NIST ARDA/DTO review2006

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 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.

Frequency dependence of Qubits

Junction area 70 mm2

Amorphous barrier

Energy splittings can give rise to energy absorbtion!Reduces the measurement fidelity

Density of splittings scales with junction size

13 um2 junction70 um2 junction

• Smaller area – Lower density, larger splitting (strong coupling)

• Larger area - Higher density, smaller splitting (weaker coupling)

Two level fluctuators in junction

Amorphous AlO tunnel barrier

• Continuum of

metastable vacancies

• Changes on thermal cycling

I

What we obtained:

Crystalline barrier-Al2O3

Poly - Al

Poly- Al

What we had:

Amorphous tunnel barrier a –AlOx – OH-

No spurious resonatorsStable barrier

Amorphous Aluminum oxide barrierSpurious resonators in junctionsFluctuations in barrier

Silicon

amorphous SiO2

Low loss substrate

Design of tunnel junctions

SC bottom electrode

Top electrode

Sapphire-Al2O3

Chose bottom superconducting electrode to stabilize crystalline tunnel barrier - Al2O3 or MgO

Elements with high melting temperature

Elements with TC > 1K

Elements that lattice match insulator

• sapphire (Al203) - Nb, Ta, Mo, Tc, Re

• MgO - V

Elements that form weaker bond with O than Al or Mg

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

LEED, RHEED, AugerRe

Sputtering

LoadLock

STM/AFM

AlO

xyge

n

O2

Tests of Junction Materials

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 nm2UHV050805.m4 400x400 nm2UHV050805.1.m6_p1

Alumina barrier grown on Mo electrode

• As-grown (RT)– Granules present

JK05.4.m2_p1 AFM 2000x2000 nm2

1000x1000 nm2JK05.3.m3_p1 AFM

• After 850C post-growth anneal– Granules gone

Pinholes in Al2O3 on MoPoor resistance to chemical etch

• Al2O3 / Re - resists SF6

Mo

Al2O3Al2O3

• 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

SF6

SF6

Re

Epitaxial growth of Re

• Low aspect hexagonal islands

• Mostly bilayer steps

• Reduces step inducedpinholes

Steps ~ 1 nm

Growth of Epi-Re/Epi-Al2O3/Poly-Al

4×10-6 Torr O2, Al 10-6 Torr O2

Epitaxial Re/Al2O3

Re

@ 850 CAl

Amorphous AlOx

@ RT

Epitaxial Al2O3

@ 800 C

Polycrystalline Al

Re/Al2O3/AlSmooth, crystalline interfaces

Re

Al

TEM cross section

Elemental resolution

2 nm

Re

Al

Josephson Junction with a single-crystal Al2O3Tunnel barrier

I(0.70mV)/I(0.35mV) = 1200V (mV)

First epitaxial junctions with low subgap conductance

• Room temperature resistances very reproducible• Low temperature results of junctions:

Bias coil

50 m

Qubit (70 m2) DC-SQUID

Flux-biased Phase Qubut with a single-crystal Al2O3Tunnel barrier

Improvement of Junction Materials

T = 25 mK

Junction area 70 mm2

Amorphous barrier

Improvement of Junction Materials

T = 25 mK

Junction area 70 mm2

Amorphous barrier

Improvement of Junction Materials

T = 25 mK

Junction area 70 m2

Spectroscopy: Epitaxial Barrier

Splitting density reduced by ~ factor of 525 => ~5 /GHz

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

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

Oxygen0.43 nm

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

Effect of Top electrodeThermally evaporated Al vs Sputtered Re

Base Epi Re Base Epi Re

Top AlTop Re

Note: In these samples, AlOx barriers are amorphous

AlOx

Base Re-AlOx

interface smooth

AlOx-Top Al interfacesmooth

AlOx-Top Re interfacerough

AlOx

Good I-V’s Bad I-V’s