PROOF OF CONCEPT THIN FILMS AND MULTILAYERS TOWARD ENHANCED FIELD - PowerPoint PPT Presentation

PROOF OF CONCEPT THIN FILMS AND MULTILAYERS TOWARD ENHANCED FIELD GRADIENTS IN SRF CAVITIES Rosa A. Lukaszew , Douglas B. Beringer, William M. Roach, College of William and Mary , Williamsburg, Virginia, USA Grigory V. Eremeev, Charles E. Reece, Anne-Marie Valente-Feliciano, Thomas Jefferson National Accelerator Facility [TJNAF], Newport News, Virginia, USA Xiaoxing Xi, Temple University , Philadelphia, Pennsylvania, USA

Nb SRF cavities • The choice of Nb for superconducting cavities has been dictated by the requirement of having a material with a high lower critical field B c1 and a large energy gap D to prevent vortex dissipation and provide a low surface resistance R s caused by thermally-activated quasiparticles at T << T c and w << D, R s = (A w 2 /T) exp(- D /T) + R i where R i is a small temperature • independent residual resistance and A depends on SC parameters and w and T

The Gurevich model A. Gurevich, Appl. Phys. Lett. 88 , 012511 (2006). • Significant improvement could be achieved if a Nb cavity is coated with a multilayer consisting of alternating superconducting S layers with higher B c , and dielectric I layers The S layer has thickness d < l , and • therefore can remain in the Meissner state at fields much higher than B c1 bulk due to the increase of the parallel B c1 in a thin film, while the insulating layer (~15 nm) is needed to prevent Josephson coupling between the SC layers. • Such structure would be particularly efficient in the case of elliptical cavities where the magnetic field is concentrated well inside the cavity and is parallel to the surface.

Thin film geometry  B c1 enhancement  Multilayer shielding Image: CERN Accelerator School

Our experimental approach and methods • In order to test the Gurevich • We monitored the model we have investigated microstructure of the films, the effect of microstructure the morphology of the and morphology on the surface and the superconducting properties superconducting properties of Nb thin films deposited as well as the DC and RF onto different ceramic transport properties. surfaces. In particular we • We explored several aspects studied a-plane sapphire in the thin film deposition and (001) MgO. parameters-space, such as • We have also investigated growth rate, substrate Nb, NbN, NbTiN and MgB 2 temperature during growth, based S/I/S trilayers. annealing treatments, etc.

Thin film growth onto various different surfaces • Growth on sapphire, magnesium oxide and copper surfaces

Nb growth on a-plane sapphire • Nb can grow epitaxially on a-plane sapphire, with Nb(110)//Al2O3(11-20) Comparison of RRR values obtained by different groups: Group Nb film thickness RRR (nm) Lukaszew 600 97 S. A. Wolf [1] 600 82 G. Wu [2] 235 50.2* * RRR values for niobium thin films is highly dependent on thickness [1]. S. A. Wolf et al. , J. Vac. Sci. Tecnol. A 4 (3), May/June 1986 [2] G. Wu et al. , Thin Solid Films, 489 (2005) 56-62

Early stages of growth Nb thickness (nm) • Using Reflection high energy 0.23 2.3 23 electron diffraction (RHEED), we 0.36 [111]Nb ll[0001]Al O 2 3 observed a hexagonal Nb 0.35 surface structure for the first 3 Lattice parameter (nm) hcp Nb atomic layers followed by a 0.34 a strained bcc Nb(110) structure and the lattice parameter relaxes after 0.33 hcp+bcc Nb [1120]Nb ll bulk Nb bcc 3 nm. [0001]Al O 2 3 0.32 • RHEED images for the hexagonal 0.31 a phase at the third atomic layer. bcc Nb Patterns repeat every 60 deg. 0.30 0.29 1 10 100 Nb atomic layers 0 deg 30 deg 60 deg

Susceptibility AC measurements • The thinner Nb film exhibits two steps in the χ’ susceptibility 0 0.1 transition accompanied by two 30 nm peaks in the χ’’ susceptibility due to strained Nb layers at the -1 0.0 interface. 0 0.1 • Growth on a-plane sapphire 100 nm  '' initially follows a hexagonal  ' surface structure to relax the 0.0 -1 strain and to stabilize the 0 subsequent growth of bcc 0.2 600 nm Nb(110) phase. • Such initial layers affect the -1 0.0 superconducting properties of the films and these effects must 7 8 9 10 7 8 9 10 Temperature (K) Temperature (K) be taken into account in the design of multilayers. χ(ω)= χ’(ω)+i χ’’(ω) Strain Effects on the Crystal Growth and Superconducting Properties of Epitaxial Niobium Ultrathin Films, C. Clavero, D. B. Beringer, W. M. Roach, J. R. Skuza, K. C. Wong, A. D. Batchelor, C. E. Reece, and R. A. Lukaszew, Cryst. Growth Des ., 12 (5), pp 2588 – 2593 (2012)

. ( a ) 30 nm Nb Subsequent growth ] 0 1 1 [ Al O [0001] b N 3 Al O [1100] 2 2 3 Nb [001] 50 nm 200 nm ( b ) 100 nm Nb ( d ) 600 nm 30 heigth (nm) 20 100 nm 10 30 nm 200 nm 0 0 500 10 00 ( c ) 600 nm Nb distance (nm) Biaxial anisotropy is observed for thicknesses up to 100 nm while uniaxial anisotropy is observed. For thicker films 400 nm

Nb growth on (001) MgO • Nb can also be epitaxially grown on (001) MgO surfaces. • Unexpected findings: We have found that depending on the deposition conditions it is possible to tailor different epitaxial possibilities.

RHEED images for Nb(110) on MgO

Scaling of surface features 50 nm 600 nm RRR = 46.5 RMS = 6.51 nm

Nb (001) on MgO 14.29 nm 400nm 0.00 nm RRR = 165 RMS = 4.06 nm >200 RRR values!

Nb (001) on MgO RHEED beam along MgO [100] MgO out of box MgO annealed at 600 °C 30 nm Nb 100 nm Nb RHEED beam along MgO [110] MgO out of box MgO annealed at 600 °C 30 nm Nb 100 nm Nb

30.00 nm 10.00 nm 400nm 1.0µm 0.00 nm 0.00 nm RMS = 2.90 nm RMS = 1.21 nm 10.00 nm 200nm 0.00 nm RMS = 1.08 nm D. B. Beringer, W. M. Roach, C. Clavero, C. E. Reece, and R. A. Lukaszew , "Roughness analysis applied to niobium thin films grown on MgO(001) surfaces for superconducting radio frequency cavity applications," Phys. Rev. ST Accel. Beams 16 , 022001 (2013).

SQUID characterization 4 5 6 7 8 9 10 0.0 -0.2 -0.4  ' -0.6 -0.8 Tc = 9.2 K! -1.0 1.0 0.8 Possible loss 0.6 due to interfacial 0.4  " strain 0.2 0.0 -0.2 4 5 6 7 8 9 10 Temperature (K)

Nb-based trilayer 30 nm Nb RHEED indicates 15 nm MgO film with high degree of (001) 250 nm Nb texture XRD confirmed RHEED results: MgO (100) Nb(200) Intensity (arb. u.) Nb(110) 38.5 39.0 55 56 2  (deg)

SQUID magnetometry • Antoine et al [1] using SQUID magnetometry as well as third harmonic analysis to validate SQUID magnetometry measured the vortex penetration field on multilayered samples and demonstrated field enhancement. • In our work, we measured hysteresis loops as well as trapped moments that appear after application and removal of the applied field, following the work of C. Bohmer et al. [2] [1] C. Z. Antoine, S. Berry, S. Bouat, J.-F. Jacquot, J.-C. Villegier, G. Lamura, and A. Gurevich, Phys. Rev. ST Accel. Beams, vol. 13, p. 121 001, 2010; C. Z. Antoine, S. Berry, M. Aurino, J.-F. Jacquot, J.- C. Villegier, G. Lamura, and A. Andreone, IEEE Trans. Appl. Supercond., vol. 3, p. 2601, 2011; [2] C. Böhmer, G. Brandstätter, and H. W. Weber, Supercond. Sci. Technol. 10 A1 (1997).

SQUID characterization 4 5 6 7 8 9 0.0 -0.2 -0.4  ' -0.6 -0.8 -1.0 1.0 0.02 0.8 4 K 0.6  " 0.4 Long Moment (emu) 0.2 0.0 4 5 6 7 8 9 Temperature (K) 0.00 -0.02 -6000 -4000 -2000 0 2000 4000 6000 Field (Oe)

Nb on Cu (111) • Growth at room temperature and annealing at 350 ºC leads to the crystallization of Nb islands in a hexagonal surface structure, even though Nb is expected to growth tetragonal (110). 3.3 Å 3.30 Å 0.00 Å 0 Å Cesar Clavero, Nathan P. Guisinger, Srivilliputhur G. Srinivasan, and R. A. Lukaszew , “Study of Nb epitaxial growth on Cu(111) at sub- monolayer level”, J. Appl. Phys. 112 , 074328 (2012).

Nb films on Cu (001) surfaces Possible Nb/Cu(100) epitaxy: (a) RHEED pattern for Nb(110)/Cu(100)/Si(100) along the Si[100] and Si[110] azimuths. (b) A representative 2 µm x 2 µm AFM scan for Nb films on the Cu template.

SC properties for different growth T • The films grown at 150 °C have a very sharp transition from the superconducting state to the normal state that begins at ~9 K while films grown at RT have a much more gradual transition. • Our results suggest that an increased deposition temperature of Nb onto Cu leads to films with higher crystalline quality (grain size) and thus improved superconducting properties (HC1). Niobium thin film deposition studies on copper surfaces for superconducting radio frequency cavity applications , W. M. Roach, D. B. Beringer, J. R. Skuza, W. A. Oliver, C. Clavero, C. E. Reece, and R. A. Lukaszew, Phys. Rev. ST Accel. Beams 15, 062002 (2012).

Nb films on Cu Nb films with quality comparable to high RRR bulk Nb (as used for SRF cavities) have been produced both on single crystal and polycrystalline Cu substrates with energetic condensation via ECR (electron cyclotron resonance) at Jlab. SRF measurements are in progress. Substrate RRR Single crystal Cu (100) 129 Cu (110) 275 Cu (111) 242 Polycrystalline Cu fine grains 150 Hetero-epitaxial relationships between Nb and Cu verified. Cu large grains 289

Other possible SC thin films for the SIS model NbN, MgB 2 , etc.