Pulsed Laser Deposition (PLD)
Physical Vapour Deposition Techniques
and
High temperature Superconductors
Nina Heinig
WATLABS
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
Why superconductors?
Why PLD?
YBa2Cu3O7-x
crystal structure
showing Cu coordination.
12.5 kV, 1250 Amp, 3 phase
High temperature superconducting cable
RF superconducting filter
for cell phone base stations.
Bulk vs. thin film synthesis
Grain boundary
JUNCTION
YBCO FILM
SUBSTRATE
Flux-grown YBCO bicrystal
PLD-grown YBCO bicrystal
- simple growth
- very good stoichiometry
- very good crystallinity
- difficulty in controlling structure
- 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 commensurate
with 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 species
has its own “effusion cell”.
Each type of target is
vaporized independently by
i) heat, ii) e-beam, iii) laser.
Rate control is
the big challenge.
UHV MBE system for GaAs etc.
Sputtering
Ar ion
substrate
Target atoms
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.
Pulsed Laser Ablation
Lens
Excimer Laser
Mirror
Faceplate
Rotating
Target Holder
Lamps
YBa2Cu3O7-x
Target
Substrate
Plume
YBa2Cu3O7-x films on SrTiO3 substrates.
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 easily
ablated.
The challenge is minimizing
“chunks”, and maintaining
stoichometry.
chamber
Laser + optics
gas handling
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 transport
across low angle grain boundaries (LAGBs).
EBE films at 5 K, from [1].
PLD films at 77 K.
1.0
c
101
Jb (MA/cm2)
Jb/Jc
0.8
0.6
a
b
100
b
10-1
a
10-2
10-3
0.4
0
10
20
30
40
[001] θ
In YBa2Cu3O7-x bicrystals, the zero field Jc
across the grain boundary drops rapidly
with misorientation angle, θ.
0.2
0.0
0
5
10
15
20
25
30
[001] θ
Zero field Jb/Jc vs. θ at 77 K decreases
rapidly with increasing θ.
35
Dimos, Chaudhari, Mannhart, LeGoues,PRL,
61, 219 (1988); PRB, 41, 4038 (1990).
Heinig et al., Appl. Phys. Lett., 69, 577, (1996).
What do PLD grown films look like?
FESEM image of 7o YBa2Cu3O7
growth spiral
a-axis grain
HRTEM image of 10o YBa2Cu3O7
High magnetic field, nV sensitive transport
data show progressive change with θ
0o
100
7T
Log(V) - log(I) show
increasing GB influence,
at low and high voltage,
as θ increases.
µV
10
1
6
7T
6
1
5
4
3
0.01
103
0.1
2
104
4
1
105
3 2
0.01
0.5
0T
106
100
107
103
100
(c)
104
100
10
10
10
µV
77 K, H||c
8T
1
1
0.5
105
0T
106
107
(d)
20o
15o
1
0.1
1
5o
(b)
10
5
0.1
10o
100
(a)
0.01
1
0.5
0T
101 102 103 104 105
0.1
7
0.1
6
0.01
102
3
54
103
2 1
0.5
104
A/cm
0.01
0T
105
2
106
100
A/cm2
5
2
4
1
3
101
102
0.5
103
2
A/cm
0T
104
Different 7o thin film bicrystals can have very
different extended V-I characteristics.
100
100
o
(a)
Y692s-b(7 )
10
8
µV
1
5
7
6
4
3
0.1
2
104
5
0.1
1
0.01
103
(b)
10
6
1
o
Y727s-b(7 )
105
0.5
4
3
0T
106
0.01
103
104
1
0.5
2
105
0T
106
A/cm2
In one bicrystal, there is little evidence
of the GB in the V-I characteristic.
The other bicrystal shows evidence
of 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 by
FESEM, 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 WisconsinWisconsin-Madison
Applied Superconductivity Center