1. NIST ARDA/DTO review 2006 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 Al 2 O 3 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 mm 2 Amorphous barrier Energy splittings can give rise to energy absorbtion! Reduces the measurement fidelity

4. Density of splittings scales with junction size 13 um 2 junction 70 um 2 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. What we obtained: Crystalline barrier  -Al 2 O 3 Poly - Al What we had: Amorphous tunnel barrier a –AlO x – OH - No spurious resonators Stable barrier Amorphous Aluminum oxide barrier Spurious resonators in junctions Fluctuations in barrier Silicon amorphous SiO 2 Low loss substrate Design of tunnel junctions SC bottom electrode Top electrode Sapphire  -Al2O3

7. Chose bottom superconducting electrode to stabilize crystalline tunnel barrier - Al 2 O 3 or MgO Elements with high melting temperature

8. Elements with T C > 1K

9. Elements that lattice match insulator sapphire (Al 2 0 3 ) - 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. LEED, RHEED, Auger Re Sputtering Load Lock STM/AFM Al Oxygen O2O2 Tests of Junction Materials

13. Molybdenum film grown on a-plane sapphire As-grown (850C) –Narrow terraces –Some step bunching Post-growth anneal (1000C) –Broad terraces –More step bunching 400x400 nm 2 UHV050805.m4 400x400 nm 2 UHV050805.1.m6_p1

14. Alumina barrier grown on Mo electrode As-grown (RT) –Granules present JK05.4.m2_p1 AFM 2000x2000 nm 2 1000x1000 nm 2 JK05.3.m3_p1 AFM After 850C post-growth anneal –Granules gone

15. Pinholes in Al 2 O 3 on Mo Poor resistance to chemical etch Al 2 O 3 / Re - resists SF 6 Mo Al 2 O 3 Al 2 O 3 /Mo is etched rapidly in SF 6 Conclusion: Al 2 O 3 grown on Mo has high pinhole density Agrees with electrical tests – poor electrical properties => Try different template for growth - Re SF 6 Re

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

17. Growth of Epi-Re/Epi-Al2O3/Poly-Al 4×10 -6 Torr O 2,Al10 -6 Torr O 2 Epitaxial Re/Al 2 O 3 Re @ 850  C Al Amorphous AlO x @ RT Epitaxial Al 2 O 3 @ 800  C Polycrystalline Al

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

19. Re Al Josephson Junction with a single-crystal Al 2 O 3 Tunnel barrier I(0.70mV)/I(0.35mV) = 1200 V (mV) First epitaxial junctions with low subgap conductance Room temperature resistances very reproducible Low temperature results of junctions:

20. Bias coil 50  m Qubit (70  m 2 ) DC-SQUID Flux-biased Phase Qubut with a single-crystal Al 2 O 3 Tunnel barrier

21. Improvement of Junction Materials T = 25 mK Junction area 70 mm 2 Amorphous barrier

22. Improvement of Junction Materials T = 25 mK Junction area 70 mm 2 Amorphous barrier

23. Improvement of Junction Materials T = 25 mK Junction area 70  m 2 Spectroscopy: Epitaxial Barrier 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 SiO 2 ) 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 splitting density 2 nm epi-Re interface non-epi Al interface Oxygen 0.43 nm

26. 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 3.03 Å Vanadium MgO 4.13 Å

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