1. University of Illinois at Chicago RRC - Electron Microscopy Service X-Ray Photoelectron Spectroscopy: Theory and Practice PHYS-481 (Fall 2009)

2. University of Illinois at Chicago RRC - Electron Microscopy Service Contact Information for EMS in RRC-East Alan Nicholls, PhD Director of Research Service Facility - Electron Microscopy Research Resources Center-East 845 West Taylor Street SES Building, Room 110 Email:nicholls@uic.edu Office:(312) 996-1227 Ke-Bin Low, PhD Senior Research Specialist - Electron Microscopy Research Resources Center-East 845 West Taylor Street SES Building, Room 112 Email:kebinlow@uic.edu Office:(312) 355-2087

3. University of Illinois at Chicago RRC - Electron Microscopy Service Outline of Lecture (1)Background (2)Vacuum 101 (3)Analytical Capabilities (4)Instrumentation (5)Spectrum Simulation (6)Summary

4. University of Illinois at Chicago RRC - Electron Microscopy Service Background Photoelectric effect discovered by Albert Einstein Nobel Prize 1921 Photoemission as an analytical tool demonstrated by Kai Siegbahn (Electron Spectroscopy for Chemical Analysis – ESCA) Nobel Prize 1981

5. University of Illinois at Chicago RRC - Electron Microscopy Service specimen Background

6. University of Illinois at Chicago RRC - Electron Microscopy Service Background

7. Quantum NumbersSpectroscopist Notation nlsjnl j 10± 1/21/21s 1/2 20± 1/21/22s 1/2 21+ 1/23/22p 3/2 21- 1/21/22p 1/2 30± 1/21/23s 1/2 31+ 1/23/23p 3/2 31- 1/21/23p 1/2 32+ 1/25/23d 5/2 32- 1/23/23d 3/2 j = |l ± s| j: Total angular momentum l: Orbital angular momentum s: Spin angular momentum Background

8. University of Illinois at Chicago RRC - Electron Microscopy Service XPS probes core-levels → Binding energies in the range of 10 – 10 3 eV → Kinetic energies of similar magnitudes when Al-Kα or Mg- Kα radiation is used → Electrons with such low KE easily scattered (REMEMBER THIS) Background

9. University of Illinois at Chicago RRC - Electron Microscopy Service 1000 10 1 IMFP, λ (nm) 100 Kinetic Energy (eV) ‘Universal’ IMFP vs. KE Curve Background ‘Universal Curve’ shows that photoelectrons with KE in the 10 – 10 3 eV range have inelastic mean-free-paths (IMFPs) from 1 – 3.5 nm IMFP depends on: (1) Material (atomic #, density) (2) Kinetic energy

10. University of Illinois at Chicago RRC - Electron Microscopy Service 95% of all photoelectrons detected are generated within 3λ of the surface = Sampling Depth (65% within 1λ). ‘3λ’ is used as the ‘benchmark’ definition for Sampling Depth in XPS. So the sampling depth for XPS is typically 3 - 10 nm → Surface-Sensitive! Instrument must be run under ultra-high vacuum! Background

11. University of Illinois at Chicago RRC - Electron Microscopy Service VISCOUS FLOW - When diameter of tube > 100  gas molecules more likely to bump into each other. Molecules in general move towards lower pressure end of tube. Unlikely to get backstreaming. MOLECULAR FLOW - When diameter of tube is <  gas molecules are more likely to collide with the tube wall than each other. There is free movement of molecules in either direction, the numbers directly related to ratio of pressures at each end of tube. At high vacuum this ratio is likely to be close to 1. Backstreaming a concern. TRANSITIONAL FLOW - Intermediate between Viscous and Molecular. Vacuum 101

12. University of Illinois at Chicago RRC - Electron Microscopy Service So for an 60mm diameter tube VISCOUS > 0.1 Torr > TRANSITIONAL > 1 mTorr > MOLECULAR PUMPS Used from atmosphere down to 0.1Pa Problems with corrosive of condensable gases (H 2 O) Potential source of oil contamination of vacuum system if pressure in line to system is not kept in viscous flow regime. At or near atmospheric pressure an oil mist is ejected through outlet valve. Must vent outside or through a filter. Used from 1Pa to 10 -7 Pa Historically most widely used high vacuum pump, really a vapor jet pump. Pumping speed virtually constant below 10 -1 Pa Major problem - Backstreaming; minimised by using a low vapour pressure oil. BAD NEWS - do not let air into a diffusion pump! Used from 10 -1 Pa to 10 -9 Pa Gas molecules that are pumped are trapped inside pump by the gettering action of the sputtered Ti - limited lifetime. Absolute freedom from oil contamination with no moving parts. Ideal for high vacuum systems but are not well suited on systems that are cycled frequently to atmosphere. Used from 10 Pa to 10-8 Pa Extremely high speed (10,000rpm) mechanical pumps typically with magnetic levitation bearings for EM use. Works efficiently in Molecular Flow region - needs to be backed. No backstreaming of oil when operating at full speed. Major concern - preventing physical damage to pump! ROTARYDIFFUSIONION PUMPTurbo Molecular PumpOTHER PUMPS Cryosorption - oil free, capture, Atmosphere to 10 -1 Pa Diaphragm - oil free, transfer, Atmosphere to 1Pa Claw pump - dry, transfer, Atmosphere to 10Pa Molecular Drag - dry, transfer, 10Pa to 10 -6 Pa Sublimation - oil free, capture, 10 -1 Pa to 10 -9 Pa REMEMBER -- No Pump exerts a force that drags or pulls gas molecules to it. Pumping is purely diffusion of gas molecules from high pressure to low pressure regions Vacuum 101

13. University of Illinois at Chicago RRC - Electron Microscopy Service Analytical Capabilities of XPS (1)Identify elements/compounds (except H and He) (2)Determine oxidation states (e.g. Ti 3+ or Ti 4+ ) (3)Identify types of chemical bonds (e.g. Si-O or Si-C) (4)Semi-quantitative analysis (10-15% error) (5)Determine adsorbate/film thickness (6)Highly surface-sensitive (3 – 10 nm from the surface) →Detection limit 0.1 to 1 at% →Ultra-high vacuum required!!! →Minimize/delay surface reactions and contaminations

14. University of Illinois at Chicago RRC - Electron Microscopy Service KINETIC ENERGY, eV Analytical Capabilities of XPS Survey spectrum for element identification

15. University of Illinois at Chicago RRC - Electron Microscopy Service Nickel Analytical Capabilities of XPS

16. University of Illinois at Chicago RRC - Electron Microscopy Service XPS spectra show characteristic "stepped" background. Due to inelastic processes (extrinsic losses) from deep in bulk. Electrons deeper in surface loose energy and emerge with reduced KE, apparent background increase at higher BE Analytical Capabilities of XPS

17. University of Illinois at Chicago RRC - Electron Microscopy Service Typical Features and ‘Artifacts’ of Core-Level Peaks Can get multiple peaks from core levels – must be aware of where they come from in order to carry out chemical analysis (not all may be present) 1.Spin orbit splitting leads to additional peaks (no splitting for s, splitting for p,d,f etc.) 2.Additional peaks due to, for example, chemical shifts and oxidation states 3.Ghost peaks at lower binding energies (achromatic X-ray only) – no useful info! 4.Shake up/ off peaks at higher binding energies (result of energy being transferred from the ejected photoelectron electron to a valence electron). 5.Plasmon loss peaks (due to electron excitations) 6.Photon-induced Auger peaks 7.Effects of charging of non conductive specimens Analytical Capabilities of XPS

18. University of Illinois at Chicago RRC - Electron Microscopy Service 537.0 534.0 531.0 528.0 525.0 Binding Energy (eV) O 1s No spin-orbital splitting for s Spin-orbital splitting for p, d, f Analytical Capabilities of XPS

19. University of Illinois at Chicago RRC - Electron Microscopy Service Ti 2p 1/2 and 2p 3/2 chemical shift for Ti and Ti4+. Charge withdrawn Ti → Ti4+ so 2p orbital relaxes to higher BE Analytical Capabilities of XPS

20. University of Illinois at Chicago RRC - Electron Microscopy Service Analytical Capabilities of XPS

21. University of Illinois at Chicago RRC - Electron Microscopy Service Schematic of Ghost and Shake-up peaks Binding Energy Kinetic Energy Main peak Shake-up Peak Ghost Peak Analytical Capabilities of XPS

22. University of Illinois at Chicago RRC - Electron Microscopy Service Electrical insulators cannot dissipate charge generated by photoemission Process. Surface picks up excess positive charge - all peaks shift to higher BE Can be reduced by exposing surface to neutralizing flux of low energy electrons - "flood gun" or "neutralizer“. BUT must have good reference peak. Analytical Capabilities of XPS

23. University of Illinois at Chicago RRC - Electron Microscopy Service I s = I o exp (-d / λcosθ) I s : Intensity at surface I o : Intensity from infinitely-thick sample d: depth λ: Inelastic mean-free- path (IMFP) θ: Spectrometer take-off angle Beer-Lambert relationship (Numerical expression Describing the photoelectron Intensity generated from a material) Analytical Capabilities of XPS

24. University of Illinois at Chicago RRC - Electron Microscopy Service SiO 2 Surface Layer Si Substrate Analytical Capabilities of XPS Using the Beer-Lambert expression to estimate film thickness…

25. University of Illinois at Chicago RRC - Electron Microscopy Service I Si : 1411.6 I SiO2 : 241.4 When λ, θ and all the respective intensities are known, film thickness can be determined by taking the ratio of I film to I substrate and solve for d Analytical Capabilities of XPS

26. University of Illinois at Chicago RRC - Electron Microscopy Service Semi-Quantitative Analysis Photoelectron intensity from a homogeneous material is also dependent on instrumental factors, and can be alternatively-described by I = JCσζTλ J:X-ray flux C:Concentration of the element-of-interest σ:Ionization cross-section ζ:Spectrometer angular acceptance T:Spectrometer transmission function λ:IMFP of the element Analytical Capabilities of XPS

27. University of Illinois at Chicago RRC - Electron Microscopy Service C = I/(JσζTλ) = I/JF F is termed the atomic sensitivity factor: - incorporates all the terms associated with the spectrometer and material - empirically-determined by XPS manufacturer - values are normalized against Fluorine Analytical Capabilities of XPS

28. University of Illinois at Chicago RRC - Electron Microscopy Service Atomic fraction of an element (A) in a multi-component material (ABCD…) can be estimated using the following, Atomic % A = (I A /F A ) /  (I n /F n ) Quantification using this expression is valid only if: (1) Material is homogeneous, (2) Material surface is smooth and flat. Analytical Capabilities of XPS

29. University of Illinois at Chicago RRC - Electron Microscopy Service Ion Gun Charge Neutralizer (built into lenses) Instrumentation

30. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Kratos Axis-165 XPS system in RRC-East

31. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Total resolution (i.e. peak’s full-width-half-maximum) of instrument is convolution of: (1) X-ray energy spread, (2) Spectrometer broadening, and (3) Intrinsic line-width of the element-of-interest. Total FWHM = {FWHM x-ray 2 + FWHM spectrometer 2 + FWHM intrinsic 2 } 1/2 Intensity Energy 50 % 100 % Total FWHM

32. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Binding energy shifts due to different chemical states or bonding configurations can be subtle (1eV or less) for certain elements. Say, if achievable total FWHM of a peak is 2 eV, the XPS instrument will not be able to resolve 2 peaks that are separated only by 0.5eV!!! Total FWHM can only be decreased by minimizing the FWHM of: (1) X-ray; (2) Spectrometer * Intrinsic line-widths is a non-controllable term!

33. University of Illinois at Chicago RRC - Electron Microscopy Service Ideal Candidates for X-ray source: Al- and Mg-Kα (1)High energy (2) Narrow energy spread Instrumentation

34. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Twin-Anode Achromatic X-ray source: Bombard metallic anode with 10-25kV electrons with ~10mA of current to generate X-rays. Can generate high X-ray flux producing high signal BUT specimen may be damaged by heat generated by the X-rays and continuum radiation and source emits X-ray satellites (additional weak lines at lower binding energies) Simple, relatively inexpensive High flux (10 10 -10 12 photons·s - 1 ) Beam size ~ 1cm

35. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Rowland Circle Quartz Crystal X-ray Source Sample Electron Spectrometer Monochromatic X-Ray source: Diffraction from bent SiO 2 crystal focusing primary λ at specimen. Other λ 's focused at different points in space (filtered). Always use Al Kα which is diffracted from quartz (no equivalent crystal for Mg Kα) Beam size ~ 1 cm to 50 mm Eliminates satellites peaks – simpler spectra Decreases FWHM of X-ray energy Flux decreases at least an order of magnitude leading to less damage, improves S/N (no X-ray continuum) but lower signal More complicated and expensive

36. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Most common type of electrostatic deflection-type analyzer: Concentric Hemispherical Analyzer (CHA) or spherical sector analyzer Energy resolution dependant on radius. Capable of collecting photoelectrons of larger angular distribution. Photoelectrons of a specific energy are focused by the lens at the slit of the spectrometer. Lens also controls sampling area. Photoelectrons travel through a circular path and exit into a series of channeltrons (electron multipliers).

37. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Negative potential on two hemispheres V 2 > V 1 Potential of mean path, R o through analyzer is V o = (V 1 R 1 +V 2 R 2 )/2R o An electron of kinetic energy eV = V o will travel a circular orbit through hemispheres at radius R o Since R o, R 1 and R 2 are fixed, in principle changing V 1 and V 2 will allow electrons of different KE to be detected.

38. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation But  E/E o = S/2R o  E = (S/2R o )E o Peak FWHM Spectrometer Term (constant) Photoelectron KE Spectrometer broadening (i.e. FWHM spectrometer ) is a function of photoelectron KE entering spectrometer… i.e. Resolution is non-uniform across XPS energy spectrum!

39. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation - Hollow glass tube with semiconducting layer on inner surface - Electrons/ions entering Channeltron produce secondary electrons (SEs) - ‘Avalanche’ effect as SEs accelerate down the tube under HV - Signal amplified by ~ 10 6 to 10 8 Single-Channel Electron Multipliers (Channeltron)

40. University of Illinois at Chicago RRC - Electron Microscopy Service To circumvent the problem… (1) Hemisphere potentials are fixed to allow electrons with a fixed KE (the pass energy, E p ) to reach the electron detectors (Channeltrons); (2) Electrostatic lens before the slit decelerate photoelectrons of a particular KE to E p ; (3) Magnetic lens focus these electrons with E p at the slit, so that only electrons with that pass energy are allowed to enter the slit. Scanning of the energy-scale is achieved by varying the decelerating potential on the electrostatic lens instead of the hemisphere potentials → Fixed energy-resolution across the energy-scale!!! Something to Ponder… Should an infinitesimally-small pass energy yield an extremely mild spectrometer broadening??? Instrumentation

41. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Most XPS systems also come equipped with an ion gun. Purpose: (1) Removes surface contaminants (usually oxides and hydrocarbons); (2) Allows depth-profiling study. How does it ‘clean’ a surface? (1) Ionizes an inert gas (usually Ar) (2) Focuses and accelerates the ions towards the specimen surface (3) Rasters the ion beam across the surface (4) Ions impart energy to surface contaminants → Sputtering them away!

42. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Atomic Fraction of Cd Atomic Fraction of Te Cd: Te Ratio As-received0.650.351.86 10 mins Sputter 0.510.491.04 20 mins Sputter 0.550.451.22 Example: Successful removal of Cr-contamination from CdTe

43. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Potential Problems Associated with Sputter-Cleaning: Surface roughening; Alter oxidation states of some elements; Selective ‘etching’ of surface due to different sputtering rates in a multi-component material.

44. University of Illinois at Chicago RRC - Electron Microscopy Service Instrumentation Example: Oxidation-state changed in Pd after sputtering…

45. University of Illinois at Chicago RRC - Electron Microscopy Service Spectrum Simulation (Peak-Fitting) Simulation of XPS spectra (i.e. peak fitting) to match experimentally- observed spectra. Purpose: (1) Background noise subtraction to reveal true peak intensities; (2) Precise determination of peak position; (3) Deconvolute spectra into individual components when 2 or more peaks are in close proximity.

46. University of Illinois at Chicago RRC - Electron Microscopy Service Select Background Model Adding Synthetic Peak Providing Initial-Guess For Peak Parameters: (1)Line position (2)Area (3)FWHM (4)Gaussian-Lorentzian Ratio (5)Asymetry (6)S.O.S. Software Fitting End of Fitting Modify Peak Parameters Background Quality? Good Poor Fit Quality? Good Poor Option 1 Option 2 Spectrum Simulation (Peak-Fitting)

47. University of Illinois at Chicago RRC - Electron Microscopy Service Types of Background: (1) Linear (2) Shirley (typically-used) (3) Tougaard Shirley Tougaard Linear Spectrum Simulation (Peak-Fitting)

48. University of Illinois at Chicago RRC - Electron Microscopy Service Types of Line-Shapes: (1) Gaussian Function → Describes the measurement process (e.g. instrumental response, X-ray line-shape, Doppler and thermal broadening) (2) Lorentzian Function → Describes lifetime broadening (intrinsic line-width) XPS peaks can usually be described by varying G-L ratio Spectrum Simulation (Peak-Fitting)

49. University of Illinois at Chicago RRC - Electron Microscopy Service Peaks associated with pure metals may tend to be asymmetic… → Need to introduce a ‘Tail Modifier’ term to the G-L functions Spectrum Simulation (Peak-Fitting)

50. University of Illinois at Chicago RRC - Electron Microscopy Service XPSPEAK version 4.1 is a free windows-based XPS peak fitting program available online. Installation file and user manual downloadable at: http://www.phy.cuhk.edu.hk/~surface/XPSPEAK You will need this software to complete the XPS assignments! Spectrum Simulation (Peak-Fitting)

51. University of Illinois at Chicago RRC - Electron Microscopy Service Summary X-ray Photon Spectroscopy (XPS) (Originally called ESCA) is a surface- sensitive technique that probes the chemical properties of the top ~10 nm of a solid surface that must be UHV-compatible – no wet specimens! It is the most versatile and quantifiable of all the surface chemical analysis methods: elemental ID, oxidation-states, chemical bonds, film thickness, depth-profiling, semi-quantitative analysis… Limitations: (1) does not detect H or He; (2) Radiation damage possible (worse for achromatic sources); (3) Charge neutralization needed for insulating material; (4) Chemical analysis can be limited to functional groups and in some cases chemical shifts are not resolvable.

52. University of Illinois at Chicago RRC - Electron Microscopy Service Electron Microscopy Service @ UIC JEM-1220JEM-3010JEM-2010FHB601UXJSM-6320F S-3000NAXIS-165 XPSRamascope 2000VT-SPM