MgBMgB2 2 thin films: thin films: growth techniques and peculiar propertiesgrowth techniques and peculiar properties
C. FerdeghiniCNR-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 SUPERCONDUCTIVITYLegnaro National Lab, Padova, October 9-12, 2006
OutlineOutline
• 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
MgBMgB22 properties-I properties-ICrystalline structure Fermi surface
E2g phonon mode
Tc 40 K
• Simple layered structure• Covalent bonding between B atoms• Conventional superconductivity (isotopic effect)• Coupling with vibrational modes of B atoms ( bands)
3Dbands
2Dbands
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
J.Nagamatsu et al. Nature (2001) 410
MgBMgB22 properties-II properties-II
Two distinct energy gaps
closing at the same Tc
2 meV ; 2 KTC 1.6
7 meV ; 2 /KTC 4
M.Iavarone et al., PRL 89, 187004 (2002) F.Bouquet et al., PRL 87 (2001)
047001
Two gaps from the specific heat
Two gaps from STM
MgB2 NbTc (K) 39 9.2
0 (cm) 0.1-10 0.05
RRR 3-30 300
(meV) 2, 7 1.2
KBTc (meV)
1.6, 4 3.9
(nm) 50,12 40
(nm) 85 80
0Hc2 (T) 6-50 0.2
RBCSs @ 4K,
500MHz (n)2.5/2.3x
10-5 69
A comparison with conventional SC A comparison with conventional SC for RF applicationsfor RF applications
cKTGHz
BCSs e
TnR
2510
1)( from
F.Collings et al. SUST 17 (2004)
Challenges in MgBChallenges in MgB22 thin films growth thin films growthoptimal T for epitaxial growth ~ Tmelt/2
For MgB2 , 540°C → it requires PMg ~11 Torr
Too high for UHV deposition techniques (PLD, MBE...)
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
gas +MgB2 : Mg excess does not
condense on the film surface and MgB2 is
stable
Z.-K. Liu et al., APL 78(2001) 3678.
M. Naito and K. Ueda, SUST 17 (2004) R1
Mg
MgB2
Kinetically limited Mg
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.
Kinetic of Mg is also important
At low Mg pressure only extremely low deposition temperatures can be used
Deposition techniquesDeposition techniques
Two-step method
In-situ techniques
Deposition of an amorphous precursor(boron or mg+B) at room temperature
Post-annealing in Mg atmosphere (usually ex-situ) +
Advantages: • Possibility to use high temperatures for the phase crystallization• High Tc, good structural properties
Disadvantages: • Difficult to extend to large area
Growth of MgB2 at low temperature
Two main problems in depositing MgB2 thin films: 1. sensitivity of Mg to oxydation2. High Mg vapour pressure
required for phase stability
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 methodsTwo step methods
Group and reference
Growth techniqu
e
Substrate
Precursor
Reaction temperature(°C) and time
Tc (K)
Kang et al. Science 292, 1521 (2001)
PLDSrTiO3 (100),
Al2O3 r cutboron 900, 10-30 min 37-39
Eom et al. Nature 411,558 (2001)
PLDSrTiO3 (111)
Mg+B 850, 15 min 34-36
Ferdeghini et al. Physica C 378, 56
(2002)Ferrando et al. SUST
16, 241(2003)
PLDAl2O3 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-beamAl2O3 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, 1h39,25-
28
Vaglio et al. SUST 15,1236 (2002)
Magnetron sputtering
Al2O3 r cut, MgO
Mg+B stoich.
830,10 min in situ 35
P.C.Canfield et al. PRL 86, 2324 (2001)
A feasible two step methodA feasible two step method
Two stage CVDTwo stage CVDReaction of a boron coating in Mg vapor
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). 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
B filament
Reacted filament:MgB2
•High critical temperature; TC onset= 39.4 K and ΔTc=0.9K•Low resistivity (0=0.38µΩcm)•High RRR (25)
In-situIn-situ techniques techniques
Group and reference
MethodComment
sSource
Substrate
Growth temperatur
e (°C)
Tc (K)
Ueda et al. APL 79, 2046 (2001), JAP
93,2113 (2003)
MBECo-
depositionMg and B
metalAl2O3, STO,
Si280 33-36
Saito et al. J.JAP 41,L127
(2002)Sputtering
Carousel sputtering
MgB2 target Al2O3 250 28
Grassano et al. SUST 14,762 (2001)
PLD Blue plumeMg+B
pressed target
Al2O3, MgO 400 25
Jo et al. APL 80, 3563 (2002)
MBECo-
depositionMg and B
metalAl2O3 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 evaporati
on
Large area films
Mg and B metal
Single and poly
crystals, metallic
400-600 38-39
Zeng et al.Nat.Mat. 1,35 (2002)
HPCVDClean
epitaxial films
Mg and B2H6 SiC, Al2O3 720 39-41
Boron Plume
Deposition zone
Magnesium vapor
Heater pocketRotating shaft
B. Moeckly et al. SUST 19, L21(2006)
A promising A promising in situin situ method methodUse 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.
Advantages:Advantages:1. high Mg pressure provided locally near the substrates2. Mg temperature independent of the substrate
temperature3. double-sided deposition4. Growth of large area films5. Growth on metallic substrates
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.
Superconductor Tecnologies Inc.
Two steps deposition process @ LAMIATwo steps deposition process @ LAMIA
followed by
PLD deposition of an MgB2 precursor layer from stoichiometric target (prepared with pure 11B)
at room temperature in UHV
annealing in Mg vapour In Ar atmosphere in a sealed Ta tube at 850-900 °C
The reaction temperature is crucial for the quality of the samples Best samples at 900 °C
MgQuartz tube
Vacuum pump
FurnaceTa crucible
Films
Ta case
The PLD apparatus
Properties of the films grown @ LAMIAProperties of the films grown @ LAMIA
25 30 35 40 45 50 55 601
10
100
(111
) M
gO
(001
) M
gB2
(002
) M
gB2
AlA
l
Inte
nsity
[A.U
.]
2 [Degrees]
24 25 26 27 280
100
200
300
400
500
FWHM=1.2°
Inte
nsity
[A.U
.]
[degrees]0 60 120 180 240 300 360
0
5
10
15
(101) MgB2
[Deg]
0
1000
2000
(024) Sapphire
scanRocking curve-2 scan
50 100 150 200 250 3000
2
4
6
8
10
12
Tc= 38.8 K
(
cm)
T (K)
1.0 1.2 1.4 1.6 1.8
0.6
0.7
0.8
0.9
1.0
MgO
B
A
H
K
1000
1.680E4
3.260E4
4.840E4
6.420E4
8.000E4
Good structural properties: c axis orientation, single in plane orientationPresence of an epitaxial interlayer of MgOTc close to the bulk value
MgB2 has two bands, weak interband scattering two channels conducting in
parallel:
20
200
000
111
Hc2(0) is determined by the band with the lowest
diffusivity
Hc2 does not depend on resistivity
Similar Hc2 in samples with very different 0
V.Ferrando et al. Phys. Rev.B 68, 094517 (2003)
Tuning MgBTuning MgB22 properties by disorder properties by disorder
0 5 10 15 20 25 30 35 400
5
10
15
20
25
30
Film 1 0=40cm
Film 2 0=50cm
Film 3 0=20cm
Film 4 0=5cm
0Hc2
[T]
T [K]
Four samples whose resistivity differs of more than one order of magnitude
Introducing selectively disorder in or band one
can have samples with low 0
and high upper critical field
Interesting for RF applications
ConclusionsConclusions
•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.