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What makes Surface Science “surface” science ?

What makes Surface Science “surface” science ?. R. J. Smith Physics Department, Montana State Univ. Work supported by NSF (DMR) http://www.physics.montana.edu. Outline. Motivation for surface sensitivity - thin film devices General comments on surface sensitive techniques

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What makes Surface Science “surface” science ?

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  1. What makes Surface Science “surface” science ? R. J. Smith Physics Department, Montana State Univ. Work supported by NSF (DMR) http://www.physics.montana.edu CMP Seminar MSU 10/18/ 2000

  2. Outline • Motivation for surface sensitivity - thin film devices • General comments on surface sensitive techniques • Electron spectroscopy • define the attenuation length (AL) • empiracle results for attenuation length • simple model for overall shape of AL • Recent analyses based on attenuation lengths CMP Seminar 10/18/ 2000

  3. Metal-metal Interface Structure • Understand overlayer growth and alloy formation • Chemical composition and structure of the interface • Applications: magnetoresistive devices, spin electronics • Surface energy (broken bonds) • Chemical formation energy • Strain energy B interface A CMP Seminar 10/18/ 2000

  4. Metal-metal systems studied... • Substrates: Al(111), Al(100), Al(110) • Metal overlayers studied so far: • Fe, Ni, Co, Pd (atomic size smaller than Al) • Ti, Ag, Zr (atomic size larger than Al) • All have surface energy > Al surface energy • All form Al compounds with Hform < 0 • Use resistively heated wires ( ~ML/min) • Deposit on substrate at room temperature CMP Seminar 10/18/ 2000

  5. Comments on surface sensitive techniques... • Sensitivity to the surface is intrinsic to the technique - not based only on probing depth • Electron spectroscopy: Distance traveled by the electron before losing characteristic information (Attenuation length) is short • Low energy ions - neutralization and strong Coulomb interation • High energy ions - geometric shadowing CMP Seminar 10/18/ 2000

  6. Define the attenuation length • Measure decrease in beam intensity dI(x) after transmission through a film of thickness dx I(x) < I  I dx • 86% of the signal originates from depth of 2 • What is value of ? What determines this value? CMP Seminar 10/18/ 2000

  7. Other discussion…and surprises Idzerda, Journal Vac. Sci Technol A7(3), 1341 (1989) S. Ossicini, et al. JVST A3, 387 (1985) - growth models S. Tanuma, et al. JVST A8, 2213 (1990) - attenuation lengths CMP Seminar 10/18/ 2000

  8. Observations of Attenuation Length for electrons in solids • Depends on electron kinetic energy (KE) • KE depends on parameters of the technique:Auger, XPS, SEM • Varies with materials CMP Seminar 10/18/ 2000

  9. E Low Energies: 1 eV < E < 100 eV Empty e- states E = Kinetic Energy; Elastic Scattering E’’+(E-E’) Transition rate (like Golden Rule) E’ EF W=(Transition matrix element)2 Filled e-states =density of filled or empty states E’’ Total scattering probability/sec [1/]

  10. E Distance between collisions is then empty states E’’+(E-E’) E’ EF To go further might assume free-electron form for the density of states filled states  E’’ No Simple model (for homework) Attenuation Length at low energies

  11. E High Energies: E > few 100 eV Empty e- states Consider scattering of point charges, i.e. Coulomb Scattering E’’+(E-E’) E’ EF Unscreened Coulomb Potential has cross section  ~ 1/E2 Filled e-states E’’ • For small E phonon scattering may dominate • Insulators and semiconductors have energy gap so have long AL as approach twice the gap energy

  12. Ion scattering chamber • High precision sample goniometer • Hemispherical VSW analyzer (XPS, ISS) • Ion and x-ray sources • LEED • Metal wires for film deposition CMP Seminar 10/18/ 2000

  13. FM growth:layer-by-layer • Non-linear growth curves • Rapid attenuation of substrate signal • Breaks in slope • =6,6 (green) =20,20 (red) substrate CMP Seminar 10/18/ 2000

  14. VW mode: Island growth • Linear growth in signals • Relatively slow decrease in substrate signal • =6,6 (green) =20,20 (red) substrate CMP Seminar 10/18/ 2000

  15. Co on Al (100): He+ backscattering • Ion channeling • Only near-surface Al atoms are visible to the ion beam • Increase of Al peak means Co causes Al atoms to move off lattice sites • Coverage from Co peak area (RBS) CMP Seminar 10/18/ 2000

  16. Co on Al (100): HEIS intensities • HEIS Al surface peak vs. Co coverage • Number of visible Al increases up to 3 ML • Slope of 2:1 suggests stoichiometry Al2Co for the interface but Al2Co not in phase diagram CMP Seminar 10/18/ 2000

  17. Co on Al (100): XPS intensities • Measured (o , ) Simulation (lines) • Use HEIS for coverage • 0-3: layered CoAl • 3-10: Co islands • CoAl particle density is ~ 1.4 x Al density CMP Seminar 10/18/ 2000

  18. LEED patterns for Co on Al(100) • LEED at 42.8 eV • (a) Clean Al(001) • (b) 0.5 ML Co destroys pattern completely • (c) 7.6 ML A hint of some long range order (1x1) is seen. • Co coverage from RBS CMP Seminar 10/18/ 2000

  19. Co on Al(110): HEIS intensities • Number of visible Al increases to 5 ML • Slope 2.3:1, suggests stoichiometry of Al2Co or Al5Co2 • Al5Co2 exists in bulk phase diagram, but crystal structure is complex so less likely to form at 30 oC. CMP Seminar 10/18/ 2000

  20. Co on Al(110): XPS intensities • Measured (o , ) Simulation (lines) • Use HEIS for coverage (change at 5 ML) • 0-5: layered CoAl growth • 3-10: layered Co metal growth CMP Seminar 10/18/ 2000

  21. Comparison of XPS Intensities for Al(100) and Al(110) • Contract (110) coverage to (100) MLs •  (100) Co  (110) Co  (100) Al  (110) Al • Co overlap • Al don’t ! CMP Seminar 10/18/ 2000

  22. Snapshots from MC simulations • MC (total energy) using EAM potentials for Ni, Al (Voter) • Equilibrate then add Ni in 0.5 ML increments (solid circles) • Ion scattering simulations (VEGAS) Clean Al(110) Al(110)+0.5 ML Ni Al(110)+2.0 ML Ni CMP Seminar 10/18/ 2000

  23. Ion scattering simulations using VEGAS and the MC snapshots • Measured (o) Simulation () • Slopes agree • Change of slope at 2 ML correct • Good agreement so use snapshots for more insight CMP Seminar 10/18/ 2000

  24. XPS chemical shifts for Ni 2p • Shifts in BE • Shifts in satellite • Compare with XPS for bulk alloys (BE) (sat) NiAl3 1.05eV Ni2Al 0.75eV (8.0 eV) NiAl 0.2 eV (7.2 eV) Ni3Al 0.0 eV (6.5 eV) Ni 0.0 eV (5.8 eV) CMP Seminar 10/18/ 2000

  25. Simulated XPS intensity for Ni using EAM snapshots • Ni coverage from RBS • Fit using model with exponential attenuation • See a change in slope for all values of  • Best fit:Ni= 5.2Å CMP Seminar 10/18/ 2000

  26. XPS: Comparison of Calculated and Measured Al Intensities • XPS intensity vs Ni coverage • Best agreement with data for Ni = 5.2 Å Al = 15 Å • Universal curve Ni = 13.5 Å Al = 20.2 Å • Equilibrium? CMP Seminar 10/18/ 2000

  27. Summary • Surface sensitivity associated with short attenuation length for electrons in solids • Long AL at low energy associated with decreased availability of final states for scattering • Long AL at high energy associated with decreasing scattering cross section for point charges • Minimum AL for KE ~ 150 eV • Modeling of film morphology can be helpful for complex interface and alloy formation CMP Seminar 10/18/ 2000

  28. XPS: Comparison of Calculated and Measured Ni 2p Intensities • XPS intensity vs Ni coverage • Best agreement with data for Ni = 5.2 Å Al = 15 Å • Universal curve Ni = 13.5 Å Al = 20.2 Å • Equilibrium? CMP Seminar 10/18/ 2000

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