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

2. ● Introduction to Organic Semiconductors ● Inorganic Semiconductor surfaces ● Metal-Inorganic Semiconductor interfaces ● Metal-Organic Semiconductor interfaces ● Conclusion Contents:

3. Organic Semiconductors ● Small Organic Molecules ● Polymers AFM (200x200nm) PTCDA polycrystalinic structure on Si(100) Small molecule example:

4. Organic Semiconductors Polymer example: SEM of Polyaniline thin film deposited in vacuum on mica, silicon and mcroporous silicon

5. Organic Semiconductors Electronic Polarization cloud - Electronic polaron

6. Organic Semiconductors Molecular polaron Lattice polaron

7. Energy diagram of dinamic polaron states in anthracene type crystals Organic Semiconductors

8. Space-Charge Layers Tight-binding model: - smaller overlap integral - surface state levels - donor states: empty positive - acceptor states: full negative - generally states are mixed

9. Space-Charge Layers Charge neutrality Depletion layer Acceptor states

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

11. Band Bending due to Space-Charge

12. Schottky Depletion Space-Charge Layer Band bending V(surface)>>kT Approximation of space charge density Electric field Electric potential energy Band bending:

13. 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

14. Ideal* Metal-Inorganic Semiconductor *known as Schottky model

15. Ideal Metal-Inorganic Semiconductor

16. 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

17. 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 E B

18. 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

19. Metal-Organic semiconductors Band model of semiconductor: ● No depletion layers ● Space Charge Limited Currents ● Image potential is important

20. 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

21. 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.