High Purity MgB2 Thin Films

October 10, 2006Thin 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

MgB2: An Exciting SuperconductorSCIENCE— Tc = 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 (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

-1.0

-0.5

0.0

0.5

1.0

-0.4 -0.2 0.0 0.2 0.4

-2 dBm

-9 dBm

V (mV)

I (m

A)

no RF

MgB2/TiB

2

planar junctionT = 28 KRF f = 29.5 GHz

0 10 20 30 400

10

20

30

40

50

60

NbTi Nb3Sn

MgB2

Fie

ld (

T)

Temperature (K)

MgB2

//

0 10 20 30 400

10

20

30

40

50

60

NbTi Nb3Sn

MgB2

Fie

ld (

T)

Temperature (K)

MgB2

//

MgB2: Two Superconducting Gaps

Choi et al. Nature 418, 758 (2002)

σ States

π States

E2g Phonon

Two Superconducting Gaps

Gaps vs. T

el-ph Coupling

λσσ=1.017 λσπ=0.213

λπσ=0.155 λππ=0.448

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

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

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.

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.

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 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/(cm2·s), or 0.5 mm/s

Too high for most vacuum deposition techniques

0.12 Tmelt ~ 50 °C.

F P

2 m kB T

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)

400300200

1.0

0.8

0.6

0.4

0.2

0

Temperature (°C)

Mg

Stic

king

Coe

ffic

ient

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.

400 600 800 1000 1200 1400

-1x106

-1x106

-9x105

-8x105

-7x105

-6x105

Gib

bs E

nerg

y (J

/mol

e O

2)

Temperature (K)

Si

Mg

1 atm O2

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 Tc

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 °Cin Mg Vapor

Epitaxial 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 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 °Cin situ

Nanocrystalline Films

B, Mg

Mg

MgB2 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 TEMSuperconducting 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

TexturedFilms

B, Mg

Ueda & Makimoto, JJAP 45, 5738 (2006)

MgB2 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

EpitaxialFilms

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, 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 Deposition

Deposition procedure and parameters:

• Purge with N2, H2

• Carrier gas: H2

• Ptotal = 100 Torr.

• Inductively heating susceptor, AND Mg, to 550–760 °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.

H2, B2H6

Mg

Susceptor

Schematic View rid of oxygenprevent oxidation

make high Mgpressure possible

generate high Mg pressure

pure source of B

control growth rate

low Mg sticking no Mg deposit

high enough TFor epitaxy

Hybrid Physical-Chemical Vapor Deposition

Velocity Distribution

(Dan Lamborn)

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/SiC (0001) MgB2/Al2O3 (0001) MgB2

a = 3.086 Å

Al2O3

a = 4.765 Å

4H-SiCa = 3.07 Å

MgB2

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, 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

0 50 100 150 200 250 3000

2

4

6

8

39.5 40.0 40.5 41.0 41.50.00

0.05

0.10

(cm

)

T (K)

Res

istiv

ity (

cm)

Temperature (K)

053105aMgB

2/sapphire

Thickness 770 nm

Mean free length is limited by the film thickness.

0.0 5.0x10-4 1.0x10-30.0

0.5

1.0

1.5

Thickness (Å)4000 1000

(

cm

)

1/Thickness (1/Å)

2000

Clean HPCVD MgB2 Films: Potential Low Rs (BCS)

Pickett, Nature 418, 733 (2002)

Rs (BCS) versus (ρ0, Tc)

π Gap σ Gap

Vaglio, Particle Accelerators 61, 391 (1998)

ρ

Rowell Model of Connectivity

0

0A

A

— 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. 0 50 100 150 200 250 3000

2

4

6

8

R

esis

tivity

(

cm)

Temperature (K)

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

High-T Annealed Film

HPCVD Film

0

2

4

6

8

10

0 50 100 150 200 250 300

M03044a

Resistivity

Res

istiv

ity (

c

m)

Temperature (K)

MgB2 on polycrystalline aluminaREC Film

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

Intermediate-T AnnealingLow-T In Situ Film

Films with Poor Connectivity

0 5 10 15 20 25 30 35 40

104

105

106

107

108

Pure MgB2/6H-SiC

4

3

2

1

0.5

0.20.1

00.05

H(T)

Temperature (K)

J c (A

/cm

2 )

0 10 20 30 400

5

10

15

20

Hc2

(T)

T (K)

H // ab H // c

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

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

μ0H (T)

J c (

A/c

m2 )

0 2 4 6 8 10104

105

106

107

pure 7.4% C 12% C 15% C

4.2 K, H ab

Jin et al, SC Sci. Tech. 18, L1 (2005)

Good Microwave Properties in Clean Films

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.

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 λL: 34.5 nm

Surface Morphology with N2 Addition

100 sccm: RMS = 8.21 nm30 sccm: RMS = 5.58 nm15 sccm: RMS = 1.73 nm

10 sccm: RMS = 1.01 nm5 sccm: RMS = 0.96 nmPure MgB2: RMS = 3.64 nm

N2 Addition in HPCVD Reduces Roughness

Thickness: 1000 Å

0 20 40 60 80 1000

2

4

6

8

10

R

MS

Ro

ug

hn

ess

(n

m)

N2 Flow Rate (sccm)

Total flow rate: 700 sccm

0 20 40 60 80 10039.0

39.5

40.0

40.5

41.0

Tc(0

) (K

)

N2 Flow Rate (sccm)

0 20 40 60 80 1000

2

4

6

8

10

12

RR

R

N2 Flow Rate (sccm)

0 20 40 60 80 1000

2

4

6

8

10

12

14

0 (c

m)

N2 Flow Rate (sccm)

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 Films

Flux Entry Remnant State

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

Absence of Dendritic Magnetic InstabilityIn 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

0 50 100 150 200 250 3000

5

10

15

20

Re

sist

ivity

(

cm)

T(K)

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

500 550 600 650 7000.0

0.5

1.0

1.5

2.0

2.5

FW

HM

(de

g)

Deposition Temperature(oC)

500 550 600 650 70038

39

40

41

42

Tco

(K)

Deposition Temperature (oC)

500 550 600 650 7000

1

2

3

4

Ris

istiv

ity(

cm)

Deposition Temperature(oC)

Possible Substrates or Buffer layersfor MgB2 Films

Result of Thermodynamic Calculations: Reactivity

Polycrystalline MgB2 Coated-Conductor Fiber

a

b

30 40 50 60 70 80 9010

100

1000

Inte

nsity (

a.u

.)

2 (degrees)

*

*

*

**

*

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)

SEM X-ray diffraction

5 μm

(a)

50 μm

50 μm

W

SiC

MgB2

(b)

(c)

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)

0 10 20 30 400

20

40

60

Alloyed fiber #2

Alloyed fiber #1

0Hc2

(T

)

T (K)

Clean fiber

0 10 20 30 400

10

20

30

40

Alloyed fiber #2

Alloyed fiber #1

0H

irr (

T)

T (K)

Clean fiber

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)

0 5 10 15 20 25 30 35 40104

105

106

107

J c (

A/c

m2 )

Temperature (K)

MgB2/YSZ

flexible

070705a transport 070705b6 bent, transport 050306b magnetization

HPCVD MgB2 Films on Metal Substrates

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

0 50 100 150 200 250 3000.000

0.002

0.004

0.006

0.008

36 37 38 39 40 410.000

0.002

0.004

R ()

T (K)

Re

sist

ance

(O

hm

s)

Temperature (K)

MgB2/Stainless Steel

0 50 100 150 200 250 3000.0

0.5

1.0

1.5

2.0

36 37 38 39 40 410.00

0.05

0.10

0.15

R (

x 1

04

)

T (K)

Res

ista

nce

( x

104

)

Temperature (K)

MgB2/Nb

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

― 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 Temperature

0°C

36 38 40 42 44

0.01

0.1

1

10

Re

sis

tan

ce

()

Temperature (K)

0 min3060

90

120

150 minIn water, RT

36 38 40 42 44

0.01

0.1

1

10

Re

sis

tan

ce

()

Temperature (K)

0 min3060

90

120

150 minIn water, RT

0 1 2 3 4 5 6 70

5

10

15

20

R/R

(0)

Time (hour)

(Brian Moeckly. STI)

Stability of RCE MgB2 Films in Water

Compared to the HPCVD films, MgB2 films deposited by reactive co-evaporation are much more stable against degradation in water.

0

5

10

15

20

25

30

0 50 100 150 200 250 300

M03049d

Res

istiv

ity (

cm

)

Temperature (K)

As grownt = 550 nm

After 20 hrst =440 nm

After 42 hrst =400 nm

Tc = 38.0 K

Tc = 38.5 KTc = 38.9 K

0

5

10

15

20

25

30

0 50 100 150 200 250 300

M03049d

Res

istiv

ity (

cm

)

Temperature (K)

As grownt = 550 nm

After 20 hrst =440 nm

After 42 hrst =400 nm

Tc = 38.0 K

Tc = 38.5 KTc = 38.9 K

(Park and Greene, Rev. Sci. Instr. 77, 023905 (2006))

Point-Contact Spectroscopy on MgB2 Films

HPCVD film: Andreev-Reflection-like.

Metallic surface.

RCE film: tunneling-like.

Surface with tunnel barrier.

Integrated HPCVD System

CVD #1

CVD #2

Sputtering

TransferChamber

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 (<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