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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.
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