arXiv subjects

Davide Degli Esposti

Publications and source records attributed to Davide Degli Esposti.

22 records · Page 2Linked to original sources

arXiv2024

Low disorder and high valley splitting in silicon

The electrical characterisation of classical and quantum devices is a critical step in the development cycle of heterogeneous material stacks for semiconductor spin qubits. In the case of silicon, properties such as disorder and energy separation of conduction band valleys are commonly investigated individually upon modifications in selected parameters of the material stack. However, this reductionist approach fails to consider the interdependence between different structural and electronic properties at the danger of optimising one metric at the expense of the others. Here, we achieve a significant improvement in both disorder and valley splitting by taking a co-design approach to the material stack. We demonstrate isotopically-purified, strained quantum wells with high mobility of 3.14(8)$\times$10$^5$ cm$^2$/Vs and low percolation density of 6.9(1)$\times$10$^{10}$ cm$^{-2}$. These low disorder quantum wells support quantum dots with low charge noise of 0.9(3) $μ$eV/Hz$^{1/2}$ and large mean valley splitting energy of 0.24(7) meV, measured in qubit devices. By striking the delicate balance between disorder, charge noise, and valley splitting, these findings provide a benchmark for silicon as a host semiconductor for quantum dot qubits. We foresee the application of these heterostructures in larger, high-performance quantum processors.

cond-mat.mes-hall↗

arXiv2023

Germanium wafers for strained quantum wells with low disorder

We grow strained Ge/SiGe heterostructures by reduced-pressure chemical vapor deposition on 100 mm Ge wafers. The use of Ge wafers as substrates for epitaxy enables high-quality Ge-rich SiGe strain-relaxed buffers with a threading dislocation density of (6$\pm$1)$\times$10$^5$ cm$^{-2}$, nearly an order of magnitude improvement compared to control strain-relaxed buffers on Si wafers. The associated reduction in short-range scattering allows for a drastic improvement of the disorder properties of the two-dimensional hole gas, measured in several Ge/SiGe heterostructure field-effect transistors. We measure an average low percolation density of (1.22$\pm$0.03)$\times$10$^{10}$ cm$^{-2}$, and an average maximum mobility of (3.4$\pm$0.1)$\times$10$^{6}$ cm$^2$/Vs and quantum mobility of (8.4$\pm$0.5)$\times$10$^{4}$ cm$^2$/Vs when the hole density in the quantum well is saturated to (1.65$\pm$0.02)$\times$10$^{11}$ cm$^{-2}$. We anticipate immediate application of these heterostructures for next-generation, higher-performance Ge spin-qubits and their integration into larger quantum processors.

cond-mat.mes-hall↗

arXiv2023

Hotter is easier: unexpected temperature dependence of spin qubit frequencies

As spin-based quantum processors grow in size and complexity, maintaining high fidelities and minimizing crosstalk will be essential for the successful implementation of quantum algorithms and error-correction protocols. In particular, recent experiments have highlighted pernicious transient qubit frequency shifts associated with microwave qubit driving. Workarounds for small devices, including prepulsing with an off-resonant microwave burst to bring a device to a steady-state, wait times prior to measurement, and qubit-specific calibrations all bode ill for device scalability. Here, we make substantial progress in understanding and overcoming this effect. We report a surprising non-monotonic relation between mixing chamber temperature and spin Larmor frequency which is consistent with observed frequency shifts induced by microwave and baseband control signals. We find that purposefully operating the device at 200 mK greatly suppresses the adverse heating effect while not compromising qubit coherence or single-qubit fidelity benchmarks. Furthermore, systematic non-Markovian crosstalk is greatly reduced. Our results provide a straightforward means of improving the quality of multi-spin control while simplifying calibration procedures for future spin-based quantum processors.

cond-mat.mes-hall↗

arXiv2022

Wafer-scale low-disorder 2DEG in $^{28}$Si/SiGe without an epitaxial Si cap

We grow $^{28}$Si/SiGe heterostructures by reduced-pressure chemical vapor deposition and terminate the stack without an epitaxial Si cap but with an amorphous Si-rich layer obtained by exposing the SiGe barrier to dichlorosilane at 500 °C. As a result, $^{28}$Si/SiGe heterostructure field-effect transistors feature a sharp semiconductor/dielectric interface and support a two-dimensional electron gas with enhanced and more uniform transport properties across a 100 mm wafer. At T = 1.7 K we measure a high mean mobility of (1.8$\pm$0.5)$\times$10$^5$ cm$^2$/Vs and a low mean percolation density of (9$\pm$1)$\times$10$^{10}$ cm$^{-2}$. From the analysis of Shubnikov-de Haas oscillations at T = 190 mK we obtain a long mean single particle relaxation time of (8.1$\pm$0.5) ps, corresponding to a mean quantum mobility and quantum level broadening of (7.5$\pm$0.6)$\times$10$^4$ cm$^{2}$/Vs and (40$\pm$3) $μ$eV, respectively, and a small mean Dingle ratio of (2.3$\pm$0.2), indicating reduced scattering from long range impurities and a low-disorder environment for hosting high-performance spin-qubits.

cond-mat.mes-hall↗