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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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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


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


MgB2: An Exciting Superconductor

  • 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
  • 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



MgB2: Two Superconducting Gaps

Two Superconducting Gaps

E2g Phonon

σ States

Gaps vs. T

el-ph Coupling

λσσ=1.017 λσπ=0.213


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

π States

Choi et al.Nature 418, 758 (2002)


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


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)


Pressure-Temperature Phase Diagram

  • 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)


Sticking Coefficient of Mg




Mg Sticking Coefficient







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)



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


High-Temperature Ex-Situ Annealing





~ 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


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)


Intermediate-Temperature In-Situ Annealing

B, 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


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)


Low-Temperature In-Situ Deposition

B, Mg





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)


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)


High- and Intermediate-Temperature In-Situ Deposition

B, Mg

High and





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)


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))


MgB2 Films by Reactive Co-Evaporation

4” MgB2 film on polycrystalline alumina

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


Hybrid Physical-Chemical Vapor Deposition

Schematic View

H2, B2H6



  • 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


low Mg sticking no Mg deposit


Hybrid Physical-Chemical Vapor Deposition

(Dan Lamborn)

Velocity Distribution


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

MgB2/SiC (0001)

MgO Regions

Epitaxial Growth on Sapphire and SiC


a = 3.086 Å


a = 4.765 Å


a = 3.07 Å

MgB2/Al2O3 (0001)


No MgO



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)


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)


Very Clean HPCVD MgB2 Films: RRR > 80

Mean free length is limited by the film thickness.


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)


Rowell Model of Connectivity


REC Film

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


  • 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)


Films with Poor Connectivity

Intermediate-T Annealing

Low-TIn Situ Film


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


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

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)


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


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.

Absence of Dendritic Magnetic Instability

in Clean HPCVD Films

Flux Entry

Remnant State

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


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.

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


Deposition Temperature Dependence

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

Possible Substrates or Buffer layers

for MgB2 Films

Result of Thermodynamic Calculations: Reactivity



50 μm





50 μm




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)


MgB2 (1,1,2)


Polycrystalline MgB2 Coated-Conductor Fiber


X-ray diffraction


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

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)


HPCVD MgB2 Films on Metal Substrates

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


Morphology of MgB2 Films on Stainless Steel

Higher deposition temperature. Lower growth rate.

Lower deposition temperature. Higher growth rate.


Degradation of HPCVD MgB2 Films in Water

Room Temperature


  • 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.

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.


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))


Integrated HPCVD System

CVD #2




CVD #1



  • 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



  • 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