Researchers at Univ. Grenoble Alpes, CEA, Grenoble INP, IRIG, PHELIQS, and LETI have measured an interface transparency of 0.88 in a germanium quantum device, reporting a value that suggests remarkably efficient electron transfer between the semiconductor and a nearby aluminum superconductor. The work centers on Ge/SiGe quantum well heterostructures, which utilize a two-dimensional hole gas and are emerging as a promising platform for hybrid superconductor-semiconductor quantum devices. This precise measurement is in excellent agreement with theoretical expectations and reports evidence of a gate-tunable induced superconducting gap.

Andreev Reflection Enhances Conductance Quantization to 0.88

A measured interface transparency of 0.88 was reported by researchers at Univ. Grenoble Alpes, CEA, Grenoble INP, IRIG, PHELIQS, and LETI, in a Ge/SiGe heterostructure, a platform increasingly utilized for building these advanced quantum systems. The heterostructure confines a two-dimensional hole gas (2DHG), providing a high-mobility environment essential for observing delicate quantum phenomena. The team investigated split-gate quantum point contacts (QPCs) fabricated within this Ge/SiGe material, positioning them at different distances from an aluminum superconducting contact.

Ballistic one-dimensional transport was observed within these QPCs, evidenced by clear conductance quantization featuring at least four distinct plateaus. This quantization, a hallmark of pristine electron flow, was then demonstrably enhanced by Andreev reflection occurring at the interface between the superconductor and the semiconductor. Specifically, conductance steps increased by 40% compared to measurements taken under a 100-mT out-of-plane magnetic field, indicating the significant impact of this interfacial effect.

This result is in excellent agreement with the theoretical expectation for an interface transparency of 0.88. By operating the QPCs in the tunneling regime, the researchers probed the local density of states of the proximitized 2DHG, reporting direct experimental evidence of an induced superconducting gap and demonstrating that its magnitude can be tuned by a gate voltage acting on the carrier density in the 2DHG.

Gate-Tunable Superconducting Gap in Proximitized 2DHG

Hybrid quantum devices leveraging germanium and silicon-germanium heterostructures are gaining prominence as platforms for exploring superconductivity, and recent work at Univ. Grenoble Alpes, CEA, Grenoble INP, IRIG, PHELIQS, and LETI demonstrates a high degree of control over induced superconducting properties within these systems. Researchers, led by Elyjah Kiyooka, have reported an interface transparency of 0.88 between aluminum superconducting contacts and a two-dimensional hole gas (2DHG) confined within the Ge/SiGe structure, a figure suggesting remarkably efficient electron transfer across the interface.

This precise measurement is in excellent agreement with the theoretical expectation and validates the experimental setup and material quality. The high interface transparency facilitates the extension of superconductivity from the aluminum into the semiconductor, a phenomenon known as the proximity effect, and is a key factor in realizing novel quantum functionalities.

Importantly, the conductance steps exhibited a 40% enhancement due to Andreev reflection at the superconductor-semiconductor interface, a process where an electron entering the superconductor is converted into a hole, effectively doubling the current. The ability to observe such well-defined ballistic transport and Andreev reflection is essential for creating robust and predictable quantum devices. This gate-tunable control over the superconducting gap represents an advancement, offering a pathway towards fully controllable superconducting circuits and devices.

Interface Transparency Measured via Conductance Step Scaling

Researchers are meticulously characterizing the interface between superconductors and semiconductors, with a recent focus on germanium-based quantum devices. This precise measurement, obtained through analysis of split-gate quantum point contacts positioned at different distances from an aluminum superconducting contact, provides critical insight into building stable and high-performing quantum circuits.

The team’s approach involved defining quantum point contacts within the Ge/SiGe heterostructure and varying their distance from the aluminum contact, allowing for a detailed examination of proximity effects. This enhancement directly correlates with the reported interface transparency, validating theoretical models and demonstrating the quality of the material interfaces created.

The observed conductance quantization, a hallmark of ballistic transport, further confirms the high degree of control achieved over the quantum system. To verify the transparency value, the researchers employed a fitting procedure on both normal-type and Andreev-enhanced conductance plateaus, utilizing the transmission coefficient as the sole adjustable parameter, solidifying the initial measurement. This result is in line with theoretical expectations, providing strong support for the experimental methodology and data interpretation.

The equal transmission probability observed across the one-dimensional channels likely originates from backscattering associated with the high, yet finite, transparency of the interface. Further investigation involved examining the bias-voltage dependence of the differential conductance, revealing a clear enhancement around zero bias, consistent with the aluminum superconducting gap. Color-scale plots of this differential conductance demonstrated the proportionality relation between Andreev-enhanced and normal conductance, even in high-bias, nonlinear regimes.

The experimental setup included two quantum point contacts, a left QPC and a right QPC, positioned at different distances from the superconducting aluminum contact. This configuration was deliberately chosen to isolate the effects of distance on the proximity effect, minimizing the influence of device-to-device variations. By comparing measurements from the two QPCs, the team aimed to determine how the proximity effect is affected by distance, providing a more complete understanding of the superconducting proximity effect.

The findings demonstrate a clear experimental verification of theoretical expectations, a significant achievement in the field of superconductor-semiconductor hybrid devices. The high interface transparency of 0.88,

Fabrication of Split-Gate QPCs in Ge/SiGe Devices

Ge/SiGe heterostructures are now central to building hybrid superconductor-semiconductor quantum devices, offering a platform that leverages the unique properties of a two-dimensional hole gas (2DHG). This deliberate arrangement allows researchers to isolate the effects of proximity on the quantum behavior of the semiconductor material, a critical step towards realizing more complex quantum circuits.

The fabrication process begins with a 16-nanometer thick strained germanium quantum well sandwiched between relaxed silicon-germanium barrier layers, with an upper barrier thickness of 22 nanometers. Superconducting contacts are then defined using calibrated dry etching followed by the deposition of a 50-nanometer thick aluminum layer, a process refined through earlier publications detailing growth parameters.

Two overlapping gate layers are created with intervening steps of aluminum oxide atomic-layer deposition, enabling precise control over the carrier density within the quantum well. Hall bar devices fabricated alongside the QPCs allow for measurement of the gate dependence of hole carrier density and mobility, providing crucial data for characterizing the material’s properties. A key finding from this work is the reported interface transparency of 0.88, a value indicating a high degree of electron transfer at the interface between the aluminum superconductor and the germanium quantum well.

This value, derived from analyzing the enhancement of conductance steps due to Andreev reflection, suggests a high-quality interface with minimal impedance to electron flow. “We find that conductance steps are enhanced by Andreev reflection at the S/Sm interface, in agreement with the theoretical expectation for an interference transmission close to unity,” the study reports, highlighting the strong correlation between experimental results and theoretical predictions. This level of transparency is particularly noteworthy as it facilitates ballistic transport, a condition where electrons travel without scattering, essential for certain quantum computing applications.

This gate-tunable induced superconducting gap is a significant advancement, allowing for electrostatic control over the carrier density in the proximitized 2DHG. The experimental setup positioned QPCs at different distances from the superconducting interface.

The observed modulation of the gap with gate voltage demonstrates the potential for manipulating superconducting properties within the semiconductor, opening avenues for designing novel quantum devices. “We report direct evidence of an induced superconducting gap and present a study of its spatial and gate-voltage dependence, providing new insight on the superconducting proximity effect,” the paper states, emphasizing the importance of understanding these fundamental interactions. The high mobility and low percolation density of the 2DHG within the Ge/SiGe heterostructure are crucial to achieving these results.

👉 More information
🗞 Andreev-Enhanced Conductance Quantization and Gate-Tunable Induced Superconducting Gap in Germanium
✍️ Elyjah Kiyooka et al.
🧠 DOI: http://link.aps.org/doi/10.1103/zd45-rvtk

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