Hydrogen bonds in Al2O3 as dissipative two-level systems in superconducting qubits

Gordon, Luke; Abu-Farsakh, Hazem; Janotti, Anderson; Walle, Chris G. Van de

doi:10.1038/srep07590

Cited by 52 publications

(54 citation statements)

References 39 publications

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“…2(a) for the most stable V O model. As for crystalline oxides, [8][9][10][11][12] the neutral and the +2 charge states are the most stable ones with a +2/0 charge transition level occurring in the band gap of a-Al 2 O 3 at ∼5 eV from the VBM.…”

Section: E Qmentioning

confidence: 99%

“…[1][2][3][4][5] For this reason, native defects in Al 2 O 3 have been the subject of extensive experimental and theoretical investigations aiming at verifying the presence of deep-level traps in the dielectric. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] The vast majority of theoretical studies performed so far have focused on crystalline phases of Al 2 O 3 . However, Al 2 O 3 in MOS devices is usually deposited via atomic layer deposition, which results in an amorphous phase with electronic properties differing considerably from those of crystalline phases.…”

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confidence: 99%

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Oxygen defects in amorphous Al2O3: A hybrid functional study

Guo

Ambrosio

Pasquarello

2016

Applied Physics Letters

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The electronic properties of the oxygen vacancy and interstitial in amorphous Al 2 O 3 are studied via ab initio molecular dynamics simulations and hybrid functional calculations. Our results indicate that these defects do not occur in amorphous Al 2 O 3 , due to structural rearrangements which assimilate the defect structure and cause a delocalization of the associated defect levels. The imbalance of oxygen leads to a nonstoichiometric compound in which the oxygen occurs in the form of O 2− ions. Intrinsic oxygen defects are found to be unable to trap excess electrons. For low Fermi energies, the formation of peroxy linkages is found to be favored leading to the capture of holes. The relative +2/0 defect levels occurs at 2.5 eV from the valence band. in MOS devices is usually deposited via atomic layer deposition, which results in an amorphous phase with electronic properties differing considerably from those of crystalline phases. [21][22][23][24] Furthermore, the experimental characterization of defect states in a-Al 2 O 3 has led to conflicting interpretations. The oxygen vacancy has been invoked as an electron trap in oxygen-deficient a-Al 2 O 3 films on the basis of electron energy loss, photoluminescence, and capacitance measurements, and this suggestion has received support from theoretical studies based on density-functional-theory calculations. 6,21 At variance,Århammar et al. 7 synthesized a-Al 2 O 3 via physical vapour deposition and investigated the defect states in the gap using X-ray absorption and resonant inelastic scattering techniques. Supplementing their experimental results with computational studies, they assigned the observed defect states to peroxy linkages between two oxygen atoms (O-O) rather than to oxygen vacancies.In order to elucidate the nature of oxygen-related defects in a-Al 2 O 3 , we here investigate the structural and electronic properties of oxygen vacancies (V O ) and interstitials (O i ) in a-Al 2 O 3 through ab initio molecular dynamics (MD) simulations and hybrid functional calculations. We find that structural rearrangements in the amorphous can assimilate oxygen vacancies in the +2 charge state and oxygen interstitials in the −2 charge state removing any electronic state from the band gap. While excess electrons cannot be trapped by intrinsic oxygen defects, the presence of excess holes results in the formation of peroxy linkages, with defect states in the band gap at 2.5 eV from the valence band.All the calculations are carried out with the freely available CP2K suite of codes, 25 which is based on the use of atomic a) Electronic mail: zhendong.guo@epfl.ch basis sets and of a plane-wave expansion for the electron density. Analytical Goedecker-Teter-Hutter pseudopotentials 26 are used to account for core-valence interactions. We use a triple-ζ correlation-consistent polarized basis set 27 for O atoms, and the shorter range molecularly optimized doublezeta basis set with one polarization function 28 for Al atoms. For the plane-waves, a cutoff of 500 Ry is employed. The ...

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Section: E Qmentioning

confidence: 99%

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confidence: 99%

Oxygen defects in amorphous Al2O3: A hybrid functional study

Guo

Ambrosio

Pasquarello

2016

Applied Physics Letters

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“…In recent years bulk defects have been generally ruled out as the dominant source, and the search has focused on surfaces and interfaces [2, 3]. Despite a wide range of theoretical models [4, 5, 6,7,8,9,10,11,12] and experimental efforts [2,13,14,15], the origin of these surface TLS still remains largely unknown, making further mitigation of TLS induced decoherence extremely challenging.Here we use a recently developed [16] on-chip electron spin resonance (ESR) technique that allows us to detect spins with a very low surface coverage. We combine this technique with various surface treatments specifically to reveal the nature of native surface spins on Al 2 O 3the mainstay of almost all solid state quantum devices [1].…”

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confidence: 99%

“…Similarly, a wide range of models seek to explain the electrically coupled TLSs in resonators and charge qubits where the latest results indicate similar mechanisms at interfaces [2, 3, 6,12,14]. Ab inito studies have suggested several possible candidates for charge coupled TLS, such as hydrogen and hydroxyl defects [5], or tunnelling of protons [11].To shed more light on the nature of noise and decoherence in Al 2 O 3 -based devices we use on-chip electron spin resonance (ESR) spectroscopy in combination with various surface treatments. ESR is a non-destructive technique used to probe the nature and concentration of paramagnetic centers and their interaction with the environment.…”

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confidence: 99%

Direct Identification of Dilute Surface Spins on Al2O3 : Origin of Flux Noise in Quantum Circuits

et al. 2017

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An on-chip electron spin resonance technique is applied to reveal the nature and origin of surface spins on Al_{2}O_{3}. We measure a spin density of 2.2×10^{17} spins/m^{2}, attributed to physisorbed atomic hydrogen and S=1/2 electron spin states on the surface. This is direct evidence for the nature of spins responsible for flux noise in quantum circuits, which has been an issue of interest for several decades. Our findings open up a new approach to the identification and controlled reduction of paramagnetic sources of noise and decoherence in superconducting quantum devices.

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“…Thus TLSs are found as defects in various dielectric structures: deposited insulating films [1,5,6], Josephson Junction (JJ) tunneling barriers [1,7,8], imperfect interfaces between superconducting films with crystalline substrates [9], and the native oxides on materials [10]. Recent modeling has predicted possible structures and values for the TLS dipole moment [11][12][13]. While these TLSs are generally known to be charged atomic configurations that spatially tunnel, their microscopic structure and elemental composition are generally unknown.…”

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confidence: 99%

Projected Dipole Moments of Individual Two-Level Defects Extracted Using Circuit Quantum Electrodynamics

et al. 2016

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Material-based two-level systems (TLSs), appearing as defects in low-temperature devices including superconducting qubits and photon detectors, are difficult to characterize. In this study we apply a uniform dc-electric field across a film to tune the energies of TLSs within. The film is embedded in a superconducting resonator such that it forms a circuit quantum electrodynamical (cQED) system. The energy of individual TLSs is observed as a function of the known tuning field. By studying TLSs for which we can determine the tunneling energy, the actual pz, dipole moments projected along the uniform field direction, are individually obtained. A distribution is created with 60 pz. We describe the distribution using a model with two dipole moment magnitudes, and a fit yields the corresponding values p = p1 = 2.8 ± 0.2 Debye and p = p2 = 8.3 ± 0.4 Debye. For a strong-coupled TLS the vacuum-Rabi splitting can be obtained with pz and tunneling energy. This allows a measurement of the circuit's zero-point electric field fluctuations, in a method that does not need the electric-field volume.PACS numbers: 03.67.-a, 03.67. Pp, 33.15.Kr, 66.35.+a Dielectric two-level systems (TLSs) have attracted the attention of the quantum computing community ever since they were identified as a major source of decoherence in superconducting qubits [1]. Subsequent studies found that TLSs were also a performance-limiting source of noise in photon detectors used for astronomy [2,3]. This motivation has led to quantum characterization of TLSs in the tunneling barrier of superconducting qubits [1,4] and both noise and loss characterization in high quality superconducting resonator circuits. Thus TLSs are found as defects in various dielectric structures: deposited insulating films [1,5,6], Josephson Junction (JJ) tunneling barriers [1,7,8], imperfect interfaces between superconducting films with crystalline substrates [9], and the native oxides on materials [10]. Recent modeling has predicted possible structures and values for the TLS dipole moment [11][12][13]. While these TLSs are generally known to be charged atomic configurations that spatially tunnel, their microscopic structure and elemental composition are generally unknown.In a qubit made from an anharmonic oscillator the interaction energy is observed as a spectroscopic splitting in the qubit state. However, this quantity is not only dependent on circuit element parameters, but also on the thickness of the tunneling barrier [1]. The barrier has a thickness variation of 1-2 nm [14], such that this lack of accuracy is also present in the electric field amplitude and the measured TLS dipole moment. In general, individual measurements of TLSs within JJs find the transition dipole moment p tr = p z ∆ 0 /E, where p z ≡ |p z | is the absolute value of the dipole moment projected in the field direction, and the TLS tunneling energy over the total energy ∆ 0 /E is generally unknown. As a result only a lower bound of p z is determined from those measurements (∆ 0 /E ≤ 1). However, in studie...

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Hydrogen bonds in Al2O3 as dissipative two-level systems in superconducting qubits

Cited by 52 publications

References 39 publications

Oxygen defects in amorphous Al2O3: A hybrid functional study

Oxygen defects in amorphous Al2O3: A hybrid functional study

Direct Identification of Dilute Surface Spins on Al2O3 : Origin of Flux Noise in Quantum Circuits

Projected Dipole Moments of Individual Two-Level Defects Extracted Using Circuit Quantum Electrodynamics

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