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

MgB 2 : An Exciting Superconductor SCIENCE — T c = 40 K, BCS superconductor (2001) — Two bands with weak inter-band scattering: 2D σ band and 3D π band — Two gaps: A superconductor with two order parameters — Low material cost, easy manufacturing — High performance in field (H c2 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 MgB 2 — 25 K operation, much less cryogenic requirement than LTS Josephson junctions — Superconducting digital circuits HIGH FIELD

MgB 2 : Two Superconducting Gaps Choi et al. Nature 418, 758 (2002) σ States π States E 2g Phonon Two Superconducting Gaps Gaps vs. T el-ph Coupling λ σσ =1.017 λ σπ =0.213 λ πσ =0.155 λ ππ =0.448 (Golubov et al. J. Phys.: Condens. Matter 14, 1353 (2002).)

Oates, Agassi, and Moeckly, ASC 2006 Proceeding, submitted MgB 2 : Promising at Microwave Frequency — Higher T c, low resistivity, larger gap, higher critical field than Nb. — It has been predicted theoretically that nonlinearity in MgB 2 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+MgB 2. If deposition is to take place at 850°C, Mg partial pressure has to be above 340 mTorr to keep the MgB 2 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 MgB 2 composition. Pressure-Composition Phase Diagram P-x Phase Diagram at 850°C Liu et al., APL 78, 3678 (2001)

PHASE STABILITY — Mg pressure for the process window is very high — Typically, optimal epitaxy T sub ≈ 0.5 T melt (Yang and Flynn, PRL 62, 2476 (1989)) — Minimum T sub for metal epitaxy is T sub ≈ 0.12 T melt (Flynn, J. Phys. F 18, L195 (1988)) — For MgB 2  0.5 T melt ~ 1080 °C. Requires 11 Torr Mg vapor pressure Or Mg flux of 2x10 21 Mg atoms/(cm 2 ·s), or 0.5 mm/s Too high for most vacuum deposition techniques  0.12 T melt ~ 50 °C. Pressure-Temperature Phase Diagram Automatic composition control: P-T diagram the same for all Mg:B ratio above 1:2. Liu et al., APL 78, 3678 (2001)

Temperature (°C) Mg Sticking Coefficient Sticking Coefficient of Mg Kim et al, IEEE Trans. Appl. Supercond. 13, 3238 (2003) Mg sticking coefficient drops to near zero above 300°C. Not many Mg available to react with B.

Contaminations Mg 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 Oxygen Carbon contamination reduces T c

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 °C in Mg Vapor Epitaxial Films

Kang et al, Science 292, 1521 (2001) Berenov et al, APL 79, 4001 (2001) MgB 2 Films by High-T Ex-Situ Annealing — Epitaxial films — Good superconducting properties

Intermediate-Temperature In-Situ Annealing 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) Low Temperature ~ 600 °C in situ Nanocrystalline Films B, Mg Mg

MgB 2 Films by Intermediate-T In-Situ Annealing Zeng 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 TEM Superconducting Transition

Low-Temperature In-Situ Deposition 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) Low Temperature Textured Films B, Mg

Ueda & Makimoto, JJAP 45, 5738 (2006) MgB 2 Films by Low-T In-Situ Deposition Ueda & Naito, APL 79, 2046 (2001) — UHV conditions — Superconducting films below about 300°C — Good superconducting properties

High- and Intermediate-Temperature In-Situ Deposition 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) High and Intermediate Temperature Epitaxial Films B, Mg

(Moeckly & Ruby, SC Sci Tech 19, L21 (2006)) 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, LaAlO 3, NdGaO 3, LaGaO 3, LSAT, SrTiO 3, YSZ, etc.

4” MgB 2 film on polycrystalline alumina (Moeckly & Ruby, SC Sci Tech 19, L21 (2006)) MgB 2 Films by Reactive Co-Evaporation

Hybrid Physical-Chemical Vapor Deposition Deposition procedure and parameters: Purge with N 2, H 2 Carrier gas: H 2 P total = 100 Torr. Inductively heating susceptor, AND Mg, to 550–760 °C. P Mg = ? (44 mTorr is needed at 750 °C according to thermodynamics) Start flow of B 2 H 6 mixture (1000 ppm in H 2 ): sccm. Film starts to grow. Total flow: 400 sccm - 1 slm Deposition rate: Å/sec Switch off B 2 H 6 flow, turn off heater. H 2, B 2 H 6 Mg Susceptor Schematic View rid of oxygen prevent oxidation make high Mg pressure possible generate high Mg pressure pure source of B control growth rate low Mg sticking no Mg deposit high enough T For epitaxy

Hybrid Physical-Chemical Vapor Deposition Velocity Distribution (Dan Lamborn)

Epitaxial Growth of MgB 2 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 MgB 2 /SiC (0001) MgB 2 /Al 2 O 3 (0001) MgB 2 a = Å Al 2 O 3 a = Å 4H-SiC a = 3.07 Å MgB 2 6H-SiC No MgO MgO Regions

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 TEM Pogrebnyakov et al. PRL 93, (2004)

Volmer-Weber Growth Mode of MgB 2 Films

Coalescence of Islands in MgB 2 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 MgB 2 Films: RRR > 80 Mean free length is limited by the film thickness.

Clean HPCVD MgB 2 Films: Potential Low R s (BCS) Pickett, Nature 418, 733 (2002) R s (BCS) versus (ρ 0, T c ) π Gapσ Gap Vaglio, Particle Accelerators 61, 391 (1998)

ρρ Rowell Model of Connectivity — Residual resistivity: impurity, surface, and defects — Δρ ≡ ρ(300K) - ρ(50K): electron-phone coupling, roughly 8 μΩcm — If Δρ 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 Film HPCVD Film REC Film Rowell, SC Sci. Tech. 16, R17 (2003)

Intermediate-T Annealing Low-T In Situ Film Films with Poor Connectivity

Clean MgB 2 : Weak Pinning and Low H c2 J c (0 K) ~3.5 x 10 7 A/cm 2 is nearly 0.1J d (0 K), which is 4 x 10 8 A/cm 2

C-Alloyed MgB 2 : Strong Pinning and High H c2 — Carbon alloying: mixing (C 5 H 5 ) 2 Mg in the carrier gas. — Pinning enhanced by carbon alloying. — H c2 enhanced to over 60 T, due to modification of interband and intraband scattering μ 0 H (T) J c (A/cm 2 )

Jin et al, SC Sci. Tech. 18, L1 (2005) Good Microwave Properties in Clean Films Surface 18 GHzπ-Band Gap — Surface resistance decreases with residual resistivity. Clean HPCVD films show low surface resistance. — Interband scattering makes π 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 λ L : 34.5 nm

Surface Morphology with N 2 Addition 100 sccm: RMS = 8.21 nm 30 sccm: RMS = 5.58 nm 15 sccm: RMS = 1.73 nm 10 sccm: RMS = 1.01 nm 5 sccm: RMS = 0.96 nm Pure MgB 2 : RMS = 3.64 nm

N 2 Addition in HPCVD Reduces Roughness Thickness: 1000 Å

Johanson et al. Europhys. Lett. 59, 599 (2002) Dendritic Magnetic Instability in MgB 2 Films — Flux jumps observed at low temperature and low field in many MgB 2 films. — Dendritic magnetic instability observed by magneto-optical imaging.

Absence of Dendritic Magnetic Instability in Clean HPCVD Films Flux EntryRemnant State (Ye et al. APL 85, 5285 (2004))

Absence of Dendritic Magnetic Instability In Clean MgB 2 Films Measurement by Prof. Tom Johansen (Oslo): — Measurement down to 3.5 K — Spacer between the MgB 2 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 MgB 2 films.

Epitaxial MgB 2 Film Grown at 550 °C — Film is epitaxial, but with a broader rocking curve — There is a small amount of 30° in- plane twinning — T c remains high, but residual resistivity is higher than the standard films T c =40.3 K

Deposition Temperature Dependence — T c does not change much with deposition temperature — Residual resistivity increases at lower temperature — Crystallinity degraded at lower temperature

Possible Substrates or Buffer layers for MgB 2 Films Result of Thermodynamic Calculations: Reactivity

Polycrystalline MgB 2 Coated-Conductor Fiber a b * * * * * * MgB 2 (1,0,1) MgB 2 (0,0,2) MgB 2 (1,0,0) MgB 2 (1,1,2) Mg 2 Si (2,2,0) Mg 2 Si (4,0,0) Mg 2 Si (4,2,2) Mg 2 Si (4,4,0) SEMX-ray diffraction 5 μm (a) 50 μm W SiC MgB 2 (b) (c)

MgB 2 Coated Conductors: High H c2 and H irr — Similar to H c2 and H irr in parallel field in thin films. — No epitaxy or texture necessary Upper Critical Field (0.9R 0 )Irreversibility Field (0.1R 0 )

Polycrystalline MgB 2 Films on Flexible YSZ — T c = 38.9 K. — J c high. Insensitive to bending — Low R s similar to epitaxial films on sapphire substrate observed. R s measured by A. Findikoglu (LANL)

HPCVD MgB 2 Films on Metal Substrates High T c has been obtained in polycrystalline MgB 2 films on stainless steel, Nb, TiN, and other substrates.

Morphology of MgB 2 Films on Stainless Steel Higher deposition temperature. Lower growth rate. Lower deposition temperature. Higher growth rate.

Degradation of HPCVD MgB 2 Films in Water ― Film properties degrade with exposure to air/moisture: resistance goes up, T c goes down ― Experiments show that MgB 2 degrades quickly in water, and is sensitive to temperature. Room Temperature 0°C

(Brian Moeckly. STI) Stability of RCE MgB 2 Films in Water Compared to the HPCVD films, MgB 2 films deposited by reactive co- evaporation are much more stable against degradation in water.

(Park and Greene, Rev. Sci. Instr. 77, (2006)) Point-Contact Spectroscopy on MgB 2 Films HPCVD film: Andreev-Reflection- like. Metallic surface. RCE film: tunneling-like. Surface with tunnel barrier.

Integrated HPCVD System CVD #1 CVD #2 Sputtering Transfer Chamber

Conclusion ― Keys to high quality MgB 2 thin films:  high Mg pressure for thermodynamic stability of MgB 2  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 MgB 2 thin films have excellent properties:  low resistivity (<0.1 μΩ) and long mean free path  high T c ~ 42 K (due to tensile strain), high J c (10% depairing current)  low surface resistance, short penetration depth  smooth surface (RMS roughness < 10 Å with N 2 addition)  good thermal conductivity (free from dendritic magnetic instability) Mean free path can be adjusted by carbon doping ― Polycrystalline films maintain good properties ― MgB 2 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