Progress in Investigation of MgB2 Thin Films for SRF Cavity Applications

Teng Tan, Narendra Acharya, Matthaeus Wolak, Nam Hoon Lee, Ke Chen, Alex Krick, Steve May, Evan Johnson, Michael Hambe, and Xiaoxing Xi

Department of Physics Temple University, Philadelphia, PA

October 8, 2014Thin Film SRF 2014

Padua, ItalySupported by DOE/HEP, ANL

CollaboratorsMitra Taheri (Drexel), Enzo Palmieri (INFN), Tsuyoshi Tajima and Leonardo Civale (LANL), Ale Lukaszew (W&M), Charlie Reese (JLab), Ali Nassiri and

Thomas Proslier (ANL)

MgB2: Potential Low RF loss and High Gradient

Niobium MgB2

Tc/K 9 40

ρ0 /(μΩ cm) 5 0.1

Energy gap/meV 1.5 7 (σ), 2(π)

Bc(0)/T 0.20 > 0.30

Bc1(0)/T 0.17 <0.1

― RF surface resistance depends on energy gap and residual resistivity. Larger gap and lower resistivity indicate potential low RF loss than in Nb.

― Field gradient is ultimately limited by thermodynamic critical field. For MgB2, Bc(0) could be as high as 800 mT, vs. 200 mT for Nb.

― Lower critical field Bc1(0) may be important.

Surface Resistance: MgB2 vs Nb

Oates et al., SUST 23, 034011 (2010)

Nb on sapphire

MgB2 on LAO

MgB2 on sapphire

Stripline resonatorscaled to 1.5 GHz

Lower surface resistance comparable to Nb film.

HPCVD Reactors at Temple University

• Two HPCVD reactors with resistive heaters

• Smaller reactor for 15mm x 15mm films

• Larger reactor for 2” diameter films

Mg

ResistiveHeater

Hybrid Physical-Chemical Vapor Deposition

B2H6, H2

2” MgB2 Films Grown by HPCVD

200 nm 2’’ MgB2 film on sapphire

AFM image of film surface

1 2 3 4 5 635

36

37

38

39

RR

R

T

c (0)

RRR

Tc

0 (K

)

Position

MB20, 40 sccm B2H

6 for 4' at 730oC

center strip diced into 6 pcs 8 x 8 mm2 0

1

2

3

4

5

6

7

8

9

10

Surface Resistance Compared to Large Grain Nb

Surface resistance of 2” dia. 350 nm MgB2 film on sapphire comparable to the best large grain Nb at 4 K.

7.4 GHz, measured at JLab

Xiao et al., SUST 25, 095006 (2012)

Enhancing Bc1 by Multilayering

• When vortices enter the superconductor, their motion driven by the RF field can contribution to RF loss.

• When film thickness d < λ, Bc1 is larger than the bulk Bc1

Bc1 = (2f0/πd2)[ln(d/ξ)]

• Vortex entrance field an be enhanced by coating a superconducting cavity with several thin film superconductors with d < λ.

Gurevich, APL 88, 012511 (2006)

60 80 100 120 140200

300

400

500

600

700

800

900

0Hc1

(m

T)

Thickness (nm)

=5nm

Measurement of Penetration Depth of MgB2

Fraunhofer Pattern in MgB2/I/Pb Josephson Junctions

Cunnane et al., APL 102, 109904 (2013)

Voltage Modulation in DC SQUID Using MgB2/MgO/MgB2 Josephson Junctions

Vortex Penetration Field Bvp

Tajima et al., Proc SRF2013, Paris, France

Vortex penetration field higher than bulk Bc1 (and higher than bulk Nb) has been observed in some MgB2 films.

Thickness Dependence of Hc1: Below 100 nm

Beringer et al., IEEE Trans. Appl. Supercond. 23, 7500604 (2013)

SQUID magnetometer measurement shows enhancement of Hc1 to above 600 mT at 4 K in 60 nm MgB2 film.

Alternating MgB2-insulator structures have been fabricated on sapphire substrate. Sputtering MgO are used as insulating layer.

Top MgB2 layers amorphous.

SapphireMgB2

MgOMgB2

MgOMgB2

MgB2-MgO Multilayer Films

1

2

3

4

5

SiC

5. a-MgB2

4. poly-MgO

3. a-MgB2

2. poly-MgO

1. Epitaxial MgB2

Epitaxial and Polycrystalline Films: Hc1 vs Thickness

0 50 100 150 200 250 300 3500

2

4

6

8

10 ACMS VSM SQUID Fitting curve =59 nm

Hc1

/500 O

e

Thickness (nm)

T=5K

Epitaxial and polycrystalline MgB2 films both show increase in Hc1(0) with decreasing film thickness.

2 21

2 201

(tanh ( / ) 1){1 } / (1 sech )

2(ln 0.5) ( / )c

c b

k d dkH d

H k d

0 5 10 15 20 25 30 350

1000

2000

3000

Hc1

(O

e)

Temprature (K)

100nm 120nm 150nm 180nm 200nm 250nm 300nm

(b)

0 5 10 15 20 25 30 350

1000

2000

3000

Hc1

(O

e)

Temprature (K)

100nm 120nm 150nm 180nm 200nm 250nm 300nm

(a) Epitaxial MgB2/SiC

Polycrystalline MgB2/MgO

6 GHz Nb Cavity, Mock Cavity, and Coating System

(a) 6 GHz Nb cavity provided by Enzo Palmieri, INFN. (b) Mock stainless steel cavity used to test deposition conditions.

In Situ Coating of 6 GHz Cavity

• Good superconducting property obtained in films on sapphire substrates mounted at different locations of the cavity.

Two-Step Coating of 6 GHz Cavity

• First step: deposition of B film by CVD.• Second step: annealing in Mg vapor to

convert the film to MgB2.• Good superconducting property

obtained in films on sapphire substrates mounted at different locations of the cavity.

Need to Scale up to 3 GHz Cavity

In-Situ Coating of MgB2: 3 GHz Cavity

• Scale up the 6 GHz coating system

• More space between diborane supply line/Mg oven and cavity tubes than in the 6 GHz setup

• Better control of gas flow inside the cavity

• Largest size that can be accommodated by the existing vacuum chamber

Vacuum Chamber

Diborane Supply Line

Mg Oven

Heat Shield

Clam-Shell Heater

Cavity

Cryocooler-Cooled MgB2-Coated Cu Cavities

Nassiri et al., Proc SRF2013, Paris, France

• Coating of MgB2 on Cu cavity makes it possible to operate at 8-12 K• This temperature range can be achieved with efficient cryocoolers,

providing significant benefit with reduced cost.• Goal: 500 MHz MgB2-coated Cu cavity

675 C

650 C

620 C

• The SEM and AFM images show a large number of cracks or pinholes at higher temperatures (650 C and 675 C). At lower temperature (620 C) a more uniform growth and lower number of cracks can be seen

• This is most likely a result of the formation of an Mg-Cu alloy at steps in the substrate at higher temperatures

Deposition of MgB2 on Cu with MgO Buffer Layer

• MgB2 grown at 620C on MgO buffered Cu substrates shows a critical temperature of around 37.8 K

• Due to the small sample size (5x5mm), the pickup coil in the mutual inductance setup was not completely shielded, leading to a residual signal

MgB2 on Cu with MgO Buffer Layer

MgB2 layer was grown on top of the Nb buffered Cu substrate using HPCVD

An Nb layer ( 80 nm) was sputtered on the unpolished Cu substrate using DC sputtering

Unpolished Cu substrate

• To prevent interdiffusion of Mg through the MgO layer, an Nb buffer layer has been employed

Sample grown 700 C Sample grown 650 C Sample grown 630 C

Deposition of MgB2 on Cu with Nb Buffer Layer

• MgB2 grown at 630C on Nb buffered Cu substrates shows a critical temperature of around 37.7 K

• The MgB2 films show a slightly higher crystallinity, although a Mg-Cu alloy was still observed

MgCu2

Cu

MgCu2 and MgB2

MgB2 on Cu with Nb Buffer Layer

MgB2 layer grown on 2” Cu disk at 650 C

• In order to improve the adhesion of the Nb buffer layer to the Cu substrate, the substrate was ion milled in situ before Nb was sputter deposited

• This process results in the most uniform coverage

MgB2 on Cu with Ion Milling/Nb Buffer Layer

Summary

― MgB2 films show low surface resistance

― Enhancement of Hc1 observed in thin epitaxial and polycrystalline films

― Coating of 6 GHz cavity by both in situ and two-step annealing processes show promising results

― Efforts underway to coat 3 GHz cavity

― MgB2 films with high Tc deposited on Cu substrate with MgO or Nb buffer layers.