ph413 lasers & photonics · advantages of pulsed laser deposition (pld) • any material can be...
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Laser matter interaction
PH413 Lasers & PhotonicsPH413 Lasers & PhotonicsLecture 26
A.K.Sharma
13.10.11
Department of Physics, IIT Guwahati
Why study laser matter interaction?
• Fundamental physics
• Chemical analysis
• Material processing
• Biomedical applications
• Deposition of novel structures• Deposition of novel structures
A.K.Sharma
13.10.11
2Department of Physics, IIT Guwahati
What is a Plasma?
A quasi-neutral gas of charged and neutral particles
which exhibits collective behavior
(F. F. Chen)(F. F. Chen)
• Fourth state of matter
• 99% Universe
A.K.Sharma
13.10.11
3Department of Physics, IIT Guwahati
Debye length
• High density
Q
2/12
0 )(ne
Tk eBD
ελ =
εεεεo = permittivity of free space
k = Boltzmann constant
• Low density Debye sphere
Q
kB = Boltzmann constant
Te = electron temperature
n = electron number density
e = electric charge
A.K.Sharma
13.10.11
4Department of Physics, IIT Guwahati
Criteria for plasma
1. λλλλD << L
2. ND >> 1
3. ωτωτωτωτn > 1
A.K.Sharma
13.10.11
5Department of Physics, IIT Guwahati
Plasma Production
• Low pressure cold cathode discharge
• Thermionic arc discharge
• RF produced plasma
• Solar plasma
• Laser-produced plasma
A.K.Sharma
13.10.11
6Department of Physics, IIT Guwahati
Laser-ablated plasma
TargetPlasma Converging
Laser
Characteristics
1. High temperature (∼∼∼∼ KeV)
2. High density (∼∼∼∼ 1021 cm-3)
3. High velocity (∼∼∼∼ 107 cm s-1)
Plasma Converging
lens
A.K.Sharma
13.10.11
7Department of Physics, IIT Guwahati
What is Laser Ablation ?
When a short-pulsed, high-peak-power laser beam is focused onto any solid target,
a portion of the material instantaneously explodes into vapor. The name "laser
ablation" is used generally to describe the explosive laser-material interaction, a
more appropriate definition that does not imply a mechanism. Laser-material
interactions involve coupling of optical energy into a solid, resulting in
vaporization; ejection of atoms, ions, molecular species, and fragments; shock
waves; plasma initiation and expansion; and a hybrid of these and other processes.
A.K.Sharma
13.10.11
8Department of Physics, IIT Guwahati
Various processes in plasma
• Collisional excitation/de-excitation/ionization
• Photo-excitation/ionization
• Bremsstrahlung (recombination)
• Inverse-bremsstrahlung (absorption of photon)
A.K.Sharma
13.10.11
9Department of Physics, IIT Guwahati
Laser-matter interaction regime
1. Evaporation regime (Laser-target interaction)
2. Isothermal regime (Laser-plasma interaction)
3. Adiabatic regime (Plasma expansion after the termination of the
laser pulse)
A.K.Sharma
13.10.11
10Department of Physics, IIT Guwahati
A.K.Sharma
13.10.11
11Department of Physics, IIT Guwahati
A.K.Sharma
13.10.11
12Department of Physics, IIT Guwahati
Adiabatic regime
A.K.Sharma
13.10.11
13Department of Physics, IIT Guwahati
Measurement Techniques for Plasma Parameters
Diagnostics Plasma parameters
Optical Emission Spectroscopy
Absorption Spectroscopy
Fast Photography & Imaging
Ion Probe Diagnostics
Electron temperature, electron density
Ground state electron density
Plume front velocity, vapour pressure,
vapour temperature
Electron temperature, electron density Ion Probe Diagnostics
Laser-Induced Fluorescence
Time-of-Flight Mass Spectroscopy
Interferometry
Laser Beam Deflection Method
Electron temperature, electron density
Ground state electron density
Velocity of species, states of ionization
Electron density
Density gradient, ablation threshold
A.K.Sharma
13.10.11
14Department of Physics, IIT Guwahati
Experimental setup for laser ablation and deposition
A.K.Sharma
13.10.11
15Department of Physics, IIT Guwahati
Formation of CN band: Temporal & spatial dependence
C + N2 ⇔⇔⇔⇔ CN + N - 2 eV
Fluence: 20 Jcm-2
(violet B2 ΣΣΣΣ+ −−−− X2 ΣΣΣΣ+ band system):
(0-1) at 421.6 nm, (1-2) at 419.7 nm,
(2-3) at 418.1 nm, (3-4) at 416.8 nm
(4-5) at 415.6 nm, and (5-6) at 415.2
nm.
A.K.Sharma
13.10.11
16Department of Physics, IIT Guwahati
Fast photography of expanding plasma
• Plume dynamics of the plasma.
• Conservation of mass, momentum, and energy equations to
estimate physical parameters of interest.
• Plume length (optimized distance for thin film deposition).
• Interesting features which otherwise are not possible to• Interesting features which otherwise are not possible to
obtain/discuss using other techniques (plume splitting, instability).
A.K.Sharma
13.10.11
17Department of Physics, IIT Guwahati
ICCD images of expanding Al plasma in N2 at 88 mJ
0 ns
20 ns
40 ns
60 ns
0 100 200 300 400 5000
1
2
3
4
5
6
7
R(t) = 3.2(1-e-0.04t
)
10 Torr
R(t) = 0.87t0.33
1 Torr
R(t) = 1.5t0.3
0.01 Torr
(d)
(c)
Dis
tan
ce R
(m
m)
Delay time (ns)
(b)
0 20 40 60 800.0
0.5
1.0
1.5
2.0
2.5
3.0
0 10 20 30 40 50
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Dis
tan
ce R
(m
m)
Delay time (ns)
Dis
tance R
(m
m)
Al in vacuum (< 10-5 Torr)
λ = 1064 nm E = 88 mJ
R(t) = 0.035t
Inset R(t) = 0.065t
(Ablation at early times)
80 ns
100 ns
120 ns
140 ns
160 ns
0.01 Torr 0.1 Torr
Vac
Delay time (ns)Delay time (ns)
Drag Model
R = Rmax(1-e-βt)
5/2
5/1
o
o tE
R
ρξ=
Shock wave model
A.K.Sharma
13.10.11
18Department of Physics, IIT Guwahati
Distribution of various atomic species within the plasma
D’Alessio et al, Appl. Surf. Sci. 208-209, 113 (2003)
Shifted Maxwellian velocity distribution
A.K.Sharma
13.10.11
19Department of Physics, IIT Guwahati
Lateral dimensions of plasma using nano- and picosecond
pulses
(a) 0.1 Torr nitrogen ambient
(b) 1 Torr nitrogen ambient
A.K.Sharma
13.10.11
20Department of Physics, IIT Guwahati
Nano-, pico- and femto-second laser ablation
100 µm thick steel foil
80 ps, 3.7 J cm-2 3.3 ns, 4.2 J cm-2 200 fs, 0.5 J cm-2
Chichkov et al, Appl. Phys. A 63, 109 (1996)
A.K.Sharma
13.10.11
21Department of Physics, IIT Guwahati
Thermal diffusion length
2/1)2( pthth DL τ=Diffusion length Pulse width
1.6 µm 8 ns
100 nm 35 ps
6 nm 100 fs
Al
Energy absorbed in diffusion length
pIRF τ)1( −= Reflectivity Wavelength
0.99 1.064 µm
0.3 248 nm
Si
A.K.Sharma
13.10.11
22Department of Physics, IIT Guwahati
Advantages of Pulsed Laser Deposition (PLD)
• Any material can be ablated.
• Pulsed nature of PLD means that film growth can be controlled to any
desired amount.
• Laser is outside the vacuum chamber and therefore provides greater
flexibility in geometrical arrangements.
• Compositional fidelity is often retained between the target material and
the deposited film and hence is attractive for fabricating stoichiometric
multicomponent films.
• Amount of evaporated source material is localized to the area on which
the laser is focused.
• Kinetic energies of the ablated species lie in a range that promotes
surface mobility and avoid bulk displacement.
A.K.Sharma
13.10.11
23Department of Physics, IIT Guwahati
Drawbacks of PLD
• Formation of droplets.
• Impurities in the target
material.
• Crystallographic defects in the
films caused by bombardment Droplets/splashing
by high kinetic energy ablation
particles.
• Inhomogeneous flux and
angular energy distributions
within the ablated plume.
Flux/distribution
A.K.Sharma
13.10.11
24Department of Physics, IIT Guwahati
What structures can be grown by PLD ?
• Nanoparticles
• Quantum well
• Nano-rods /wires
• Heterostructures, p-n junction
• Superlattices
A.K.Sharma
13.10.11
25Department of Physics, IIT Guwahati
Nanorods/wires
Kwok et al, Appl. Phys. Lett. 87,
223111 (2005)
Jie et al, Appl. Phys. Lett. 86, 031909 (2005)
ZnO nanorods on ZnO/Si film
Various shapes of ZnO
nanostructures
A.K.Sharma
13.10.11
26Department of Physics, IIT Guwahati