High Purity MgB2 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
MgB2: An Exciting Superconductor
SCIENCE
— 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
ELECTRONICS
HIGH FIELD
1.0
60
MgB2/TiB2
planar junction
T = 28 K
RF f = 29.5 GHz
no RF
50
MgB2 //
40
0.0
Field (T)
I (mA)
0.5
-2 dBm
-9 dBm
MgB2
30
20
-0.5
NbTi
10
-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 MgB2
— 25 K operation, much less cryogenic
requirement than LTS Josephson junctions
— Superconducting digital circuits
0
0
Nb3Sn
10
20
30
40
Temperature (K)
— Low material cost, easy manufacturing
— High performance in field (Hc2 over 60 T)
— High field magnets for NMR/MRI; highenergy physics, fusion, MAGLEV, motors,
generators, and transformers
MgB2: Two Superconducting Gaps
Two Superconducting Gaps
E2g Phonon
σ States
Gaps vs. T
el-ph Coupling
π States
λσσ=1.017
λσπ=0.213
λπσ=0.155
λππ=0.448
(Golubov et al. J. Phys.:
Condens. Matter 14, 1353
(2002).)
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.
P-x Phase Diagram at 850°C
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.
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
F
P
2 m kB T
Mg flux of 2x1021 Mg atoms/(cm2·s),
or 0.5 mm/s
Too high for most vacuum deposition
techniques
Automatic composition control: P-T
diagram the same for all Mg:B ratio
above 1:2.
 0.12 Tmelt ~ 50 °C.
Liu et al., APL 78, 3678 (2001)
Sticking Coefficient of Mg
Mg Sticking Coefficient
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
C-doped single crystals
Gibbs Energy(J/mole O2)
5
-6x10
5
-7x10
1 atm O2
Si
5
-8x10
5
-9x10
6
-1x10
Mg
6
-1x10
400 600 800 1000 1200 1400
Temperature (K)
(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
Mg
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)
Low
Temperature
~ 850 °C
in Mg Vapor
Epitaxial
Films
MgB2 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
MgB2 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)
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
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
Deposition procedure and
parameters:
• Purge with N2, H2
H2, B2H6
• Carrier gas: H2
• Ptotal = 100 Torr.
Mg
• Inductively heating susceptor, AND
Mg, to 550–760 °C. PMg = ? (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 B2H6 mixture (1000
ppm in H2): 25 - 250 sccm. Film starts
to grow.
pure source of B
•Total flow: 400 sccm - 1 slm
control growth
rate
• Deposition rate: 3 - 57 Å/sec
Susceptor
• Switch off B2H6 flow, turn off heater.
low Mg sticking
no Mg deposit
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/Al2O3 (0001)
MgB2
a = 3.086 Å
MgB2/SiC (0001)
Al2O3
a = 4.765 Å
4H-SiC
a = 3.07 Å
MgB2
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 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.
0.10
 (cm)
6
0.05
0.00
39.5
4
40.0
40.5
41.0
41.5
T (K)
053105a
MgB2/sapphire
2
1.5
4000
Thickness (Å)
2000
1000
Thickness 770 nm
0
0
50
100
150
200
Temperature (K)
250
300
(cm)
Resistivity (cm)
8
1.0
0.5
0.0
0.0
-4
5.0x10
-3
1.0x10
1/Thickness (1/Å)
Clean HPCVD MgB2 Films: Potential Low Rs (BCS)
Rs (BCS) versus (ρ0, Tc)
Pickett, Nature 418, 733 (2002)
π Gap
Vaglio, Particle Accelerators 61, 391 (1998)
σ Gap
Rowell Model of Connectivity
Rowell, SC Sci. Tech. 16, R17 (2003)
HPCVD Film
Resistivity (cm)
8
— Residual resistivity: impurity, surface, and
defects
— Δρ ≡ ρ(300K) - ρ(50K): electron-phone
coupling, roughly 8 μΩcm
6
ρ
4
   0  
 
A
  0   
A
— If Δρ is larger : actual area A’ smaller than
total area A
2
0
0
50
Resistivity100 150 200 250 300
HPCVD films: grains well connected.
Temperature (K)
High-T Annealed Film
10
REC Film
MgB on polycrystalline alumina
2
Resistivity ( cm)
8
6
4
2
M03044a
0
0
50
100
150
200
Temperature (K)
250
300
Bu et al., APL 81, 1851 (2002)
Films with Poor Connectivity
Low-T In Situ Film
Intermediate-T Annealing
Clean MgB2: Weak Pinning and Low Hc2
10
10
20
8
H(T)
0
0.05
0.1
7 0.2
Pure MgB2/6H-SiC
H // ab
H // c
15
10
2
6
Hc2(T)
2
Jc (A/cm )
0.5
1
3
10
5
10
4
4
10
5
0
0
5
10
15
20
25
30
Temperature (K)
35
40
0
10
20
30
T (K)
Jc (0 K) ~3.5 x 107 A/cm2 is nearly 0.1Jd (0 K), which is 4 x 108 A/cm2
40
C-Alloyed MgB2: Strong Pinning and High Hc2
7
Jc (A/cm2)
10
4.2 K, H ab
6
10
pure
7.4% C
12% C
15% C
5
10
4
10
0
2
4
6
μ0H (T)
8
10
— 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.
π-Band Gap
Surface Resistance @ 18 GHz
— 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
Pure MgB2: 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
N2 Addition in HPCVD Reduces Roughness
41.0
Thickness: 1000 Å
8
40.5
6
Tc(0) (K)
RMS Roughness (nm)
10
4
40.0
39.5
2
Total flow rate: 700 sccm
0
39.0
0
20
40
60
80
100
0
20
60
80
100
80
100
N2 Flow Rate (sccm)
N2 Flow Rate (sccm)
12
14
10
12
8
10
0 (cm)
RRR
40
6
4
8
6
4
2
0
2
0
20
40
60
N2 Flow Rate (sccm)
80
100
0
0
20
40
60
N2 Flow Rate (sccm)
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° inplane twinning
— Tc remains high, but residual
resistivity is higher than the standard
films
20
Resistivity (cm)
Tc=40.3 K
15
10
5
0
0
50
100
150
T(K)
200
250
300
Deposition Temperature Dependence
— Tc does not change much with
deposition temperature
2.5
FWHM(deg)
2.0
— Residual resistivity increases at
lower temperature
— Crystallinity degraded at lower
temperature
1.5
1.0
0.5
0.0
500
42
Risistivity(cm)
4
41
Tco(K)
550
600
650
700
o
Deposition Temperature( C)
40
39
38
500
550
600
650
700
Deposition Temperature (oC)
3
2
1
0
500
550
600
650
700
o
Deposition Temperature( C)
Possible Substrates or Buffer layers
for MgB2 Films
Result of Thermodynamic Calculations: Reactivity
Polycrystalline MgB2 Coated-Conductor Fiber
SEM
X-ray diffraction
a
50 μm
MgB2
W
SiC
(a)
50 μm
b
(b)
*
*
(c)
Mg2Si (4,4,0)
*
Mg2Si (4,2,2)
Mg2Si (4,0,0)
*
MgB2 (1,1,2)
5 μm
MgB2 (0,0,2)
*
MgB2 (1,0,1)
Mg2Si (2,2,0)
100
MgB2 (1,0,0)
Intensity (a.u.)
1000
*
10
30
40
50
60
2 (degrees)
70
80
90
MgB2 Coated Conductors: High Hc2 and Hirr
Upper Critical Field (0.9R0)
Irreversibility Field (0.1R0)
60
40
Alloyed fiber #2
Alloyed fiber #2
40
irr
Alloyed fiber #1
20
10
20
Alloyed fiber #1
10
Clean fiber
0
0
0 H
0H
c2
(T)
(T)
30
Clean fiber
20
30
40
0
0
10
T (K)
— Similar to Hc2 and Hirr in parallel field in thin films .
— No epitaxy or texture necessary
20
T (K)
30
40
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.
7
10
6
Jc (A/cm2)
10
5
10
MgB2/YSZflexible
070705a transport
070705b6 bent, transport
050306b magnetization
4
10
0
5
10
15
20
25
30
35
40
Temperature (K)
Rs measured by A. Findikoglu (LANL)
HPCVD MgB2 Films on Metal Substrates
MgB2/Stainless Steel
0.15
2.0
1.5
0.10
0.05
4
Resistance ( x 10 )
0.006
0.004
0.004
R ()
Resistance (Ohms)
4
R ( x 10 )
0.008
0.002
0.002
0.000
0.00
36 37 38 39 40 41
T (K)
1.0
0.5
MgB2/Nb
36 37 38 39 40 41
T (K)
0.000
0
50
100
150
200
Temperature (K)
250
300
0.0
0
50
100
150
200
250
300
Temperature (K)
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
20
10
In water, RT
150 min
0°C
Resistance ()
R/R(0)
15
10
5
120
1
90
60
30
0 min
0.1
0.01
0
0
1
2
3
4
Time (hour)
5
6
7
36
38
40
42
44
Temperature (K)
― 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
30
Tc = 38.0 K
Resistivity ( cm)
25
After 42 hrs
t =400 nm
20
After 20 hrs
t =440 nm
15
10
Tc = 38.5 K
Tc = 38.9 K
As grown
t = 550 nm
5
M03049d
0
0
50
100
150
200
250
300
(Brian Moeckly. STI)
Temperature (K)
Compared to the HPCVD films, MgB2 films deposited by reactive coevaporation are much more stable against degradation in water.
Point-Contact Spectroscopy on MgB2 Films
HPCVD film: Andreev-Reflectionlike.
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 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