1. CMP Seminar September 22, 2003 1 Growth and Structure of Thin Fe Films on the Ti-Al Interface C. V. Ramana Montana State University http://www.physics.montana.edu/Ionbeams/ionbeams.html

2. CMP Seminar September 22, 20032 Stabilizing Metal-Metal Interfaces Metal thin film devices have layers ~ nm thicknessMetal thin film devices have layers ~ nm thickness Diffusion occurs frequently: interface ~ nm thickDiffusion occurs frequently: interface ~ nm thick Need to stabilize the interface, provide a template for epitaxial growth, minimize interdiffusionNeed to stabilize the interface, provide a template for epitaxial growth, minimize interdiffusion Applications: magnetoresistive devices, spin electronicsApplications: magnetoresistive devices, spin electronics Surface energy (broken bonds)Surface energy (broken bonds) Chemical formation energyChemical formation energy Strain energyStrain energy A B interface Smith et al, Appl. Surf. Sci. 219 (2003) 28

3. CMP Seminar September 22, 20033 Experiment Substrate: Al(100)Substrate: Al(100) Metal overlayers:Metal overlayers: Fe Fe Ti Ti Surface energy > Al surface energySurface energy > Al surface energy Form Al compounds with  H form < 0Form Al compounds with  H form < 0 Use resistively heated wires ( ~ML/min)Use resistively heated wires ( ~ML/min) Deposition at room temperatureDeposition at room temperature

4. CMP Seminar September 22, 20034 Techniques and Goals Rutherford backscattering and channeling (RBS/c)Rutherford backscattering and channeling (RBS/c) Low-energy electron diffraction (LEED)Low-energy electron diffraction (LEED) Low-energy ion scattering (LEIS)Low-energy ion scattering (LEIS) Fe Ti Al

5. CMP Seminar September 22, 20035 Overview of Rutherford Backscattering and Channeling MeV He + ionsMeV He + ions Yield = Q   (Nt)Yield = Q   (Nt) Fe (Ti) peak for coverageFe (Ti) peak for coverage Al peak for structureAl peak for structure

6. CMP Seminar September 22, 20036 Fe Growth on Al(100) – Does Ti Interlayer Makes Any Difference? Ramana et al, Phys. Rev. Lett. 90 (2003) 66101

7. CMP Seminar September 22, 20037 Fe-Fe Shadowing

8. CMP Seminar September 22, 20038 Angular Ion Yield Curves 0o0o FCC Al 45 o FCC Fe [010] [100] If it is fcc Fe

9. CMP Seminar September 22, 20039 Angular Ion Yield Curves Cont… FCC Al 45 o [010] [100] 54.7 o BCC Fe If it is bcc Fe

10. CMP Seminar September 22, 200310 Structure of Fe: Off-Normal Ion Channeling Measurements

11. CMP Seminar September 22, 200311 Structure of Fe contd.. θ min = 54.55 o

12. CMP Seminar September 22, 200312 Lattice Parameters & Calculations c)TiAl: Tetragonal a = 4.0155 Å c = 4.0625 Å a) Fe: bcc 2.8665 Å b) Al: fcc (4.0496) 2.8635 Å Lattice Parameters 2.8560 Å

13. CMP Seminar September 22, 200313 Unrelaxed bcc Fe 54.74 o Volume is conserved V Fe = C Fe A = C Fe X 2 C Fe = (2.8665) 3 /X 2

14. CMP Seminar September 22, 200314 Fe (bcc) relaxed to Al substrate θ 54.74 o θ 54.65 o θ = tan -1 (X / C Fe ) Volume is conserved V Fe = C Fe A = C Fe X 2 C Fe = (2.8665) 3 /X 2

15. CMP Seminar September 22, 200315 Fe (bcc) relaxing to Ti-Al interface 54.74 o θ θ 54.44 o

16. CMP Seminar September 22, 200316 Calculations cont.… X C Fe θ Fe2.8665-54.74 Al2.86352.872554.65 TiAl2.85602.887654.44

17. CMP Seminar September 22, 200317 Why does it work? Bimetallic Formation Energies (kJ/mole-atom)Bimetallic Formation Energies (kJ/mole-atom) Fe/Al: 25Ti/Al: 38Fe/Ti: 20 Ti-Al bond stronger than Fe-Al or Fe-Ti so Ti prefers to stay near the Al interfaceTi-Al bond stronger than Fe-Al or Fe-Ti so Ti prefers to stay near the Al interface Lattice matching: bcc Fe (100) unit cell has a = 2.86 ÅLattice matching: bcc Fe (100) unit cell has a = 2.86 Å fcc Al (100) has Al-Al distance 2.86 Å fcc Al (100) has Al-Al distance 2.86 Å hcp Ti (0001) has Ti-Ti distance 2.95 Å Fe (small atom) more easily accommodated by Al(100) than Ti (larger atom). Formation of Ti-Al interface “stiffens” theFe (small atom) more easily accommodated by Al(100) than Ti (larger atom). Formation of Ti-Al interface “stiffens” the surface, restrains movement of Fe into the substrate. surface, restrains movement of Fe into the substrate.

18. CMP Seminar September 22, 200318 Summary and Conclusion Demonstrated the use of a metallic interlayer to stabilize a metal-metal interface and promote epitaxial growthDemonstrated the use of a metallic interlayer to stabilize a metal-metal interface and promote epitaxial growth Fe growth occurs in slightly distorted bcc structure on the Al(100) surface with Ti interlayer at the interface.Fe growth occurs in slightly distorted bcc structure on the Al(100) surface with Ti interlayer at the interface.

19. CMP Seminar September 22, 200319 Acknowledgements Prof. Richard J. SmithProf. Richard J. Smith Prof. Bum-Sik Choi (Korea)Prof. Bum-Sik Choi (Korea) Ion BeamersIon Beamers NSF (DMR-0077534)NSF (DMR-0077534)