Author: Egon Pavlica

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Comparision of Metal-Organic Semiconductor
interfaces to Metal-Semiconductor interfaces
Author Egon Pavlica
Nova Gorica Polytechic
May 2003
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Contents

• Introduction to Organic Semiconductors
• Inorganic Semiconductor surfaces
• Metal-Inorganic Semiconductor interfaces
• Metal-Organic Semiconductor interfaces
• Conclusion

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Organic Semiconductors

• Small Organic Molecules
• Polymers

Small molecule example
AFM (200x200nm) PTCDA polycrystalinic structure
on Si(100)
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Organic Semiconductors
Organic Semiconductors
Organic Semiconductors
Polymer example
SEM of Polyaniline thin film deposited in vacuum
on mica, silicon and mcroporous silicon
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Organic Semiconductors
Electronic Polarization cloud - Electronic polaron
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Organic Semiconductors
Lattice polaron
Molecular polaron
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Energy diagram of dinamic polaron states in
anthracene type crystals
Organic Semiconductors
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Space-Charge Layers
Tight-binding model - smaller overlap integral -
surface state levels - donor states empty
positive - acceptor states full negative -
generally states are mixed
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Space-Charge Layers
Depletion layer
Acceptor states
Charge neutrality
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Depletion - low major carr.conc. Inversion -
high minor carr.conc. Accumulation - high Ds
states - free charge
Space-Charge Layers
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Band Bending due to Space-Charge
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Schottky Depletion Space-Charge Layer
Band bending V(surface)gtgtkT
Approximation of space charge density
Electric field
Electric potential energy
Band bending
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Band bending - Inorganic semiconductors
Weak space-charge layer
Strong space-chare layer
Schottky layer
Calculated band bending due to acceptor/donor
surface state level for GaAs
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Ideal Metal-Inorganic Semiconductor
known as Schottky model
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Ideal Metal-Inorganic Semiconductor
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Bardeen model

• Model approximations
• Interface region
• Surface states of clean semiconductor persist and
  pin Fermi level

• Facts
• Metal atoms in close contact with semiconductor
  form chemical bonds
• Charge flow in bonds....formation of dipole layer
• Interdiffusion
• Formation of new electronic interface states
• Both model fails to explain the barrier height
  dependence on metal work function

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VIGS and MIGS
Deposited metals produce interface states
Virtualy Induced Gap States in semiconductor are
matched to Conduction band of metal
Induced surface states are of mixed
acceptor/donor character
Fermi level near cross-over energy EB
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Metal-Organic semiconductors

• Band model of semiconductor
• Neglible doping
• No intrinsic carriers
• Wide band gap 2 eV
• No band bending
• Low mobility lt 0.1cm2/Vs
• Dielectric constat low 3

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Metal-Organic semiconductors

• Band model of semiconductor
• No depletion layers
• Space Charge Limited Currents
• Image potential is important

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Metal-Organic semiconductors

• Hopping model
• Interfaces currently relevant only to charge
  transport simulations
• Monte Carlo simulations
• Gaussian Distribution of state energies

An succesful attempt to understand
current-voltage characteristics included inteface
dipoles, image charge effects and phonons in bulk
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Conclusions

• No theory of metal-organic semiconductor
  interfaces, since too specific.
• Band models are based on different structure, so
  are fundamentally incorrect.
• The hopping models and localized states are
  promising theory for metal-organic semiconductor
  interfaces.

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