Introduction to Surface Physics Introduction By Prof. N. Tabet

1. Introduction to Surface Physics Introduction By Prof. N. Tabet

2. Objectives of the Course • To understand the properties of surfaces and their effects on many physical and chemical processes • Understand the working principle and capabilities of Surface Techniques

3. The importance of surfaces • A surface is the boundary between identifiable phases of matter: solid/air, liquid /solid, solid/solid,… In this course, we focus on air(vacuum)/Solid (surfaces), and solid/solid interfaces • Surfaces and interfaces play an important role in tribology (friction, wear,..), microelectronics (surface and junction electron states), catalysis, nanotechnology…

4. Surface and Bulk properties • The density of atoms: Surface: NS~1015cm-2 Bulk : NB~ 1022cm-3 But ratio NS/NB increases as the dimensions of the solid decrease • Loss of periodicity along the direction normal to the surface • Relaxation, re arrangement of atoms • Modified Composition because of exposure to ambient atmosphere

5. Relaxation and Reconstruction • Surfaces are not formed at zero temperature : Heat leads to atomic re-arrangement. • Atoms at a surface have a lower co-ordination than those in the bulk – the origin of surface tension (strictly surface stress in a solid) Relaxation Reconstruction

6. A concrete example: Si(100) [001] [110] [110] x • Covalent bonding is very directional

7. Notice that after reconstruction, there is still one dangling bond per atom. See article : M. Tao et al. Appl. Phys. Lett. Vol. 82, No.10, (2003)1559

8. An STM image of the Si(100)-2x1 reconstruction

9. Grain Boundary: S 7

10. Surface States in semiconductors and insulators • Energy levels in the band Gap: • Electrical activity: centers of recombination, reduction of the carrier lifetime, ... • 2. Pinning of the Fermi level: • Control of the energy barrier of Schottky contacts • 3. Induce energy barriers at Grain boundaries: • Varistor effect in polycrystals • 4. Control the electrical activity of grain boundaries: • Lower the energy conversion efficiency of solar cells • 5. Effect on activity of Phocatalysts • 6. Possibility of passivation: Hydrogen ,….

11. GB Structure S 9 [011] <122> J. R. Morris et al.

12. Nanomaterials : Surface Domination

13. Photocatalysis: Cleaning polluted water Xenon lamp 250 ml Monolinuron (4mg/l) + ZnO nanpowder ZnO Nanopowder N. Tabet et al . 2005

14. Bulk Plasmons: Classical electromagnetic theory: Free electron gas with an applied oscillatory electric field Equation of motion Solution:

15. Dipole moment of an electron parallel to the x-axis is The polarisation, ( dipole moment density ( per unit volume The Displacement is given by wp ~ 1015Hz For a longitudinal electromagnetic wave to exist See Kittel, Chapter 10, 7th edition

16. Surface Plasmons Surface Plasmons: Charge oscillations at a surface Vacuum Surface Bulk

17. Displacement Field: If no external charges, then, Hence But for a “zero thickness” surface e0 vacuum ee0 metal So, for the oscillation to exist we need Thus Conclusion: We have opened a new mode of excitation, a localised Surface plasmon, with frequency wsp

18. Recombination Activity of Grain Boundaries in Germanium N. Tabet , Doctorat es Sciences thesis, Univ. Paris-Sud, Orsay, 1988

20. Energy band diagram of the selected metals and 4H-SiC.

21. Table 3.1: Work function of selected metals and their measured and calculated barrier height on n-type 4H-SiC.

24. Band Bending at Surfaces and Interfaces Depletion region (~100nm) Escape depth (~3nm) Incident Photon Ec Fermi Level Eb= Eb (surface) Ev surface Eb(Bulk) Photoelectrons Core level

25. Oxidized Ge(011) surfaceT= 380ºC, t = 25min. PO2 = 400Torrs hn = 300eV 29.5eV 1: As oxidized surface 2-8: After successive Ar+ Sputtering cycles, hn= 650eV. N. Tabet et al. Surface Science, 523, (2003) 68

26. Pressure ranges

27. Keeping your surface clean We can appreciate the need for good vacuum when analysing the surface of a sample by considering a simple example: It is desired to measure the properties of a clean surface for a period of 12 hours after production. If the maximum coverage of contamination which is acceptable is q = 0.2 monolayers (ML), what pressure must the sample be kept under? From the kinetic theory of gases the flux incident upon a surface is given by: Over a time t a total number of gas molecules JI t are incident per unit area of surface.

28. We require that a maximum of qn atoms per unit area bind to contaminants over time t, where q is the required coverage and n is the number of atoms per unit area of the surface under consideration. Substituting and rearranging gives:

29. Surface Analysis Techniques

30. Electron Escape Depth • The inelastic mean free path indicates depth from which photoelectrons can convey information • With conventional laboratory sources the bulk of the information comes from the near-surface region. M.P. Seah, and W.A. Dench, Surf, Intef. Anal. 1, 2 (1979).

31. References • ‘Surface Analysis – The Principal Techniques’ , J.C. Vickerman (ed.), (Wiley, Chichester, 1998). • ‘Modern techniques of surface science’, D.P. Woodruff and T.A. Delchar, (CUP, Cambridge, 1986). • ‘Surface Physics, 2nd Edition’, M. Prutton (OUP, Oxford, 1984). • ‘Low Energy Electrons and Surface Chemistry, 2nd Edition’, G. Ertl and J. Küppers, (VCH, Weinheim, 1985) • ‘Surfaces and Interfaces of Solid Materials, 3rd Edition’, H.Lüth, (Springer Verlag, Berlin, 1997) • ‘Electron Spectroscopy for Surface Analysis’, H. Ibach (ed.), (Springer-Verlag, Berlin, 1977). - Lecture notes,Surface Physics, Dr Hunt, Durham University, UK