Transcript Author: Egon Pavlica Nova Gorica Polytechic Comparision of Metal

Comparision of Metal-Organic
Semiconductor interfaces to MetalSemiconductor interfaces
Author: Egon Pavlica
Nova Gorica Polytechic
May 2003
Contents:
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Introduction to Organic Semiconductors
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Inorganic Semiconductor surfaces
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Metal-Inorganic Semiconductor interfaces
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Metal-Organic Semiconductor interfaces
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Conclusion
Organic Semiconductors
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Small Organic Molecules
Polymers
Small molecule example:
AFM (200x200nm)
PTCDA polycrystalinic structure on Si(100)
Organic Semiconductors
Polymer example:
SEM of Polyaniline thin film deposited in vacuum on mica,
silicon and mcroporous silicon
Organic Semiconductors
Electronic Polarization cloud - Electronic polaron
Organic Semiconductors
Molecular polaron
Lattice polaron
Energy diagram of
dinamic polaron states
in anthracene type crystals
Organic Semiconductors
Space-Charge Layers
Tight-binding model:
- smaller overlap integral
- surface state levels
- donor states: empty positive
- acceptor states: full negative
- generally states are mixed
Space-Charge Layers
Depletion layer
Acceptor states
Charge neutrality
Depletion:
- low major carr.conc.
Inversion:
- high minor carr.conc.
Accumulation:
- high Ds states
- free charge
Space-Charge Layers
Band Bending due to Space-Charge
Schottky Depletion Space-Charge Layer
Band bending V(surface)>>kT
Approximation of space charge density
Electric field
Electric potential energy
Band bending:
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
Ideal* Metal-Inorganic Semiconductor
*known as Schottky model
Ideal Metal-Inorganic Semiconductor
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
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
Metal-Organic semiconductors
Band model of semiconductor:
●Neglible doping
●No intrinsic carriers
●Wide band gap ~ 2 eV
●No band bending
●Low mobility < 0.1cm2/Vs
●Dielectric constat low ~ 3
Metal-Organic semiconductors
Band model of semiconductor:
●No depletion layers
●Space Charge Limited Currents
●Image potential is important
Metal-Organic semiconductors
Hopping model
Interfaces currently
relevant only to charge
transport simulations
●Monte Carlo
simulations
●Gaussian Distribution
of state energies
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An succesful attempt to understand current-voltage characteristics included
inteface dipoles, image charge effects and phonons in bulk
Conclusions
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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.