Constructingab initiomodels of ultra-thin Al–AlO_x–Al barriers

Abstract

The microscopic structure of ultra-thin oxide barriers often plays a major role in modern nanoelectronic devices. In the case of superconducting electronic circuits, their operation depends on the electrical non-linearity provided by one or more such oxide layers in the form of ultra-thin tunnel barriers (also known as Josephson junctions). Currently available fabrication techniques manufacture an amorphous oxide barrier, which is attributed as a major noise source within the device. The nature of this noise is currently an open question and requires both experimental and theoretical investigation. Here, we present a methodology for constructing atomic scale computational models of Josephson junctions using a combination of molecular mechanics, empirical and ab initio methods. These junctions consist of ultra-thin amorphous aluminium-oxide layers sandwiched between crystalline aluminium. The stability and structure of these barriers as a function of density and stoichiometry are investigated, which we compare to experimentally observed parameters. arXiv:1503.01859v2 [cond-mat.mtrl-sci] 22 Aug 2015 * tim.dubois@tcqp.science 1 J. Clarke, A. N. Cleland, M. H. Devoret, D. Esteve, and J. M. Martinis, Science 239, 992 (1988).

Reduced resistance drift in tunnel junctions using confined tunnel barriers

“…8,24,33 Consequently, the aging is less severe, due to the initial oxygen profile being more homogeneous and the stoichiometry closer to that of Al 2 O 3 . 11,19,37 …”

“…Our confinement strategy is another method to increase the as-formed oxygen concentration. 11,14,18,19 …”

Structural and nanochemical properties ofAlOxlayers inAl/AlOx/Al-layer systems

“…However, an amorphous phase also contains 1-, 2-, 3-and 5-fold coordinated atoms[34], preventing straightforward determination from EELS spectra. Moreover, the fraction of the differently coordinated Al atoms depends strongly on the density and stoichiometry of the AlOx layer[54].Nevertheless, the pronounced tetrahedral peak indicates a shift towards low-coordinated atoms and lower oxygen content in EBPlas, EBPlas-UV and EBPlas-70|0.3 compared to the crystalline -Al2O3 phase. With increasing oxygen pressure for EBPlas-70|9.5 (yellow line in Figure 4a), the tetrahedral peak decreases in intensity and a weak intermediate-range order peak appears.…”

Structural details of Al/Al2O3 junctions and their role in the formation of electron tunnel barriers

“…Although the structure of Al/Al 2 O 3 systems has actively been explored over the decades both theoretically [12][13][14][15][16][17][18][19] and * manana.koberidze@aalto.fi experimentally [4,5,9,[20][21][22][23][24], an in-depth understanding of the interface properties is still an ongoing issue. Experimental observations have provided valuable information, for example, about the most probable crystallographic orientation relationship between Al and Al 2 O 3 at the interface [21,23], Al substrate roughness [4,20], the chemical state of the ions [25], oxide thickness distribution [9], bond lengths and coordinations of the atoms [5] at the interface, Al/O ratios for different oxide thicknesses [24], and different oxidation times and temperatures [25].…”

“…Pioneering DFT works addressed the atomic structures of differently constructed Al/Al 2 O 3 interfaces, bonding at the interface, adhesion energies, and the most stable terminations of the oxide at the interface [12][13][14]27,28]. More recent DFT studies are focused on improving junction models and on screening possibilities for improved adhesion, which is the measure of the structural stability of the system [16,17,19].…”