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