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 Mazumder F. Vurpillot, A. Vella, B. Deconihout & G. Martel. G roupe de P hysique des M atériaux / Coria 29th April 2009. Plan. Introduction to Atom Probe Tomography

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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

Investigation of Semiconducting materials using Ultrafast Laser assisted Atom Probe Tomography

Baishakhi Mazumder

F. Vurpillot, A. Vella, B. Deconihout & G. Martel

Groupe de Physique des Matériaux / Coria

29th April 2009


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Plan

  • Introduction toAtom Probe Tomography

  • Ultra-short Pulse Laser Assisted Atom Probe

  • Applications

  • Silicon Field evaporation

  • Theoretical interpretation

  • Conclusion & Perspectives


Atom probe tomography

Position Sensitive

Detector (X,Y,TOF)

Radius

R<100 nm

Y

X

Atom Probe Tomography

  • APT = FIM + TOF

  • Tip subjected to field F~V/R and the evaporation

  • rate 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

L

V


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Femtosecond laser assisted atom probe

τpulse

DSpot

Laser beam

  • 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

Tip

B. Gault, et al. Rev. Sci. Instrum. 77, 043705 (2006)


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Laser Assisted Tomography Atom Probe

R<100nm

R

Ion

tip

-10

P < 10

Pa

T < 20-80K

PSD

V

< 20 kV

0

Femtosec laser,100kHz

500fs

fs laser

pulse

Green

UV

3 Colour box

Stop

IR

signal

Start

signal

Time of flight

Specimen

Needle

Shape


Applications of different aspects

Applications of Different Aspects

SiCo

CoFeTb multilayer

FeMgOFe

MgO

Fe

100 nm

M.Gilbert et al. Ultramicroscopy 107,767,2007

A. Grenier et al. JAP 102,033912 2007

Talaat Al Kassab, IJMR 99,5,2008

Chemical nature of the material mass to charge ratio obtained by TOF measurement

Wide range of materials

- All metallic materials

- Alloys

- Multiple quantum well

- Nano wires

m mass of the ion,V the DC voltage

L,flight length,t flight time,k constant


Mechanism for field evaporation

Mechanism for Field evaporation

eEx

CB

vacuum

2

hn

hn

3

1

VB

Thermal evaporation

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

Photo ionisation

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

Tsong et al J. Chem. Phys., 65(6) 1976

Tsong, PRB 30(9) 1984


Condition for good mass resolution

Mass spectra of Silicon under Infra Red Femtosecond Laser at 80K

Condition for good mass resolution

photon energy(1.2eV)

 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:

Metal

Silicon

Zone 2

Zone 1

n, number of photon

One photon

Best Poster Award, IFES 2008

B.Mazumder,A.Vella,M.Gilbert,B.Deconihout,G.Schimtz Submitted to Surface Science

Intensity GW/cm2


Condition for good mass resolution1

Mass spectra of Silicon under Infra Red Femtosecond Laser at 80K

Condition for good mass resolution

Metal

Bad mass resolution with higher laser energy

Loosing events close to Si mass

There is a saturation after a certain laser energy

Silicon

Zone 2

Zone 1

Intensity (GW/cm2 )


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Study of Si mass spectra with different wavelength at 80K

Photon energy 1.2 eV

(IR)

There is a hump appeared with increasing

laser energy with photon energy of near band

gap energy.

Photon energy 2.45eV

(Green)

Non existence of the hump in mass

spectrum by using laser energy with photon

energy higher than the band gap energy.


Existence of hump in sic using photon energy of near band gap energy

Existence of hump in SiC using photon energy of near band gap energy

Existence of hump in SiC using photon energy of near band gap energy

(Green)

Photon energy - 2.45eV

Evidence of hump with photon energy of

near band gap energy

Photon energy - 3.62eV

(UV)

No evidence of hump, even by increasing laser

energy; and no variation in mass spectra.

CONCLUSION

The hump seems to appear only using photons with near-band gap energies


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Model

Y

Initial conditions:

S(z)-

S(z)+

diameter <<1000 nm Absorption ~10 cm-1

dV

Z

I/I0~1  Homogeneous absorption

 Localized injected carrier density

2-steps transition

Temporal evolution:

 N2 (z,t), injected electron density with a relaxation time 2

Relaxation time 2

 N1 (z,t), thermalised electron density with a relaxation time 1

E2=0.1 eV

Total energy given

to the lattice 1.2 eV

Relaxation time 1

Spatial evolution:

E1=1.1 eV

Using simple Fourier equation with a generation term

and an approximation on time evolution of Cv(T)

with:


Results from simulation

Results from Simulation

Band structure of Si at 300 K

Laser intensity

Photon energy 1.2 ev, K=100 W/mK, Heated zone 200 nm

h=1.2 eV

0.1 eV

1.1 eV


Results from simulation1

Results from Simulation

Laser intensity

Band structure of Si at 300 K

Photon energy 1.2 ev, K=100 W/mK, Heated zone 200 nm

Laser intensity

1.35 eV

h=2.45 eV

1.1 eV

Photon energy 2.2 ev,K=100 W/mK, Heated zone 200 nm


Conclusion perspectives

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


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

THANK YOU

FOR YOUR ATTENTION


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Questions


Sample preparation

Sample preparation

  • Two steps for sample preparation

  • Lift out method (CAMECA)

  • Annular milling

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


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Annular Milling

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.

1 m

h

d

Si

Rough Mill Sharpening Final

0.5-1nA,30 keV 20-100pA, 30keV few pA, minimum Ga

acceleration

h > 2 x d


Investigation of semiconducting materials using ultrafast laser assisted atom probe tomography

Model

Y

Initial conditions:

S(z)+

S(z)-

Z

diameter <<1000 nm Absorption ~10 cm-1

dV

I/I0~1  Homogeneous absorption

 Localized injected carrier density

2-steps transition

Temporal evolution:

 N2 (z,t), injected electron density with a relaxation time 2

Relaxation time 2

E2=0.1 eV

Total energy given

to the lattice 1.2 eV

 N1 (z,t), thermalized electron density with a relaxation time 1

Relaxation time 1

E1=1.1 eV

Spatial evolution:

Using simple Fourier equation with a generation term

and an approximation on time evolution of Cv(T)


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