1. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-1 Energetic Condensation Growth of Nb 3 Sn, MoRe and MgB 2 films for SRF Accelerators * M. Krishnan, C. James and B. Bures Alameda Applied Sciences Corporation (AASC), San Leandro, California 94577 presented at Thin Films and New Ideas in RF Superconductivity July 18-20, 2012 Collaborators: Jefferson Science Associates, LLC Los Alamos National Laboratory Helmholtz-Zentrum Berlin für Materialien und Energie GmbH  This research is supported at AASC by DOE SBIR Grants DE-FG02-08ER85162, DE-SC0004478 and DE-SC0007679  The JLab effort was provided by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177, including supplemental funding provided by the American Recovery and Reinvestment Act.

2. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-2 Outline  Motivation  AASC’s Energetic Condensation Coaters  MoRe films  Nb 3 Sn films  MgB 2 films  Discussion

3. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-3 Motivation  The accelerator technology R&D research area of DOE/HEP develops the next generation of particle accelerators and related technologies for discovery science; and also for possible applications in industry, medicine and other fields  SRF accelerators are a key part of this future development, as they can significantly reduce costs  SRF technology also has many commercial applications:  More than 1000 particle accelerators worldwide; most use normal cavities  Superconducting RF (SRF) cavities offer a ~10x improvement in energy efficiency over normal cavities, even accounting for cryogenic costs at 2K  Operating at higher temperatures (~10K) would further improve accelerator energy efficiency as the cryogenic cooling becomes less demanding and moves away from liquid He and towards off the shelf cryo-coolers such as those used in cryo-pumps  Replacing bulk Nb with Nb coated Cu cavities would also reduce costs  The ultimate payoff would be from cast Al SRF cavities coated with higher temperature superconductors (Nb 3 Sn, MoRe, MgB 2, Oxipnictides) Our thin film superconductor development is aimed at these broad goals SRF uses less power than normal SRF with cheaper cavity materials (Cu) would be better SRF with cheaper cavity materials and high-T c thinfilms would be better still

4. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-4 Coating Facilities available at AASC

5. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-5 Coaxial Energetic Deposition (CED) Coater at AASC Coaxial Energetic Deposition (CED TM )  CED coater uses “welding torch” technology  Arc source is scalable to high throughputs for large scale cavity coatings  Present version deposits ≈1 monolayer/pulse in ≈1ms

6. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-6 CAD: Pulsed Biased deposition; Dual Targets Dual Target source for Nb 3 Sn, Mo 3 Re, MgB 2 etc. Cathodic Arc Deposition (CAD) Pulsed Bias capability The questions posed are: (1) will use of dual targets help optimize film stoichiometry? (2) will energetic condensation (~100eV ions) allow growth of the correct phase at lower temperatures? CAD: Grows a monolayer (≈4Å) of Nb in ≈2ms Pulse Forming Network is easily adjusted to vary growth rate from 0.1-3 monolayers/pulse

7. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-7 Energetic Condensation is different from sputtering and PVD Comparison of Stress build-up in low energy deposition, energetic condensation, and energetic condensation plus high voltage bias Relief of compressive stress by pulsed ion impact [*] [*] M. M. Bilek, R N. Tarrant, D. R. McKenzie, S H. N. Lim, and D G. McCulloch “Control of Stress and Microstructure in Cathodic Arc Deposited Films” IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 31, NO. 5, OCTOBER 2003 (A. Anders) (A. Bendavid, CSIRO) (M.M. Bilek) Nb * A. BENDAVID, P. J. MARTIN, R. P. NETTERFIELD, G. J. SLOGGETT, T. J. KINDER, C., ANDRIKIDIS, JOURNAL OF MATERIALS SCIENCE LETTERS 12 (1993) 322-323

8. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-8 MoRe films from single (composite) Mo 3 Re targets  MoRe serves a template for other more complex A-15 superconductors A-M. Valente-Feliciano. “Overview: New Materials for SRF Cavities” Presentation to SRF Materials Workshop, May 23-24 2007

9. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-9 MoRe films from a composite {Mo 3 Re 1 } target: Stoichiometry  Measured film thickness allows erosion rates of Mo & Re to be estimated  Deduced erosion rates allow film stoichiometry to be predicted and compared with EDX data Composite Mo 3 Re 1 target/cooled anode mesh Heated substrate holderPulse forming networkCurrent vs. time Mo 2.8 Re 1

10. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-10 MoRe films from a composite {Mo 75% Re 25% } target: T c & RRR 700C/800C S.M. Deambrosis, G. Keppel, V. Ramazzo, C. Roncolato, R.G. Sharma, V. Palmieri, Physica C 441 (2006) 108–113

11. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-11 MoRe films from a composite {Mo 75% Re 25% } target: nomogram analysis  Our samples fall into the same BCS region (16-32nΩ) as estimated by Deambrosis for films of two compositions: Mo 60 Re 40 and Mo 75 Re 25. Nomogram reproduced from Deambrosis et al lines of constant BCS resistance in the ρ n -T c phase space, for materials at 4.2 K, 500MHz and a strong coupling factor of 4. [Deambrosis] (Mo 60 Re 40 : T c =12.13 K, RRR=1.3, ρn~30µΩcm; Mo 75 Re 25 :T c =11.82 K, RRR=1.71, ρ n ~10µΩcm. (The red star is our PhI data point: ρ n ~10µΩ- cm; T c =10.5K). V. Palmieri, in: 10th Workshop on RF Superconductivity Proceedings, Tsukuba 2001 (Noguchi). S.M. Deambrosis, G. Keppel, V. Ramazzo, C. Roncolato, R.G. Sharma, V. Palmieri, Physica C 441 (2006) 108–113

12. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-12 MoRe films from a dual target {Mo 75% Re 25% } target: crystal structure SEM image of ≈10x10µm region of Mo/Re film from dual-target source, CAD-66. Deposited at <100C!!  The SEM image shows crystallites that are triangular.  The typical macro-particles seen in films from less refractory metals are conspicuous by their absence!  Could the mono-crystallites be ReO 3 ?

13. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-13 MoRe films from a dual target {Mo 75% Re 25% } target: serendipity?  C. Keenan, S. Chandril, T. H. Myers, D. Lederman, E. Ramos-Moore, and A. L. Cabrera, “Properties of YMnO 3 self-assembled nanocrystalline prisms on GaN,” Applied Physics Letters, vol. 92, no. 1, p. 013119, 2008.  D. Myung et al [Phys. Status Solidi RRL 4, No. 12, 365–367 (2010) / DOI 10.1002/pssr.201004417]  ReO 3 is the only known binary metal oxide with both metallic conductivity and negative thermal expansion AFM image of 10x10µm region of YMnO 3 self- assembled nanocrystalline prisms (from Keenan 2008); grown using MBE at 850C SEM image of ≈10x10µm region of Mo/Re film from dual-target source, CAD-66. Deposited at <50C!!

14. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-14 ~MoRe films from separate Mo and Re targets

15. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-15 MoRe films from separate Mo and Re targets: Stoichiometry  Measured film thickness and stoichiometry determine the erosion rates of the two separate cathodes. The reduced Mo erosion rate implied Mo plasma flow away from substrate. This was confirmed by examination of the target after a long run Dual Mo and Re targets/cooled anode mesh Separate Currents vs. time

16. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-16 MoRe films from separate Mo and Re targets: T c  Measured film thickness and stoichiometry revealed that in this case, the Re plasma was emitted from the rim of the target. The T c data show a transition at 10.6K even with poor stoichiometry 900C/900C

17. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-17 MoRe films from separate Mo and Re targets: SIMS  Presence of Mg-24 and Al-27 (from sapphire substrate) show influence of high annealing and deposition temperatures  Fairly constant Mo-98 and Re-185 counts suggest that stoichiometry is constant across the film CAD-40 900C/900CCAD-61 700C/800C

18. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-18 ~Nb3Sn films from separate Nb and Sn targets

19. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-19 NbSn films from separate Nb and Sn targets: questions Two key questions that must be answered:  Must the stoichiometry of the incident ions be controlled at the lattice level? In other words, must the incident ratio Nb/Sn be 3:1 on every 4x4 Å square region across the substrate? Or will it suffice to make this ratio 3:1 on a larger footprint?  Must the ratio be 3:1 (within some acceptable margin of error) on every pulse? Or would it suffice to meet this requirement when averaged over many pulses?

20. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-20 NbSn films from separate Nb and Sn targets: Langmuir probe  Langmuir probe data allow determination of expansion cone

21. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-21 NbSn films from separate Nb and Sn targets: stoichiometry  Stoichiometry was globally correct, but what about T c ?  Langmuir probe data showed that a current ratio of 7:1 (Nb:Sn) should produce Nb 3 Sn at substrate. Deposition done at 500 C!

22. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-22 NbSn films from separate Nb and Sn targets: T c  Stoichiometry was globally correct, but macro-particles might have spoiled T c  Langmuir probe insensitive to neutral macro-particles Average Sn concentration: 24.99% T c of 13K low relative to Nb 3 Sn value of ≈18K Deposition done at 500 C!

23. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-23 NbSn films grow at lower temperatures (Frances Hellman)  Hellman showed that Nb 3 Sn could be grown even at 750C but coexisted with Nb 6 Sn 5 phase and had more disorder Frances Hellman Ph.D. thesis, Stanford Univ., 1985

24. ALAMEDA APPLIED SCIENCES CORPORATION aasc12vp03-24 Summary/Discussion  AASC has used composite and dual-target cathodic arc sources to grow thin films of MoRe, NbSn and MgB 2  Measurements of film thickness, stoichiometry and ion fluxes (Langmuir probe) allow us to infer erosion rates and plasma expansion plumes, from which we can ‘design’ source conditions that would create specified film stoichiometry  Mo 3 Re films showed a serendipitous result: mono-crystalline prisms (possibly of ReO 3 ) formed on ~50C substrates. Should pursue this tangent as a novel method of synthesizing binary metal oxides  Nb 3 Sn films have been grown at low temperatures (~500C) using a dual- target source with controlled currents, but the RRR and T c need improvement  Discussion