MgB2 thin films grown by hybrid physical-chemical vapor deposition (HPCVD) have been investigated for SRF cavity applications. I will present our recent results of research in three directions: enhancement of Hc1 in thin MgB2 films, large area MgB2 films on Cu, and the effort on coating of RF cavities. By reducing the thickness of the MgB2 film from 300 nm to 100 nm, Hc1(0) increases systematically from 38 mT to about 200 mT in both epitaxial and polycrystalline films. The HPCVD process has been successfully applied on 2” diameter Cu substrate. Both the in-situ and two-step processes have been used for the coating of a 6 GHz cavity. Samples from various locations of the cavity show good superconducting properties. Effort is underway to coat 3 GHz RF cavities.

1. 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
Collaborators
Mitra Taheri (Drexel), Enzo Palmieri (INFN), Tsuyoshi Tajima and Leonardo
Civale (LANL), Ale Lukaszew (W&M), Charlie Reese (JLab), Ali Nassiri and
Thomas Proslier (ANL)
October 8, 2014
Thin Film SRF 2014
Supported by DOE/HEP, ANL Padua, Italy

2. 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.

3. Surface Resistance: MgB2 vs Nb
Oates et al., SUST 23, 034011 (2010)
Nb on sapphire
MgB2 on
LAO
MgB2 on
sapphire
Stripline resonator
scaled to 1.5 GHz
Lower surface resistance comparable to Nb film.

4. HPCVD Reactors at Temple University
Hybrid Physical-Chemical Vapor Deposition
Mg
B2H6, H2
Resistive
Heater
• Two HPCVD reactors with
resistive heaters
• Smaller reactor for 15mm x
15mm films
• Larger reactor for 2” diameter
films

5. 2” MgB2 Films Grown by HPCVD
200 nm 2’’ MgB2 film on sapphire
AFM image of film surface
MB20, 40 sccm B
H
2
6
for 4' at 730oC
center strip diced into 6 pcs 8 x 8 mm2
1 2 3 4 5 6
39
38
37
36
35
10
RRR
T
c
(0)
RRR
0 (K)
c
T
Position
9
8
7
6
5
4
3
2
1
0

6. Surface Resistance Compared to Large Grain Nb
7.4 GHz, measured at JLab
Surface resistance of 2” dia. 350 nm MgB2 film on sapphire comparable
to the best large grain Nb at 4 K.
Xiao et al., SUST 25, 095006 (2012)

7. 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)
900
800
700
600
500
400
300
200
=5nm
60 80 100 120 140
(mT)
c1
H

0
Thickness (nm)

8. Measurement of Penetration Depth of MgB2
Fraunhofer Pattern in
MgB2/I/Pb Josephson
Junctions
Voltage Modulation in DC SQUID
Using MgB2/MgO/MgB2 Josephson
Junctions
Cunnane et al., APL 102, 109904 (2013)

9. 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.

10. Thickness Dependence of Hc1: Below 100 nm
x = 5 nm
x = 7 nm
40 60 80 100 120
10000
8000
6000
4000
2000
0
Field (Oe)
Film Thickness (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.

11. MgB2-MgO Multilayer Films
Alternating MgB2-insulator structures
have been fabricated on sapphire
substrate. Sputtering MgO are used as
insulating layer.
Top MgB2 layers amorphous.
MgB2
MgO
MgB2
MgO
MgB2
Sapphire
5
4
3
2
1
SiC
5. a-MgB2
4. poly-MgO
3. a-MgB2
2. poly-MgO
1. Epitaxial
MgB2

12. Epitaxial and Polycrystalline Films: Hc1 vs Thickness
10
8
6
4
2
0
ACMS
VSM
SQUID
Fitting curve =59 nm
0 50 100 150 200 250 300 350
/500 Oe
c1
H
Thickness (nm)
T=5K
2 2
H  (tanh k  ( d / 
) 
1)
dk d
c
1
  
H k d
{1 }/ (1 sech )
(ln 0.5) ( / ) 2
  

0 2 2
c 1
b
 
Epitaxial and polycrystalline MgB2 films
both show increase in Hc1(0) with
decreasing film thickness.
3000
2000
1000
0 5 10 15 20 25 30 35
3000
2000
1000
0
0 5 10 15 20 25 30 35
(Oe)
c1
(Oe)
c1
H
Temprature (K)
100nm
120nm
150nm
180nm
200nm
250nm
300nm
(b)
0
H
Temprature (K)
100nm
120nm
150nm
180nm
200nm
250nm
300nm
(a) Epitaxial
MgB2/SiC
Polycrystalline
MgB2/MgO

13. 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.

14. In Situ Coating of 6 GHz Cavity
• Good superconducting property obtained in films on sapphire
substrates mounted at different locations of the cavity.

15. 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.

16. Need to Scale up to 3 GHz Cavity

17. 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

18. 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

19. Deposition of MgB2 on Cu with MgO Buffer Layer
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

20. 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

21. Deposition of MgB2 on Cu with Nb Buffer Layer
• To prevent interdiffusion of Mg through the MgO layer, an Nb buffer layer has
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
been employed
Sample grown 700 C Sample grown 650 C Sample grown 630 C

22. 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

23. MgB2 on Cu with Ion Milling/Nb Buffer Layer
• 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 layer grown on 2” Cu disk at 650 C

24. 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.