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Atomic structure and oxygen deficiency of the ultrathin aluminium oxide barrier in Al/AlOx/Al Josephson junctions.

Zeng L, Tran DT, Tai CW, Svensson G, Olsson E - Sci Rep (2016)

Bottom Line: The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier.In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide.Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise in superconducting quantum circuits.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Physics, Chalmers University of Technology, 41296, Göteborg, Sweden.

ABSTRACT
Al/AlOx/Al Josephson junctions are the building blocks of a wide range of superconducting quantum devices that are key elements for quantum computers, extremely sensitive magnetometers and radiation detectors. The properties of the junctions and the superconducting quantum devices are determined by the atomic structure of the tunnel barrier. The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier. Here we show that the miniaturized dimension of the barrier and the interfacial interaction between crystalline Al and amorphous AlOx give rise to oxygen deficiency at the metal/oxide interfaces. In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide. Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise in superconducting quantum circuits.

No MeSH data available.


Related in: MedlinePlus

Al-O coordination in the aluminium oxide barrier in the Al/AlOx/Al junction.(a) the refined atomic structure model of aluminium oxide tunnel barrier with the visualization of Al (coloured)–O (grey) coordination. Al atoms with different colours have different O coordination numbers. Dashed lines indicate the positions of the Al/AlOx interfaces. The profile of 2 Å-column averaged coordination number of Al atoms across the AlOx barrier is shown below the atomic structure model. The coordination number is averaged along the direction parallel to the Al/AlOx interfaces and with a 2 Å step along the direction perpendicular to the interfaces. (b) Al-O coordination number distribution in the barrier oxide.
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f5: Al-O coordination in the aluminium oxide barrier in the Al/AlOx/Al junction.(a) the refined atomic structure model of aluminium oxide tunnel barrier with the visualization of Al (coloured)–O (grey) coordination. Al atoms with different colours have different O coordination numbers. Dashed lines indicate the positions of the Al/AlOx interfaces. The profile of 2 Å-column averaged coordination number of Al atoms across the AlOx barrier is shown below the atomic structure model. The coordination number is averaged along the direction parallel to the Al/AlOx interfaces and with a 2 Å step along the direction perpendicular to the interfaces. (b) Al-O coordination number distribution in the barrier oxide.

Mentions: The main difference between the atomic structure of ultrathin barrier oxide and the bulk amorphous aluminium oxide lies in the difference in Al-O coordination. The cut-off distances for structural statistics of the NBED-RMC AlOx were set for Al-O, Al-Al and O-O as 2.5 Å, 4 Å and 3.5 Å, respectively. The AlOx has an average Al-O coordination number of ~3.4 (Fig. 5a,b, Supplementary Fig. 3 and Table 1) while the MD simulations on bulk Al2O3 gave the values of 4.1 and 4.25 for the liquid and amorphous phases respectively. The low [5]Al fraction (9.4%) in the ultrathin amorphous AlOx tunnel barrier is consistent with the observation that the fraction of Al sites with coordination number 5 decreases with the film thickness in amorphous aluminium oxide thin films26. There are almost no octahedrally coordinated Al atoms in the barrier oxide (Fig. 5b and Table 1). In contrast, there are quite many Al atoms in the nanosized AlOx being coordinated with less than 4 O atoms (39.2% Al atoms coordinated with 3 O atoms, 15.6% Al atoms coordinated with 1-2 O atoms). There are only ~35% Al atoms having fully 4 O coordination while in the MD Al2O3 bulk models it is 66% (liquid) and 76% (amorphous) (Table 1).


Atomic structure and oxygen deficiency of the ultrathin aluminium oxide barrier in Al/AlOx/Al Josephson junctions.

Zeng L, Tran DT, Tai CW, Svensson G, Olsson E - Sci Rep (2016)

Al-O coordination in the aluminium oxide barrier in the Al/AlOx/Al junction.(a) the refined atomic structure model of aluminium oxide tunnel barrier with the visualization of Al (coloured)–O (grey) coordination. Al atoms with different colours have different O coordination numbers. Dashed lines indicate the positions of the Al/AlOx interfaces. The profile of 2 Å-column averaged coordination number of Al atoms across the AlOx barrier is shown below the atomic structure model. The coordination number is averaged along the direction parallel to the Al/AlOx interfaces and with a 2 Å step along the direction perpendicular to the interfaces. (b) Al-O coordination number distribution in the barrier oxide.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4940733&req=5

f5: Al-O coordination in the aluminium oxide barrier in the Al/AlOx/Al junction.(a) the refined atomic structure model of aluminium oxide tunnel barrier with the visualization of Al (coloured)–O (grey) coordination. Al atoms with different colours have different O coordination numbers. Dashed lines indicate the positions of the Al/AlOx interfaces. The profile of 2 Å-column averaged coordination number of Al atoms across the AlOx barrier is shown below the atomic structure model. The coordination number is averaged along the direction parallel to the Al/AlOx interfaces and with a 2 Å step along the direction perpendicular to the interfaces. (b) Al-O coordination number distribution in the barrier oxide.
Mentions: The main difference between the atomic structure of ultrathin barrier oxide and the bulk amorphous aluminium oxide lies in the difference in Al-O coordination. The cut-off distances for structural statistics of the NBED-RMC AlOx were set for Al-O, Al-Al and O-O as 2.5 Å, 4 Å and 3.5 Å, respectively. The AlOx has an average Al-O coordination number of ~3.4 (Fig. 5a,b, Supplementary Fig. 3 and Table 1) while the MD simulations on bulk Al2O3 gave the values of 4.1 and 4.25 for the liquid and amorphous phases respectively. The low [5]Al fraction (9.4%) in the ultrathin amorphous AlOx tunnel barrier is consistent with the observation that the fraction of Al sites with coordination number 5 decreases with the film thickness in amorphous aluminium oxide thin films26. There are almost no octahedrally coordinated Al atoms in the barrier oxide (Fig. 5b and Table 1). In contrast, there are quite many Al atoms in the nanosized AlOx being coordinated with less than 4 O atoms (39.2% Al atoms coordinated with 3 O atoms, 15.6% Al atoms coordinated with 1-2 O atoms). There are only ~35% Al atoms having fully 4 O coordination while in the MD Al2O3 bulk models it is 66% (liquid) and 76% (amorphous) (Table 1).

Bottom Line: The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier.In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide.Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise in superconducting quantum circuits.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Physics, Chalmers University of Technology, 41296, Göteborg, Sweden.

ABSTRACT
Al/AlOx/Al Josephson junctions are the building blocks of a wide range of superconducting quantum devices that are key elements for quantum computers, extremely sensitive magnetometers and radiation detectors. The properties of the junctions and the superconducting quantum devices are determined by the atomic structure of the tunnel barrier. The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier. Here we show that the miniaturized dimension of the barrier and the interfacial interaction between crystalline Al and amorphous AlOx give rise to oxygen deficiency at the metal/oxide interfaces. In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide. Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise in superconducting quantum circuits.

No MeSH data available.


Related in: MedlinePlus