Investigation of Semiconducting materials using Ultrafast Laser assisted

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 29 th April 2009

Plan • Introduction to Atom 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) • 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 L • Ions projected on a PSD Y • TOF mass spectrometry X • 3 D reconstruction of the atomic distribution • Volume ~100 x 100 nm 3 V Radius R<100 nm • Spatial Resolution - 0. 2 nm in depth 0. 5 nm laterally

Femtosecond laser assisted atom probe DSpot τpulse 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 k. Hz Tip B. Gault, et al. Rev. Sci. Instrum. 77, 043705 (2006)

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

Applications of Different Aspects Co. Fe. Tb multilayer Si. Co Fe. Mg. OFe Fe Mg. O 100 nm M. Gilbert et al. Ultramicroscopy 107, 767, 2007 A. Grenier et al. JAP 102, 033912 2007 Talaat Al Kassab, IJMR 99, 5, 200 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 Wide range of materials - All metallic materials - Alloys - Multiple quantum well - Nano wires

Mechanism for Field evaporation 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 x CB Photo ionisation vacuum 2 hn e. E hn 3 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 VB 1 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 80 K photon energy(1. 2 e. V) Metal Silicon Zone 2 Zone 1 Intensity GW/cm 2 Measured flux is linearly dependent on laser intensity For the first time we have demonstrated that it is a single-photon process. n, number of photon I. e. the rate of evaporation can written Onebe photon as: Best Poster Award, IFES 2008 B. Mazumder, A. Vella, M. Gilbert, B. Deconihout, G. Schimtz Submitted to Surface Science

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

Study of Si mass spectra with different wavelength at 80 K Photon energy 1. 2 e. V (IR) There is a hump appeared with increasing laser energy with photon energy of near band gap energy. Photon energy 2. 45 e. V (Green) Non existence of the hump in mass spectrum by using laser energy with photon energy higher than the band gap energy.

of using humpphoton in Si. Cenergy using of photon Existence of. Existence hump in Si. C near energy of near band gap energy Photon energy - 2. 45 e. V (Green) Evidence of hump with photon energy of near band gap energy Photon energy - 3. 62 e. V (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

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

Results from Simulation Laser intensity Band structure of Si at 300 K Photon energy 1. 2 ev, K=100 W/m. K, Heated zone 200 nm h =1. 2 e. V 0. 1 e. V 1. 1 e. V

Results from Simulation Laser intensity Band structure of Si at 300 K Photon energy 1. 2 ev, K=100 W/m. K, Heated zone 200 nm Laser intensity 1. 35 e. V h =2. 45 e. V 1. 1 e. V Photon energy 2. 2 ev, K=100 W/m. K, Heated zone 200 nm

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

Sample preparation Two steps for sample preparation (a) Lift out method (CAMECA) (b) 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

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 0. 5 -1 n. A, 30 ke. V 20 -100 p. A, 30 ke. V h > 2 x d Final few p. A, minimum Ga acceleration

Model S(z)- S(z)+ Y Initial conditions: Z d. V diameter <<1000 nm Absorption ~10 cm-1 I/I 0~1 Homogeneous absorption Localized injected carrier density 2 -steps transition Temporal evolution: Relaxation time 2 N 2 (z, t), injected electron density with a relaxation time E 2=0. 1 e. V 2 Total energy given to the lattice 1. 2 e. V Relaxation time 1 E 1=1. 1 e. V N 1 (z, t), thermalized electron density with a relaxation time 1 Spatial evolution: Using simple Fourier equation with a generation term and an approximation on time evolution of Cv(T)