Strained Silicon Quantum Wells Offer Path to Faster Quantum Computing

A germanium-silicon germanium heterostructure cultivates a high-mobility two-dimensional hole gas with a mobility of 1.33 x 10⁶ cm²/Vs and low percolation density. Quantum transport reveals density-dependent effective mass and significant heavy-hole-light-hole mixing, suggesting potential for rapid quantum computing and hybrid systems.

The pursuit of robust and scalable quantum computing necessitates materials platforms that balance high performance with manufacturability. Current silicon-germanium heterostructures, while enabling advanced spin-based quantum processors, often rely on substrates containing structural imperfections. Researchers at QuTech and the Kavli Institute of Nanoscience, Delft University of Technology – Davide Costa, Karina Hudson, Patrick Del Vecchio, Lucas E. A. Stehouwer, Alberto Tosato, Davide Degli Esposti, Mario Lodari, Stefano Bosco, and Giordano Scappucci – detail in their recent work, “Buried unstrained Ge channels: a lattice-matched platform for quantum technology”, a novel approach utilising unstrained germanium channels embedded within a strained silicon-germanium barrier. This architecture, grown on a pristine germanium substrate, demonstrates a high-mobility two-dimensional hole gas and exhibits characteristics suggesting potential for fast quantum operations and integration into complex quantum systems.

High-Mobility Hole Gases in Ge/SiGe Heterostructures Advance Quantum Technologies

Recent progress in spin-based quantum computing increasingly focuses on semiconductor heterostructures incorporating germanium and silicon. Conventional designs often utilise silicon-germanium substrates, which inherently introduce structural imperfections. Researchers are now investigating an alternative heterostructure – comprising unstrained germanium and a strained silicon-germanium barrier, lattice-matched to a germanium substrate – to establish a platform supporting a two-dimensional hole gas (2DEG) exhibiting high carrier mobility and minimal disorder. Experiments demonstrate a 2DEG with a mobility reaching 1.33 x 10⁶ cm²/Vs and a low percolation density of 1.4 x 10¹⁰ cm^-2, indicating a substantial improvement in material quality and potential for advanced applications.

A 2DEG is a system where electrons (or, in this case, holes – the absence of an electron, behaving as a positive charge carrier) are confined to move freely in two dimensions, forming a quasi-two-dimensional electron gas. The ‘mobility’ of these carriers dictates how easily they move through the material under an electric field; higher mobility translates to faster and more efficient devices. Percolation density refers to the minimum concentration of charge carriers required to form a continuous conducting path.

Quantum transport measurements reveal a density-dependent in-plane effective mass and out-of-plane g-factor for the confined holes, providing critical insights into the electronic properties of the 2DEG. Analysis confirms significant mixing between heavy and light hole states, aligning with theoretical predictions and demonstrating a unique band structure. This mixing influences the overall electronic properties and offers opportunities to tailor the material’s characteristics. The effective mass describes how a charge carrier responds to an external force, while the g-factor quantifies the interaction between the carrier’s spin and a magnetic field. Heavy and light holes are different energy states for holes, arising from the band structure of the semiconductor.

Theoretical calculations corroborate these experimental findings, confirming the strong heavy-hole–light-hole mixing and validating the understanding of the band structure. Researchers employ a numerical method to determine the energies of Landau levels – discrete energy levels formed when a 2DEG experiences a magnetic field – allowing for precise extraction of effective mass and g-factor. The calculations truncate the system to a finite number of Landau levels to manage computational complexity, yet maintain accuracy and robustness.

The observed strong spin-orbit interaction, coupled with the potential for isotopic purification, positions this Ge/SiGe heterostructure as a promising platform for developing fast quantum hardware and integrating hybrid quantum systems. Spin-orbit interaction arises from the coupling between an electron’s spin and its orbital motion, influencing its behaviour in a magnetic field. Isotopic purification reduces scattering, leading to even higher mobilities and enhancing device performance. The combination of these properties enables the creation of robust and efficient quantum systems with improved coherence times and scalability.

This study demonstrates a 2DEG formed at the interface between unstrained germanium and a strained silicon-germanium heterostructure, grown on a germanium substrate, offering a viable alternative to existing platforms. The use of a germanium substrate avoids the defects associated with metamorphic silicon-germanium substrates commonly used in previous designs, resulting in improved material quality.

Calculations employ a numerical method to determine the energies of Landau levels. By analysing the energy differences between these levels, researchers extract the effective mass and g-factor, confirming the theoretical predictions and validating the experimental results. The calculations demonstrate gauge independence, confirming the robustness of the model and ensuring the reliability of the extracted parameters.

The observed strong spin-orbit interaction, potential for isotopic purification, and possibility of inducing pairing correlations position this Ge/SiGe heterostructure as a promising platform for developing fast quantum hardware and integrating hybrid quantum systems. The strong spin-orbit interaction is crucial for manipulating the spin of the holes, a key requirement for quantum information processing, while isotopic purification further enhances the material’s properties. The ability to host a high-mobility 2DEG with tunable properties offers significant advantages for future quantum device fabrication and performance optimisation.