Chemical/Compositional Analysis

1. Chemical/Compositional Analysis • Incident electron undergoes inelastic scattering causing ejection of core (inner shell) electron • Energy loss of incident electron, or energy of ejected electron or x-ray, is characteristic of the target element • Can determine compositional & chemical bonding information incident electron ejected orbital electron scattered primary electron (EELS)  Electron Relaxation   Auger electron emitted (AES) x-ray photon emitted (EDX)

2. Chemical/Compositional Analysis • Incident electrons undergo inelastic scattering with sample (X-rays, Auger electrons, secondary electrons) Electron Beam Secondary electrons (1-10 nm) Auger electrons (1 nm) Backscattered electrons (0.1 – 1 mm) X-rays (0.2 – 2 mm)

3. Electron Spectroscopy Electron, x-ray Spectroscopy EDX AES EELS PES Compositional Information Compositional & Chemical (Bonding) Information

4. EDX • EDX: Energy dispersive x-ray analysis • EDAX: Energy dispersive analysis of x-rays • EPM or EPMA: Electron probe micro-analysis • EDS: energy dispersive spectrometry • EMP: electron microprobe

5. EDX • Incident electron has sufficient energy (critical ionization energy) to eject a core shell electron From Ohring, Fig. 6-14(a) & (b), p. 278

6. EDX • Requires electron energy ~ keV Critical Ionization Energy for Pt

7. EDX • Outer shell electron fills the inner shell vacancy producing an x-ray photon (x-ray fluorescence) From Ohring, Fig. 6-14(c), p. 278

8. Kb La Ka EDX • X-ray nomenclature: • Ka : L → K • Kb : M → K • La : M →L Energy transition Terminating energy level • Letters denote principal quantum numbers (K: n = 1, L: n=2, etc.) adapted from Loretto, Fig. 2.3, p. 30

9. Kb La Ka EDX • Energy of emitted x-ray is determined by difference in electron energy levels : • hn = E(Ka) = EK- EL adapted from Loretto, Fig. 2.3, p. 30

10. EDX • X-ray energies are characteristic of the element From Ohring, Fig. 6-16, p. 281

11. EDX • Can identify the element from the x-ray lines emitted (Ti) From Ohring, Fig. 6-15(b), p. 280

12. EDX • X-ray detectors • EDS • Si p-i-n junction diode & MCA • Resolution ~ 150 eV • Based on energy of x-rays (EDS) From Williams, Fig. 1.16, p. 10

13. EDX • X-ray detectors • WDS • Uses Bragg reflection from crystal with known interplanar spacings to select l • Resolution ~ 5 eV • Based on l of x-rays from Schroder, Fig. 10.15, p. 669

14. EDX from Schroder, Table 10.1, p. 671 from Schroder, Fig. 10.16, p. 672

15. EDX • Can perform EDX using SEM or STEM (AEM) • Can produce elemental maps From Schroder, Fig. 10.4, p. 655

16. EDX • Can form compositional maps

17. EDX • EDX-SEM : • Lateral resolution ~ 1 mm • Depth resolution ~ 1 mm • EDX-AEM : • Lateral resolution ~ beam diameter (~0.5-2 nm) • Depth resolution ~ sample thickness (~2000 Å) Electron Beam Secondary electrons (1-10 nm) Auger electrons (1 nm) Backscattered electrons (0.1 – 1 mm) X-rays (0.2 – 2 mm)

18. EDX Quantification • Can determine amount of element present (to within ~ 0.1 at %)by measuring x-ray line intensity • Method 1: Calculation • Intensity of x-rays from a depth d is : • I = Ie(d)cswxe-md/cosq e dW/4p • Ie(d) = intensity of e-beam at depth d • c = atomic concentration • s = ionization cross-section • wx = x-ray yield (fluorescence yield) • = x-ray absorption coefficient q = detector angle wrt e-beam • e = detector efficiency • dW = detector solid angle

19. EDX Quantification • Method 2: Comparison with known standards • Compare x-ray intensity of sample with x-ray intensity from standard with known composition

20. EDX • X-ray emission competes with Auger process • Fluorescence yield is low below Na • EDX can detect elements above Na from Schroder, Fig. 10.14, p. 668

21. Can also use incident x-rays instead of electron beam • XRF: X-ray Fluorescence • XRFS: X-ray fluorescence spectroscopy

22. AES • Auger electron spectroscopy • Incident electron (few keV) ejects core electron from sample • Energy from electron transition is transferred to another electron (the Auger electron) causing it to be ejected incident electron ejected orbital electron scattered primary electron (EELS)  Electron Relaxation   Auger electron emitted (AES) x-ray photon emitted (EDX)

23. AES • Incident electron has sufficient energy to eject a core shell electron from Ohring, Fig. 6-14(a) & (b), p. 278

24. AES • Outer shell electron fills the inner shell vacancy causing ejection of Auger electron from Ohring, Fig. 6-14(d), p. 278

25. AES • Auger process requires 3 electrons (incident, core, Auger) • Can detect all elements except H & He from Ohring, Fig. 6-14(d), p. 278

26. AES • Auger nomenclature: • KLL Level of first ejected electron Initial level of Auger electron Level of electron that moves from outer to inner shell to fill electron vacancy

27. AES • Energy of Auger electron is determined by difference in electron energy levels : • E(KL1L2) = (EK- EL1) – (EL2+ f) Work function Energy released Energy required for Auger electron to escape surface

28. AES • Auger electron energies (~ 30 – 3000 eV) are characteristic of the element • Can detect all elements except H & He from Ohring, Fig. 6-17, p. 281

29. AES • Usually Auger signals, N(E), are differentiated, dN(E)/dE, to accentuate them from the background direct Auger spectra differentiated Auger spectra From Schroder, Fig. 10.10, p. 663

30. AES • Can identify the element from the AES spectrum Ti From Ohring, Fig. 6-15(c), p. 280

31. AES • Depth resolution determined by escape depth of electrons, < 20 Å • AES is a surface-sensitive technique; requires UHV • Depth profiling achieved using sputter gun from Yu & Cardona, Fig. 8.5, p. 420

32. AES • Lateral resolution ~ 10 - 50 nm (field emission source, scanning Auger) to 100 mm (non-scanning) • Cylindrical mirror analyzer (CMA) • Spectrometer resolution ~ 4 – 10 eV • Sensitivity ~ 0.1 – 1 at% • Quantification (10 % accuracy) achieved using calibrated standards from Ohring, Fig. 6-18, p. 284

33. EELS • Electron energy loss spectroscopy • Can examine energy loss of incident electron incident electron ejected orbital electron scattered primary electron (EELS)  Electron Relaxation   Auger electron emitted (AES) x-ray photon emitted (EDX)

34. EELS • Typical EELS Spectrum • Energy loss peak is characteristic of the elements DE = EK = binding energy of ejected electron Energy loss zero-loss peak (used as energy reference) energy loss due to inner shell ionization

35. EELS • EELS detector From Williams, Fig. 1.15, p. 9

36. EELS • EELS usually employed in TEM (AEM) From Williams, Fig. 1.2, p. 2

37. EELS • Fine structure is present in EELS spectrum • ELNES (energy loss near edge structure) • EXELS (extended energy loss spectroscopy) • Gives local atomic structure •  chemical bonding information •  atomic bond lengths (e.g., VCA) (EXELS) (ELNES)

38. EELS • EELS is complementary to EDX • More sensitive to low Z elements than EDX • Fine structure gives local atomic structure information

39. X-ray Absorption Spectroscopy • Can also observe fine structure in x-ray absorption • Measure transmission or fluorescence from sample as a function of incident x-ray photon energy • Core level excitations produce peaks in absorption or fluorescence • Fine structure in absorption edge gives chemical bonding information •  XANES (x-ray absorption near-edge structure) or NEXAFS (near edge x-ray absorption fine structure •  EXAFS (extended x-ray absorption fine structure) from C. Lamberti, Surf. Sci. Rep. 53 (2004) 1-197

40. X-ray Absorption Spectroscopy m(E)x = ln [Ii(E) / It(E) ] k = (2p/h) √ 2mo(hn – Eo) ; Eo = photoelectron binding energy

41. X-ray Absorption Spectroscopy • Synchrotron radiation is linearly polarized • Polarization-dependent EXAFS •  gives information on bond orientation •  can measure Da and Da in strained • layers

42. X-ray Absorption Spectroscopy • Can achieve surface sensitivity (~ 10 Å) by using grazing incidence geometry • Or detect Auger or photoelectrons (lower escape depth compared to fluorescence) •  surface EXAFS (SEXAFS)

43. Photoemission Spectroscopy (PES) • X-ray photoelectron spectroscopy (XPS) • EDX and AES use incident electrons • XPS uses incident x-rays (few keV) to cause ionization (photoelectric effect) • Measure energy of ejected electron • = electron spectroscopy for chemical analysis (ESCA) • Ultraviolet photoelectron spectroscopy (UPS) • Uses uv photons From Schroder, Fig. 10.34, p. 702

44. PES • K.E. of ejected electron is characteristic of the element : • Ephotoelectron = hn- EB - incident photon energy binding energy of ejected electron (ionization energy) • Can detect elements above Li • Requires very good spectrometer for H, He

45. PES from Ibach and Luth, Fig. V.1, p. 125

46. PES • Typical XPS spectrum From Ohring, Fig. 6-15(d), p. 280

47. PES • XPS Advantages: • X-rays less prone to damage surfaces than electrons (e.g., electrons can reduce hydrocarbons on surface to carbon) • EB is sensitive to chemical surroundings (e.g., Si versus SiO2) • Typical applications are determination of electronic states (e.g., oxides, heterojunction band alignments)

48. ARXPS • Angle-Resolved XPS: • At grazing angles of detection only electrons from top surface region can escape • Surface-sensitive technique XPS detector path length of electrons is too large for escape

49. PEEM • PEEM: • Photoelectron emission microscopy • = photoelectron spectromicroscopy • Provides laterally resolved PES

50. Summary From Ohring, Fig. 6-15, p. 280