Grazing Incidence Fast Atom Diffraction: a new tool for surface characterization

Transcription

1 Grazing Incidence Fast Atom Diffraction: a new tool for surface characterization Victor H. Etgens Institut des NanoSciences de Paris

2 Outline Introduction GIFAD : technique how it works; Experimental set-up; What can we gain or loose with it? Some experiments will be needed to ; Summary.

3 Joint program between INSP and LCAM Victor H. Etgens, Franck Vidal, Massimiliano Marangolo, Mahmoud Eddrief and Fabio Finocchi Hocine Khemliche Philippe Roncin

4 Development of a new on-line surface characterization tool for in situ and real time studies Equipment installed in a real Molecular Beam Epitaxy growth chamber Specially customized growth chamber with Time of Flight option

5 GIFAD : technique how it works Ideal experimental parameters: - Tiny He beam with small divergence (φ<500 µm,α< 1 mrad) - Low normal energy (<1 ev) - total energy 1 kev Wavelength: Thermal vibrations: λ = 0.45 pm for 1 kev He (10 pm for 15 kev e-) < u 2 > few 10 pm

6 Particle-surface interaction at grazing incidence θ inc ~2 deg u.a. Å 200 < E 0 < 10 kev E = sin 2 ( θ ) E 0 inc 10 mev < E < 10 ev no penetration below the topmost layer probes the valence electron density (few 10-3 e - /Å 3 ) high sensitivity to surface defects (ad-atoms, step-edges) Sensitive and non-invasive tool for probing the solid-vacuum interface

7 High energy grazing incidence Electrons LEED RHEED Atoms, molecules Low energy normal incidence TEAS??

8 Low energy normal incidence High energy grazing incidence Electrons LEED RHEED Atoms, molecules TEAS GIFAD?? Looks like GIFAD is naturally filling an empty slot!

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11 Surface structure analysis: state of the art Diffraction tools (reciprocal space) - LEED (low energy, normal incidence) high resolution 2D structure - RHEED (high energy, grazing incidence) real-time monitoring of growth - Grazing incidence X-Rays synchrotron radiation facilities - TEAS (thermal energy atom scattering) top layer sensitivity, very slow Microscopy tools (real space) - AFM corrugation from total electron density - STM corrugation from energy resolved density of states Compared to these common techniques, what could be gained with Grazing Incidence Fast Atom Diffraction (GIFAD)?

12 Classical vs. quantum classical quantum Peak spacing distance between atomic rows (crystallography) Peak intensities shape of the interaction potential ( surface corrugation) Egg-box Corrugated ion plate Only motion normal to the surface ( 10 mev<e <1 ev) is considered Comparable to thermal energy atom scattering! Assuming a cosine-like potential, intensities of the diffraction peaks are given by Bessel functions: I n = J n 4π. h λ 2 For He, interaction potential valence electron density

13 Experimental INSP

14 Experimental INSP Replaced by a new machine

15 Experimental INSP RIBER compact 21 R&D of last generation

16 Two set-ups will be integrated : RHEED and GIFAD

17 Universality of GIFAD insulators Schüller, Phys. Rev. Lett. 98, (2007) Rousseau, Phys. Rev. Lett. 98, (2007) Rousseau, J. of Physics: Conf. Series 133, (2008) semiconductors Alkali-halides: we see the electrons from the alkali! metals Khemliche, Appl. Phys. Lett. (2009) Bundaleski, Phys. Rev. Lett. 101, (2008) Schüller, Phys. Rev. Lett. 102, (2009) Decoherence due to electronic excitations is not a problem!

18 Application to semiconductors: ZnSe(001) GIFAD, He at 400 ev ZnSe (001) c(2x2) - [110] [110] [100] RHEED, 12 kev - [110] [110] Weigand et al., Phys. Rev. B 68, (2003) Quiz: RHEED or GIFAD? LEED

19 Corrugation of ZnSe(001) c(2x2) Eikonal approach : a 1 I( n) = exp[ i( ( kiz kf z ) h( x) ng x) ] dx a corrugation function: h( x) = 5 i i= 0 λ 2 4π x h.sin A simple fit to the data yields the shape of the interaction potential profile of the valence electron density Charge transfer Zn Se The corrugation is measured with very high resolution. (No atomically resolved STM images of ZnSe!)

20 What we hope GIFAD can do : surfaces properties: - crystalline structure and orientation - valence electron density profile - electronic structure (pulsed beam) - chemical composition at the surface (LEIS-DRS) - growth mode (multi-layers, islands, ) - number of layers Static or dynamic model dynamic model GIFAD appears to be a robust diffraction technique - can sustain high temperatures - can sustain imperfect surfaces - can sustain some inelasticity (inelastic diffraction)

21 Conclusions Advantages of GIFAD: - extreme surface sensitivity (2-5 Å above top layer) - non-invasive (well suited for insulators and other fragile materials) - quantitative interpretation (based on the simple hard-wall model) - probes the electron density within an interesting distance to the surface: far from the nuclei (unlike electrons and X- Rays) in a range where calculations are reliable (LDA vs. GGA) - high resolution AFM-like corrugation (variable distance from surface) - allows identification of atomic species present at the surface (terminal layer, adatoms) - real-time (few sec/image) - grazing geometry compatible with film growth (MBE)