1. Point Contact Tunneling as a Surface Superconductivity Probe of bulk Nb and (Nb 1-x Ti x )N Thin Films Chaoyue Cao Advisor: J. Zasadzinski ANL N. Groll, J. Klug, S. Altin, N. Becker, T. Prolier FNAL Y. Trenikhina, A. Romanenko, L. Cooley

2. Outline Introduction to point contact tunneling (PCT) technique Data analysis Introduction to atomic layer deposition (ALD) PCT measurements on (Nb 1-x Ti x )N films made by ALD

3. Point Contact Tunneling 3 Smeared BCS DOS Q depends on Surface Superconductivity, need surface probe Cannot use STM due to surface oxide

4. A typical spectrum on Nb single crystal and BTK fit Δ Gap Γ DOS Broadening Z Barrier Strength BTK (Blonder-Tinkham-Klapwijk) theory describes smooth transition from tunneling to Andreev reflection

5. PCT Spectrum Dependence on Junction Resistance NbO Nb High Z junction Low Z junction Au Tip Depairing region NbO 2 Nb 2 O 5

6. Γ collapses for Z < 1.5 Au tip scraping through surface oxide removes depairing

7. Fermi lab cold spot Significantly lower Γ values

8. Preliminary PCT measurements on (Nb 1-x Ti x )N films made by ALD

9. A thin film synthesis process based on sequential, self-limiting surface reactions between vapors of chemical precursors and a solid surface to deposit films in an atomic layer-by-layer manner. Atomic layer deposition (ALD)

10. 10 ALD thin film materials

11. Advantages:  Atomic-level control of thickness and composition  Smooth, continuous, pinhole-free coatings on large area substrates  No line-of-sight limits → excellent conformality over complex shaped surfaces Coat inside Nb SRF cavity with precise, layered structure → ALD 1 μm 200 nm ZnO Si ALD is very good at coating non-planar surfaces

12. 12 Multilayer thin films for SRF Superconductor-Insulator multilayer [ Gurevich, Appl. Phys. Lett. 88, 012511 (2006 )]  Potential path to high E acc and high Q 0 d B0B0 B i =B 0 exp(-Nd/ L )

13. Nb 1-x Ti x N Thin Films made by ALD TEM  Chemistry: (NbCl 5 :TiCl 4 ) + Zn + NH 3 at 450°C, 500°C  Can vary Ti content with NbCl 5 :TiCl 4 ratio (1:2 ~ 20% TiN)  Impurity content: 0.05 atom % Cl  21 sec/cycle 2-7-1-5-1-5 ("NH3 dose"-"purge"-"MCl x dose"-"purge"-"Zn dose"-"purge ")

14. 14 Nb 1-x Ti x N-based superconductor-insulator structures Aluminum nitride: AlN  Oxygen-free insulator, stable interface with Nb(Ti)N  Good thermal conductivity (285 W/m-K)  Similar structure to Nb(Ti)N –0.27% mismatch between in-plane spacing of (001)-oriented AlN and (111)- oriented NbN  Can be grown with AlCl 3 and NH 3 at same temperature as Nb(Ti)N

15. 49nm Nb 0.8 Ti 0.2 N on AlN T c = 12.8K by SQUID 2Δ/kT c = 3.4 - 3.6 (BCS limit) Δ(meV) Γ(meV)

16. 28nm Nb 0.8 Ti 0.2 N (without AlN layer) T c = 8.3K by SQUID Δ(meV) Γ(meV)

17. Point contact tunneling (PCT) technique is ideal for measuring the local surface superconducting energy gap and density of states (DOS) of samples with a natural barrier. Nb 1-x Ti x N on AlN gives Tc = 12.8K, Δ = 1.8-2.2 meV, 2Δ/kTc = 3.4- 3.6(BCS limit). Nb 1-x Ti x N (without AlN) Tc = 8.3K. High quality gap region DOS, low zero bias conductance. Δ = 1.8-2.2 meV. Conclusion Substrate Strained layer, low Tc 12.8 K

18. Back up

19. ALD Reaction Scheme ALD involves the use of a pair of reagents. each reacts with the surface completely each will not react with itself This setup eliminates line of site requirments Application of this AB Scheme Reforms the surface Adds precisely 1 monolayer Pulsed Valves allow atomic layer precision in growth Viscous flow (~1 torr) allows rapid growth ~1  m / 1-4 hours

20. 20 Viscous flow ALD reactor Key features:  Inconel 600 reactor tube (superior corrosion resistance) – Halide precursors (NbCl 5, TiCl 4, etc.)  All-welded precursor inlet manifold (reduced sites for potential leaks) – Oxygen contamination in nitride films Inconel 600 Welded inlet manifold

21. 21 Niobium titanium nitride: Nb 1-x Ti x N  Chemistry: (NbCl 5 :TiCl 4 ) + Zn + NH 3 at 450°C, 500°C  Can vary Ti content with NbCl 5 :TiCl 4 ratio (1:2 ~ 20% TiN) –Cubic d phase in all films With increasing TiN  Peaks shift to higher angle  Density decreases –7.2 g/cm 3 (1:0) –5.7 g/cm 3 (1:4)  RT resistivity decreases –380 mW-cm (1:0) –130 mW-cm (1:4) Impurity content: 0.05 atom % Cl Are they good superconductors?

22. 22 Nb 1-x Ti x N / AlN: X-ray diffraction  AlN changes Nb 1-x Ti x N orientation  With AlN, total integrated intensity increases by ~2x  AlN → improved Nb 1-x Ti x N crystallinity

23. 23 Nb 1-x Ti x N / AlN: X-ray reflectivity  Density ~5% higher with AlN  Roughness ~2x higher with AlN (modified crystallinity)  Change in thickness/cycles (difference in nucleation delay~100-200 cycles)

24. Th. Proslier, j. zasadzinski et al. APL 92, 212505 (2008)

25. 25 Optimized growth of Nb 1-x Ti x N  Achieved superconducting T c =14 K, 40% higher than any other ALD film  Nearly 5 K higher than Nb

26. What could be done? fast time scale 356 nm 96 nm Reduce curvature radius Reduce field emission What material?: W, TiN, Cu Copper Buttons 100nm

29. Jlab cavity hot spot Spread of gap Zero bias conductance

30. An electron (red) meeting the interface between a normal conductor (N) and a superconductor (S) produces a Cooper pair in the superconductor and a retroreflected hole (green) in the normal conductor. Vertical arrows indicate the spin band occupied by each particle.