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 • 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
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)
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 • 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)
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)
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
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
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 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
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 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)
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 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)
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 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
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 MgB2 a = 3.086 Å Al2O3 a = 4.765 Å 4H-SiC a = 3.07 Å MgB2/Al2O3 (0001) MgB2 No MgO 6H-SiC
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 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)
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 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
N2 Addition in HPCVD Reduces Roughness Thickness: 1000 Å
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
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
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 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.
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 Transfer Chamber Sputtering CVD #1