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Moutanabbir O, Assali S, Attiaoui A, Daligou G, Daoust P, Vecchio PD, Koelling S, Luo L, Rotaru N. Nuclear Spin-Depleted, Isotopically Enriched 70 Ge/ 28 Si 70 Ge Quantum Wells. Adv Mater 2024; 36:e2305703. [PMID: 38009242 DOI: 10.1002/adma.202305703] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/14/2023] [Revised: 11/15/2023] [Indexed: 11/28/2023]
Abstract
The p-symmetry of the hole wavefunction is associated with a weaker hyperfine interaction, which makes hole spin qubits attractive candidates to implement quantum processors. However, recent studies demonstrate that hole qubits are still very sensitive to nuclear spin bath, thus highlighting the need for nuclear spin-free germanium (Ge) qubits to suppress this decoherence channel. Herein, this work demonstrates the epitaxial growth of 73 Ge- and 29 Si-depleted, isotopically enriched 70 Ge/silicon-germanium (SiGe) quantum wells. The growth is achieved by reduced pressure chemical vapor deposition using isotopically purified monogermane 70 GeH4 and monosilane 28 SiH4 with an isotopic purity higher than 99.9% and 99.99%, respectively. The quantum wells consist of a series of 70 Ge/SiGe heterostructures grown on Si wafers. The isotopic purity is investigated using atom probe tomography (APT) following an analytical procedure addressing the discrepancies caused by the overlap of isotope peaks in mass spectra. The nuclear spin background is found to be sensitive to the growth conditions with the lowest concentration of 73 Ge and 29 Si is below 0.01% in the Ge well and SiGe barriers. The measured average distance between nuclear spins reaches 3-4 nm in 70 Ge/28 Si70 Ge, which is an order of magnitude larger than in natural Ge/SiGe heterostructures. The spread of the hole wavefunction and the residual nuclear spin background in APT voluminals comparable to the size of realistic quantum dots are also discussed.
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Affiliation(s)
- Oussama Moutanabbir
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Simone Assali
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Anis Attiaoui
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Gérard Daligou
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Patrick Daoust
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Patrick Del Vecchio
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Sebastian Koelling
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Lu Luo
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
| | - Nicolas Rotaru
- Department of Engineering Physics, École Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, H3C 3A7, Canada
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Paysen E, Capellini G, Talamas Simola E, Di Gaspare L, De Seta M, Virgilio M, Trampert A. Three-Dimensional Reconstruction of Interface Roughness and Alloy Disorder in Ge/GeSi Asymmetric Coupled Quantum Wells Using Electron Tomography. ACS Appl Mater Interfaces 2024; 16:4189-4198. [PMID: 38190284 DOI: 10.1021/acsami.3c15546] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/10/2024]
Abstract
Interfaces play an essential role in the performance of ever-shrinking semiconductor devices, making comprehensive determination of their three-dimensional (3D) structural properties increasingly important. This becomes even more relevant in compositional interfaces, as is the case for Ge/GeSi heterostructures, where chemical intermixing is pronounced in addition to their morphology. We use the electron tomography method to reconstruct buried interfaces and layers of asymmetric coupled Ge/Ge0.8Si0.2 multiquantum wells, which are considered a potential building block in THz quantum cascade lasers. The three-dimensional reconstruction is based on a series of high-angle annular dark-field scanning transmission electron microscopy images. It allows chemically sensitive investigation of a relatively large interfacial area of about (80 × 80) nm2 with subnanometer resolution as well as the analysis of several interfaces within the multiquantum well stack. Representing the interfaces as iso-concentration surfaces in the tomogram and converting them to topographic height maps allows the determination of their morphological roughness as well as layer thicknesses, reflecting low variations in either case. Simulation of the reconstructed tomogram intensities using a sigmoidal function provides in-plane-resolved maps of the chemical interface widths showing a relatively large spatial variation. The more detailed analysis of the intermixed region using thin slices from the reconstruction and additional iso-concentration surfaces provides an accurate picture of the chemical disorder of the alloy at the interface. Our comprehensive three-dimensional image of Ge/Ge0.8Si0.2 interfaces reveals that in the case of morphologically very smooth interfaces─depending on the scale considered─the interface alloy disorder itself determines the overall characteristics, a result that is fundamental for highly miscible material systems.
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Affiliation(s)
- Ekaterina Paysen
- Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., 10117 Berlin, Germany
| | - Giovanni Capellini
- Dipartimento di Scienze, Università degli Studi Roma Tre, 00146 Roma, Italy
- IHP─Leibniz-Institut für innovative Mikroelektronik, 15236 Frankfurt (Oder), Germany
| | | | - Luciana Di Gaspare
- Dipartimento di Scienze, Università degli Studi Roma Tre, 00146 Roma, Italy
| | - Monica De Seta
- Dipartimento di Scienze, Università degli Studi Roma Tre, 00146 Roma, Italy
| | - Michele Virgilio
- Dipartimento di Fisica "Enrico Fermi", Università di Pisa, I-56127 Pisa, Italy
| | - Achim Trampert
- Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., 10117 Berlin, Germany
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Meyer M, Déprez C, Meijer IN, Unseld FK, Karwal S, Sammak A, Scappucci G, Vandersypen LMK, Veldhorst M. Single-Electron Occupation in Quantum Dot Arrays at Selectable Plunger Gate Voltage. Nano Lett 2023; 23:11593-11600. [PMID: 38091376 PMCID: PMC10755753 DOI: 10.1021/acs.nanolett.3c03349] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/03/2023] [Revised: 12/06/2023] [Accepted: 12/07/2023] [Indexed: 12/28/2023]
Abstract
The small footprint of semiconductor qubits is favorable for scalable quantum computing. However, their size also makes them sensitive to their local environment and variations in the gate structure. Currently, each device requires tailored gate voltages to confine a single charge per quantum dot, clearly challenging scalability. Here, we tune these gate voltages and equalize them solely through the temporary application of stress voltages. In a double quantum dot, we reach a stable (1,1) charge state at identical and predetermined plunger gate voltage and for various interdot couplings. Applying our findings, we tune a 2 × 2 quadruple quantum dot such that the (1,1,1,1) charge state is reached when all plunger gates are set to 1 V. The ability to define required gate voltages may relax requirements on control electronics and operations for spin qubit devices, providing means to advance quantum hardware.
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Affiliation(s)
- Marcel Meyer
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The
Netherlands
| | - Corentin Déprez
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The
Netherlands
| | - Ilja N. Meijer
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The
Netherlands
| | - Florian K. Unseld
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The
Netherlands
| | - Saurabh Karwal
- QuTech
and Netherlands Organisation for Applied Scientific Research (TNO), PO Box 155, 2600 AD Delft, The Netherlands
| | - Amir Sammak
- QuTech
and Netherlands Organisation for Applied Scientific Research (TNO), PO Box 155, 2600 AD Delft, The Netherlands
| | - Giordano Scappucci
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The
Netherlands
| | - Lieven M. K. Vandersypen
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The
Netherlands
| | - Menno Veldhorst
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The
Netherlands
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Varley JB, Ray KG, Lordi V. Dangling Bonds as Possible Contributors to Charge Noise in Silicon and Silicon-Germanium Quantum Dot Qubits. ACS Appl Mater Interfaces 2023; 15:43111-43123. [PMID: 37651689 DOI: 10.1021/acsami.3c06725] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/02/2023]
Abstract
Spin qubits based on Si and Si1-xGex quantum dot architectures exhibit among the best coherence times of competing quantum computing technologies, yet they still suffer from charge noise that limit their qubit gate fidelities. Identifying the origins of these charge fluctuations is therefore a critical step toward improving Si quantum-dot-based qubits. Here, we use hybrid functional calculations to investigate possible atomistic sources of charge noise, focusing on charge trapping at Si and Ge dangling bonds (DBs). We evaluate the role of global and local environment in the defect levels associated with DBs in Si, Ge, and Si1-xGex alloys, and consider their trapping and excitation energies within the framework of configuration coordinate diagrams. We additionally consider the influence of strain and oxidation in charge-trapping energetics by analyzing Si and GeSi DBs in SiO2 and strained Si layers in typical Si1-xGex quantum dot heterostructures. Our results identify that Ge dangling bonds are more problematic charge-trapping centers both in typical Si1-xGex alloys and associated oxidation layers, and they may be exacerbated by compositional inhomogeneities. These results suggest the importance of alloy homogeneity and possible passivation schemes for DBs in Si-based quantum dot qubits and are of general relevance to mitigating possible trap levels in other Si, Ge, and Si1-xGex-based metal-oxide-semiconductor stacks and related devices.
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Affiliation(s)
- Joel B Varley
- Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94550, United States
| | - Keith G Ray
- Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94550, United States
| | - Vincenzo Lordi
- Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94550, United States
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Meyer M, Déprez C, van Abswoude TR, Meijer IN, Liu D, Wang CA, Karwal S, Oosterhout S, Borsoi F, Sammak A, Hendrickx NW, Scappucci G, Veldhorst M. Electrical Control of Uniformity in Quantum Dot Devices. Nano Lett 2023; 23:2522-2529. [PMID: 36975126 PMCID: PMC10103318 DOI: 10.1021/acs.nanolett.2c04446] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/11/2022] [Revised: 03/20/2023] [Indexed: 06/18/2023]
Abstract
Highly uniform quantum systems are essential for the practical implementation of scalable quantum processors. While quantum dot spin qubits based on semiconductor technology are a promising platform for large-scale quantum computing, their small size makes them particularly sensitive to their local environment. Here, we present a method to electrically obtain a high degree of uniformity in the intrinsic potential landscape using hysteretic shifts of the gate voltage characteristics. We demonstrate the tuning of pinch-off voltages in quantum dot devices over hundreds of millivolts that then remain stable at least for hours. Applying our method, we homogenize the pinch-off voltages of the plunger gates in a linear array for four quantum dots, reducing the spread in pinch-off voltages by one order of magnitude. This work provides a new tool for the tuning of quantum dot devices and offers new perspectives for the implementation of scalable spin qubit arrays.
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Affiliation(s)
- Marcel Meyer
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Corentin Déprez
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Timo R. van Abswoude
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Ilja N. Meijer
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Dingshan Liu
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Chien-An Wang
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Saurabh Karwal
- QuTech
and Netherlands Organisation for Applied Scientific Research (TNO), PO Box 155, 2600 AD Delft, The Netherlands
| | - Stefan Oosterhout
- QuTech
and Netherlands Organisation for Applied Scientific Research (TNO), PO Box 155, 2600 AD Delft, The Netherlands
| | - Francesco Borsoi
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Amir Sammak
- QuTech
and Netherlands Organisation for Applied Scientific Research (TNO), PO Box 155, 2600 AD Delft, The Netherlands
| | - Nico W. Hendrickx
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Giordano Scappucci
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Menno Veldhorst
- QuTech
and Kavli Institute of Nanoscience, Delft
University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
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6
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Paquelet Wuetz B, Degli Esposti D, Zwerver AJ, Amitonov SV, Botifoll M, Arbiol J, Vandersypen LMK, Russ M, Scappucci G. Reducing charge noise in quantum dots by using thin silicon quantum wells. Nat Commun 2023; 14:1385. [PMID: 36914637 DOI: 10.1038/s41467-023-36951-w] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2022] [Accepted: 02/25/2023] [Indexed: 03/16/2023] Open
Abstract
Charge noise in the host semiconductor degrades the performance of spin-qubits and poses an obstacle to control large quantum processors. However, it is challenging to engineer the heterogeneous material stack of gate-defined quantum dots to improve charge noise systematically. Here, we address the semiconductor-dielectric interface and the buried quantum well of a 28Si/SiGe heterostructure and show the connection between charge noise, measured locally in quantum dots, and global disorder in the host semiconductor, measured with macroscopic Hall bars. In 5 nm thick 28Si quantum wells, we find that improvements in the scattering properties and uniformity of the two-dimensional electron gas over a 100 mm wafer correspond to a significant reduction in charge noise, with a minimum value of 0.29 ± 0.02 μeV/Hz½ at 1 Hz averaged over several quantum dots. We extrapolate the measured charge noise to simulated dephasing times to CZ-gate fidelities that improve nearly one order of magnitude. These results point to a clean and quiet crystalline environment for integrating long-lived and high-fidelity spin qubits into a larger system.
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Thomas McJunkin, Benjamin Harpt, Yi Feng, Merritt P. Losert, Rajib Rahman, J. P. Dodson, M. A. Wolfe, D. E. Savage, M. G. Lagally, S. N. Coppersmith, Mark Friesen, Robert Joynt, M. A. Eriksson. SiGe quantum wells with oscillating Ge concentrations for quantum dot qubits. Nat Commun 2022; 13:7777. [PMID: 36522370 DOI: 10.1038/s41467-022-35510-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2022] [Accepted: 12/07/2022] [Indexed: 12/23/2022] Open
Abstract
Large-scale arrays of quantum-dot spin qubits in Si/SiGe quantum wells require large or tunable energy splittings of the valley states associated with degenerate conduction band minima. Existing proposals to deterministically enhance the valley splitting rely on sharp interfaces or modifications in the quantum well barriers that can be difficult to grow. Here, we propose and demonstrate a new heterostructure, the "Wiggle Well", whose key feature is Ge concentration oscillations inside the quantum well. Experimentally, we show that placing Ge in the quantum well does not significantly impact our ability to form and manipulate single-electron quantum dots. We further observe large and widely tunable valley splittings, from 54 to 239 μeV. Tight-binding calculations, and the tunability of the valley splitting, indicate that these results can mainly be attributed to random concentration fluctuations that are amplified by the presence of Ge alloy in the heterostructure, as opposed to a deterministic enhancement due to the concentration oscillations. Quantitative predictions for several other heterostructures point to the Wiggle Well as a robust method for reliably enhancing the valley splitting in future qubit devices.
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