Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology

Sammak, Amir; Sabbagh, Diego; Hendrickx, Nico W.; Lodari, Mario; Wuetz, Brian Paquelet; Tosato, Alberto; Yeoh, L. A.; Bollani, Monica; Virgilio, Michele; Schubert, Markus Andreas; Zaumseil, P.; Capellini, Giovanni; Veldhorst, Menno; Scappucci, Giordano

doi:10.1002/adfm.201807613

Cited by 122 publications

(149 citation statements)

References 49 publications

(87 reference statements)

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“…The α values confirm that the mobility is limited by scattering from the dielectric/semiconductor interface, as previously observed in Si/SiGe and Ge/SiGe H-FETs. [10,24,29,30] Despite the close proximity to the dielectric interface, the shallower quantum well has a remarkable peak mobility of 1.64 × 10 5 cm 2 /Vs at p = 1.05 × 10 12 cm −2 , 2.4× larger than previous reports for quantum wells positioned at a similar distance from the surface. [24] At higher density the mobility starts to drop, possibly due to occupation of the second subband or to different scattering mechanisms becoming dominant.…”

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confidence: 86%

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Light effective hole mass in undoped Ge/SiGe quantum wells

et al. 2019

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We report density-dependent effective hole mass measurements in undoped germanium quantum wells. We are able to span a large range of densities (2.0 − 11 × 10 11 cm −2 ) in top-gated field effect transistors by positioning the strained buried Ge channel at different depths of 12 and 44 nm from the surface. From the thermal damping of the amplitude of Shubnikov-de Haas oscillations, we measure a light mass of 0.061me at a density of 2.2 × 10 11 cm −2 . We confirm the theoretically predicted dependence of increasing mass with density and by extrapolation we find an effective mass of ∼ 0.05me at zero density, the lightest effective mass for a planar platform that demonstrated spin qubits in quantum dots.Holes are rapidly emerging as a promising candidate for semiconductor quantum computing.[1-3] In particular, holes in germanium (Ge) bear favorable properties for quantum operation, such as strong spin-orbit coupling enabling electric driving without the need of microscopic objects,[1-3] large excited state splitting energies to isolate the qubit states, [4] and ohmic contacts to virtually all metals for hybrid superconducting-semiconducting research [5][6][7][8][9]. Furthermore, undoped planar Ge quantum wells with hole mobilities µ > 5 × 10 5 cm 2 /Vs were recently developed[10] and shown to support quantum dots [11,12] and single and two qubit logic,[3] providing scope to scale up the number of qubits.

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confidence: 86%

“…Details of the heterostructure growth, device fabrication and operation, and magnetotransport measurements are reported in Ref. [10]. Figure 1(a) shows scanning transmission electron microscopy with energy dispersive X-ray (STEM-EDX) analysis of the shallow Ge quantum well (t = 12 nm) under the gate stack.…”

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confidence: 99%

Light effective hole mass in undoped Ge/SiGe quantum wells

et al. 2019

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“…[ 9 ] Different material combinations ranging from II–VI to IV–IV have been widely used. Among them, Ge is attracting more and more attention due to its high hole mobility, [ 10–13 ] low effective mass in 2D hole gases, [ 14,15 ] good contacts with metals, [ 16–18 ] strong spin–orbit interactions, [ 19–22 ] capability of isotopic purification, [ 23 ] and compatibility with Si. These attractive features make Ge a promising candidate not only as a transistor channel material but also as a host for spin [ 6,24,25 ] and even topological qubits.…”

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confidence: 99%

Site‐Controlled Uniform Ge/Si Hut Wires with Electrically Tunable Spin–Orbit Coupling

et al. 2020

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attention due to its high hole mobility, [10][11][12][13] low effective mass in 2D hole gases, [14,15] good contacts with metals, [16][17][18] strong spin-orbit interactions, [19][20][21][22] capability of isotopic purification, [23] and compatibility with Si. These attractive features make Ge a promising candidate not only as a transistor channel material but also as a host for spin [6,24,25] and even topological qubits. [26,27] Excitingly, the first hole spin qubit [6] and proximity-induced superconductivity with a hard gap [28] have been realized recently in 1D Ge.Despite much progress that has been made so far, it remains a formidable challenge to have individuals as well as arrays of NWs with a high degree of addressability and scalability for the next generation of NW-based quantum devices. For example, in the field of group III-V semiconductors, precisely positioned NW networks have been achieved with predefined metal islands. [29] The out-of-plane grown NW structures, however, need to be transferred from the growth wafer to a second substrate for device fabrication, which limits their scalability. [7,29] Very recently, high-quality inplane NW networks have been successfully demonstrated with Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site-controlled nanowires is a prerequisite toward the next generation of devices that will require addressability and scalability. Here, combining top-down nanofabrication and bottom-up self-assembly, the growth of Ge wires on prepatterned Si (001) substrates with controllable position, distance, length, and structure is reported. This is achieved by a novel growth process that uses a SiGe strain-relaxation template and can be potentially generalized to other material combinations. Transport measurements show an electrically tunable spin-orbit coupling, with a spin-orbit length similar to that of III-V materials. Also, charge sensing between quantum dots in closely spaced wires is observed, which underlines their potential for the realization of advanced quantum devices. The reported results open a path toward scalable qubit devices using nanowires on silicon.

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“…Fabrication is based on silicon substrates and standard manufacturing materials. We grow strained germanium quantum wells, measured to have high hole mobilities µ > 500.000 cm 2 /Vs and a low effective hole mass m h = 0.09 m e [22,24], and predicted to reach m h = 0.05 m e at zero density [25,26]. This allows us to define quantum dots of comparatively large size and we find excellent control over the exchange interaction between the two dots.…”

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Fast two-qubit logic with holes in germanium

Hendrickx

Franke

Sammak

et al. 2020

Nature

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The promise of quantum computation with quantum dots has stimulated widespread research. Still, a platform that can combine excellent control with fast and high-fidelity operation is absent. Here, we show single and two-qubit operations based on holes in germanium. A high degree of control over the tunnel coupling and detuning is obtained by exploiting quantum wells with very low disorder and by working in a virtual gate space. Spin-orbit coupling obviates the need for microscopic elements and enables rapid qubit control with Rabi frequencies exceeding 100 MHz and a singlequbit fidelity of 99.3 %. We demonstrate fast two-qubit CX gates executed within 75 ns and minimize decoherence by operating at the charge symmetry point. Planar germanium thus matured within one year from a material that can host quantum dots to a platform enabling two-qubit logic, positioning itself as a unique material to scale up spin qubits for quantum information.Gate-defined quantum dots were recognized early on as a promising platform for quantum information [1] and a plethora of materials stacks has been investigated as host material. Initial research mainly focused on the low disorder semiconductor gallium arsenide [2,3]. Steady progress in the control and understanding of this system culminated in the initial demonstration and optimization of spin qubit operations [4,5] and the realization of rudimentary analog quantum simulations [6]. However, the omnipresent hyperfine interactions in group III-V materials seriously deteriorate the spin coherence, despite attempts to mitigate this by nuclear polarization [7]. Drastic improvements to the coherence times could be achieved by switching to the group IV semiconductor silicon, in particular when defining spin qubits in isotopically purified host crystal with vanishing concentrations of nonzero nuclear spin [8]. This enabled single qubit rotations with fidelities beyond 99.9% [9] and the execution of two-qubit logic gates with fidelities up to 98% [10-13], underlining the potential of spin qubits for quantum computation. Nevertheless, quantum dots in silicon are often formed at unintended locations and control over the tunnel coupling determining the strength of two-qubit interactions is limited. Moreover, the absence of a sizable spin-orbit coupling for electrons requires the inclusion of microscopic components such as onchip striplines or nanomagnets close to each qubit, which seriously complicates the design of large and dense 2D-structures. This, combined with the limited control over the location and coupling of the dots, remains an outstanding challenge for the scalability of these systems and a platform that can overcome these limitations would be highly desirable. * These two authors contributed equally to this work Hole states in semiconductors typically exhibit strong spinorbit coupling, which has enabled the demonstration of fast single qubit rotations [14,15] and additionally, unlike electrons, holes do not suffer from nearby valley states. In silicon, unfavorable band alignment...

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Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology

Cited by 122 publications

References 49 publications

Light effective hole mass in undoped Ge/SiGe quantum wells

Light effective hole mass in undoped Ge/SiGe quantum wells

Site‐Controlled Uniform Ge/Si Hut Wires with Electrically Tunable Spin–Orbit Coupling

Fast two-qubit logic with holes in germanium

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