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NIST ARDA/DTO review 2006

NIST ARDA/DTO review 2006. Materials David P. Pappas Seongshik Oh Jeffrey Kline. Materials Milestones. 3.1 First Year 3.1.1 Fabricated epitaxial aluminum for aluminum oxide junctions AlOx barrier still amorphous 3.2 Second Year

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NIST ARDA/DTO review 2006

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  1. NIST ARDA/DTO review2006 Materials David P. Pappas Seongshik Oh Jeffrey Kline

  2. Materials Milestones • 3.1 First Year • 3.1.1 Fabricated epitaxial aluminum for aluminum oxide junctions • AlOx barrier still amorphous • 3.2 Second Year • 3.2.1 Investigate new materials for superconducting electrodes • Lattice commensurability to the crystalline Al2O3 tunnel • Refractory metals (Nb, Ta, Mo, W) – found Re • engineered the epitaxial growth of Al2O3 • 3.3.1 Epitaxial barriers for junctions • All-epitaxial tunnel junctions will be fabricated in this phase of the program • Sputter-deposited films, recrystallized by high temperature annealing on sapphire substrates. • The tunnel barriers formed using the technique from second year • Trilayers will then be processed into qubit devices • Other barrier materials, such as nitrides, carbides and semiconductor materials.

  3. Frequency dependence of Qubits Junction area 70 mm2 Amorphous barrier Energy splittings can give rise to energy absorbtion! Reduces the measurement fidelity

  4. Density of splittings scales with junction size 13 um2 junction 70 um2 junction • Smaller area – Lower density, larger splitting (strong coupling) • Larger area - Higher density, smaller splitting (weaker coupling)

  5. Two level fluctuators in junction Amorphous AlO tunnel barrier • Continuum of • metastable vacancies • Changes on thermal cycling I

  6. Design of tunnel junctions What we obtained: What we had: Amorphous Aluminum oxide barrier Spurious resonators in junctions Fluctuations in barrier No spurious resonators Stable barrier Top electrode Poly - Al Crystalline barrier a-Al2O3 Amorphous tunnel barrier a –AlOx – OH- SC bottom electrode Poly- Al amorphous SiO2 Sapphire a-Al2O3 Low loss substrate Silicon

  7. Chose bottom superconducting electrode to stabilize crystalline tunnel barrier - Al2O3 or MgO Elements with high melting temperature

  8. Elements with TC > 1K

  9. Elements that lattice match insulator • sapphire (Al203) - Nb, Ta, Mo, Tc, Re • MgO - V

  10. Elements that form weaker bond with O than Al or Mg

  11. Elements that are not radioactive • Mo or Re for Al2O3 barrier • V for MgO tunnel barrier

  12. Load Lock LEED, RHEED, Auger Re Sputtering STM/AFM O2 Al Oxygen Tests of Junction Materials

  13. Molybdenum film grown on a-plane sapphire • As-grown (850C) • Narrow terraces • Some step bunching UHV050805.m4 400x400 nm2 UHV050805.1.m6_p1 400x400 nm2 • Post-growth anneal (1000C) • Broad terraces • More step bunching

  14. Alumina barrier grown on Mo electrode • As-grown (RT) • Granules present JK05.4.m2_p1 AFM 2000x2000 nm2 JK05.3.m3_p1 AFM 1000x1000 nm2 • After 850C post-growth anneal • Granules gone

  15. Pinholes in Al2O3 on MoPoor resistance to chemical etch SF6 SF6 • Al2O3 / Re - resists SF6 Al2O3 Al2O3 Re Mo • Al2O3/Mo is etched rapidly in SF6 • Conclusion: • Al2O3 grown on Mo has high pinhole density • Agrees with electrical tests – poor electrical properties • => Try different template for growth - Re

  16. Epitaxial growth of Re • Low aspect hexagonal islands • Mostly bilayer steps • Reduces step inducedpinholes Steps ~ 1 nm

  17. Polycrystalline Al Epitaxial Al2O3 Amorphous AlOx Epitaxial Re/Al2O3 @ 800 C @ RT @ 850 C Al Al 10-6 Torr O2 4×10-6 Torr O2, Re Growth of Epi-Re/Epi-Al2O3/Poly-Al

  18. Re/Al2O3/AlSmooth, crystalline interfaces TEM cross section Al 2 nm Re Elemental resolution

  19. Al Re V (mV) Josephson Junction with a single-crystal Al2O3Tunnel barrier • Room temperature resistances very reproducible • Low temperature results of junctions: I(0.70mV)/I(0.35mV) = 1200 First epitaxial junctions with low subgap conductance

  20. Flux-biased Phase Qubut with a single-crystal Al2O3Tunnel barrier Qubit (70 m2) DC-SQUID 50 m Bias coil

  21. Improvement of Junction Materials Junction area 70 mm2 Amorphous barrier T = 25 mK

  22. Improvement of Junction Materials Junction area 70 mm2 Amorphous barrier T = 25 mK

  23. Improvement of Junction Materials Spectroscopy: Epitaxial Barrier Junction area 70 mm2 T = 25 mK Splitting density reduced by ~ factor of 5 25 => ~5 /GHz

  24. Rabi oscillations from epi-qubits • T*2 comparable to qubits of same design (max SiO2) • Illustrates that the insulator is limiting decoherence source for this design

  25. Reduction of splitting density • Interfacial effect • ~1 in 5 oxygens at Al interface • Agrees with reduced splittingdensity non-epi Al interface Oxygen 0.43 nm 2 nm epi-Re interface

  26. 3.03 Å 4.13 Å Vanadium MgO Next year’s materials milestones • Grow epi-top layers • Eliminate splittings? • V/MgO junctions • Better lattice match • Lower temperatures • Lower barrier (thicker films) • Better manufacturability • Integrate with min-SiO2 &vacuum crossover designs

  27. Effect of Top electrode Thermally evaporated Al vs Sputtered Re AlOx-Top Re interface rough AlOx-Top Al interface smooth Top Al Top Re AlOx AlOx Base Epi Re Base Epi Re Good I-V’s Base Re-AlOx interface smooth Bad I-V’s Note: In these samples, AlOx barriers are amorphous

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