Author: Egon Pavlica

Nova Gorica Polytechic

Comparision of Metal-Organic Semiconductor interfaces to Metal-

Semiconductor interfaces

May 2003

● Introduction to Organic Semiconductors

● Inorganic Semiconductor surfaces

● Metal-Inorganic Semiconductor interfaces

● Metal-Organic Semiconductor interfaces

● Conclusion

Contents:

Organic Semiconductors

● Small Organic Molecules● Polymers

AFM (200x200nm)PTCDA polycrystalinic structure on Si(100)

Small molecule example:

Organic SemiconductorsOrganic SemiconductorsOrganic 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 statesin 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

Charge neutrality

Depletion layer

Acceptor states

Space-Charge Layers

Depletion:- low major carr.conc.Inversion:- high minor carr.conc.Accumulation:- high Ds states- free charge

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 levelfor GaAs

Ideal* Metal-Inorganic Semiconductor*known as Schottky model

Ideal Metal-Inorganic Semiconductor

Bardeen model

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

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

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

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

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.