Transcript Diapositiva 1 - LNL-INFN

MgB2 thin films:
growth techniques and peculiar properties
C. Ferdeghini
CNR-INFM Lamia, Genova, Italy
Coworkers:
V.Ferrando, C.Tarantini, I.Pallecchi, M.Putti
The International Workshop on:
THIN FILMS APPLIED TO SUPERCONDUCTING RF AND NEW IDEAS FOR
PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY
Legnaro National Lab, Padova, October 9-12, 2006
Outline
• Magnesium diboride and its intriguing properties
• Thermodynamic of MgB2
• Challenges in MgB2 thin film deposition
• Two steps methods: examples
• In situ methods: examples
• Results obtained @ Lamia on MgB2 thin films
MgB2 properties-I
Crystalline structure
Tc  40 K
Fermi surface
J.Nagamatsu et al. Nature (2001) 410
3D p bands
2D s bands
•
•
•
•
Simple layered structure
Covalent bonding between B atoms
Conventional superconductivity (isotopic effect)
Coupling with vibrational modes of B atoms (s bands)
Weak interband scattering
due to different symmetry of the
two bands
The two bands are two conducting channels in parallel:
crucial role of disorder in coupling them
E2g phonon mode
MgB2 properties-II
Two distinct energy gaps
Dp2 meV ; 2 Dp /KTC  1.6
Ds7 meV ; 2 Ds /KTC  4
closing at the same Tc
Dp
Ds
Two gaps from STM
Two gaps from the
specific heat
M.Iavarone et al., PRL 89, 187004 (2002)
F.Bouquet et al., PRL 87 (2001) 047001
A comparison with conventional SC
for RF applications
MgB2
Nb
Tc (K)
39
9.2
r0 (mWcm)
0.1-10
0.05
RRR
3-30
300
Dp,s (meV)
2, 7
1.2
2 Dp,s /KBTc (meV)
1.6, 4
3.9
x p,s (nm)
50,12
40
l (nm)
85
80
m0Hc2 (T)
6-50
0.2
RBCSs @ 4K,
500MHz (nW)
2.5/2.3x
10-5
69
from
F.Collings et al. SUST 17 (2004)
1
2
RsBCS (nW)   105 GHz
e  D
T 
KTc 
Challenges in MgB2 thin films growth
optimal T for epitaxial growth ~ Tmelt/2
For MgB2 , 540°C → it requires PMg ~11 Torr
Too high for UHV deposition techniques (PLD, MBE...)
gas +MgB2 : Mg excess
does not condense on
the film surface and
MgB2 is stable
At PMg = 10-4-10-6 Torr, compatible with MBE, Tsub ~ 400°C
MgB2 is stable, but no MgB2 formation:
Mg atoms re-evaporate before reacting with B
Kinetic of Mg is also important
Mg
Z.-K. Liu et al., APL 78(2001) 3678.
evaporation Mg pressure from MgB2 < decomposition
curve of MgB2 < Mg vapor pressure
At P=10-6 Torr and T> 250°C no accumulation of Mg
will take place on the substrate and the growth of
the superconducting phase is very slow due to a
large kinetic energy barrier.
MgB2
Kinetically limited Mg
M. Naito and K. Ueda, SUST 17 (2004) R1
At low Mg pressure only extremely low deposition temperatures can be used
Deposition techniques
Two main problems in depositing MgB2 thin films:
1. sensitivity of Mg to oxydation
2. High Mg vapour pressure required
for phase stability
Two-step method
Deposition of an amorphous precursor
(boron or mg+B) at room temperature
Advantages:
+
Post-annealing in Mg atmosphere
(usually ex-situ)
• Possibility to use high temperatures for the phase crystallization
• High Tc, good structural properties
Disadvantages: • Difficult to extend to large area
In-situ techniques
Growth of MgB2 at low temperature
Advantages: for some of them, possibility to deposit large area films
Disadvantages:
• low growth temperature (except for HPCVD, see next talk)
• sometimes low Tc, poor structural properties
Substrates for MgB2 growth:
• Single crystalline: c-cut Al2O3, 4H and 6H SiC, (111) MgO
Two step methods
Precursor
Reaction
temperature(°C)
and time
Tc (K)
PLD
SrTiO3
(100), Al2O3
r cut
boron
900, 10-30 min
37-39
Eom et al. Nature
411,558 (2001)
PLD
SrTiO3 (111)
Mg+B
850, 15 min
34-36
Ferdeghini et al. Physica
C 378, 56 (2002)
Ferrando et al. SUST 16,
241(2003)
PLD
Al2O3 c cut,
MgO(111)
Mg+B
stoich.
850-900, 30 min
35-38
Paranthaman et al APL
78,3669 (2001)
E-beam
Al2O3 r cut
boron
890, 10-20 min
38-39
Moon et al.APL 79, 2429
(2001)
E-beam
Al2O3 c cut,
MgO(111)
boron
700-950, 30 min
39
Zhai et al. J.Mater.Res.
16, 2759 (2001)
E-beam, PLD
Al2O3 r cut
B, Mg+B
900, 1h
39,2528
Vaglio et al. SUST
15,1236 (2002)
Magnetron
sputtering
Al2O3 r cut,
MgO
Mg+B
stoich.
830,10 min in situ
35
Group and
reference
Growth
technique
Substrate
Kang et al. Science 292,
1521 (2001)
A feasible two step method
Two stage CVD
Reaction of a boron coating in Mg vapor
•High critical temperature; TC onset= 39.4 K
and ΔTc=0.9K
•Low resistivity (r0=0.38µΩcm)
•High RRR (25)
The B fibers are made by a CVD technique:
drawing of a W filament (the ‘substrate’),
heated to 1200 °C , through a Boron gaseous
compound (mixture of H2 and BCl3).
B filament
Reacted filament:MgB2
A similar approach could
be applied to the formation of a MgB2
film on the surface
of an RF cavity previously coated with
B using established
CVD technology
P.C.Canfield et al. PRL 86, 2324 (2001)
In-situ techniques
Substrate
Growth
temperature
(°C)
Tc (K)
Mg and B
metal
Al2O3, STO, Si
280
33-36
Carousel
sputtering
MgB2 target
Al2O3
250
28
PLD
Blue plume
Mg+B pressed
target
Al2O3, MgO
400
25
Jo et al. APL
80, 3563
(2002)
MBE
Co-deposition
Mg and B
metal
Al2O3
300
34
Erven et al.
APL 81, 4982
(2002)
MBE
Co-deposition
Mg and B
metal
Si + MgO seed
layer
300
35
Moeckly et al.
SUST 19, L21
(2006)
Reactive
evaporation
Large area
films
Mg and B
metal
Single and
poly crystals,
metallic
400-600
38-39
Zeng et
al.Nat.Mat.
1,35 (2002)
HPCVD
Clean
epitaxial films
Mg and B2H6
SiC, Al2O3
720
39-41
Group and
reference
Method
Comments
Source
Ueda et al.
APL 79, 2046
(2001), JAP
93,2113 (2003)
MBE
Co-deposition
Saito et al.
J.JAP 41,L127
(2002)
Sputtering
Grassano et al.
SUST 14,762
(2001)
A promising in situ method
Use of a rotating pocket heater
( similar to that developed for deposition of
large area HTS thin films) containing a rotating
platter that holds the substrates and spins
them through a quasiblack-body radiative oven.
1. The substrate is exposed to the vacuum
chamber via a window and hence to the
evaporated flux of boron.
2. Then it passes through the heater and
is exposed to a pocket with Mg vapour
only into the interior of the heater.
Heater pocket
Superconductor Tecnologies Inc.
Rotating shaft
Deposition zone
Magnesium
vapor
Boron Plume
B. Moeckly et al. SUST 19, L21(2006)
The Mg vapour is relatively well sealed inside the heater pocket by means of a small gap
between the platter and the heater body, and the single B e-beam source is therefore free
to operate in a vacuum environment.
Advantages:
1. high Mg pressure provided locally near the substrates
2. Mg temperature independent of the substrate temperature
3. double-sided deposition
4. Growth of large area films
5. Growth on metallic substrates
Two steps deposition process @ LAMIA
The PLD apparatus
PLD deposition of an MgB2 precursor layer from
stoichiometric target (prepared with pure 11B)
at room temperature in UHV
followed by
annealing in Mg vapour
In Ar atmosphere in a sealed Ta tube at 850-900 °C
Furnace
Ta crucible
Ta case
Vacuum pump
Mg
Quartz tube
Films
The reaction temperature is crucial for the quality of the samples
Best samples at 900 °C
Properties of the films grown @ LAMIA
-2 scan
f scan
Rocking curve
(002) MgB2
Intensity [A.U.]
Al
100
Al
(111) MgO
(001) MgB2
Intensity [A.U.]
2000
(024) Sapphire
500
10
1000
400
300
0
15
200
(101) MgB2
FWHM=1.2°
10
100
5
0
24
1
25
30
35
40
45
50
55
1000
1.680E4
3.260E4
0.9
4.840E4
MgO
0.8
27
H
1.6
1.8
r (mWcm)
1.4
120
180
240
300
360
12
Tc= 38.8 K
10
0.6
1.2
60
Good structural properties:
c axis orientation, single in plane orientation
Presence of an epitaxial interlayer of MgO
Tc close to the bulk value
B
1.0
0
 [Deg]
8.000E4
0.7
0
28
6.420E4
A
K
26
 [degrees]
60
2 [Degrees]
1.0
25
8
6
4
2
0
50
100
150
T (K)
200
250
300
Tuning MgB2 properties by disorder
MgB2 has two bands, weak interband scattering
 two channels conducting in parallel:
Similar Hc2 in samples with very
different r0
1
r0

1
r0p

1
r0s

  0 02p p  02s s
Four samples whose resistivity differs of
more than one order of magnitude
Hc2 does not depend on resistivity
Hc2(0) is determined by the
band with the lowest diffusivity
Introducing selectively disorder in
s or p band one can have samples
with low r0 and high upper
critical field
Interesting for RF applications

V.Ferrando et al. Phys. Rev.B 68, 094517 (2003)
Conclusions
•Due to its high critical temperature and its non granular
behaviour, MgB2 is an emerging superconducting material for
applications.
•In principle thin film deposition is not an easy task.
•Nevertheless, several different techniques were developed in
the last years. Some of them can be suitable for deposition on
large areas.
•In form of thin film, this material can present very low
resistivity along with considerable Hc2.