An overview of Pulsed Laser Ablation or PLD

1. Introduction to Pulsed Laser Deposition
Arun Aravind
19th July 2012

2.  The lasers having a higher repetition
rate
 Reliable electronic Q-switches lasers -
very short optical pulses
 Successfully to grow high-temperature
superconducting films in 1987
Historical back ground
Crystalline thin films with epitaxy
quality.
Ceramic oxide
nitride films,
Metallic multilayers,
superlattices
nanotubes,
Nanopowders
quantum dots
Production-related issues
- reproducibility
 large-area scale-up
Multiple-level have begun
.

3.  Pulsed-laser deposition (PLD) widely used
 Its ease of use
 Depositing materials of complex stoichiometry
 PLD was the first technique - superconducting YBa2Cu3O7 thin film.
 Multi-element oxides, have been successfully deposited by PLD.
Schematic diagram of PLD

4. Advantages of PLD
 Flexible, easy to implement
 Growth in any environment
 Exact transfer of complicated materials (YBCO)
 Variable growth rate
 Epitaxy at low temperature
 Resonant interactions possible (i.e., plasmons in metals,
absorption peaks in dielectrics and semiconductors)
 Atoms arrive in bunches, allowing for much more
controlled deposition
 Greater control of growth (e.g., by varying laser
parameters)

5. Laser used in the present study: Nd:YAG (Spectra Physics INC,GCR 150)
Fourth harmonic of the fundamental with 266 nm wavelength
Pulsed Laser Deposition in our lab
Schematic diagram of the pulsed
laser deposition system

6. Laser energy of 642 mJ
Pre-deposition vacuum of 8.3X10-3
mTorr
Target-to-Substrate of 4-8 cm
Pulse Repetition Rate of 10 Hz
2. Excimer Laser System

7. 1. Laser systems

8. Typical spatial beam
distributions of high pulse
energy excimer (left) and
Nd:YAG lasers (right).
The rippled beam profile of a
266nm flashlamp
pumped Nd:YAG laser
Typical target ablation track
cross section obtained
with a 248 nm excimer
laser in polyimide

9. 2. Deposition Systems/Chambers
T= target port flange to beam focal plane
Z= substrate port flange to substrate distance
S=target to substrate distance
L= laser port length
Θ=angle between plume direction and laser beam
Four deposition systems
and associated optics
L, ɵ(~45°) and T+S+Z is fixed in commercial systems

10. 3. Substrate holders and Heaters

11. Mechanism of PLD
Laser-target interaction
Effect of Background gas
Helps in slowing down the ablated species.
Helps in the formation of compound oxide thin films.
 Radiant energy from laser focused to the target
 Materials are ablated from the target surface stoichiometry as in the target
 A directed plume of ablated material is formed
 Film grows after a thermalized region is formed
 Due to laser – plasma interaction ions in the plasma are accelerated
( 100 – 1000 eV )

12. CCD /PMT
spectromete
r
Target
Substrates
or
Faraday
cup
laser beam
Pulsed Laser Deposition
Target: Just about anything! (metals, semiconductors…)
Laser: Typically excimer (UV, 10 nanosecond pulses)
Vacuum: Atmospheres to ultrahigh vacuum
 Laser radiation interaction with the target
 Dynamic of the ablation materials
 Decomposition of the ablation materials onto the substrate
 Nucleation and growth of a thin film on the substrate surface

13. Processes in PLD
Laser pulse
1. Interaction of laser
radiation with target material

14. Processes in PLD
e-
e-
e-
e-e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
Electronic excitation

15. Processes in PLD
e-
e-
e-
e-e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
Energy relaxation to lattice (~1 ps)
lattice

16. Processes in PLD
Heat diffusion (over microseconds)
lattice

17. Processes in PLD
Melting (tens of ns), Evaporation, Plasma Formation
(microseconds), Resolidification
lattice
2. Dynamics of ablated material

18. Processes in PLD
lattice
If laser pulse is long (ns) or repetition rate
is high, laser may continue interactions
3. Deposition of the ablated material on the surface
4. Nucleation and growth of the thin film on the substrate surface

19. Processes in Pulsed Laser Deposition
1. Absorption of laser pulse in material
Qab=(1-R)Ioe-aL
(metals, absorption depths ~ 10 nm, depends on l)
2. Relaxation of energy (~ 1 ps) (electron-phonon interaction)
3. Heat transfer, Melting and Evaporation
when electrons and lattice at thermal equilibrium (long pulses)
use heat conduction equation:
(or heat diffusion model)
abp QTK
t
T
C 


)(

20. Processes in Pulsed Laser Deposition
4. Plasma creation
threshold intensity:
goverened by Saha equation:
5. Absorption of light by plasma, ionization
(inverse Bremsstrahlung)
6. Interaction of target and ablated species with plasma
7. Cooling between pulses
(Resolidification between pulses)
pulse
threshold
t
cmWsx
I
22/14
104 






 



kTmm
mm
Q
QQ
n
nn ion
ie
ie
n
ie
n
ie exp

21. Incredibly Non-Equilibrium!!!
At peak of laser pulse, temperatures on target can reach
>105 K (> 40 eV!)
Electric Fields > 105 V/cm, also high magnetic fields
Plasma Temperatures 3000-5000 K
Ablated Species with energies 1 –100 eV

22.  Laser energy density and pulse repetition rate
 The targets used in PLD are small large size required for other
sputtering techniques
 Stoichiometry of the target can be retained in the deposited
films
Film growth Mechanism
Plumes obtained due to laser matter interaction

23. Thin Film Deposition
Transfer atoms from a target to a vapor (or plasma) to a substrate
After an atom is on surface, it diffuses according to: D=Doexp(-eD/kT)
eD is the activation energy for diffusion ~ 2-3 eV
kT is energy of atomic species.
Want sufficient diffusion for atoms to find best sites.
Either use energetic atoms, or heat the substrate.

24. The ejected high energy - substrate surface
- damage to the substrate
These energetic species sputter - the
surface atoms
A collision region is established between
the incident flow and the sputtered atoms
Film grows immediately after this
thermalized region (collision region) forms
When the condensation rate is higher than the rate of particles supplied by the
sputtering, thermal equilibrium condition can be reached quickly and film grows on
the substrate surface at the expense of the direct flow of the ablation particles.

25. Supersaturation, Dm = kT ln(R/Re)
k is the Boltzmann constant,
R is the actual deposition rate,
Re is the equilibrium value at temperature T
The nucleation process depends -
interfacial energies (substrate, the
condensing material and the vapour)
The critical size of the nucleus depends on the driving force, i.e.
the deposition rate and the substrate temperature

26. The crystalline film growth depends on
the surface mobility of the adatom
(vapour atoms)
 Volmer - Weber Nucleation and growth – 3D island growth
 Frank- van der Merve growth – 2D full monolayer growth
 Stranski- Krastinov growth – Monolayer growth (1-5), then 3D clusters
layer-by-layer nucleation is favoured and ultra-thin and smooth film can be produced.
The rapid deposition of the energetic ablation species helps to raise the substrate
surface temperature.
PLD tends to demand a lower substrate temperature for crystalline film growth.

27. Effect of back ground gas
The energy of the ablated species
can be reduced
Compound semiconductor
preparation

28. Effect of Laser Ablation wavelength - target
Metal -
Penetration is larger in UV than IR

29. Effect of ablation wavelength - film
 Smoother films obtained at lower laser wavelength
 Photofragmentation occurs at lower wavelength

30.  One of the major problems is the splashing or the particulates
deposition on the films
 The physical mechanisms leading to splashing include the sub-
surface boiling, expulsion of the liquid layer by shock wave recoil
pressure and exfoliation
 The narrow angular distribution of the ablated species, which is
generated by the adiabatic expansion of laser, produced plasma
plume and the pitting on the target surface
 These features limit the usefulness of PLD in producing large area
uniform thin films
Splashing……

32.  Most versatile .
 Operate in wide pressure range.
 Almost any kind of materials can be ablated.
 Instantaneous control of the evaporation process .
 Reactive gas can be used as ambient gas.
 Highly oriented or epitaxial films can be obtained.
 Splashing causes micron-sized
particulates
 Plume highly directional
 Uniform only over a small area
 Mass production hindered
Disadvantages
Advantages