Pulsed Laser Deposition (PLD)

Physical Vapour Deposition Techniquesand

High temperature Superconductors

Nina HeinigWATLABS

Summary• why study superconductors?• thin film vs bulk synthesis• issues to consider in film growth:

– stoichiometry, epitaxy, impurities, strain, “second phases”

• types of thin film synthesis• pulsed laser ablation• example: the grain boundary problem in YBaCuO

12.5 kV, 1250 Amp, 3 phaseHigh temperature superconducting cable

RF superconducting filterfor cell phone base stations.

YBa2Cu3O7-xcrystal structureshowing Cu coordination.

Why superconductors?Why PLD?

Bulk vs. thin film synthesis

YBCO FILMGrain boundaryJUNCTION

SUBSTRATE

Flux-grown YBCO bicrystal

- simple growth- very good stoichiometry- very good crystallinity- difficulty in controlling structure

PLD-grown YBCO bicrystal

- growth in a vacuum chamber- can control stoichiometry- can control crystallinity- well-defined final structure.

Thin film growth techniques• evaporation/ MBE• sputtering • pulsed laser deposition• CVD (chemical vapour deposition)

• electrochemical• sol-gel• spin-coating• spray pyrolysis

vacuum- one step epitaxy

“bulk” techniques- post-anneal needed for epitaxy

Film growth issues:

1. Stoichiometry: chemical composition of the film.

2. Epitaxy: the film is single crystal, and is related to the substrate crystal orientation. Achieved by heating the substrate, so the film atoms have energy to relax into a crystal structure commensuratewith the substrate.

3. Impurities: at high temperatures, the substrate and film atoms can interdiffuse. Other impurity sources: target holders, walls etc.

4. Strain: the crystal spacing of film and substrate material is usually not the same. This means the film has intrinsic strain,which can lead to defects, film roughness, delamination.

5. Second phases: the film may phase-separate into different regions, with different properties.

1. Island growth (Volmer - Weber)•form three dimensional islands •film atoms more strongly bound to each other than to substrate •and/or slow diffusion

2. Layer by layer growth (Frank - van der Merwe)•generally highest crystalline quality •film atoms more strongly bound to substrate than to each other •and/or fast diffusion

•3. Mixed growth (Stranski - Krastanov)•initially layer by layer, then forms three dimensional islands

Evaporation/ Molecular Beam Epitaxy

Each target atomic specieshas its own “effusion cell”.

Each type of target is vaporized independently byi) heat, ii) e-beam, iii) laser.

Rate control is the big challenge.

UHV MBE system for GaAs etc.

Sputtering

Ar ion

Target atomssubstrate

A sputtering gas is used to excite a charged plasma which coats the substrate.Each type of atom has a different sputtering energy.Controlling the charged plasma, and preventing resputtering from the substrate are the challenges here.

YBa2Cu3O7-x films on SrTiO3 substrates.

Lamps

RotatingTarget Holder

Excimer Laser Mirror

Lens

YBa2Cu3O7-xTarget

SubstratePlume

Faceplate

Pulsed Laser Ablation

A high-power excimer laser is focused on the target.

The target is ablated to form a plume of atoms, molecules and“chunks”.

The advantage of PLD is that complex materials can be easilyablated.The challenge is minimizing “chunks”, and maintaining stoichometry.

Laser + optics

gas handling

chamber

PVD Laser deposition system

Advantages and Disadvantages of PLD

Advantages:1. New technique.2. Simple (fast, and easiest to study new chemical systems).3. Compatible with oxygen and other reactive gases.

Disadvantages:1. Particulates.2. Composition and thickness depend on deposition conditions. Difficult scale-up to large wafers?

Goal: Understanding electromagnetic transportacross low angle grain boundaries (LAGBs).

[001] θ0 5 10 15 20 25 30 35

J b/Jc

0.0

0.2

0.4

0.6

0.8

1.0EBE films at 5 K, from [1].PLD films at 77 K.

[001] θ

0 10 20 30 40

J b (M

A/c

m2 )

10-3

10-2

10-1

100

101

ab

c a

b

In YBa2Cu3O7-x bicrystals, the zero field Jcacross the grain boundary drops rapidly with misorientation angle, θ.

Dimos, Chaudhari, Mannhart, LeGoues,PRL,61, 219 (1988); PRB, 41, 4038 (1990).

Heinig et al., Appl. Phys. Lett., 69, 577, (1996).Zero field Jb/Jc vs. θ at 77 K decreases

rapidly with increasing θ.

What do PLD grown films look like?

FESEM image of 7o YBa2Cu3O7 HRTEM image of 10o YBa2Cu3O7

a-axis grain

growth spiral

High magnetic field, nV sensitive transport data show progressive change with θ

(d)

103 104 105 106 107

µV0.01

0.1

1

10

1007T

0T

6

0.51

23

4

5

103 104 105 106 107

0.01

0.1

1

10

100

7T

6

5

43 2 0.5

1

0T

A/cm2

102 103 104 105 106

µV

0.01

0.1

1

10

100

8T

7

6

5 4

32 1 0.5 0T

A/cm2

100 101 102 103 104

0.01

0.1

1

10

100

5

34 2 1

0.5 0T

A/cm2101 102 103 104 105

0.01

0.1

1

10

100(c)

(b)(a)

20o

0 T0.51

0o

10o

5o

15o

Log(V) - log(I) show increasing GB influence, at low and high voltage,as θ increases.

77 K, H||c

Different 7o thin film bicrystals can have very different extended V-I characteristics.

A/cm2

103 104 105 106

0.01

0.1

1

10

100

103 104 105 106

µV

0.01

0.1

1

10

100Y692s-b(7o) Y727s-b(7o)(a) (b)

0T0.5

12

34

5

6

8

0T0.51

23

45

6

7

In one bicrystal, there is little evidenceof the GB in the V-I characteristic.

The other bicrystal shows evidenceof weak coupling and flux flow along the GB at higher V.

• Low angle [001] tilt YBa2Cu3O7-x bicrystals were grown by laser ablation, and characterized byFESEM, TEM, and zero and high magnetic field transport.

• A progression from strongly coupled, flux-pinning limited transport behavior to weakly coupled, Josephson behavior was seen between 3o and 15o.

• Variation between bicrystals with the same θ shows that extrinsic factors modify the GB transition region.

Acknowledgments

Ron Redwing, George Daniels, J.E. Nordman,A.A. Polyanskii, A.E. Pashitski, A. Gurevich,

I-Fei Tsu, S.E. Babcock,D.C. Larbalestier

Work supported by NSF-MRSEC and EPRI

University of WisconsinUniversity of Wisconsin--MadisonMadisonApplied Superconductivity CenterApplied Superconductivity Center