Tc. The theoretically predicted conductance and electroluminescence spectra based on Cooper-pair injection into the semiconductor correspond well to our experimental results. Our findings pave the way for practical superconductor-semiconductor quantum light sources.

Hybridization at Superconductor-Semiconductor Interfaces

Hybrid superconductor-semiconductor devices are currently one of the most promising platforms for realizing Majorana zero modes. Their topological properties are controlled by the band alignment of the two materials, as well as the electrostatic environment, which are currently not well understood. Here, we pursue to fill in this gap and address the role of band bending and superconductor-semiconductor hybridization in such devices by analyzing a gated single Al-InAs interface using a self-consistent Schrodinger-Poisson approach. Our numerical analysis shows that the band bending leads to an interface quantum well, which localizes the charge in the system near the superconductor-semiconductor interface. We investigate the hybrid band structure and analyze its response to varying the gate voltage and thickness of the Al layer. This is done by studying the hybridization degrees of the individual subbands, which determine the induced pairing and effective $g$-factors. The numerical results are backed by approximate analytical expressions which further clarify key aspects of the band structure. We find that one can obtain states with strong superconductor-semiconductor hybridization at the Fermi energy, but this requires a fine balance of parameters, with the most important constraint being on the width of the Al layer. In fact, in the regime of interest, we find an almost periodic dependence of the hybridization degree on the Al width, with a period roughly equal to the thickness of an Al monolayer. This implies that disorder and shape irregularities, present in realistic devices, may play an important role for averaging out this sensitivity and, thus, may be necessary for stabilizing the topological phase.

RETRACTED ARTICLE: Epitaxy of advanced nanowire quantum devices

Semiconductor nanowires are ideal for realizing various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasiparticles (such as anyons) can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought into contact with a superconductor. To exploit the potential of non-Abelian anyons-which are key elements of topological quantum computing-fully, they need to be exchanged in a well-controlled braiding operation. Essential hardware for braiding is a network of crystalline nanowires coupled to superconducting islands. Here we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks with a predefined number of superconducting islands. Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire 'hashtags' reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase-coherent system with strong spin-orbit coupling. In addition, a proximity-induced hard superconducting gap (with vanishing sub-gap conductance) is demonstrated in these hybrid superconductor-semiconductor nanowires, highlighting the successful materials development necessary for a first braiding experiment. Our approach opens up new avenues for the realization of epitaxial three-dimensional quantum architectures which have the potential to become key components of various quantum devices.

Soft Superconducting Gap in Semiconductor Majorana Nanowires

We theoretically consider the ubiquitous soft gap measured in the tunneling conductance of semiconductor-superconductor hybrid structures, in which recently observed signatures of elusive Majorana bound states have created much excitement. We systematically study the effects of magnetic and nonmagnetic disorder, temperature, dissipative Cooper pair breaking, and interface inhomogeneity, which could lead to a soft gap. We find that interface inhomogeneity with moderate dissipation is the only viable mechanism that is consistent with the experimental observations. Our work indicates that improving the quality of the superconductor-semiconductor interface should result in a harder induced gap.

Superconductor-semiconductor magnetic microswitch

A hybrid superconductor-two-dimensional electron gas microdevice is presented. Its working principle is based on the suppression of Andreev reflection at the superconductor-semiconductor interface caused by a magnetic barrier generated by a ferromagnetic strip placed on top of the structure. Device switching is predicted with fields up to some mT and working frequencies of several GHz, making it promising for applications ranging from microswitches and storage cells to magnetic field discriminators.

Reflectionless tunneling due to Andreev reflection in a gated superconductor–semiconductor junction

We have studied the differential resistance in a gated superconductor–semiconductor junction in the ballistic transport regime. The zero-bias resistance shows a minimum as a function of the gate voltage, while the junction normal resistance measured at a high bias voltage shows a monotonic increase. This is explained as follows: the conductance enhancement due to coherent Andreev reflection is controlled by the gate voltage which changes the transparency of the gate barrier. We have also fabricated a two-gated junction with a high transparency at the superconductor–semiconductor interface and measured its resistance as functions of two gate voltages. An increase in one gate voltage results in the resistance minimum as a function of the other gate voltage. The obtained results agree well with the so-called reflectionless tunneling theory.

Experimental determination of the Andreev reflection probability using ballistic point contact spectroscopy

In this article, we present an experimental technique to determine the Andreev reflection probability—a quantity of central importance for superconductor/semiconductor-hybride structures. This is achieved with a point contact (PC) formed in a two-dimensional electron gas (2DEG) in front of a superconductor/semiconductor (SN) interface. The PC in the InGaAs/InP heterostructure emits electrons towards the SN interface and detects the reflected carriers. Due to the specific sample setup, the vast majority of the detected carriers are retroreflected holes and the signal is a direct measurement of the Andreev reflection probability. A( E) is found to be up to 20% by measuring the point contact conductance. The experimental results are compared with the predictions of two theoretical models. It is found that the Andreev reflection probability significantly differs from theoretical expectations. The main advantage of the presented technique is the direct way of measuring the Andreev reflection probability, independent of any model describing the SN interface.

Formation of the superconductor-semiconductor interface between Bi2Sr2CaCu2O8−x and indium-tin oxide

Abstract Planar heterostructures consisting of semiconducting highly textured indium-tin oxide (ITO) overlayers and c-axis oriented superconducting Bi2Sr2CaCu2O8−x (2212) underlayers have been formed onto MgO(100) substrates using dc magnetron sputtering. Annealing of the structures in air at 500 °C resulted in the appearance of both the interface resistivity anomaly at superconductive transition temperature—Tc and nonlinear current vs voltage behavior characteristics for a S-N junction. Meanwhile gradual change of the electrical properties towards S-I-N type behavior have been indicated with subsequent heat treatment in air at 50–100 °C. The origin of the insulating I-interlayer we associate with reversible oxygen removal in the 2212 compound at superconductor-semiconductor interface.

Enhanced conductance in superconductor semiconductor junctions at zero voltage.

An enhancement of the zero-voltage conductance of a niobium-silicon junction is found at very low temperatures. This enhancement is a result of quantum interference of electrons, near the superconductor semiconductor interface, which results in an increase of Andreev reflection probability.

Low temperature current transport of Sn-GaAs contacts

We measure low temperature current transport properties of superconducting Sn contacts to p+-GaAs. For contacts alloyed at 450 °C, the current-voltage characteristics show a strong dependence on alloying time. The critical temperature of Sn near the superconductor-semiconductor interface decreases from 3.8 to 1.8 K as the alloying time increases from 0 to 120 s. On the other hand, a long-time alloying increases the transparency of the interface. Using the Blonder, Tinkham, and Klapwijk model [Phys. Rev. B 25, 4515 (1982)], we find that the transmission coefficient of the interface increases from 0.2 to 0.7 by alloying. However, the normal state resistance calculated using the model is much smaller than the experimental value.

Carrier transport in mesoscopic silicon-coupled superconducting junctions.

An overview is presented of experimental results on supercurrent flow and transport at finite voltages in a well-characterized, sandwich-type superconductor-semiconductor-superconductor junction. Carrier transport through the structure is found to be dominated by the interfaces. At low temperatures, inelastic scattering is negligible in the intermediate silicon layer and the system is mesoscopic. Application of a voltage results in a strongly nonthermal carrier distribution in the bulk of the semiconductor. The details of the distribution depend on elastic and Andreev scattering at the interfaces and are directly reflected by the measured current-voltage characteristics. The supercurrent is well described by a recent theory for the proximity effect that takes into account the reduction of the superconducting order parameter across the superconductor-semiconductor interface. Supercurrent measurements to identify a possible interplay between the superconductor phase and the single carrier phase in the semiconductor are discussed.

Transistors based on proximity effect control of the critical current of a superconductor

The critical current of a bilayer consisting of a thin superconductor in contact with a normal conductor depends on the thickness of the normal layer due to the proximity effect. Using one electrode of a semiconductor pn junction as the normal material, it is possible to vary the normal layer thickness by applying a voltage to the pn junction. The author discusses the feasibility of transistors based on such structures. He concludes that there is no fundamental impediment to operating a transistor based on proximity effect control of a superconductor. However, the proposed device requires that both superconductor and semiconductor layers be no thicker than roughly a coherence length. The large mismatch at the superconductor-semiconductor interface reduces the size of the proximity effect, possibly making the desired effect too small to be useful. It is difficult to construct a nonlatching device, or one with voltage gain. Thus, the proposed device has major drawbacks which prevent it from being considered as more than a scientific curiosity. >

Excess conductance of superconductor-semiconductor interfaces due to phase conjugation between electrons and holes.

A semiclassical description is given of charge transport through a superconductor-semiconductor interface. As a result of the presence of a potential barrier both Andreev and normal reflection occur. Elastic scatterers in the semiconductor generate multiple reflections at the interface. The constructive quantum interference which results from the phase conjugation between electrons and holes enhances the (differential) conductance above its classical value. This excess conductance is suppressed by a magnetic field, or by a finite energy. The latter can be due to a finite voltage bias or a finite temperature.

Static properties of the superconducting FET: Numerical analysis

A study of the feasibility of the superconducting FET using the high-T/sub c/ superconductors. Static properties are calculated using an equivalent circuit model. All superconductors are assumed to behave like BCS superconductors. The proximity effect is introduced from the phenomenological equation. Although the Schottky barrier effect at the superconductor-semiconductor interface is taken into account, this effect is found to be negligible. For the statics, a SFET made of all-copper-oxide materials seems to give a much longer transit time than that of a Nb-Si-Nb SFET due to low mobility of the normal conducting copper oxide assumed in this analysis. It is found, however, that a transit time of less than 1 ps can be attained if a material having a mobility as high as Si is used for the semiconductor layer. >