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Surface Characterization Techniques: LEED and RHEED in Thin Film Growth

Understand Low Energy Electron Diffraction (LEED) & Reflection High Energy Electron Diffraction (RHEED) for surface analysis during thin film growth. Learn principles, patterns, applications, and comparisons with X-ray Diffraction (XRD). References included.

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Surface Characterization Techniques: LEED and RHEED in Thin Film Growth

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  1. LEED • Low energy electron diffraction • Based on electron diffraction from Ni in the early experiments of Davisson and Germer (1925)

  2. LEED • Low energy electron beam (~10-1000 eV) impinges at near normal incidence From Ohring, Fig. 7-25, p. 343

  3. LEED • Low energy electrons are diffracted from near surface region • LEED pattern consists of spots where Ewald sphere intersects lattice rods From Ohring, Fig. 7-27, p. 346

  4. LEED • e.g., Si surface From Ohring, Fig. 7-26(a), p. 345

  5. LEED • UHV required • Eliminate electron scattering by gas molecules • Eliminate sensitivity of diffraction to adsorbed impurities • Cannot be used during film growth due to obstruction of fluorescent screen From Ohring, Fig. 7-27, p. 346

  6. RHEED • Reflection high energy electron diffraction • High energy electron beam (~5-100 keV) at grazing angle of incidence (~ few degrees) • Can be used during film growth From Ohring, Fig. 7-25, p. 343

  7. RHEED • Glancing incidence electrons  interact with near surface region • High energy electrons → large diameter Ewald sphere • RHEED patterns consist of streaks for smooth surfaces From Ohring, Fig. 7-27, p. 346

  8. RHEED • Streak pattern indicates surface periodicity

  9. RHEED • Surface reconstruction phase diagram from Semicond. Sci. Technol. 9, 123 (1994)

  10. RHEED from Semicond. Sci. Technol. 9, 123 (1994)

  11. RHEED • RHEED patterns consist of spots for rough surfaces or streaks for smooth surfaces From Herman et al, Fig. 10.4, p. 229

  12. RHEED From Ohring, Fig. 7-28, p. 347

  13. RHEED • RHEED oscillations From Ohring, Fig. 7-22, p. 340

  14. RHEED • Vicinal surfaces From Tsao, Fig. 6.5, p. 208

  15. RHEED Step-flow growth Layer-by-layer growth From Herman et al, Fig. 1.7, p. 10

  16. RHEED • References : • D.K. Biegelson, R.D. Bringans, J.E. Northrup, and L.-E. Swartz, “Surface Reconstruction of GaAs(100) Observed by Scanning Tunneling Microscopy”, Phys. Rev. B 41, 5701 (1990). • H.H. Farrell and C.J. Palmstrom, “Reflection High Energy Electron Diffraction Characteristic Absences in GaAs (100) (2x4)-As : A Tool for Determining the Surface Stoichiometry”, J. Vac. Sci. Technol. B 8, 903 (1990). • S. Clarke and D. D. Vvedensky, “Growth Kinetics and Step Density in Reflection High-Energy Electron Diffraction During Molecular-Beam Epitaxy”, J. Appl. Phys. 63, 2272 (1988). • M. D. Pashley, “The Application of Scanning Tunneling Microscopy to the Study of Molecular Beam Epitaxy”, J. Cryst. Growth 99, 473 (1990).

  17. RHEED References : 5. B. A. Joyce, “The Evaluation of Growth Dynamics in MBE Using Electron Diffraction”, J. Cryst. Growth 99, 9 (1990). 6. J. H. Neave, B.A. Joyce, P.J. Dobson and N. Norton, “Dynamics of Film Growth of GaAs by MBE from RHEED Observations”, Appl. Phys. A 31, 1 (1983). 7. L. Daweritz and K. Ploog, “Contribution of Reflection High-Energy Electron Diffraction to Nanometre Tailoring of Surfaces and Interfaces by Molecular Beam Epitaxy”, Semicond. Sci. Technol. 9, 123 (1994). 8. J. H. Neave, P. J. Dobson, B. A. Joyce, and J. Zhang, “Reflection High-Energy Electron Diffraction Oscillations from Vicinal Surfaces – a New Approach to Surface Diffusion Measurements”, Appl. Phys. Lett. 47, 100 (1985).

  18. X-ray Diffraction (XRD) • Structural characterization technique: • Composition • Lattice constant • Film thickness • Strain and relaxation • Crystalline perfection • Interface quality • Non-destructive • Best strain sensitivity, 10-7

  19. XRD • Collimated (parallel beam), monoenergetic beam of x-rays diffract from sample From Panish & Temkin, Fig. 6.1(a), p. 174

  20. XRD • Atoms scatter incident x-rays • Constructive interference occurs in specific directions lateral resolution ~ mm to mm ahkl Penetration depth ~ 10 mm …

  21. Large penetration depth compared to electrons •  narrower diffraction peaks •  better strain sensitivity •  less surface sensitivity • Use glancing incidence x-ray scattering (GIXS) for surface sensitivity (similar to RHEED) • Difficult to focus x-rays •  electrons are better for imaging •  electrons have higher resolution (nm’s) compared to x-rays (mm’s to mm’s) XRD

  22. XRD • Crystal planes act as diffraction grating • Constructive interference occurs when • 2ahkl sinqb = ml • (Bragg’s law) • qb = Bragg angle • m = diffraction order = 1, 2, 3, … • Caution: q defined relative to surface plane not the surface normal (as in optics) qb ahkl

  23. XRD • 2ahkl sinqb = ml • sinqb≤ 1 • l < 2ahkl (first order, m = 1) • Need l ~ Å

  24. XRD • X-ray source • Electrons are produced by thermionic emission from a filament • Electrons are accelerated by a potential ~ 30 kV towards a solid target (usually Cu) • Electrons produce core shell ionization, resulting in characteristic x-ray emission From Dunlap, Fig. 13.5, p. 328

  25. XRD • X-ray detectors • Proportional counters • x-ray ionizes gas in a tube; ion current is measured by negative electrode • Scintillation counters • scintillator converts x-rays to optical photons; PMT detects optical photons • Solid-state detectors • electron-hole pairs created in reverse-biased p-n junction

  26. Kb La Ka XRD • 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

  27. XRD From Dunlap, Fig. 13.2, p. 325 From Dunlap, Table 13.1, p. 326

  28. XRD • X-ray detector • Scintillator-PMT • Solid-state detector (Si p-n junction diode) From Panish & Temkin, Fig. 6.1(a), p. 174

  29. XRD • Usually use (hkl) = (400) for III-V epilayers • e.g., a(400) for InP = 5.869 Å/4 = 1.467 Å a From Sze, Fig. 3(a), p. 5

  30. XRD • 2ahkl sinqb = ml • lCuKa1 = 1.541 Å • a(400) InP = 1.467 Å • qb = 31.67 º • qb varies with ahkl • Can measure qb to determine ahkl

  31. XRD • Sample and detector are rotated through the Bragg angle • Maintain q – 2q geometry From Panish & Temkin, Fig. 6.1(a), p. 174

  32. XRD • Measure x-ray intensity versus angle for a specific Bragg reflection •  x-ray rocking curve From Panish & Temkin, Fig. 6.2, p. 176

  33. XRD 2a┴ sinqb = ml • qb is different for substrate and epilayers due to strain (remember the Posisson effect) qb,epilayer qb,substrate epilayer (compression) substrate

  34. XRD 2a┴ sinqb = ml • a┴ increases with compressive strain • → qb decreases (layer peak left of substrate peak) • a┴ decreases with tensile strain • → qb increases (layer peak right of substrate peak) From Ohring, Fig. 7-1, p. 308

  35. XRD • Measure diffraction angle of film relative to substrate, Dq • Differentiate Bragg’s law to derive perpendicular mismatch from Dq : • (Da/a)┴ = -Dq / tan q • Can use Poisson’s ratio to determine natural (unstrained) lattice constant of film : (Da/a)┴ = [(1+n)/(1-n)] (Da /a)o

  36. XRD • Strain sensitivity : • Diffraction peaks have finite width From Panish & Temkin, Fig. 6.2, p. 176

  37. XRD • Strain sensitivity : • Diffraction peaks have finite width • Source is not perfectly monochromatic (Cu Ka dispersion ~ 0.00046 Å = difference between Ka1 and Ka2 lines) • Source is not perfectly collimated (require angular divergence < few arcseconds) • Layers have finite thickness • Layers may be bent due to strain • Layers may be partially relaxed

  38. DCXRD • Double crystal XRD or High resolution XRD (HRXRD) • Uses a reference crystal to filter and collimate the incident x-rays • Only x-rays satisfying the Bragg condition are reflected to sample • e.g., InP crystal aligned to 31.67º fixes l at 1.541 Å From MRS Short Course (1990)

  39. DCXRD Double Crystal XRD or High Resolution XRD (HRXRD) Acts as monochromator From Panish & Temkin, Fig. 6.1(b), p. 174

  40. DCXRD • Linewidth of substrate peak ~ 12 arcsecs • Strain sensitivity, Da/a ~ 10-4 From Panish & Temkin, Fig. 6.3, p. 176

  41. DCXRD • Pendellosung fringes From Panish & Temkin, Fig. 6.3, p. 176

  42. DCXRD • Pendellosung fringes : • Diffraction corresponding to total film thickness • 2t sinqb = ml • lDm = 2tcosqb • t = l / (2Dqcosqb) for Dm = 1 qb t

  43. DCXRD t = l / (2Dqcosqb) • Can measure spacing of fringes to determine film thickness Dq From Panish & Temkin, Fig. 6.3, p. 176

  44. HRXRD • Can use more than one crystal to further improve resolution → multi-crystal diffractometers From Panish & Temkin, Fig. 6.8(b), p. 181

  45. Superlattices From Panish & Temkin, Fig. 6.6, p. 179

  46. Superlattices • Satellite peaks are produced corresponding to the periodicity of the superlattice Experiment Model From Panish & Temkin, Fig. 6.10(b), p. 183

  47. Superlattices • Monolayer resolution from HRXRD (> 2 crystals) From Panish & Temkin, Fig. 6.10(b), p. 183

  48. Superlattices • Interface roughness and grading From Panish & Temkin, Fig. 6.17(a), p. 193

  49. Superlattices From Panish & Temkin, Fig. 6.15, p. 191

  50. Superlattices • The intensities of the peaks contains information on the interface composition • The satellite peaks correspond to the Fourier transform of the superlattice structure •  sinusoidal structure: only 1 pair of satellite peaks •  perfect square wave: infinite number of odd satellite peaks • Higher order peaks (corresponding to higher order Fourier components) are sensitive to the interface composition • Varying the compositions is like varying the structure factors in the unit cells

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