High Purity MgB

2

Thin Films

Xiaoxing Xi

Department of Physics and Department of Materials Science and Engineering Penn State University, University Park, PA Supported by ONR, NSF

October 10, 2006 Thin Film RF Workshop Padua, Italy

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

ELECTRONICS

1.0

0.5

MgB 2 /TiB 2 planar junction T = 28 K RF f = 29.5 GHz

0.0

no RF

HIGH FIELD MgB 2 MgB 2

-9 dBm -2 dBm -0.5

Nb 3 Sn

-1.0

-0.4

-0.2

0.0

V

(mV) 0.2

0.4

— 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 — — — 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

MgB 2 : Two Superconducting Gaps Two Superconducting Gaps E 2g Phonon σ States π States

Choi

et al. Nature

418, 758 (2002)

el-ph Coupling λ σσ =1.017

λ πσ =0.155

λ λ σπ ππ =0.213

=0.448

(Golubov et al.

J. Phys.: Condens. Matter

14, 1353 (2002).)

Gaps vs. T

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.

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

Pressure-Composition Phase Diagram P-x Phase Diagram at 850 °C 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

.

Liu

et al

., APL 78, 3678 (2001)

Pressure-Temperature Phase Diagram

Automatic composition control:

P-T

diagram the same for all Mg:B ratio above 1:2.

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 0.12

T

melt (Flynn,

J. Phys. F T

sub 18, L195 (1988)) ≈ — For MgB 2  0.5

T

melt ~ 1080 ° C.

Requires

11 Torr

Mg vapor pressure Or

F



P

2 

m k B T

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.

Liu

et al

., APL 78, 3678 (2001)

Sticking Coefficient of Mg

1.0

0.8

0.6

0.4

0.2

0 200 300 Temperature ( °C) 400

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

-6x10 5 -7x10 5 -8x10 5 -9x10 5 -1x10 6 -1x10 6 1 atm O 2 Si Mg 400 600 800 1000 1200 1400 Temperature (K) (Zi-Kui Liu, PSU) Mg reacts strongly with oxygen: — — reduces Mg vapor pressure forms MgO - small grain size, insulating grain boundaries

C-doped single crystals

Lee et al. Physica C397, 7 (2003) Carbon contamination reduces

T c

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

MgB 2 Films by High-T Ex-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

MgB 2 Films by Intermediate-T In-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)

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 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, LaAlO 3 , NdGaO 3 , LaGaO 3 , LSAT, SrTiO 3 , YSZ, etc.

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

MgB 2 Films by Reactive Co-Evaporation 4” MgB 2 film on polycrystalline alumina

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

Hybrid Physical-Chemical Vapor Deposition Schematic View H 2 , B 2 H 6 Mg Susceptor

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)

rid of oxygen prevent oxidation make high Mg pressure possible generate high Mg pressure high enough T For epitaxy

• Start flow of B 2 H 6 ppm in H to grow.

2 ): mixture (1000 25 - 250 sccm. Film starts

pure source of B

•Total flow: 400 sccm - 1 slm

control growth rate

• Deposition rate: 3 57 Å/sec • Switch off B 2 H 6 flow, turn off heater.

low Mg sticking no Mg deposit

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 / Al 2 O 3 (0001) MgB 2 a = 3.086 Å MgB 2 / SiC (0001) Al 2 O 3 a = 4.765 Å 4H-SiC a = 3.07 Å MgB 2

MgO Regions 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)

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)

6 8

Very Clean HPCVD MgB 2 Films: RRR > 80

0.10

Mean free length is limited by the film thickness.

0.05

4 2 0 0 0.00

39.5

40.0

40.5

T

(K) 41.0

41.5

053105a MgB 2 /sapphire Thickness 770 nm 50 100 150 200 Temperature (K) 250 300 1.5

4000 Thickness (Å) 2000 1000 1.0

0.5

0.0

0.0

5.0x10

-4 1.0x10

-3 1/Thickness (1/Å)

Clean HPCVD MgB 2 Films: Potential Low R

s

(BCS)

R s

(BCS) versus (

ρ 0

, T

c

) π Gap

Vaglio,

Particle Accelerators

61, 391 (1998) Pickett,

Nature

418, 733 (2002)

σ Gap

6 4 8 10 8 6 4 2

HPCVD Film Rowell Model of Connectivity

Rowell,

SC Sci. Tech

. 16, R17 (2003) — Residual resistivity: impurity, surface, and defects —

Δρ ≡ ρ(300K) - ρ(50K):

coupling,

roughly 8 μΩcm

electron-phone 

ρ

   0      

A A

   0     — If

Δρ

is larger : actual area

A’

total area

A

smaller than 0 0 100 150 200 250 300 Temperature (K)

HPCVD films: grains well connected.

High-T Annealed Film REC Film

2 2 0 0 50 100 150 200 Temperature (K) 250

M03044a

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

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

Clean MgB 2 : Weak Pinning and Low H

c2

10 8 10 7 H(T) 0 0.05

0.1

0.2

0.5

1 10 6 2 3 10 5 4 Pure MgB 2 /6H-SiC 20 15 10 H // ab H // c 5 10 4 0 5 10 15 20 25 30 Temperature (K) 35 40 0 0 10 20

T

(K) 30

J c

(0 K) ~3.5 x 10 7 A/cm 2 is nearly 0.1

J d

(0 K), which is 4 x 10 8 A/cm 2 40

10 7 4.2 K,

H



ab

10 6

C-Alloyed MgB 2 : Strong Pinning and High H

c2

pure 7.4% C 12% C 15% C 10 5 10 4 0 2 4 6

μ 0 H

(T) 8 10 — — Pinning enhanced by carbon alloying.

—

Carbon alloying:

H c2

scattering mixing (C 5 H 5 ) 2 Mg in the carrier gas. enhanced to over 60 T, due to modification of interband and intraband

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

λ L

: 34.5 nm Jin

et al

, SC Sci. Tech. 18, L1 (2005)

Surface Morphology with N 2 Addition

Pure MgB 2 : RMS = 3.64 nm 5 sccm: RMS = 0.96 nm 10 sccm: RMS = 1.01 nm 15 sccm: RMS = 1.73 nm 30 sccm: RMS = 5.58 nm 100 sccm: RMS = 8.21 nm

12 10 8 6 4 2 0 0 10

N 2 Addition in HPCVD Reduces Roughness

41.0

Thickness: 1000 Å 8 40.5

6 40.0

4 39.5

2 0 0 Total flow rate: 700 sccm 20 40 60 N 2 Flow Rate (sccm) 80 100 39.0

0 20 40 60 N 2 Flow Rate (sccm) 80 20 40 60 N 2 Flow Rate (sccm) 80 100 14 12 10 8 6 4 2 0 0 20 40 60 N 2 Flow Rate (sccm) 80 100 100

Dendritic Magnetic Instability in MgB 2 Films

Johanson

et al. Europhys. Lett.

59, 599 (2002) — 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 Entry Remnant 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 20

T c

=40.3 K 15 10 5 0 0 50 100 150 T(K) 200 250 300

Deposition Temperature Dependence

—

T c

does not change much with deposition temperature — Residual resistivity increases at lower temperature — Crystallinity degraded at lower temperature 2.5

2.0

1.5

1.0

0.5

0.0

500 550 600 650 700 Deposition Temperature(oC) 42 4 41 3 40 2 39 1 38 500 550 600 650 Deposition Temperature ( o C) 700 0 500 550 600 650 700 Deposition Temperature( o C)

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

50 μm Polycrystalline MgB 2 Coated-Conductor Fiber SEM X-ray diffraction a MgB 2 W 5 μm (a) 50 μm SiC (b)

1000

b

*

(c)

100 * * * * * 10 30 40 50 60 2  (degrees) 70 80 90

60

MgB 2 Coated Conductors: High H

c2

and H

irr

Upper Critical Field (0.9R

0 ) Irreversibility Field (0.1R

0 )

40 Alloyed fiber #2 Alloyed fiber #2 40 30 20 20 Alloyed fiber #1 10 Clean fiber Clean fiber 0 0 10 20

T

(K) 30 40 0 0 10 — Similar to

H c2

and

H irr

in parallel field in thin films .

Alloyed fiber #1 20

T

(K) 30 40 — No epitaxy or texture necessary

10 7

Polycrystalline MgB 2 Films on Flexible YSZ

— —

T c J c

= 38.9 K.

high. Insensitive to bending — Low

R s

similar to epitaxial films on sapphire substrate observed.

10 6 10 5 10 4 0 MgB 2 /YSZ flexible 070705a transport 070705b6 bent, transport 050306b magnetization 5 10 15 20 25 30 Temperature (K) 35 40

R s

measured by A. Findikoglu (LANL)

0.008

0.006

0.004

HPCVD MgB 2 Films on Metal Substrates

MgB 2 /Stainless Steel 0.004

2.0

1.5

1.0

0.15

0.10

0.05

0.00

36 37 38 39 40 41

T

(K) 0.002

0.002

0.000

0 50 100 0.000

36 37 38 39 40 41

T

(K) 150 200 Temperature (K) 250 300 0.5

MgB 2 /Nb 0.0

0 50 100 150 200 Temperature (K) 250 300 High

T c

has been obtained in polycrystalline MgB 2 steel, Nb, TiN, and other substrates.

films on stainless

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

20 15 10 0 °C 5 0 0 1 2 3 4 Time (hour) 5 6 7 ― 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.

Stability of RCE MgB 2 Films in Water

T c

After 42 hrs t =400 nm After 20 hrs t =440 nm As grown t = 550 nm

T c T c

300 (Brian Moeckly. STI) Compared to the HPCVD films, MgB 2 films deposited by reactive co evaporation are much more stable against degradation in water.

Point-Contact Spectroscopy on MgB 2 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

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

smooth surface (RMS roughness < 10 (10% depairing current) low surface resistance, short penetration depth 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