Magnesium Diboride Thin Films for Superconducting RF Cavities

Xiaoxing Xi

Department of Physics Temple University, Philadelphia, PA

October 5, 2010Workshop on Thin Film RF

Padua, ItalySupported by DOE/HEP

ContributorsChenggang Zhuang, Alex Krick, Teng Tan, Ke Chen, Ed Kaczanowicz

CollaboratorsEnzo Palmieri (INFN), Som Tyagi (Drexel), Steven Alage (Maryland)

Outline

• Why is MgB2 promising– Rs

– Gradient – Successful fabrication of MgB2 films

• Progress report– DOE/HEP supported project: “Magnesium Diboride Thin Films for

Superconducting RF Cavities” – Film deposition and coating of cavity– Microwave characterization

• Summary

MgB2: An Outstandting SuperconductorSCIENCE— BCS, Tc = 40 K— Two bands with weak inter-band scattering: σ band and π band— Two gaps (∆σ~7meV, ∆π~2meV): two order parameters

HIGH FIELD

— High performance in field (Hc2 over 60 T)— Cryogen free, mobile MRI, high field magnets, etc.

0 10 20 30 400

10

20

30

40

50

60

NbTi Nb3Sn

MgB2

Fie

ld (

T)

Temperature (K)

MgB2

//

ELECTRONICS

— 25 K operation, much less cryogenic requirement than LTS Josephson junctions— Superconducting digital circuits

— High Tc, low resistivity— High Q, high Eacc

— Possibly replacing Nb— ILC, light sources

SRF CAVITY

39.5 40.0 40.5 41.0 41.50.00

0.05

0.10

(

cm)

T (K)

Potential Low BCS Rs in MgB2 for RF Cavity

Rs (BCS) versus (ρ0, Tc)

Rs from π Gap Rs from σ Gap

Nb

T = 4.2 K, f = 0.5 GHz

Nb3Sn

Vaglio, Particle Accelerators 61, 391 (1998)

MgB2

18 GHz

• 230 μΩ at 15 K and 18 GHz• Scale to 0.5 GHz: 177 nΩ at 15 K.• ~20 time reduction in Rs for 15 K to

4 K has been shown for MgB2

MgB2 films from STI by reactive co-evaporation A stripline resonator at 2 GHz for films on dielectric substrates A dielectric resonator at 10.7 GHz for films on metallic substrates.

Jin et al, SC Sci. Tech. 18, L1 (2005)

Surface Resistance vs. Mean Free Length

A minimum in the surface resistance when the mean free path is comparable to the coherence length.

Nb , 1.5 GHz

Nb /Cu

Padamsee, SC Sci. Tech. 14, R28 (2001)

MgB2

Jin et al, SC Sci. Tech. 18, L1 (2005)

Penetration Depth

= /≈ 6

Hc = Hc2/√2 ≈ 820 mT

Hsh ≈ 0.75 Hc ≈ 620 mT

Hc2(0) = 0/2πab(0)2

ab(0) ≈ 7 nm

Upper Critical Field

Potential High Maximum Eacc in MgB2 for RF Cavity

Xi, SC Sci. Tech. 22, 043001 (2009)

Issues to be studied:• Effects of existence of two gaps. • is larger for π band than for σ band (Koshelev and Golubov, PRL 90, 177002

(2003)).• is larger for π band than for σ band (Eisterer et al, PRB 72, 134525 (2005)).

(Nb: Hc ≈ 200 mT)

Single Crystal, MagnetizationPowder, Specific Heat

Experimental Measurement of Hc of MgB2

Zehetmayer et al, PRB 66, 052505 (2002)

Bouquet et al, PRL 87, 047001 (2001)

Dahm & Scalapino, APL 85, 4436 (2004)

Effects of Two Gaps on Microwave NonlinearityNonlinear Coefficient of MgB2

YBCO, MgB2, & 40-K BCS SCMgB2 of Different Intraband

Scattering

— It has been predicted theoretically that • nonlinearity in MgB2 is larger than 40 K BCS superconductor due to existence of two bands.• compares favorably with HTS at low temperature

— Manipulation of interband and intraband scattering could improve nonlinearity.

Microwave Nonlinearity of HPCVD MgB2 Films

theoretical d wavetheoretical one-band s wave

theoretical two-band s waveГπ/Гσ=2

YBCONb

MgB2

Cifariello et al, APL 88, 142510 (2006)

— Result in agreement with Dahm – Scalapino prediction.

— Modification of interband and intraband scattering key to low nonlinearity.

Larbalestier et al., Nature 414, 368 (2001)

Properties of Various Superconductors

40 0.1

Nb 9.2 0.4 40 2NbN 16.2 15 200 70

7

223.6

Keys to Growth of MgB2 Films

— Keep a high Mg pressure for phase stability

For example, at 600°C Mg vapor pressure of 0.9 mTorr or Mg flux of 500 Å/s is needed

— No need for composition control ， as long as the Mg:B ratio is above 1:2.

— Keep oxygen away: Mg reacts strongly with oxygen - forms MgO, reduces Mg vapor pressure.

— Avoid carbon: Carbon doping reduces Tc and increases resistivity

Liu et al., APL 78, 3678 (2001)

Hybrid Physical-Chemical Vapor Deposition

get rid of oxygenprevent oxidation

make high Mgpressure possible

generate high Mg pressure: required by thermodynamics

pure source of B

B supply (B2H6 flow rate) controls growth rate

Pure source of Mghigh enough T

for epitaxy

Schematic View

Substrate

H2 (~100 Torr)B2H6 (~ 5 - 250 sccm)

Mg

Susceptor

550–760 °C

Clean, Epitaxial HPCVD MgB2 Films

0 50 100 150 200 250 3000

2

4

6

8

39.5 40.0 40.5 41.0 41.50.00

0.05

0.10

(cm

)

T (K)

Res

istiv

ity (

cm)

Temperature (K)

on (0001) sapphireThickness 770 nm

Xi et al, Physica C 456, 22 (2007)

SiC

MgB2

interface

Wu et al., APL 85, 16 (2004)

Jin et al, SC Sci. Tech. 18, L1 (2005)

Microwave Properties vs Cleanness of MgB2 FilmsSurface Resistance @ 18 GHz Penetration Depth

π-Band Gap

— Dielectric single-crystal sapphire puck resonator at 18 GHz.

— Cleaner films (low resistivity) leads to lower surface resistance, shorter penetration depth, and smaller πband gap (less interband scattering).

Technical Approach

Advantages: • Localized source of high-pressure Mg vapor• Negligible MgO in the pocket• B evaporation in vacuum• No need to control Mg/B flux ratio• Ability to separate substrate T from Mg T• Uniform growth on multiple large-area substrates• Non-contact: Double-sided deposition possible• Can try a variety of substrate materials at once

Rotating blackbody heater

Reactive evaporation using the pocket heaterDirectly addresses MgB2 growth difficulties

MgB2 Film by Reactive Co-Evaporation

Moeckly and Ruby, SUST 19, L21 (2006)

Large Area HPCVD Films Using STI Pocket Heater

Differences from reactive co-evaporation:— B2H6 used as boron source instead of e-beam evaporation— Hydrogen used as the carrier gas instead of HV— Deposition temperature in broader rangeAdvantages:— Large area and double sided films — Potential for scale up for wires

B2H6

Wang et al. Supercond. Sci. Tech. 21, 085019 (2008)

0 50 100 150 200 250 3000

2

4

6

8

10

39.0 39.5 40.0 40.5 41.00.0

0.4

0.8

1.2

(

cm

)

Temperature (K)(

cm

)

Temperature (K)

(a)

High-Temperature Ex-Situ Annealing

Kang et al, Science 292, 1521 (2001)Eom et al, Nature 411, 558 (2001)Ferdeghini et al, SST 15, 952 (2001)Berenov et al, APL 79, 4001 (2001)Vaglio et al, SST 15, 1236 (2001)Moon et al, APL 79, 2429 (2001)Fu et al, Physica C377, 407 (2001)

B

Mg

Low Temperature

~ 850 °Cin Mg Vapor

Epitaxial Films

MgB2 Film by Reaction of CVD B Film

Clean B precursor layer leads to clean MgB2 film.

0 50 100 150 200 250 3000

2

4

6

8

10

38 39 40 41 420.0

0.5

1.0

1.5

Tc

onset=41.0 K

Tc

zero =40.6 K

RRR=7.8

cm

)

Temperature (K)

cm

)

Tc

onset=41.0 K

Tc

zero =40.6 K

RRR=7.8

cm

)

T (K)

Outline

• Why is MgB2 promising– Rs

– Gradient – Successful fabrication of MgB2 films

• Progress report– DOE/HEP supported project: “Magnesium Diboride Thin Films for

Superconducting RF Cavities” – Film deposition and coating of cavity– Microwave characterization

• Summary

“Magnesium Diboride Thin Films for Superconducting RF Cavities”

― Supported by DOE HEP, “Fundamental Research in Superconducting RF Cavity Design” Program

― Project period: 6-15-10 to 6-14-13

― Project objective: Develop HPCVD technology for MgB2 SRF cavities

― Research activities: Study of MgB2 film properties related to the RF cavity application Coating single-cell RF cavities with MgB2 film

― Film research: Deposit large-area (2" - 4" diameter) films using both in situ HPCVD and two-step annealing process Deposit on sapphire and metal substrates, such as Nb, Mo, and Cu Effects of smoothness, thermal expansion mismatch, hydrogen, film stability, two-gap nature of MgB2, and multilayer structure on RF properties

― Coating cavity: Both in situ HPCVD and two-step annealing process First step: 6 GHz cavity from INFN Next step: larger cavities

Scaling up HPCVD for Large Area Films

Penn State

Mg

Susceptor

550–760°C

Mg

ResistiveHeater

Peking University

Mg

ResistiveHeater

Temple University

• Inductive heater• Successful• 5mm x 5mm

• Resistive heater• Successful• 15mm x 15mm

• Resistive heater• Designed for 2" dia.

B2H6 B2H6B2H6

HPCVD Reactors at Temple

• Two HPCVD reactors with resistive heaters• Smaller reactor for 15mm x 15mm films• Larger reactor for 2” diameter films

First Small MgB2 Films Made at Temple

0 50 100 150 200 250 3000

2

4

6

8

(

cm)

T(K)

09132010A MgB2/SiC

10sccm for 2'' @ 745Cthickness ~ 80nmRRR~ 8, T

c(0) ~40.8K

(42K) ~ 1 cm

overall size 3’’x 5’’

B2H6 inlet

Mg oven

2’’ wafer

Top heater

Bottom heater

New Design of Pocket Heater

• Inspired by Michio Naito of Tokyo University of Agriculture and Technology

• Much more compact than the STI pocket heater – the dimension of the heater for 2” films is less than 5”

• Under construction

4"

B2H6

Mg

In Situ Coating of 6 GHz Nb Cavity• 6 GHz Nb cavity provided by Enzo Palmieri, INFN. Characterization capability exists at INFN.• Small size: Can be coated using existing equipment in the lab; Low cost for optimization; First step towards coating of larger cavities.• Setup design near finish

Cavity Coating Setup at Temple

Resistive tube furnace will be installed in the existing HPCVD system

Integrated HPCVD system: Two HPCVD systems, one sputtering system, and one distribution chamber connected together.

Cavity Up Cavity Down

Coating SRF Cavity with a Two-Step Process

Coating cavity with B layer at ~400-500°C using CVD

Reacting with Mg to form MgB2 at ~ 850-900 °C in Mg vapor

H2, B2H6 Mg vapor

Low-Field Microwave Magnetoabsorption

• Energy loss caused by the vortex movement leads to increase in microwave reflected power P from a resonated cavity

• Hmw ⊥ film surface Hext // film surface

9.3 GHz source

Power meter

Circulator

Helmholtzcoil

Microwave CavityQ~2500

H // ab

Cryo-cooler Head

Waveguide

Quartz Rod

Quartz Tube

Bai, Patanjali, Bhagat, and Tyagi, J. Supercond. 8, 299 (1995).

Prof. Som Tyagi, Drexel University

Low-Field Microwave Magnetoabsorption

20 30 40Temperature (K)

Abs

orpt

ion

1000 G

1500 G

0 G

9.3 GHzThickness: 100 nm

T (K) Hc1 (G)

28 1500

30 1000

32 0

Value much higher than experimental and predicted value (~ 20 mT)

To be understood.

dHc1/dT=25 mT/K

Microwave Surface Impedance Measurement

Advantages: Measures transition and homogeneity of sampleFast: built for a continuous flow cryostatCan be used for quantitative Rs measurements if required

Disadvantages: sample is pressed against a plate (this could be relaxed)The dielectric puck is attached to the sample with grease

24.7 GHz resonance

A resonance of the dielectric puck is excited. It’s properties are modifiedby the superconducting film underneath

Prof. Steven Anlage, U. of Maryland

Summary

― Clean MgB2 thin films have excellent properties: low resistivity (<0.1 cm) and high Tc promise low BCS surface resistance long coherence length and short penetration depth promise high Hc: possibly as high as ~ 820 mT Two gap effects need to be investigated.

― Current status of MgB2 coating for SRF: July 2009: Move from Penn State University to Temple University June 2010: Start of DOE/HEP SRF project HPCVD at Temple for small films (15mm x 15mm) operational Scaling up for in situ deposition of 2" films underway Coating of 6 GHz cavity by in situ processes being designed

― Low-Field Microwave Magnetoabsorption at Drexel University shows possible high Hc1

― Dielectric puck resonator from University of Maryland installed.