Si-MOSFET Achieves 90% Intervalley Mixing in Silicon Quantum Hall Channels

Researchers at NTT, Inc. have demonstrated a high degree of interaction within silicon quantum Hall channels, with a transition probability close to 1, indicating nearly complete equilibration between two valley edge channels. This result, obtained using a double-layer-gated silicon metal-oxide-semiconductor field-effect transistor, brings the creation of compact silicon quantum Hall interferometers closer to realization. The experiment brought spin-polarized valley edge channels into close proximity, revealing that interchannel transport between channels with differing spin orientations showed negligible transition probability, confirming the suppression of spin-flip scattering expected in silicon. These findings demonstrate that intervalley coupling at a silicon/silicon dioxide interface can function as a beam-splitter, establishing a key component for future quantum devices.

Silicon MOSFETs for Quantum Hall Effect Studies

Nearly complete equilibration between valley edge channels has been demonstrated within a silicon metal-oxide-semiconductor field-effect transistor (Si-MOSFET). Researchers at NTT, Inc. have detailed observations of strong intervalley mixing, the interaction between electrons in different valleys of the silicon, in a configuration designed to mimic a beam splitter for quantum information. This work builds upon decades of quantum Hall effect studies traditionally conducted in gallium arsenide, but leverages the unique properties of silicon for potentially more stable and scalable quantum systems. The team, led by Gento Yamahata, fabricated a double-layer-gated Si-MOSFET on a silicon-on-insulator substrate, employing a process similar to that used in their prior work on single-electron pumps. The device architecture allows for precise electrostatic control over the two-dimensional electron system, enabling the creation of spin-polarized valley edge channels.

By carefully tuning the voltages applied to local gates, the researchers brought these channels into close proximity near a depleted side gate, setting the bulk filling factor to a value to facilitate interaction. Unlike GaAs, silicon exhibits weak spin-orbit interaction, suppressing unwanted spin-flip scattering, a major source of decoherence in quantum devices. The NTT team confirmed this advantage, finding negligible transition probability when measuring interchannel transport between edge channels with differing spin orientations. This suppression of spin-flip scattering is consistent with previous studies of high-mobility Si/SiGe quantum Hall systems, but this work demonstrates the phenomenon directly within a Si-MOSFET structure. As the paper notes, previous studies have investigated spin- and valley-resolved edge transport in silicon quantum Hall systems, but this work achieves strong intervalley coupling within the Si-MOSFET platform.

The implications extend beyond fundamental physics; the demonstrated intervalley coupling establishes an important building block toward compact valley-based electron interferometers. The ability to manipulate valley states, coupled with the inherent stability of silicon, positions Si-MOSFETs as a promising candidate for realizing compact and robust quantum Hall interferometers, potentially surpassing the limitations of current architectures based on GaAs/AlGaAs heterostructures. The researchers emphasize that this work contributes to the development of advanced quantum technologies.

Copropagating Edge Channels and Interferometry Concepts

The pursuit of compact, silicon-based quantum interferometers has intensified, building upon decades of research into quantum Hall edge transport initially dominated by gallium arsenide heterostructures. While GaAs offered early advantages in mobility and gate control, silicon presents a compelling alternative due to its compatibility with existing semiconductor manufacturing and, crucially, the presence of the valley degree of freedom. Recent work from NTT, Inc. details significant progress in harnessing this property, demonstrating a pathway toward miniaturized interferometers that sidestep the limitations of spin-based approaches in silicon. Researchers, led by Gento Yamahata, have focused on copropagating quantum Hall edge channels within a specifically engineered silicon metal-oxide-semiconductor field-effect transistor. The device architecture, featuring a double-layer-gated structure, allows for precise control over the proximity of these channels. We observe strong intervalley mixing, with a transition probability close to 1, indicating nearly complete equilibration between the two valley edge channels.

This strong intervalley mixing contrasts with the behavior observed with spin-resolved channels. This suppression of spin-flip scattering is a key benefit, promising greater stability and coherence in quantum devices. The team’s methodology involved applying an alternating current excitation and meticulously measuring the resulting currents using a dilution refrigerator at 280 mK and a magnetic field in the device and measurement setup. The detailed measurement configuration, as illustrated in their published work, allowed for precise control and analysis of the edge channel behavior. Beyond demonstrating strong intervalley coupling, by leveraging the valley degree of freedom and minimizing unwanted spin-related effects, silicon-based interferometers could become a viable platform for advanced quantum technologies.

NTT, Inc. researchers are refining silicon-based quantum Hall devices, focusing on harnessing the unique properties of electron flow within these systems. Gento Yamahata and colleagues are demonstrating increasingly precise control over valley-based electron manipulation, a critical step toward building more compact quantum circuits. This level of coupling is notable given silicon’s inherent material properties. By carefully tuning local gates, they created a configuration where two spin-polarized valley edge channels converge near a depleted side gate. This precise electrostatic control is key to observing and manipulating the quantum behavior of electrons. This arrangement allows researchers to investigate how electrons transition between these channels, revealing fundamental insights into valley transport. Crucially, the NTT team confirmed a significant advantage of silicon over materials like gallium arsenide.

The pursuit of compact, robust quantum devices has led researchers to explore silicon as a promising material, leveraging its unique valley properties for electron manipulation. Unlike gallium arsenide, silicon’s weak spin-orbit interaction minimizes unwanted spin-flip scattering, a critical advantage for maintaining quantum coherence. Recent work at NTT, Inc. focused on a specific configuration where the bulk filling factor was set to a value that enabled the observation of these edge channels. This level of coupling is crucial for creating a beam-splitter-like operation, effectively directing and splitting electron flow within the device. Yamahata explains in the published work that they find intervalley mixing is sufficiently strong to produce nearly complete equilibration between the two valley edge channels. Measurements were conducted at extremely low temperatures, 280 mK, and a magnetic field was applied in the device and measurement setup.

Si-MOSFET Fabrication and Intervalley Transport Measurement Setup

The pursuit of compact quantum devices often hinges on material properties assumed to be straightforward, yet silicon presents a surprising advantage in the realm of quantum Hall systems. While gallium arsenide has traditionally dominated research in this area, silicon’s weak spin-orbit interaction, a characteristic often considered a limitation, actually suppresses unwanted spin-flip scattering, a crucial benefit for maintaining stable quantum states. Researchers at NTT, Inc. The NTT team’s device is not a simple structure; it’s a double-layer-gated Si-MOSFET built on a silicon-on-insulator substrate. The fabrication process, mirroring techniques used in their previous work on silicon single-electron pumps, involved defining a micrometer-scale silicon mesa using electron-beam lithography and dry etching.

A 30-nanometer thermal oxide layer was grown, followed by patterned polycrystalline silicon gates and subsequent ion implantation to create phosphorus-doped contact regions. The researchers note that the device was fabricated using the same fabrication process as in their previous study on silicon single-electron pumps, highlighting the refinement of their fabrication techniques. Central to the experiment was the ability to manipulate the bulk filling factor, using a combination of upper and lower gates. This configuration is critical, as it facilitates the observation of intervalley mixing, where electrons transition between different valleys within the silicon.