1. 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

2. Plan Introduction to Atom Probe Tomography Ultra-short Pulse Laser Assisted Atom Probe Applications Silicon Field evaporation Theoretical interpretation Conclusion & Perspectives

3. 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 nm 3 Spatial Resolution - 0.2nm in depth 0.5nm laterally Position Sensitive Detector (X,Y,TOF) Radius R<100 nm V L X Y Atom Probe Tomography

4. Femtosecond laser assisted atom probe Tip D Spot Laser beam τ pulse  Energy used ~ 0.1 – 100 μJ /pulse € D spot ~ 100-800 μm  τ pulse ~40-500 fs  on-demand wavelength (infrared-visible-UV)  repetition rate 1-100 kHz B. Gault, et al. Rev. Sci. Instrum. 77, 043705 (2006)

5. Laser Assisted Tomography Atom Probe Start signal V 0 < 20 kV PSD R Ion P < 10 -10 Pa T < 20-80K tip 3 Colour box Stop signal R<100nm fs laser pulse Femtosec laser,100kHz 500fs Time of flight UV Green IR Specimen Needle Shape

6. 100 nm Applications of Different AspectsMgO Fe Chemical nature of the material mass to charge ratio obtained by TOF measurement m mass of the ion,V the DC voltage L,flight length,t flight time,k constant CoFeTb multilayer SiCo FeMgOFe A. Grenier et al. JAP 102,033912 2007 Talaat Al Kassab, IJMR 99,5,2008 M.Gilbert et al. Ultramicroscopy 107,767,2007 Wide range of materials - All metallic materials - Alloys - Multiple quantum well - Nano wires 

7. Thermal evaporation Photo ionisation Mechanism for Field evaporation I las 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 CBvacuum eEx VB Tsong et al J. Chem. Phys., 65(6) 1976 Tsong, PRB 30(9) 1984

8. Condition for good mass resolution Mass spectra of Silicon under Infra Red Femtosecond Laser at 80K Intensity GW/cm 2 Metal Silicon Best Poster Award, IFES 2008 B.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 photon n, number of photon Zone 1 Zone 2

9. Condition for good mass resolution Mass spectra of Silicon under Infra Red Femtosecond Laser at 80K Bad mass resolution with higher laser energy Loosing events close to Si mass There is a saturation after a certain laser energy Intensity (GW/cm 2 ) Metal Silicon Zone 2 Zone 1

10. 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 80K There is a hump appeared with increasing laser energy with photon energy of near band gap energy. (IR) (Green)

11. 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 Photon energy - 2.45eV Photon energy - 3.62eV No evidence of hump, even by increasing laser energy; and no variation in mass spectra. (Green) (UV) Evidence of hump with photon energy of near band gap energy CONCLUSION The hump seems to appear only using photons with near-band gap energies Existence of hump in SiC using photon energy of near band gap energy

12. Relaxation time  2 Total energy given to the lattice 1.2 eV E 1 =1.1 eV E 2 =0.1 eV 2-steps transition Z Y dV S(z)- S(z)+ diameter <<1000 nm Absorption  ~10 cm -1 I/I 0 ~1  Homogeneous absorption Relaxation time  1 N 2 (z,t), injected electron density with a relaxation time  2 N 1 (z,t), thermalised electron density with a relaxation time  1 Model Using simple Fourier equation with a generation term and an approximation on time evolution of C v (T) Localized injected carrier density Initial conditions: Temporal evolution: Spatial evolution: with:

13. 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 1.1 eV 0.1 eV h =1.2 eV

14. 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 Photon energy 2.2 ev,K=100 W/mK, Heated zone 200 nm 1.1 eV 1.35 eV h =2.45 eV

15. 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

18. Sample preparation Sample 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 (CAMECA) (a)Lift out method (CAMECA) (b)Annular milling

19. 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.

20. Relaxation time  2 Total energy given to the lattice 1.2 eV E 1 =1.1 eV E 2 =0.1 eV 2-steps transition Z Y dV S(z)- S(z)+ diameter <<1000 nm Absorption  ~10 cm -1 I/I 0 ~1  Homogeneous absorption Relaxation time  1 N 2 (z,t), injected electron density with a relaxation time  2 N 1 (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 C v (T) Localized injected carrier density Initial conditions: Temporal evolution: Spatial evolution: