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

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

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

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

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

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

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

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

E E’ E’’+(E-E’) E’’ EFEF filled states empty states To go further might assume free- electron form for the density of states Attenuation Length at low energies Distance between collisions is then Simple model (for homework)  NoNo

E E’ E’’+(E-E’) E’’ EFEF Filled e - states Empty e- states High Energies: E > few 100 eV Consider scattering of point charges, i.e. Coulomb Scattering Unscreened Coulomb Potential has cross section  ~ 1/E 2 For small E phonon scattering may dominate Insulators and semiconductors have energy gap so have long AL as approach twice the gap energy

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

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

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

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

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

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

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

CMP Seminar 10/18/ Co on Al(110): HEIS intensities ä Number of visible Al increases to 5 ML ä Slope 2.3:1, suggests stoichiometry of Al 2 Co or Al 5 Co 2 ä Al 5 Co 2 exists in bulk phase diagram, but crystal structure is complex so less likely to form at 30 o C. ä

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

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

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

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

CMP Seminar 10/18/ XPS chemical shifts for Ni 2p ä Shifts in BE ä Shifts in satellite ä Compare with XPS for bulk alloys (BE) (sat) (BE) (sat) NiAl eV Ni 2 Al 0.75eV (8.0 eV) NiAl 0.2 eV (7.2 eV) Ni 3 Al 0.0 eV (6.5 eV) Ni 0.0 eV (5.8 eV)

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

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

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

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