Abner de Siervo (16.11.2004)

1. DELTA – Winter Semester 2004 “An Introduction to Auger Electron Spectroscopy : Applications and Fundamental Studies on Electronic Structure of Atoms Molecules and Solids” Abner de Siervo (16.11.2004)

2. Outline • Second Part: Fundamental Studies • Motivation • Theoretical simulation for Auger process: • - coupling schemes and selection rules • - multiplets calculation • - transition probabilities (intensities) • - examples of line shape calculation • Different Mechanisms associated with • Auger emission: • - satellites: Coster-Kronig (C-K), Shake-up, • plasmons • Examples of opened possibilities with synchrotron radiation and XAES: • - shake-up versus C-K • - Sudden and Adiabatic Approximation • - Auger Cascade and Screening mechanisms First Part: • Historical Introduction • Basics principles - Auger emission - Energies determination - Nomenclature - Analysis Volume - Advantages and Disadvantages • Experimental Setups - RFA, CMA, HA - Simple methods for quantification • Applications: some examples. - chemical shift analysis - Auger Depth Profile - SAM – Scanning Auger Microscopy

3. Historical Introduction AES means Auger Electron Spectroscopy – This spectroscopy technique uses Auger electrons as probes for surface science analysis: chemical and elemental characterization. TheAuger phenomenon is a not irradiative de-excitation process for excited atoms. The de-excitation occur by a Columbic interaction where the atom loss energy by emission of one or more electrons. This ejected electron to one continuum state is named Auger electron. 1923 or (1925) - This effect was discovered independently by Lise Meitner (1923 -Journal Zeitschrift fur Physik) and Pierre Auger (1925 -‘Radium’ ) 1953 - J. Lander uses electron to excited Auger electrons to study surface impurities. 1968 - L. Harris demonstrates usefulness of technique when he differentiates the energy distribution of Auger electrons emitted from a bombarded surface. About the same time, Weber and Peria employ LEED optics as Auger spectrometers. 1969 - Palmberg et. al invent the cylindrical mirror analyzer (CMA), greatly improving speed and sensitivity of the technique. The mid-80’s saw the implementation of Schottky field emitters as electron sources, allowing analysis of features ~20 nm in size. Improvements in analyzers and sources have pushed this limit to the 10 nm regime. Lise Meitner Pierre Auger

4. The Auger Process or photons or … (PE) Auger recombination and e- transport : Final State Ionization: Initial State Ground State • IMPORTANT to Remember: In the Auger process doesn’t exist a REAL photon intermediating the transition. Conservation Laws -U Observe: Auger electron energy is independent of the excitation energy ! U= Electron-Electron interaction in the final state + Relaxation energies U is known as Auger parameter

5. Nomenclature for Auger Transitions From the X-Ray techniques nlj K, (L1, L2, L3), M1, … Spectroscopy Nomenclature (example : XPS) nlj  1s; (2s, 2p1/2,2p3/2), 3s, … Conventionally is used the X-Ray type in the nomenclature of Auger transition. In this example: KL1L23 . When the electronic levels are energetically well distinguishable is common to use more sub-indices, for example L1,2,3M2,3M4,5. For a group of transition, the sub-indices are in many times omitted (KLL, LMM, MVV) and for transition involving level(s) in the valence band is common to use V instead (L,M,N,O ..): Example M4,5VV.

6. Excited with Ti K=4511 eV XPS peaks Kinetic Energy (eV) For a given element, several lines of Auger emissions can be observed. Auger Transitions lines Intensity (a.u.) LMM + LMN A. de Siervo (MSc. Thesis University of Campinas, 1988)

7. Auger Transitions lines for different elements Red dots are indicating the most intensity lines

8. Analysis Volume • Depending on the spot size of the e-gun is possible to have spatial resolution in the (nm) range. • In the direction perpendicular to the surface the analysis volume depends on the electron mean free path.

9. Advantages and Limitations Advantages: • Surface sensitive • Elemental and chemical composition analysis by comparison with standard samples of known composition • Detection of elements heavier than Li. Very good sensitivity for light elements. • Depth profiling analysis: quantitative compositional information as a function of depth below the surface (destroy the sample) • Spatial distribution of the elements (SAM): Elemental or even chemical Auger maps analysis in lines, points and areas. Disadvantages / Limitations: • Samples must be compatible with UHV in most of cases. • For samples not prepared in-situ is normally necessary cleaning procedures such as sputtering, heating or scraping of the surface (some times, it is not possible) • Samples must be conductive. In some cases is possible to avoid charging effects also for non-conductive samples • Possibility of beam damage of some surfaces, for example some organic samples and polymers • Hydrogen and helium are not detectable (only by indirect ways when they are present in the compounds or physically adsorbed). • Quantitative detection is dependent on the element: light elements > 0.1%; heavier elements > 1%. • Accuracy of quantitative analysis depending on the availability of adequate sensitivity factors (or standards). Typical accuracy ± 10%.

10. Analyzer Setups 1) RFA in 4-grid LEED optics Pre Amplifier Phase shifter 2f f f Frequency doubler Signal generator Lock in Amplifier Isolated transformer f Retarding H.V. Supply computer signal Seah and Briggs in “ Pratical Surface Analysis”

11. 2) CMA – Cylindrical Mirror Analyzer • Important Characteristics: • Energy resolution scales with Ep. • coaxial designing eliminates shadowing • Better transmission than an Hemispherical Analyzers • Relative Short work distance • Normally uses the lock-in amplifier to get the differential distribution dN(E)/dE.

12. 3) HA – Hemispherical Analyzer • Important Characteristics : • Better Energy Resolution • Long work distance possible • -Angle-dependent measurements possible

13. Sensitive factor Quantification in AES Quantification analysis using first principle is possible but rarely done due the large differences between coupling schemes that govern the Auger transitions in a multi ionized atom. The most common analysis use sensitive factors derived from pure materials or standards. This method also have a lot of imprecision and it should be judiciously used. Auger electron intensity: Simplified formula for Homogeneous materials:

14. Relative Sensitivity Factor for primary e= 3KeV PHI analyzers The most important message is:AES is very useful, probably one of the best way to surface analysis, but be careful when you start to write “ % “ for your sample !

15. Examples for AES P. Weightman, (review article) 1) Chemical Analysis • AES is one of the best complementary technique for XPS in the chemical analysis. Depending on the kinetic energy of the Auger electrons, AES is much more sensitive to the surface that conventional XPS. • Chemical shifts and Auger lineshape can be used to determine the chemical state for a given element in the sample, and in studies as charge transfer in alloys. Differences in the line shape and peak Position for the C Auger (KVV) in different CxHy compounds

16. Auger Depth Profiling • Sources of artifacts • sample charging • topographical features resulting of non-uniform sputtering of the sample • preferential sputtering • beam effects • Ion beam mixing R.Nix, http://www.chem.qmw.ac.uk/surfaces/scc/

17. SAM Conventional SEM image SAM http://www.aquila.infn.it/infm/Casti/Tech/Sam/Examples.html

18. Second Part: Fundamental Studies in AES Motivation: • Understanding the electronic structure: -Chemical bonds, charge transfer, material properties,… • Possibilities to verify simple models: -Atomic Theory, Complete Screening Model, - helpful in the development of other techniques example AED • AES is a ”laboratory of excited states” - theoretical determinations of branching ratios, fluorescent yields, ...

19. Theoretical simulation of Auger process electronic conf. of the atom . Atomistic approach: Hamiltonian of the : system (Leighton,R.B. “Principles of Modern Physics”) Robert D. Cowan, “ The theory of Atomic Structure And Spectra” (more approximations: Close shell approximation, Central potential) “Average Energy”: • Russell-Saunders ou LS: Coulomb >> Spin-Orbit [ Astrophs. J. 61,38 (1925)] • jj: Spin-Orbit >> Coulomb [ Condon and Shortley- “The theory of Atomic Spectra”] • Intermediate Coupling ( IC ): Coulomb  Spin-Orbit [ Condon and Shortley ...] Coupling schemes: jj coupling (normally for the initial state) LS coupling (normally in the final State)

20. Spin Orbit Coulomb Interaction R.D. Cowan in “ The Theory of Structure and Spectra” Transition Probabilities (Auger Intensities) (Fermi Golden Rule) For IC coupling The complete equation, also including open shell cases can be found in : E.J.McGuire, “Atomic Inner-Shell Processes-I: Ionization and Transition Probabilites” Chapter 7 (Academic Press, NY, 1975)

21. Practical Examples : 1) Auger Lineshape calculation A. de Siervo, R. Landers, G.G. Kleiman, et al. ; Phy. Rev. B 60 (1999)15790 A. de Siervo, R. Landers, G.G.Kleiman, et al.; J. Elec. Spec. Rel. Phen. 103 (1999) 751 G.G.Kleiman, R.Landers,S.G.C. de Castro, et al.; Phy. Rev. B 58 (1998)16103 R. Landers, S.G.C. de Castro, A. de Siervo, et al.; J. Elec. Spec. Rel. Phen. 94 (1998) 253

22. Open possibilities for XAES using synchrotron radiation Selecting channels for transition changing the photon energies: shake-up vs CK J. Marais, A. de Siervo, R. Landers,et al.; Surface Science 435 (1999) 878

23. From the Adiabatic to the Sudden Approximation Sudden Approx. Adiabatic Approx. No satellites J. Morais, A. de Siervo, R. Landers, et al. 103 (1999) 661 T.D.Thomas, PRL 52 (1984) 417

24. More Complex Auger Transitions : cascade Process

25. MVV excited below and above L3 threshold 1. Enormous increase in normal MVV emission - attributed to combination of normal MVV + fluorescence + Auger cascade. Describe observed intensities. 2. Pd seems to behave as though it had a full d-band induced by core hole. In Short, first observation of unambiguous quasiatomic spectral structure produced purely by screening mechanisms A. de Siervo, R. Landers and G.G. Kleiman, PRL 86, (2001) 1362. A. de Siervo, R. Landers, M.F. Carazzolle, et al. J.E.S.R.P 114 (2001) 679

26. Vielen Danke Muito Obrigado :-)