High Purity MgB2 Thin FilmsOctober 10, 2006Thin Film RF WorkshopPadua, ItalyDepartment of Physics and Department of Materials Science and EngineeringPenn State University, University Park, PAXiaoxing XiSupported 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, junctionsJoan Redwing Group (Materials Sci & Eng): HPCVD growth, modelingQi Li Group (Physics): Junctions, transport and magnetic measurementsDarrell Schlom Group (Materials Sci & Eng): structural analysis Zi-Kui Liu Group (Materials Sci & Eng): ThermodynamicsXiaoqing Pan Group (U. Michigan): Cross-Section TEM

John Spence Group (ASU): TEMN. Klein Group (Jlich): Microwave measurementA. Findikoglu (LANL): Microwave measurementQiang Li Group (Brookhaven National Lab): Magneto-optic measurementTom Johansen Group (U Oslo): Magneto-optic measurement Qing-Rong Feng Group (Peking University): SiC fiberChang-Beom Eom Group (U Wisconsin): Structural analysisJ. B. Betts and C. H. Mielke (LANL): High field measurement

MgB2: An Exciting SuperconductorSCIENCE Tc = 40 K, BCS superconductor (2001) Two bands with weak inter-band scattering: 2D band and 3D 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 transformersELECTRONICS

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 circuitsHIGH FIELD

MgB2: Two Superconducting GapsChoi et al. Nature 418, 758 (2002) States StatesE2g PhononTwo Superconducting GapsGaps vs. Tel-ph Coupling

=1.017 =0.213

=0.155 =0.448

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

Oates, Agassi, and Moeckly, ASC 2006 Proceeding, submittedMgB2: 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.

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

If deposition is to take place at 850C, 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.

Pressure-Composition Phase DiagramP-x Phase Diagram at 850CLiu et al., APL 78, 3678 (2001)

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 pressureOr

Mg flux of 2x1021 Mg atoms/(cm2s), or 0.5 mm/s

Too high for most vacuum deposition techniques

0.12 Tmelt ~ 50 C. Pressure-Temperature Phase DiagramAutomatic composition control: P-T diagram the same for all Mg:B ratio above 1:2.Liu et al., APL 78, 3678 (2001)

Sticking Coefficient of MgKim et al, IEEE Trans. Appl. Supercond. 13, 3238 (2003)Mg sticking coefficient drops to near zero above 300C.

Not many Mg available to react with B.

ContaminationsMg reacts strongly with oxygen:

reduces Mg vapor pressure forms MgO - small grain size, insulating grain boundaries(Zi-Kui Liu, PSU)Lee et al. Physica C397, 7 (2003)C-doped single crystalsReaction with OxygenCarbon contamination reduces Tc

High-Temperature Ex-Situ AnnealingKang 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)BMgLow Temperature~ 850 Cin Mg VaporEpitaxial Films

Kang et al, Science 292, 1521 (2001)Berenov et al, APL 79, 4001 (2001)MgB2 Films by High-T Ex-Situ Annealing Epitaxial films Good superconducting properties

Intermediate-Temperature In-Situ AnnealingBlank 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)Low Temperature~ 600 Cin situNanocrystalline FilmsB, MgMg

MgB2 Films by Intermediate-T In-Situ AnnealingZeng et al, APL 79, 4001 (2001) Mg vapor pressure varies with time difficult to control Nano-crystalline with oxygen contamination Superconducting properties fair.Cross-Sectional TEMSuperconducting Transition

Low-Temperature In-Situ DepositionUeda & 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)Low TemperatureTexturedFilmsB, Mg

Ueda & Makimoto, JJAP 45, 5738 (2006)MgB2 Films by Low-T In-Situ DepositionUeda & Naito, APL 79, 2046 (2001) UHV conditions Superconducting films below about 300C Good superconducting properties

High- and Intermediate-Temperature In-Situ DepositionUeda & 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)High and Intermediate TemperatureEpitaxialFilmsB, Mg

(Moeckly & Ruby, SC Sci Tech 19, L21 (2006))Reactive Co-Evaporation Deposition temperature 550C 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.

4 MgB2 film on polycrystalline alumina(Moeckly & Ruby, SC Sci Tech 19, L21 (2006))MgB2 Films by Reactive Co-Evaporation

Hybrid Physical-Chemical Vapor DepositionDeposition procedure and parameters:

Purge with N2, H2

Carrier gas: H2

Ptotal = 100 Torr.

Inductively heating susceptor, AND Mg, to 550760 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 /sec

Switch off B2H6 flow, turn off heater. rid of oxygenprevent oxidationmake high Mgpressure possiblegenerate high Mg pressurepure source of Bcontrol growth ratelow Mg sticking no Mg deposithigh enough TFor epitaxy

Hybrid Physical-Chemical Vapor DepositionVelocity 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

Epitaxial Growth on Sapphire and SiC MgB2/Al2O3 (0001) MgB2a = 3.086

Al2O3a = 4.765

4H-SiCa = 3.07 MgB26H-SiCNo MgO

Defects in Epitaxial Films on SiC There are more defects at the film/substrate interface than in the top part of the film.High-Resolution TEMLow-Resolution TEMPogrebnyakov et al. PRL 93, 147006 (2004)

Volmer-Weber Growth Mode of MgB2 Films

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) Pickett, Nature 418, 733 (2002)Rs (BCS) versus (0, Tc) Gap Gap Vaglio, Particle Accelerators 61, 391 (1998)

Rowell Model of Connectivity Residual resistivity: impurity, surface, and defects (300K) - (50K): electron-phone coupling, roughly 8 cm

If is larger : actual area A smaller than total area A

HPCVD films: grains well connected. Bu et al., APL 81, 1851 (2002)High-T Annealed FilmHPCVD FilmRowell, SC Sci. Tech. 16, R17 (2003)

Intermediate-T AnnealingLow-T In Situ FilmFilms with Poor Connectivity

Clean MgB2: Weak Pinning and Low Hc2Jc (0 K) ~3.5 x 107 A/cm2 is nearly 0.1Jd (0 K), which is 4 x 108 A/cm2

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

Jin et al, SC Sci. Tech. 18, L1 (2005)Good Microwave Properties in Clean FilmsSurface Resistance @ 18 GHz-Band Gap Surface resistance decreases with residual resistivity. Clean HPCVD films show low surface resistance.

Interband scattering makes band gap larger.Microwave measurement: sapphire resonator technique at 18 GHz.

Jin et al, SC Sci. Tech. 18, L1 (2005)Short Penetration Depth in Clean Films Penetration depth decrease with residual resistivity.

London penetration depth L: 34.5 nm

Surface Morphology with N2 Addition100 sccm: RMS = 8.21 nm30 sccm: RMS = 5.58 nm15 sccm: RMS = 1.73 nm10 sccm: RMS = 1.01 nm5 sccm: RMS = 0.96 nmPure MgB2: RMS = 3.64 nm

N2 Addition in HPCVD Reduces RoughnessThickness: 1000

Johanson et al. Europhys. Lett. 59, 599 (2002)Dendritic Magnetic Instability in MgB2 Films 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 FilmsFlux EntryRemnant State(Ye et al. APL 85, 5285 (2004))

Absence of Dendritic Magnetic InstabilityIn Clean MgB2 FilmsMeasurement 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 550C 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 filmsTc=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 layersfor MgB2 FilmsResult of Thermodynamic Calculations: Reactivity

Polycrystalline MgB2 Coated-Conductor FiberSEMX-ray diffraction

MgB2 Coated Conductors: High Hc2 and Hirr Similar to Hc2 and Hirr in parallel field in thin films .

No epitaxy or texture necessary

Upper Critical Field (0.9R0)Irreversibility Field (0.1R0)

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

Morphology of MgB2 Films on Stainless SteelHigher deposition temperature. Lower growth rate.Lower deposition temperature. Higher growth rate.

Degradation of HPCVD MgB2 Films in Water 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. Room Temperature0C

(Brian Moeckly. STI)Stability of RCE MgB2 Films in WaterCompared to the HPCVD films, MgB2 films deposited by reactive co-evaporation are much more stable against degradation in water.

(Park and Greene, Rev. Sci. Instr. 77, 023905 (2006))Point-Contact Spectroscopy on MgB2 FilmsHPCVD film: Andreev-Reflection-like.

Metallic surface.RCE film: tunneling-like.

Surface with tunnel barrier.

Integrated HPCVD SystemCVD #1CVD #2SputteringTransferChamber

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

Conclusion Clean HPCVD MgB2 thin films have excellent properties: low resistivity (