1. Epitaxial superconducting refractory metals for quantum computing
David P. Pappas
NIST - Colorado
University of California - Santa Barbara
Seongshik Oh
Raymond Simmonds
Katarina Cicak
Kevin Osborn
John M. Martinis
Ken Cooper
Matthias Steffen
Robert McDermott

2. Challenges in solid state qubits
1) Need longer T1
identify dominant loss mechanisms
Substrate & insulator– SiO2?
Kevin Osborn
John Martinis
Next session
2 ) Need higher measurement fidelity
Identify, eliminate intrinsic resonances
Junction dielectric?

3. Design of tunnel junctions
What we want:
What we have:
Spurious resonators in junctions
Fluctuations in barrier
No spurious resonators
Stable barrier
Poly - Al
Amorphous tunnel barrier
a -AlOx-OH
Rough interfaces
Unstable at room temp.
Dangling bonds
Crystalline barrier
g-Al2O3
Interfaces:
Smooth
Stable
No dangling bonds
Poly- Al
SC bottom
electrode
amorphous SiO2
dangling bonds at interface
Low loss substrate
Silicon

4. Q: Can we prepare crystalline Al2O3 on Al?
Anneal the natural oxides
Oxidize at elevated temp.
Binding energy of Al AES peak in oxide
Annealing Temp (K)
AES Energy of Reacted 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.)
A: No

5. Chose bottom superconducting electrode to stabilize crystalline Al2O3 tunnel barrier
Elements with high melting temperature

6. Elements with TC > 1K

7. Elements that lattice match sapphire (Al203)

8. Elements that form weaker bond with oxygen than Al

9. Elements that are not radioactive

10. UHV growth system Pbase< 5x10-10 Torr Sapphire c-axis substrates
Sputter deposit Re
Load
Lock
LEED, RHEED,
AES Re
Sputtering
STM

11. Morphology of Re/sapphire Room temperature growth 100 nm Re
0.5x0.5 um
3 nm RMS roughness
Mixed growth planes
c-plane
a-plane
Needs to be heated
for barrier growth

12. 100 nm Re, room temperature deposition + 750 C anneal
0.5x0.5 um
1 nm RMS roughness
Re surface begins to
crystallize between 550–650C
Need higher temperature to
crystallize Al2O3

13. Growth of epitaxial Re(0001) at high temperature
RHEED diffraction images
+ 100 nm Re
@ 850 C
Sapphire substrate
epi-Re on Sapphire

14. High temperature growth – 100 nm Re @ 850 C
500 x 500 nm
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

15. 100 nm Re, 850 C deposition – zoom in
200 x 200 nm
Step bunching on corners
Sharp dropoffs where
multiple steps come together
~100 nm wide mesas

16. 100 nm Re, 850 C deposition, 1200 C anneal
500 x 500 nm
Much large mesas
~ 200 nm diameter
Still find step bunching
Temperatures very high

17. Grow thin film at low T, anneal => add thick film with homoepitaxy @ high T
2 nm Re, R.T C anneal
+ 100 nm 850
500 x 500 nm
=> 200 nm terraces, comparable to 1200 C anneal

18. 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

19. Chose bottom superconducting electrode to stabilize crystalline Al2O3 tunnel barrier
Element with high melting temperature
TC > 1K
Epitaxial match to Al2O3 – hcp, 2.77 Å
Re - hcp (0001) < 1% lattice mismatch
Re - smaller oxidation energy (sharp interface)