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High Purity MgB 2 Thin Films. Xiaoxing Xi. Department of Physics and Department of Materials Science and Engineering Penn State University, University Park, PA. October 10, 2006 Thin Film RF Workshop Padua, Italy. Supported by ONR, NSF .

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slide1

High Purity MgB2 Thin Films

Xiaoxing Xi

Department of Physics and

Department of Materials Science and Engineering

Penn State University, University Park, PA

October 10, 2006

Thin Film RF Workshop

Padua, Italy

Supported by ONR, NSF

slide2

Xiaoxing Xi group (Physics and Materials Sci & Eng):Ke Chen, Derek Wilke, Yi Cui, Chenggang Zhuang (Beijing), Arsen Soukiassian, Valeria Ferrando (Genoa), Pasquale Orgiani (Naples), Alexej Pogrebnyakov, Dmitri Tenne, Xianghui Zeng, Baoting Liu, CVD growth, electrical characterization, junctions

Joan Redwing Group (Materials Sci & Eng):HPCVD growth, modeling

Qi Li Group (Physics): Junctions, transport and magnetic measurements

Darrell Schlom Group (Materials Sci & Eng):structural analysis

Zi-Kui Liu Group (Materials Sci & Eng):Thermodynamics

Xiaoqing Pan Group (U. Michigan): Cross-Section TEM

John Spence Group (ASU): TEM

N. Klein Group (Jülich): Microwave measurement

A. Findikoglu (LANL): Microwave measurement

Qiang Li Group (Brookhaven National Lab): Magneto-optic measurement

Tom Johansen Group (U Oslo): Magneto-optic measurement

Qing-Rong Feng Group (Peking University): SiC fiber

Chang-Beom Eom Group (U Wisconsin): Structural analysis

J. B. Betts and C. H. Mielke (LANL): High field measurement

slide3

MgB2: An Exciting Superconductor

  • SCIENCE
  • Tc = 40 K, BCS superconductor (2001)
  • Two bands with weak inter-band scattering: 2D σ band and 3Dπ band
  • Two gaps: A superconductor with two order parameters
  • Low material cost, easy manufacturing
  • High performance in field (Hc2 over 60 T)
  • High field magnets for NMR/MRI; high-energy physics, fusion, MAGLEV, motors, generators, and transformers
  • ELECTRONICS
  • No reproducible, uniform HTS Josephson junctions yet, may be easier for MgB2
  • 25 K operation, much less cryogenic requirement than LTS Josephson junctions
  • Superconducting digital circuits

HIGH FIELD

slide4

MgB2: Two Superconducting Gaps

Two Superconducting Gaps

E2g Phonon

σ States

Gaps vs. T

el-ph Coupling

λσσ=1.017 λσπ=0.213

λπσ=0.155λππ=0.448

(Golubov et al. J. Phys.: Condens. Matter 14, 1353 (2002).)

π States

Choi et al.Nature 418, 758 (2002)

slide5

MgB2: Promising at Microwave Frequency

  • Higher Tc, low resistivity, larger gap, higher critical field than Nb.
  • It has been predicted theoretically that nonlinearity in MgB2 is large due to existence of two bands.
  • Manipulation of interband and intraband scattering could improve nonlinearity.
  • Recent MIT/Lincoln Lab result on STI films very promising.

Oates, Agassi, and Moeckly, ASC 2006 Proceeding, submitted

slide6

Pressure-Composition Phase Diagram

Process window: where the thermodynamically stable phases are Gas+MgB2.

If deposition is to take place at 850°C, Mg partial pressure has to be above 340 mTorr to keep the MgB2 phase stable.

Adsorption-controlled growth: automatic composition control if Mg:B ratio is above 1:2.

You can provide as much Mg as you want above stoichiometry without affecting the MgB2 composition.

P-x Phase Diagram at 850°C

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

slide7

Pressure-Temperature Phase Diagram

  • PHASE STABILITY
  • Mg pressure for the process window is very high
  • Typically, optimal epitaxy Tsub ≈ 0.5 Tmelt(Yang and Flynn, PRL 62, 2476 (1989))
  • Minimum Tsub for metal epitaxy is Tsub ≈ 0.12 Tmelt (Flynn, J. Phys. F 18, L195 (1988))
  • For MgB2
    • 0.5 Tmelt~1080 °C.
    • Requires 11 Torr Mg vapor pressure
    • Or
    • Mg flux of 2x1021 Mg atoms/(cm2·s), or 0.5 mm/s
  • Too high for most vacuum deposition techniques
    • 0.12 Tmelt ~ 50 °C.

Automatic composition control: P-T diagram the samefor all Mg:B ratio above 1:2.

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

slide8

Sticking Coefficient of Mg

1.0

0.8

0.6

Mg Sticking Coefficient

0.4

0.2

0

200

300

400

Temperature (°C)

Mg sticking coefficient drops to near zero above 300°C.

Not many Mg available to react with B.

Kim et al, IEEE Trans. Appl. Supercond. 13, 3238 (2003)

slide9

Contaminations

Reaction with Oxygen

C-doped single crystals

(Zi-Kui Liu, PSU)

Lee et al. Physica C397, 7 (2003)

  • Mg reacts strongly with oxygen:
  • reduces Mg vapor pressure
  • forms MgO - small grain size, insulating grain boundaries

Carbon contamination reduces Tc

slide10

High-Temperature Ex-Situ Annealing

B

Low

Temperature

Mg

~ 850 °C

in Mg Vapor

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)

Epitaxial Films

slide11

MgB2 Films by High-TEx-Situ Annealing

  • Epitaxial films
  • Good superconducting properties

Kang et al, Science 292, 1521 (2001)

Berenov et al, APL 79, 4001 (2001)

slide12

Intermediate-Temperature In-Situ Annealing

B, Mg

Low

Temperature

Mg

~ 600 °C

in situ

Blank et al, APL 79, 394 (2001)

Shinde et al, APL 79, 227 (2001)

Christen et al, APL 79, 2603 (2001)

Zeng et al, APL 79, 1840 (2001)

Ermolov et al, JLTP Lett. 73, 557 (2001)

Plecenik et al, Physica C 363, 224 (2001)

Kim et al, IEEE Trans Appl. SC 13, 3238 (2003)

Nanocrystalline Films

slide13

MgB2 Films by Intermediate-TIn-Situ Annealing

Cross-Sectional TEM

Superconducting Transition

  • Mg vapor pressure varies with time – difficult to control
  • Nano-crystalline with oxygen contamination
  • Superconducting properties fair.

Zeng et al, APL 79, 4001 (2001)

slide14

Low-Temperature In-Situ Deposition

B, Mg

Low

Temperature

Textured

Films

Ueda & Naito, APL 79, 2046 (2001)

Jo et al, APL 80, 3563 (2002)

van Erven et al, APL 81, 4982 (2002)

Kim et al, IEEE Trans Appl. SC 13, 3238 (2003)

Saito et al, JJAP 41, L127 (2002)

slide15

MgB2 Films by Low-TIn-Situ Deposition

Ueda & Naito, APL 79, 2046 (2001)

  • UHV conditions
  • Superconducting films below about 300°C
  • Good superconducting properties

Ueda & Makimoto, JJAP 45, 5738 (2006)

slide16

High- and Intermediate-Temperature In-Situ Deposition

B, Mg

High and

Intermediate

Temperature

Epitaxial

Films

Ueda & Naito, APL 79, 2046 (2001)

Jo et al, APL 80, 3563 (2002)

van Erven et al, APL 81, 4982 (2002)

Kim et al, IEEE Trans Appl. SC 13, 3238 (2003)

Saito et al, JJAP 41, L127 (2002)

slide17

Reactive Co-Evaporation

  • Deposition temperature 550°C
  • Good superconducting properties
  • Large area and double sided films
  • Films stable to moisture
  • On various substrates: r-plane, c-plane, and m-plane sapphire, 4H-SiC, MgO, LaAlO3, NdGaO3, LaGaO3, LSAT, SrTiO3, YSZ, etc.

(Moeckly & Ruby, SC Sci Tech 19, L21 (2006))

slide18

MgB2 Films by Reactive Co-Evaporation

4” MgB2 film on polycrystalline alumina

(Moeckly & Ruby, SC Sci Tech 19, L21 (2006))

slide19

Hybrid Physical-Chemical Vapor Deposition

Schematic View

H2, B2H6

Mg

Susceptor

  • Deposition procedure and parameters:
  • Purge with N2, H2
  • Carrier gas: H2
  • Ptotal = 100 Torr.
  • Inductively heating susceptor, AND Mg, to550–760 °C. PMg = ? (44 mTorr is needed at 750 °C according to thermodynamics)
  • Start flow of B2H6 mixture (1000 ppm in H2): 25 - 250 sccm. Film starts to grow.
  • Total flow: 400 sccm - 1 slm
  • Deposition rate: 3 - 57 Å/sec
  • Switch off B2H6 flow, turn off heater.

rid of oxygen

prevent oxidation

make high Mg

pressure possible

generate high

Mg pressure

high enough T

For epitaxy

pure source of B

control growth

rate

low Mg sticking no Mg deposit

slide20

Hybrid Physical-Chemical Vapor Deposition

(Dan Lamborn)

Velocity Distribution

slide21

Epitaxial Growth of MgB2 Films on (0001) SiC

  • c axis oriented, with sharp rocking curves
  • in-plane aligned with substrate, with sharp rocking curves
  • free of MgO
slide22

MgB2/SiC (0001)

MgO Regions

Epitaxial Growth on Sapphire and SiC

MgB2

a = 3.086 Å

Al2O3

a = 4.765 Å

4H-SiC

a = 3.07 Å

MgB2/Al2O3 (0001)

MgB2

No MgO

6H-SiC

slide23

Defects in Epitaxial Films on SiC

Low-Resolution TEM

High-Resolution TEM

There are more defects at the film/substrate interface than in the top part of the film.

Pogrebnyakov et al.PRL 93, 147006 (2004)

slide25

Coalescence of Islands in MgB2 Films

  • Small islands grow together, giving rise to larger ones, and a flat surface for further growth.
  • The boundaries between islands are clean.

Wu et al.APL 85, 1155 (2004)

slide26

Very Clean HPCVD MgB2 Films: RRR > 80

Mean free length is limited by the film thickness.

slide27

Clean HPCVD MgB2 Films: Potential Low Rs (BCS)

Rs (BCS) versus (ρ0, Tc)

Pickett, Nature 418, 733 (2002)

π Gap

σ Gap

Vaglio, Particle Accelerators 61, 391 (1998)

slide28

Rowell Model of Connectivity

ρ

REC Film

Rowell, SC Sci. Tech. 16, R17 (2003)

HPCVD Film

  • Residual resistivity: impurity, surface, and defects
  • Δρ≡ρ(300K) - ρ(50K): electron-phone coupling, roughly8 μΩcm
  • If Δρis larger : actual area A’ smaller than total area A
  • HPCVD films:grains well connected.

High-T Annealed Film

Bu et al., APL 81, 1851 (2002)

slide29

Films with Poor Connectivity

Intermediate-T Annealing

Low-TIn Situ Film

slide30

Clean MgB2: Weak Pinning and Low Hc2

Jc (0 K) ~3.5 x 107 A/cm2 is nearly 0.1Jd (0 K), which is 4 x 108 A/cm2

slide31

Jc (A/cm2)

μ0H (T)

C-Alloyed MgB2: Strong Pinning and High Hc2

  • Carbon alloying: mixing (C5H5)2Mg in the carrier gas.
  • Pinning enhanced by carbon alloying.
  • Hc2 enhanced to over 60 T, due to modification of interband and intraband scattering
slide32

Good Microwave Properties in Clean Films

Microwave measurement: sapphire resonator technique at 18 GHz.

Surface Resistance @ 18 GHz

π-Band Gap

  • Surface resistance decreases with residual resistivity. Clean HPCVD films show low surface resistance.
  • Interband scattering makes π band gap larger.

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

slide33

Short Penetration Depth in Clean Films

  • Penetration depth decrease with residual resistivity.
  • London penetration depth λL: 34.5 nm

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

surface morphology with n 2 addition
Surface Morphology with N2 Addition

10 sccm: RMS =1.01 nm

5 sccm: RMS = 0.96 nm

Pure MgB2: RMS =3.64 nm

100 sccm: RMS =8.21 nm

30 sccm: RMS =5.58 nm

15 sccm: RMS =1.73 nm

slide36

Dendritic Magnetic Instability in MgB2 Films

Johanson et al.Europhys. Lett. 59, 599 (2002)

  • Flux jumps observed at low temperature and low field in many MgB2 films.
  • Dendritic magnetic instability observed by magneto-optical imaging.
slide37

Absence of Dendritic Magnetic Instability

in Clean HPCVD Films

Flux Entry

Remnant State

(Ye et al.APL 85, 5285 (2004))

slide38

Absence of Dendritic Magnetic Instability

In Clean MgB2 Films

  • Measurement by Prof. Tom Johansen (Oslo):
  • Measurement down to 3.5 K
  • Spacer between the MgB2 film and the ferrite garnet indicator except near the lower left corner, ensuring that there is no direct contact over a large part of the film
  • Fast ramping field
  • No dendritic flux penetration in pure MgB2 films.
slide39

Epitaxial MgB2 Film Grown at 550°C

  • Film is epitaxial, but with a broader rocking curve
  • There is a small amount of 30° in-plane twinning
  • Tc remains high, but residual resistivity is higher than the standard films

Tc=40.3 K

slide40

Deposition Temperature Dependence

  • Tc does not change much with deposition temperature
  • Residual resistivity increases at lower temperature
  • Crystallinity degraded at lower temperature
slide41

Possible Substrates or Buffer layers

for MgB2 Films

Result of Thermodynamic Calculations: Reactivity

slide42

a

50 μm

MgB2

W

SiC

b

50 μm

(a)

(b)

*

Mg2Si (4,2,2)

*

*

Mg2Si (2,2,0)

MgB2 (1,0,1)

MgB2 (1,0,0)

Mg2Si (4,0,0)

*

5 μm

*

Mg2Si (4,4,0)

MgB2 (0,0,2)

(c)

MgB2 (1,1,2)

*

Polycrystalline MgB2 Coated-Conductor Fiber

SEM

X-ray diffraction

slide43

MgB2 Coated Conductors: High Hc2 and Hirr

Upper Critical Field (0.9R0)

Irreversibility Field (0.1R0)

  • Similar to Hc2 and Hirr in parallel field in thin films .
  • No epitaxy or texture necessary
slide44

Polycrystalline MgB2 Films on Flexible YSZ

  • Tc = 38.9 K.
  • Jc high. Insensitive to bending
  • Low Rs similar to epitaxial films on sapphire substrate observed.

Rs measured by A. Findikoglu (LANL)

slide45

HPCVD MgB2 Films on Metal Substrates

High Tc has been obtained in polycrystalline MgB2 films on stainless steel, Nb, TiN, and other substrates.

slide46

Morphology of MgB2 Films on Stainless Steel

Higher deposition temperature. Lower growth rate.

Lower deposition temperature. Higher growth rate.

slide47

Degradation of HPCVD MgB2 Films in Water

Room Temperature

0°C

  • Film properties degrade with exposure to air/moisture: resistance goes up, Tc goes down
  • Experiments show that MgB2 degrades quickly in water, and is sensitive to temperature.
slide48

Stability of RCE MgB2 Films in Water

(Brian Moeckly. STI)

Compared to the HPCVD films, MgB2 films deposited by reactive co-evaporation are much more stable against degradation in water.

slide49

Point-Contact Spectroscopy on MgB2 Films

HPCVD film: Andreev-Reflection-like.

Metallic surface.

RCE film: tunneling-like.

Surface with tunnel barrier.

(Park and Greene, Rev. Sci. Instr. 77, 023905 (2006))

slide50

Integrated HPCVD System

CVD #2

Transfer

Chamber

Sputtering

CVD #1

slide53

Conclusion

  • Keys to high quality MgB2 thin films:
    • high Mg pressure for thermodynamic stability of MgB2
    • oxygen-free or reducing environment
    • clean Mg and B sources

HPCVD successfully meets these requirements

Repeated B deposition + Mg reaction is fine

  • Critical engineering considerations in HPCVD:
    • generate high Mg pressure at substrate (cold surface is Mg trap)
    • deliver diborane to the substrate (the first hot surface diborane sees should be the substrate)

Lower deposition temperature is fine

Many metal substrates are fine

Repeated B deposition + Mg reaction is fine

slide54

Conclusion

  • Clean HPCVD MgB2 thin films have excellent properties:
    • low resistivity (<0.1 μΩ) and long mean free path
    • high Tc ~ 42 K (due to tensile strain), high Jc(10% depairing current)
    • low surface resistance, short penetration depth
    • smooth surface (RMS roughness < 10 Å with N2 addition)
    • good thermal conductivity (free from dendritic magnetic instability)

Mean free path can be adjusted by carbon doping

  • Polycrystalline films maintain good properties
  • MgB2 reacts with water. Clean surface leads to degradation in water and moisture, which needs to be dealt with
  • Safety procedures for diborane exist, and must be strictly followed
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