| AVS 56th International Symposium & Exhibition | |
| Nanometer-scale Science and Technology | Thursday Sessions |
| Session NS-ThA |
| Session: | Characterization & Imaging at the Nanoscale II |
| Presenter: | F. da Silva, University of Colorado, Denver |
| Authors: | F. da Silva, University of Colorado, Denver B.P. Gorman, Colorado School of Mines M. Kaufman, Colorado School of Mines J.S. Kline, National Institute of Standards and Technology D.A. Braje, National Institute of Standards and Technology D.S. Wisbey, National Institute of Standards and Technology D.P. Pappas, National Institute of Standards and Technology |
| Correspondent: | Click to Email |
The pursuit of new low loss materials and epitaxial structures to enhance the performance of superconducting quantum bits (qubits) has heightened in recent years. The small number of defects observed in epitaxially grown structures, compared with polycrystalline and amorphous materials, accounts for several improvements reported in qubit operation such as the reduction in the density of two-level fluctuators and longer coherence times [1]. Qubit structures of interest are superconductor-insulator-superconductor (SIS) tri-layers deposited on an insulating substrate. Two candidates for the substrate and superconductor metal are sapphire (α-Al2O3) and Al respectively [2]. Complete defect removal requires a study of each layer and its corresponding interfaces. In this work we focus our attention on the interface between the substrate and the first superconductor layer. We used transmission electron microscopy (TEM) techniques to analyze the growth of Al (111) films on sapphire (0001) substrates. While the sapphire substrate induces the growth of epitaxial Al along a <111> direction as desired, the subsequent [111[ planes grow with either ABC or ACB stacking resulting in twin-related “grains” within the epitaxial film. In addition, slight (1-5°) in-plane misorientations are observed in adjacent, twin-related Al grains and appear to correspond to the slight rotations between the oxygen atoms along the c-axis of the sapphire. In other words, because the Al orients itself with the oxygen atoms on the sapphire basal planes, any miscuts of the sapphire substrate to within ±1/6 of the unit cell c-axis will slightly misorient the Al due to the slight rotation of the oxygen atoms with respect to the c-axis of the sapphire cell [3]. Finally, these twinning and misorientation effects appear to induce other growth defects in the subsequent layers used in the qubit circuit. Based on these results, we propose the use of a chemically compatible oxide buffer layer which does not have rotations between successive O layers within its unit cell.
[1] S. Oh, K. Cicak, J. S. Kline, M.A. Sillanpää, K.D. Osborn, J.D. Whittaker, R.W. Simmonds, and D.P. Pappas, Phys. Rev. B 74, 100502R (2006).
[2] J. Martinis, Quantum Information Process 8, 81 (2009).
[3] D.L. Medlin, K.F. McCarty, R.Q. Hwang, S.E. Guthrie, and M.I. Baskes, Thin Solid Films 299, 110 (1997).