Investigation of Semiconducting materials using Ultrafast Laser assisted Atom Probe Tomography...
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Investigation of Semiconducting materials using Ultrafast Laser
assisted Atom Probe Tomography
Baishakhi MazumderF. Vurpillot, A. Vella, B. Deconihout & G. Martel
Groupe de Physique des Matériaux / Coria29th April 2009
Plan
• Introduction to Atom Probe Tomography• Ultra-short Pulse Laser Assisted Atom Probe• Applications• Silicon Field evaporation • Theoretical interpretation• Conclusion & Perspectives
• APT = FIM + TOF
• Tip subjected to field F~V/R and the evaporationrate follows the Arrhenius law
• Tip pulsed field evaporated atom by atom
• Ions projected on a PSD
• TOF mass spectrometry
• 3D reconstruction of the atomic distribution
• Volume ~100x100x100 nm3
• Spatial Resolution - 0.2nm in depth 0.5nm laterally
Position SensitiveDetector (X,Y,TOF)
Radius R<100 nm
V
L
XX
YY
Atom Probe Tomography
))(
exp(0 Tk
EQ
B
Femtosecond laser assisted atom probe
TipTip
DSpot
Laser beam
ττpulsepulse
Energy used ~ 0.1 – 100 μJ /pulse Dspot~ 100-800 μm
τpulse ~40-500 fs on-demand wavelength (infrared-visible-UV) repetition rate 1-100 kHz
B. Gault, et al. Rev. Sci. Instrum. 77, 043705 (2006)
Laser Assisted Tomography Atom Probe
Startsignal
V0 < 20 kV
PSD
RIon
P < 10-10 Pa
T < 20-80K
tip
3 Colour box Stopsignal
R<100nm
fs laserpulse
Femtosec laser,100kHz500fs
Time of flight
UVGreen
IR
SpecimenNeedle Shape
1
00
nm
Applications of Different Aspects
MgOMgOFeFe
Chemical nature of the material mass to charge ratio obtained by TOF measurement
m mass of the ion,V the DC voltageL,flight length,t flight time,k constant
2
2
L
tkV
n
m
CoFeTb multilayer SiCo FeMgOFe
A. Grenier et al. JAP 102,033912 2007 Talaat Al Kassab, IJMR 99,5,2008M.Gilbert et al. Ultramicroscopy 107,767,2007
Wide range of materials- All metallic materials- Alloys- Multiple quantum well- Nano wires
nlasI
Thermal evaporation
Photo ionisation
Mechanism for Field evaporation
lasI
1log
Ilas is the intensity of laser applied to the tip. The energy deposited by the laser pulses on the specimen increases its temperature allowing the surface atoms to be ionised. Evaporation rate
n, no of photon absorbed to ionise one atom.
This process occurred only on semiconductor or oxide surfaces due to the presence of band gap
1
h
2
h
3
CB vacuumeEx
VB
Tsong et al J. Chem. Phys., 65(6) 1976Tsong, PRB 30(9) 1984
))(
exp(0 Tk
EQ
B
350 400 450 500
0
200
400
600
800
1000
30Si2+
29Si2+
Num
ber
of a
tom
s/pu
lse
TOF(nS)
28Si2+ Laser energy - 18nJ
Condition for good mass resolutionMass spectra of Silicon under Infra Red Femtosecond Laser at 80K
Intensity GW/cm2
Metal
Silicon
0.0 1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-40.0
5.0x10-5
1.0x10-4
1.5x10-4
2.0x10-4
ato
m/p
uls
e
Laser energy/pulse (mJ)
Best Poster Award, IFES 2008B.Mazumder,A.Vella,M.Gilbert,B.Deconihout,G.Schimtz Submitted to Surface Science
Measured flux is linearly dependent on laser intensity For the first time we have demonstrated that it is a single-photon process.I.e. the rate of evaporation can be written as:
photon energy(1.2eV)
One photonnlasI n, number of photonZone 1
Zone 2
360 380 400 420
0
100
200
300
400
50028Si2+
Num
ber
of a
tom
s/P
ulse
TOF(nS)
Laser energy 143nJ
350 400 450 500
0
200
400
600
800
1000
30Si2+
29Si2+
Num
ber
of a
tom
s/pu
lse
TOF(nS)
28Si2+ Laser energy - 18nJ
Condition for good mass resolutionMass spectra of Silicon under Infra Red Femtosecond Laser at 80K
Bad mass resolution with higher laser energyLoosing events close to Si massThere is a saturation after a certain laser energy
Intensity (GW/cm2 )
Metal
Silicon
Zone 2Zone 1
360 380 400 420 440
0
100
200
300
400
500
600
700
800
30Si2+
29Si2+
28Si2+
No
of
ato
ms/
pu
lse
TOF (nS)
6nJ 36.9nJ 67nJ 143nJ
Laser Energy
360 380 400 4200.1
1
10
100
Log(
No
of a
tom
s/pu
lse)
TOF (nS)
6nJ36.9nJ 67nJ 143nJ
28Si2+
29Si2+30Si2+
Laser Energy
0 20 40 60
0
5000
10000
15000
20000
25000
Laser energy ~ 100nJ
30Si2+29Si2+
28Si2+
Num
ber
of a
tom
s
TOF (nS)0 20 40 60
100
1000
10000
Log
N
TOF (nS)
28Si2+
29Si2+30Si2+
Laser energy ~ 100nJ
Photon energy 2.45eV
Photon energy 1.2 eV
Non existence of the hump in mass spectrum by using laser energy with photon energy higher than the band gap energy.
Study of Si mass spectra with different wavelength at 80KStudy of Si mass spectra with different wavelength at 80K
There is a hump appeared with increasinglaser energy with photon energy of near band gap energy.
(IR)
(Green)
-5 0 5 10 15 20
20
30
40
50
60
70
80
Log
N
TOF nS
11.7nJ 21.2nJ 30.5nJ
28Si2+
29Si2+
30Si2+
380 400 420
100
200
300
400
500
600
No
of a
tom
s/pu
lse
TOF (nS)
---33nJ---84.6nJ---98.5nJ
28Si2+
29Si2+30Si2
+
Laser Energy
Existence of hump in SiC using photon Existence of hump in SiC using photon energy of near band gap energyenergy of near band gap energy
0 10
0.0
0.5
1.0
30Si2+29Si2+
28Si2+
No
of a
tom
s/pu
lse
TOF(ns)
Photon energy - 2.45eV
Photon energy - 3.62eV
No evidence of hump, even by increasing laser energy; and no variation in mass spectra.
380 400 420
1
10
100
Log(
No
of a
tom
s/pu
lse)
TOF (nS)
---33nJ---84.6nJ---98.5nJ
28Si2+
29Si2+
30Si2+
(Green)
(UV)
Evidence of hump with photon energy of near band gap energy
CONCLUSIONThe hump seems to appear only using photons with near-band gap energies
Existence of hump in SiC using photon energy of near Existence of hump in SiC using photon energy of near band gap energyband gap energy
Relaxation time 2
Total energy given to the lattice 1.2 eV
E1=1.1 eV
E2=0.1 eV
2-steps transition
Z
Y
dV
S(z)- S(z)+
diameter <<1000 nm Absorption ~10 cm-1
yII exp0
I/I0~1 Homogeneous absorption
Relaxation time 1
2
22
N
dt
dN
N2 (z,t), injected electron density with a relaxation time
2
2
2
1
11
NN
dt
dN
N1 (z,t), thermalised electron density with a relaxation time 1
Model
1
11
2
22
),(),(),(
tzN
EtzN
EtzG
Using simple Fourier equation with a generation term and an approximation on time evolution of Cv(T)
Localized injected carrier density
Initial conditions:
2202 exp zNN
Temporal evolution:
Spatial evolution:
exchangezdVtTtTCdt
dzdVtzG )()())(()(),( v
with:
exchangestoragegenerationHeat
,
Results from Simulation
Band structure of Si at 300 K
-1 0 1 2 3 4 5 6 7 8 9 10 11 12 14
x 10-9
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
t(s)
Evap
orat
ion
prob
abilit
y (A
U)
Laser intensity
Photon energy 1.2 ev, K=100 W/mK, Heated zone 200 nm
1.1 eV0.1 eVh=1.2 eV
))(
exp(0 Tk
EQ
B
Results from Simulation
Band structure of Si at 300 K
-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 15
x 10-9
10-6
10-5
10-4
10-3
10-2
10-1
100
t(s)
relat
ive e
vapo
ratio
n pr
obab
ility (
AU)
-1 0 1 2 3 4 5 6 7 8 9 10 11 12 14
x 10-9
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
t(s)
Evap
orat
ion
prob
abilit
y (A
U)
Laser intensity
Laser intensity
Photon energy 1.2 ev, K=100 W/mK, Heated zone 200 nm
Photon energy 2.2 ev,K=100 W/mK, Heated zone 200 nm
1.1 eV
1.35 eVh=2.45 eV
Conclusion & Perspectives
• Ultra-short laser pulses have been utilized to control atom evaporation• We propose a model to explain particular evaporation flux observed with near-
resonant band gap excitation • This model can not explain the observed saturation of photon absorption• Perhaps it can be explained by band bending… Work under progress
• Are optical nonlinear absorptions an efficient process ?…Work under progress• Are diffusive transport plays a role in the evaporation process ?
• Atom probe tomography is sensitive to thermal processes in the fs range when near-resonant band gap illumination is used
Sample preparationSample preparation
Deposition of protection cap : Pt Ion deposition (~1µm)Cut a lamella by FIB “Welding” it to the micromanipulator Bringing it in contact with a support pillar Welding it and cutting a portion of tip
Two steps for sample preparation
(a) Lift out method (CAMECA)(CAMECA)(b) Annular milling
Annular Milling
Rough Mill Sharpening Final
0.5-1nA,30 keV 20-100pA, 30keV few pA, minimum Ga
acceleration
1 m
Si
h
d
h > 2 x d
The sample is aligned along the beam direction,the inner diameter of the circular mask and the milling current
are reduced after each milling stage.
Relaxation time 2
Total energy given to the lattice 1.2 eV
E1=1.1 eV
E2=0.1 eV
2-steps transition
Z
Y
dV
S(z)- S(z)+
diameter <<1000 nm Absorption ~10 cm-1
yII exp0
I/I0~1 Homogeneous absorption
Relaxation time 1
2
22
N
dt
dN
N2 (z,t), injected electron density with a relaxation time
2
2
2
1
11
NN
dt
dN
N1 (z,t), thermalized electron density with a relaxation time 1
Model
Using simple Fourier equation with a generation term and an approximation on time evolution of Cv(T)
Localized injected carrier density
Initial conditions:
2202 exp zNN
Temporal evolution:
Spatial evolution:
exchangezdVtTtTCdt
dzdVtzG )()())(()(),( v
exchangestoragegenerationHeat
,
))(()()(
))(()()(
zTgradzSTK
zTgradzSTK
)()()()()().()(
)(),( zTgradzSzTgradzSTKdt
TzdVTCdzdVtzG