Introduction to Surface Physics

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Introduction to Surface Physics
Introduction By Prof. N. Tabet
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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

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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

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Surface and Bulk properties

• The density of atoms
• Surface NS1015cm-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

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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
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• A concrete example Si(100)

001
110
110
x

• Covalent bonding is very directional

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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
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An STM image of the Si(100)-2x1 reconstruction
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Grain Boundary S 7
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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 ,.

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GB Structure S 9 011 lt122gt
J. R. Morris et al.
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Nanomaterials Surface Domination
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Photocatalysis Cleaning polluted water
Xenon lamp
250 ml Monolinuron (4mg/l) ZnO nanpowder
ZnO Nanopowder
N. Tabet et al . 2005
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• Bulk Plasmons Classical electromagnetic
  theory

Free electron gas with an applied oscillatory
electric field
Equation of motion
Solution
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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
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Surface Plasmons

• Surface Plasmons Charge oscillations at a
  surface

Vacuum
Surface
Bulk
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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
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Recombination Activity of Grain Boundaries in
Germanium
N. Tabet , Doctorat es Sciences thesis, Univ.
Paris-Sud, Orsay, 1988
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Energy band diagram of the selected metals and
4H-SiC.
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Table 3.1 Work function of selected metals and
their measured and calculated barrier height on
n-type 4H-SiC.
Al Ti Zn W Mo Cu Ni Au Pt
         4.28 4.33 4.33 4.55 4.60 4.65 5.10 5.15 5.65
        (Si-face) 1.12 1.69 1.81
        (C-face) 1.25 1.87 2.07
        (calculated) 1.01 1.06 1.06 1.28 1.33 1.38 1.63 1.68 2.08
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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
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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
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Pressure ranges
  Millibar Torr (mmHg) Micron (mmHg) Pascal (N m-2) Atmospheres
Millibar 1 0.75 750 100 9.87 x 10-4
Torr (mmHg) 1.33 1 103 133 1.32 x 10-3
Micron (mmHg) 1.3 x 103 103 1 0.133 1.32 x 10-6
Pascal (N m-2) 10-2 7.5 x 10-3 7.5 1 9.87 x 10-6
Rough vacuum 102 to 10-2 mbar
High Vacuum 10-2 to 10-8 mbar
Ultra high vacuum 10-8 mbar and below
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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.
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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
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Surface Analysis Techniques
Radiation IN Photon Photon Electron Electron Ion
Radiation OUT Electron Photon Electron Photon Ion

Surface Information
Physical Topography SEM STM
Elemental composition PES (XPS/ESCA) XRF (bulk technique) AES APS
Chemical State PES (XPS/ESCA) IR SFG AES HREELS SIMS ISS
Atomic Structure Ph.D SEXAFS XSW SEXAFS XSW LEED RHEED AED SIMS
Adsorbate Bonding PES (UPS, XPS/ESCA) NEXAFS IR HREELS ISS
Electronic Structure PES (UPS) NEXAFS IPES
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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).
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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