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.