NIST - Colorado

Seongshik OhRaymond SimmondsKatarina CicakKevin Osborn

John M. Martinis Ken Cooper Matthias SteffenRobert McDermott

University of California - Santa Barbara

David P. Pappas

Epitaxial superconducting refractory

metals for quantum computing

1) Need longer T1

– identify dominant loss mechanisms• Substrate & insulator– SiO2?

– Kevin Osborn– John Martinis

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2 ) Need higher measurement fidelity– Identify, eliminate intrinsic resonances

– Junction dielectric?

Challenges in solid state qubits

What we want:

Crystalline barrier-Al2O3

Interfaces: Smooth Stable No dangling bonds

Poly - Al

Poly- Al

What we have:

Amorphous tunnel barrier a -AlOx-OH Rough interfaces Unstable at room temp. Dangling bonds

No spurious resonatorsStable barrier

Spurious resonators in junctionsFluctuations in barrier

Silicon

amorphous SiO2

dangling bonds at interface

Low loss substrate

Design of tunnel junctions

SC bottom electrode

Q: Can we prepare crystalline Al2O3 on Al?

Binding energy of Al AES peak in oxide60

59

58

57

56

55

54

900800700600500400300Annealing Temp (K)

AE

S E

nerg

y of

Rea

cted

Al (

eV)

Al in sapphire Al203

Metallic aluminum

Aluminum Melts

68

10 Å AlOx on Al (300 K + anneal) 10 Å AlOx on Al (exposed at elevated temp.)

Anneal the natural oxides Oxidize at elevated temp.

A: No

Chose bottom superconducting electrode to stabilize crystalline Al2O3 tunnel barrier

Elements with high melting temperature

Elements with TC > 1K

Elements that lattice match sapphire (Al203)

Elements that form weaker bond with oxygen than Al

Elements that are not radioactive

LEED, RHEED, AESRe

Sputtering

LoadLock

STM

• Pbase< 5x10-10 Torr

• Sapphire c-axis substrates

• Sputter deposit Re

UHV growth system

Morphology of Re/sapphire Room temperature growth

100 nm Re

• 3 nm RMS roughness• Mixed growth planes

– c-plane– a-plane

• Needs to be heated

for barrier growth

0.5x0.5 um

100 nm Re, room temperature deposition + 750 C anneal

• 1 nm RMS roughness• Re surface begins to

crystallize between 550–650C• Need higher temperature to

crystallize Al2O3

0.5x0.5 um

Sapphire substrate epi-Re on Sapphire

Growth of epitaxial Re(0001) at high temperature

RHEED diffraction images

+ 100 nm Re @ 850 C

High temperature growth – 100 nm Re @ 850 C

• 1.5 nm RMS roughness• 2 atomic layer steps• Screw dislocations on mesas• Stranski-Krastanov growth

– Initial wetting of substrate– Formation of 3-d islands– Islands fill in gradually

• Evidence of step bunching

=> some very large steps

500 x 500 nm

100 nm Re, 850 C deposition – zoom in

• Step bunching on corners• Sharp dropoffs where

multiple steps come together

• ~100 nm wide mesas

200 x 200 nm

100 nm Re, 850 C deposition, 1200 C anneal

• Much large mesas

~ 200 nm diameter• Still find step bunching• Temperatures very high

500 x 500 nm

Grow thin film at low T, anneal=> add thick film with homoepitaxy @ high T

2 nm Re, R.T. + 850 C anneal

500 x 500 nm

+ 100 nm Re @ 850

=> 200 nm terraces, comparable to 1200 C anneal

Conclusions• Need bottom electrodes that are stable at high T

T > 700 C

• Demonstrated Re growth with large terraces

• Films need to be annealed to > 800 C to stabilize surface

• Large mesas with wide terraces can be obtained 3 ways:– High temperature growth ~850 C => 100 nm mesas– Anneal to very high temperature, ~ 1200 C => 200 nm– Low T buffer, anneal to 850, then 850 C film => 200 nm

• Need to grow epitaxial Al2O3 on these surfaces

(1) Element with high melting temperature

(2) TC > 1K

(3) Epitaxial match to Al2O3 – hcp, 2.77 Å

Re - hcp (0001) < 1% lattice mismatch

(4) Re - smaller oxidation energy (sharp interface)

Chose bottom superconducting electrode to stabilize crystalline Al2O3 tunnel barrier