An Overview of MEIS Science – Past and Future

1. An Overview of MEIS Science – Past and Future Dr Tim Noakes STFC Daresbury Laboratory, Daresbury Science and Innovation Campus, Keckwick Lane, Daresbury, Warrington, Cheshire, WA4 4AD, UK

2. Overview • The medium-energy ion scattering (MEIS) technique • Previous experiments using MEIS • Surface structure • High resolution depth profiling • Thin film characterisation • Characterisation of nanostructures • Future research areas • Summary

3. MEIS Technique Medium energy light ions (50-250 keV H+ or He+) used to probe the surface and near surface of materials • Energy losses during scattering • Elastic losses • Inelastic losses • Angular variation in scattered ion intensity • Shadowing and blocking

4. Elastic scattering • Simple ‘billiard ball’ collisions between ions and atoms • Conservation of energy and momentum relates ion energy loss to mass of target atom

5. Inelastic Energy Loss • Inelastic energy losses arise from electronic excitations as ion passes through sample • Stopping powers well known (e.g. ‘SRIM 2011’) • Resolution degrades with depth as process is stochastic (energy loss straggling)

6. Angular Intensity Variation • Shadowing effects used to select number of layers illuminated • Blocking effects reveal relative positions of the atoms(i.e. the structure!) • Shifts in blocking dips related to layer spacings (surface relaxations, strain) • Amplitudes of dips indicate additional illumination (thermal vibrations, disorder)

7. LEIS, MEIS and RBS LEIS MEIS RBS (1-5keV) (50-400keV) (0.5-4MeV) R.M.S. Vibrational Amplitude Shadow cone > vibrational amplitude Intrinsic surface specificity (1-3 atomic layers) Shadow cone vibrational amplitude Tunable surface specificity (1-100 atomic layers) Shadow cone « vibrational amplitude Low surface specificity (20-thousands atomic layers!

8. Medium Energy Ion Scattering 100 Counts > 80 Energy (keV) 60 85 110 Angle (deg) 2D image Elastic scattering gives compositional information Inelastic scattering provides depth information (and morphology!) Angular variation in the scattering intensity gives structure

9. Capabilities of MEIS • Depth selectivity, excellent structural sensitivity • Surface structure (~2 pm resolution) • Compositional sensitivity over the near surface • High resolution depth profiling (2 - 5 Å resolution) • Ability to simultaneously determine composition and structure • Full characterisation of thin film materials • Path length sensitivity • Composition, structure and morphology of nanoparticles

10. Surface Structure Metals and metal alloys • Adsorbate induced reconstruction • Model catalysts • Complex metal alloys (e.g. quasicrystals) Semiconductor materials • ‘Ideal’ Schottky Barriers • III-V growth surfaces Oxides • Catalyst supports (e.g. TiO2)

11. TiO2(100)-(1x1) Surface Structure Important support materials for catalysts No agreement between previous SXRD and LEED studies Difficult system to analyse since: • Atoms have low Z • At least 12 parameters to optimise

12. TiO2 data Data taken in two azimuths probing different aspects of the structure and fitted using ‘VEGAS’ simulation software

13. TiO2(110) Surface Structure MEIS structure similar to LEED data with surface bridging O atom relaxed outward Parkinson et al, PRB 73 (2006) 245409

14. High Resolution Depth Profiling Semiconductor device fabrication • Ion implants for semiconductor devices • High- gate dielectric materials Structural materials • Oxide layers for Corrosion protection of light metal alloys • Construction materials • Automotive, aerospace, rail and marine transport applications

15. Corrosion Protection of Light Alloys Typically dilute alloys of Aluminium used for improved corrosion resistance • Al-0.3at%Zn • Al-0.7at%W • Al-0.2at%Mn • Al-0.4at%Cu What happens to minor alloying element during oxide film growth? X-TEM image of anodized Al-0.4at%Cu sample

16. Enrichment in Al-0.4at%Cu Alloy • Anodic oxidation leads to Cu enriched layer below the grown film • Film is stripped using chromic/phosphoric acid before analysis • Data reveals constant thickness of enriched layer with anodization time • Increase in Cu content attributed to increased cluster generation Garcia-Vergara et al, App. Surf. Sci. 205 (2003) 121

17. Thin Film Characterisation Systems which benefit from the simultaneous elucidation of composition and structure • Metal-on-metal growth (giant magneto-resistance films) • Quantum well systems (III-V materials, metals) • Spintronic materials (metal/semiconductor hybrids)

18. Cu on Co(0001) Growth Cu/Co multilayers commonly used for GMR layers Many studies of Co growth on Cu(111) but the reverse system less common because of the difficulty in preparing clean well ordered Co(0001) substrates Surfactant mediated epitaxy (SME) a possible way to improve growth • 1ML of Pb pre-deposited before Cu layer growth • Data taken before and after annealing to 300ºC

19. Cu Thin Film Structure • Cu data fitted using twinned fcc plus flat signal for disordered fraction • Less disorder (9±6%) for surfactant grown sample indicating improved crystal quality • Strain also required to fit data

20. Pb Surfactant Effect For 1ML Pb surfactant grown sample • 1-2 additional epitaxial layers • Reduced strain throughout the film • Majority of Pb ‘floats’ on surface during growth • Some Pb retained in grown layer at a level of 3% ML • Sub-surface Pb mostly on lattice sites • Could strain relief contribute to the improved epitaxial growth? Noakes et al, PRB 68 (2003) 155425

21. Nanoparticle Characterisation Topographical information • Single element clusters Compositional information • Bimetallic alloys (model catalysts) • III-V quantum dots Structural Information • All the above!

22. ‘Magic’ Height Islands Ag on i-Al70Pd21Mn9 Bi on i-Al63Cu24Fe13 Bi clusters (bi-layers) Bi monolayer Si(111) substrate Fournee et al, PRL 95 (2005) 155504 4 layer islands form on top of pseudomorphic monolayer Bi on Si(111) – bilayer formation Nagao et al, PRL 93 (2004) 105501 Ag on AlNiCo – QSE’s Moras et al, PRB 74 (2006) 121405(R)

23. Structure of Bi Islands a 0.8 Å 0.8 Å b c a=cb Y Rhombohedral ‘Black-Phosphorous’ • Modelled using 25Å2 islands in 100Å2 box • Starting from bulk rhombohedral positions atoms allowed to move in Y and Z directions a b c a=b=c Large downward movement of top atom leads to bi-layer formation

24. Energy Fits to Bi Data 1ML data allows calibration, resolution, etc to be fitted 1.5ML (nominal) • 20% coverage of 4ML islands 3 ML (nominal) • 56% coverage of 4ML islands Fits improved (15% reduction in R-factor) by allowing 2 and 8 ML islands as well! Noakes et al, PRB 82 (2010) 195418

25. Self-assembled InAs Quantum Dots on GaAs Large 3D islands Quantum Dots Wetting Layer • InAs deposition on GaAs leads to: • InGaAs wetting layer • Regular well-defined quantum dots • Larger 3D islands Dot size and shape determined from AFM

26. Quantum Dots Results Wetting layer and large 3D islands included as well as quantum dots In intensity fitted using linear profile from 20% to 100% at the top of the QD First independent measurement of the composition profile of materials of this type! P.Q. Quinn et al, App. Phys. Lett. 87 (2005) 153110

27. Future Research Using MEIS Semiconductor device fabrication • Dielectric layers • Ion implantation • Metalisation Catalysts • Oxide support materials • Bimetallic nanoparticles • Adsorbate induced segregation studies Structural materials (light metal alloys) • Rail, automotive, marine and aerospace applications

28. Future Research Using MEIS Biomedical applications • Joint replacements, dental implants Photovoltaic materials • Multi-junction solar cells • II-VI quantum dot based solar cells • III-V quantum well LED’s Magnetic materials • Magnetic tunnel junctions • Novel memory materials (MRAM, race track, etc) • Spintronic materials (metal-semiconductor hybrids)

29. Future Research Using MEIS The ‘Hydrogen economy’ • Photo-catalysts • Hydrogen storage materials • Fuel cells Photocathode materials • CaAs, tellurides, antimonides Nanometrology • SIMS Calibration • Elipsometry and other optical techniques Others???

30. Summary MEIS is a fantastic technique for investigating the surface and near-surface region of materials • Simultaneous measurement of composition and structure • High sensitivity to structural parameters (~2 pm) • Virtually monolayer depth resolution (2-5 Å) • Sensitivity to nanoparticle structure, morphology and composition MEIS can be used for a wide range of applications many of which fall into RCUK priority areas • Sustainable energy • Environmental change • Life long health and well being • Nanoscience to nanoengineering

31. Acknowledgements Daresbury - P. Bailey Warwick – G.S. Parkinson, M.A. Munoz-Marquez, P.D. Quinn, M.J. Gladys, R.E. Tanner and D.P. Woodruff Manchester – S. Garcia-Vergara, P. Skeldon, G.E. Thompson, H. Habazaki and K. Shimizu Leeds – D.T Dekadjevi and M.A. Howson Liverpool/Warwick – C.F. McConville, M. Draxler, M. Walker, M.G. Brown, A. Hentz, D.P. Woodruff, J. Smerdon, L. Leung and R. McGrath Warwick – P.D. Quinn, N.R. Wilson, S.A. Hatfield, C.F. McConville, G.R. Bell, S. Al-Harthi and F. Gard