arXiv subjects

Dominique Bougeard

Publications and source records attributed to Dominique Bougeard.

At least 19 recordsLinked to original sources

arXiv2026

Fabrication-free assessment of microwave losses in germanium-based dielectrics and superconductors

We present a flip-chip-based sensing scheme to measure effective microwave losses associated with target materials for quantum technologies, without requiring any device fabrication on the material under test. Using this approach, we quantify the microwave losses of a strain-engineered Ge/SiGe quantum well heterostructure and investigate losses arising from its Ge substrate and intermediate layers. The quality factors of the fabricated microwave resonators agree with the losses of dielectric materials independently extracted from flip-chip sensing measurements. We further study the superconductor platinum silicon germanide (PtSiGe) prepared by thermal reaction with a deposited Pt film, finding high microwave losses that limit the suitability of the films studied here as the sole superconductor for high-quality resonator applications. By coating Pt with Nb prior to the reaction, we observe a substantial reduction in microwave loss and a nearly three-fold enhancement of the transport critical temperature. The temperature dependence of the microwave loss is consistent with gap inhomogeneity in both superconducting films. These results identify constraints on material choices, provide design guidance for microwave circuits on planar Ge heterostructures, and demonstrate a fast-turnaround testing method for new materials for superconducting quantum circuits.

quant-ph↗

arXiv2026

Disorder signatures emerging at millikelvin temperatures in Si/SiGe field-effect stacks

The performance and scalability of electron spin qubits based on gate-defined quantum-dots in undoped Si/SiGe field-effect stacks remain constrained by disorder originating from the gate stack. Its coupling to the quantum well can be reduced by increasing the Si quantum well depth, while electrostatic charge history, for example through interface-trap filling, can further modify the effective disorder landscape. Although such disorder is commonly benchmarked through mobility measurements using magnetotransport and Hall bar devices, dedicated investigations at millikelvin temperatures relevant for quantum-dot operation remain limited. Here, we use temperature-dependent magnetotransport on Hall bar shaped field-effect transistors to investigate how mobility-based disorder signatures depend on quantum-well depth and charge history from $1.5~\mathrm{K}$ down to the millikelvin regime. We show that magnetotransport characterization at $1.5~\mathrm{K}$ captures the dominant mobility improvement associated with reduced dielectric-interface coupling, but can underestimate disorder differences that emerge at millikelvin temperatures, particularly in the low-density regime. Our results therefore highlight that millikelvin magnetotransport characterization of Hall bar devices can provide additional insight for optimizing Si/SiGe field-effect stacks, particularly in the context of gate-defined quantum dot spin qubits.

cond-mat.mes-hall↗

arXiv2026

Multi-level π-junction in a proximitized Ge/SiGe quantum dot probed by an on-chip superconducting microwave resonator

Using on-chip microwave measurements, we investigate multilevel $π$-junctions formed by proximitized quantum dot (QD) in a germanium (Ge)/silicon-germanium (SiGe) heterostructure. In the multilevel regime, where several QD orbitals contribute simultaneously to superconducting transport, the Josephson ground state is no longer determined solely by the occupation of a single orbital. By combining DC transport and microwave techniques, we identify the qualitative signatures of multilevel $π$-junctions in both their gate-voltage dependence and microwave response. In particular, we observe combinations phase transitions that are sharp or smooth in gate voltage and which exhibit distinct inductive and dissipative signatures. Such multilevel Josephson transport has previously been observed primarily in exceptionally clean systems such as carbon nanotubes. Our results establish proximitized Ge as a platform for investigating hybrid superconductor/semiconductor physics and demonstrate the integration of gate-defined superconducting quantum devices with high-quality on-chip microwave resonators.

cond-mat.mes-hall↗

arXiv2026

Increasing valley splitting in Si/SiGe by practically achievable heterostructure profiles

Silicon spin qubits are marred by the valley degeneracy of the conduction band. In a nanodevice, the degeneracy is lifted by interfaces and alloy disorder, but the arising valley splitting is small, of order 100 $μ$eV in Si/SiGe quantum wells. Substantial efforts were invested both in theory and experiments to overcome the valley issue. Unfortunately, the existing recipes either rely on atomistic details of the interface that are beyond experimental control, or demand heterostructure profiles beyond current state-of-the-art heterostructure epitaxy. We revisit the valley splitting induced by non-trivial Ge profiles and advocate a novel view of the intervalley coupling as a backscattering on point-like impurities realized by crystal planes containing Ge atoms. This perspective reveals that enhancing the backscattering amplitude, which sets the valley splitting, requires constructive interference of multiple scatterers. % We arrive at a remarkable prediction, that the Ge content along the heterostructure growth direction does not have to have any specific periodicity, including the practically unreachable $2π/(2k_0)$ period, to significantly increase the valley splitting. This statement is corroborated with numerical evidence from tight-binding simulations and intuitive physical interpretations. We devise profiles that seem within the capabilities of current MBE growth techniques and boost the valley splitting beyond the 1\,meV scale.

cond-mat.mes-hall↗

arXiv2026

Fabrication, characterization and mechanical loading of Si/SiGe membranes for spin qubit devices

Si/SiGe heterostructures on bulk Si substrates have been shown to host high fidelity electron spin qubits. Building a scalable quantum processor would, however, benefit from further improvement of critical material properties such as the valley-splitting landscape. Flexible control of the strain field and the out-of-plane electric field $\mathcal{E}_z$ may be decisive for valley splitting enhancement in the presence of alloy disorder. We envision the Si/SiGe membrane as a versatile scientific platform for investigating intervalley scattering mechanisms which have thus far remained elusive in conventional Si/SiGe heterostructures and have the potential to yield favourable valley-splitting distributions. Here, we report the fabrication of locally etched, suspended SiGe/Si/SiGe membranes from two different heterostructures and apply the process to realize a spin-qubit shuttling device on a membrane for future valley mapping experiments. The membranes have a thickness in the micrometer range and can be metallized to form a back-gate contact for extended control over the electric field. To probe their elastic properties, the membranes are stressed by loading with a profilometer stylus at room temperature. We distinguish between linear elastic and buckling modes, each offering mechanisms through which strain can be coupled to spin qubits.

cond-mat.mes-hall↗

arXiv2025

High-quality and field resilient microwave resonators on Ge/SiGe quantum well heterostructures

Superconducting resonators integrated with Ge quantum wells (QWs) offer a promising platform for hybrid quantum devices. Yet, in the most common heterostructure architectures, they have so far been limited by sizable photon losses. Here, we report the fabrication and characterization of microwave resonators patterned in the Al thin film of an in-situ grown superconductor/semiconductor hybrid heterostructure (HS). The semiconductor part of this hybrid HS is grown on a commercial Ge substrate. We consistently achieve internal quality factors $Q_i>1000$, surpassing previous results on Ge QW heterostructures grown using the concept of a virtual Ge substrate on Si substrates. We reach $Q_i \approx 49000$ at single-photon occupation and a plateau of $Q_i \approx 20000$ at sub-one photon, an order of magnitude larger than any previously reported value of resonators on Ge QW structures at low power. We further characterize the thin Al film forming the resonator, extracting its kinetic inductance and superconducting gap, and studying its magnetic field dependence. Notably, the resonance remains well-defined up to in-plane magnetic fields of 850 mT. A hysteresis emerges in the out-of-plane magnetic field dependence, for both the resonance frequency and the quality factor, indicating an interesting interplay between vortex- and quasiparticle loss mechanisms.

cond-mat.mes-hall↗

arXiv2025

Exchange interaction in gate-defined quantum dots beyond the Hubbard model

A quantitative description of the exchange interaction in quantum dots is relevant for modeling gate operations of spin qubits. By measuring the amplitude and frequency of exchange-driven qubit state oscillations, we measure the detuning dependence of the exchange coupling in a GaAs double quantum dot over three orders of magnitude. Both 1D and 3D full configuration interaction simulations can replicate the observed behavior. Extending a Hubbard model by including excited states increases the range of detuning where it provides a good fit, thus elucidating the underlying physics.

cond-mat.mes-hall↗

arXiv2025

High yield, low disorder Si/SiGe heterostructures for spin qubit devices manufactured in a BiCMOS pilot line

The prospect of achieving fault-tolerant quantum computing with semiconductor spin qubits in Si/SiGe heterostructures relies on the integration of a large number of identical devices, a feat achievable through a scalable (Bi)CMOS manufacturing approach. To this end, both the gate stack and the Si/SiGe heterostructure must be of high quality, exhibiting uniformity across the wafer and consistent performance across multiple fabrication runs. Here, we report a comprehensive investigation of Si/SiGe heterostructures and gate stacks, fabricated in an industry-standard 200 mm BiCMOS pilot line. We evaluate the homogeneity and reproducibility by probing the properties of the two-dimensional electron gas (2DEG) in the shallow silicon quantum well through magnetotransport characterization of Hall bar-shaped field-effect transistors at 1.5 K. Across all the probed wafers, we observe minimal variation of the 2DEG properties, with an average maximum mobility of $(4.25\pm0.17)\times 10^{5}$ cm$^{2}$/Vs and low percolation carrier density of $(5.9\pm0.18)\times 10^{10}$ cm$^{-2}$ evidencing low disorder potential in the quantum well. The observed narrow statistical distribution of the transport properties highlights the reproducibility and the stability of the fabrication process. Furthermore, wafer-scale characterization of a selected individual wafer evidenced the homogeneity of the device performances across the wafer area. Based on these findings, we conclude that our material and processes provide a suitable platform for the development of scalable, Si/SiGe-based quantum devices.

cond-mat.mes-hall↗

arXiv2025

Industrially fabricated single-electron quantum dots in Si/Si-Ge heterostructures

This paper reports the compatibility of heterostructure-based spin qubit devices with industrial CMOS technology. It features Si/Si-Ge quantum dot devices fabricated using Infineon's 200 mm production line within a restricted thermal budget. The devices exhibit state-of-the-art charge sensing, charge noise and valley splitting characteristics, showing that industrial fabrication is not harming the heterostructure quality. These measured parameters are all correlated to spin qubit coherence and qubit gate fidelity. We describe the single electron device layout, design and its fabrication process using electron beam lithography. The incorporated standard 90 nm back-end of line flow for gate-layer independent contacting and wiring can be scaled up to multiple wiring layers for scalable quantum computing architectures. In addition, we present millikelvin characterization results. Our work exemplifies the potential of industrial fabrication methods to harness the inherent CMOS-compatibility of the Si/Si-Ge material system, despite being restricted to a reduced thermal budget. It paves the way for advanced quantum processor architectures with high yield and device quality.

cond-mat.mes-hall↗

arXiv2024

Impact of biased cooling on the operation of undoped silicon quantum well field-effect devices for quantum circuit applications

Gate-tunable semiconductor nanosystems are getting more and more important in the realization of quantum circuits. While such devices are typically cooled to operation temperature with zero bias applied to the gate, biased cooling corresponds to a non-zero gate voltage being applied before reaching the operation temperature. We systematically study the effect of biased cooling on different undoped SiGe/Si/SiGe quantum well field-effect stacks (FESs), designed to accumulate and density-tune two-dimensional electron gases (2DEGs). In an empirical model, we show that biased cooling of the undoped FES induces a static electric field, which is constant at operation temperature and superimposes onto the field exerted by the top gate onto the 2DEG. We show that the voltage operation window of the field-effect-tuned 2DEG can be chosen in a wide range of voltages via the choice of the biased cooling voltage. Importantly, quality features of the 2DEG such as the mobility or the temporal stability of the 2DEG density remain unaltered under biased cooling. We discuss how this additional degree of freedom in the tunability of FESs may be relevant for the operation of quantum circuits, in particular for the electrostatic control of spin qubits.

cond-mat.mes-hall↗

arXiv2024

Optimizing ToF-SIMS Depth Profiles of Semiconductor Heterostructures

The continuous technological development of electronic devices and the introduction of new materials leads to ever greater demands on the fabrication of semiconductor heterostructures and their characterization. This work focuses on optimizing Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) depth profiles of semiconductor heterostructures aiming at a minimization of measurement-induced profile broadening. As model system, a state-of-the-art Molecular Beam Epitaxy (MBE) grown multilayer homostructure consisting of $^{\textit{nat}}$Si/$^{28}$Si bilayers with only 2 nm in thickness is investigated while varying the most relevant sputter parameters. Atomic concentration-depth profiles are determined and an error function based description model is used to quantify layer thicknesses as well as profile broadening. The optimization process leads to an excellent resolution of the multilayer homostructure. The results of this optimization guide to a ToF-SIMS analysis of another MBE grown heterostructure consisting of a strained and highly purified $^{28}$Si layer sandwiched between two Si$_{0.7}$Ge$_{0.3}$ layers. The sandwiched $^{28}$Si layer represents a quantum well that has proven to be an excellent host for the implementation of electron-spin qubits.

physics.app-ph↗

arXiv2024

Atomistic compositional details and their importance for spin qubits in isotope-purified silicon-germanium quantum wells

Understanding crystal characteristics down to the atomistic level increasingly emerges as a crucial insight for creating solid state platforms for qubits with reproducible and homogeneous properties. Here, isotope composition depth profiles in a SiGe/$^{28}$Si/SiGe heterostructure are analyzed with atom probe tomography (APT) and time-of-flight secondary-ion mass spectrometry. Spin-echo dephasing times $T_2^{echo}=128 μs$ and valley energy splittings around $200 μeV$ have been observed for single spin qubits in this quantum well (QW) heterostructure, pointing towards the suppression of qubit decoherence through hyperfine interaction or via scattering between valley states. The concentration of nuclear spin-carrying $^{29}$Si is 50 ppm in the $^{28}$Si QW. APT allows to uncover that both the top SiGe/$^{28}$Si and the bottom $^{28}$Si/SiGe interfaces of the QW are shaped by epitaxial growth front segregation signatures on a few monolayer scale. A subsequent thermal treatment broadens the top interface by about two monolayers, while the width of the bottom interface remains unchanged. Using a tight-binding model including SiGe alloy disorder, these experimental results suggest that the combination of the slightly thermally broadened top interface and of a minimal Ge concentration of $0.3 \%$ in the QW, resulting from segregation, is instrumental for the observed large valley splitting. Minimal Ge additions $< 1 \%$, which get more likely in thin QWs, will hence support high valley splitting without compromising coherence times. At the same time, taking thermal treatments during device processing as well as the occurrence of crystal growth characteristics into account seems important for the design of reproducible qubit properties.

cond-mat.mes-hall↗

arXiv2023

Local laser-induced solid-phase recrystallization of phosphorus-implanted Si/SiGe heterostructures for contacts below 4.2 K

Si/SiGe heterostructures are of high interest for high mobility transistor and qubit applications, specifically for operations below 4.2 K. In order to optimize parameters such as charge mobility, built-in strain, electrostatic disorder, charge noise and valley splitting, these heterostructures require Ge concentration profiles close to mono-layer precision. Ohmic contacts to undoped heterostructures are usually facilitated by a global annealing step activating implanted dopants, but compromising the carefully engineered layer stack due to atom diffusion and strain relaxation in the active device region. We demonstrate a local laser-based annealing process for recrystallization of ion-implanted contacts in SiGe, greatly reducing the thermal load on the active device area. To quickly adapt this process to the constantly evolving heterostructures, we deploy a calibration procedure based exclusively on optical inspection at room-temperature. We measure the electron mobility and contact resistance of laser annealed Hall bars at temperatures below 4.2 K and obtain values similar or superior than that of a globally annealed reference samples. This highlights the usefulness of laser-based annealing to take full advantage of high-performance Si/SiGe heterostructures.

physics.app-ph↗

arXiv2023

Sculpting ultrastrong light-matter coupling through spatial matter structuring

The central theme of cavity quantum electrodynamics is the coupling of a single optical mode with a single matter excitation, leading to a doublet of cavity polaritons which govern the optical properties of the coupled structure. Especially in the ultrastrong coupling regime, where the ratio of the vacuum Rabi frequency and the quasi-resonant carrier frequency of light, $Ω_{\mathrm R}/ω_{\mathrm c}$, approaches unity, the polariton doublet bridges a large spectral bandwidth $2Ω_{\mathrm R}$, and further interactions with off-resonant light and matter modes may occur. The resulting multi-mode coupling has recently attracted attention owing to the additional degrees of freedom for designing light-matter coupled resonances, despite added complexity. Here, we experimentally implement a novel strategy to sculpt ultrastrong multi-mode coupling by tailoring the spatial overlap of multiple modes of planar metallic THz resonators and the cyclotron resonances of Landau-quantized two-dimensional electrons, on subwavelength scales. We show that similarly to the selection rules of classical optics, this allows us to suppress or enhance certain coupling pathways and to control the number of light-matter coupled modes, their octave-spanning frequency spectra, and their response to magnetic tuning. This offers novel pathways for controlling dissipation, tailoring quantum light sources, nonlinearities, correlations as well as entanglement in quantum information processing.

quant-ph↗

arXiv2023

Nonlinear helicity anomalies in the cyclotron resonance photoresistance of two-dimensional electron systems

Our studies of the cyclotron resonance (CR) photoresistance in GaAs-based two-dimensional electron systems (2DES) reveal an anomalously low sensitivity to the helicity of the incoming circularly polarized terahertz radiation. We find that this anomaly is strongly intensity dependent, and the ratio of the low-temperature photoresistance signals for the CR-active (CRA) and CR-inactive (CRI) polarities of magnetic field increases with lowering power, but, nevertheless, remains substantially lower than expected from conventional theory assuming interaction of the plane electromagnetic wave with the uniform 2DES. Our analysis shows that all data can be well described by the nonlinear CR-enhanced electron gas heating in both CRA and CRI regimes. This description, however, requires a source of anomalous absorption of radiation in the CRI regime. It can stem from evanescent electromagnetic fields originating from the near-field diffraction within or in the vicinity of the quantum well hosting the 2DES.

cond-mat.mes-hall↗

arXiv2023

Sensing dot with high output swing for scalable baseband readout of spin qubits

A crucial requirement for quantum computing, in particular for scalable quantum computing and error correction, is a fast and high-fidelity qubit readout. For semiconductor based qubits, one limiting factor for local low-power signal amplification, is the output swing of the charge sensor. We demonstrate GaAs and Si/SiGe asymmetric sensing dots (ASDs) specifically designed to provide a significantly improved response compared to conventional charge sensing dots. Our ASD design features a strongly decoupled drain reservoir from the sensor dot, which mitigates negative feedback effects found in conventional sensors. This results in a boosted output swing of $3\,\text{mV}$, which exceeds the response in the conventional regime of our device by more than ten times. The enhanced output signal paves the way for employing very low-power readout amplifiers in close proximity to the qubit.

cond-mat.mes-hall↗

arXiv2023

Tailoring potentials by simulation-aided design of gate layouts for spin qubit applications

Gate-layouts of spin qubit devices are commonly adapted from previous successful devices. As qubit numbers and the device complexity increase, modelling new device layouts and optimizing for yield and performance becomes necessary. Simulation tools from advanced semiconductor industry need to be adapted for smaller structure sizes and electron numbers. Here, we present a general approach for electrostatically modelling new spin qubit device layouts, considering gate voltages, heterostructures, reservoirs and an applied source-drain bias. Exemplified by a specific potential, we study the influence of each parameter. We verify our model by indirectly probing the potential landscape of two design implementations through transport measurements. We use the simulations to identify critical design areas and optimize for robustness with regard to influence and resolution limits of the fabrication process.

cond-mat.mes-hall↗

arXiv2023

Controlled rotation of electrically injected spins in a non-ballistic spin field-effect transistor

Electrically controlled rotation of spins in a semiconducting channel is a prerequisite for the successful realization of many spintronic devices, like, e.g., the spin field effect transistor (sFET). To date, there have been only a few reports on electrically controlled spin precession in sFET-like devices. These devices operated in the ballistic regime, as postulated in the original sFET proposal, and hence need high SOC channel materials in practice. Here, we demonstrate gate-controlled precession of spins in a non-ballistic sFET using an array of narrow diffusive wires as a channel between a spin source and a spin drain. Our study shows that spins traveling in a semiconducting channel can be coherently rotated on a distance far exceeding the electrons mean free path, and spin-transistor functionality can be thus achieved in non-ballistic channels with relatively low SOC, relaxing two major constraints of the original sFET proposal.

cond-mat.mes-hall↗