1. Schottky barriers and Organic semiconductors
Hubert Nguyen
Physics 211A
December 10, 2007
PFO - polymer
PTCDA - single molecule crystal

2. Overview Electronic structure Organic molecular beam deposition (OMBD)
Transport properties
Interface effects

3. Electronic structure I

4. Schottky-Mott rule Vacuum alignment at interface alignment at
thermal equilibrium
Built-in potential
Valid for inorganic
semiconductors (with some exceptions…)

5. Current transport Forward/reverse bias Thermionic emission
Field emission
Thermionic field emission
Recombination FE
Recombination - at depletion region

6. Tight binding in organic crystal
Energy gap ~ 2 eV
Bandwidth ~ 0.1 eV
No ions, only
neutral molecules
Van der Waals
forces dominate
Experiments measure:
A = electron affinity
 = work function
I = ionization potential

7. Vacuum levels at surface
V(s) ≠ V(∞)
Surface dipole layer
at metal surface
Electron cloud leaking out of surface
An additional dipole layer on top of metal surface dipole layer
Origin
Charge transfer across interface
Redistribution of charge cloud
Interfacial chemical reaction
Potential distribution at interface given by Poisson’s equation

8. Vacuum alignment for metal-organic interface
S-M rule of common
vacuum level is
broken by interfacial
dipole layer shift
“Pillow effect”
Hole barrier
Delta shows up in experimentally measured vacuum level using PES
Pillow effect - Pauli exclusion at interface compresses electron tail of metal, lowering phi_M
Electron barrier

9. Deviation from S-M limit
Slope parameter
Zero dipole, S = 1
PES eV

10. Band bending Bandwidth ~ 0.1 eV
does not always align (i.e. insufficient number of mobile charge carriers when )

11. Band gap Larger M, smaller hole-barrier
Perylene - planar aromatic; large intrinsic mobility at low temp
MPcs - planar with metal ion; high photoconductivity, electroluminescence, fairly large mobility

12. Organic molecular beam deposition (OMBD)II

13. Sample purification Gradient sublimation
Organic stacking behavior is highly sensitive to impurities
Electrical properties not as heavily affected directly by impurities due to localization
Growth very sensitive to impurities
Inpurities don’t affect electrical properties due to localization of molecular orbitals, but they do affect stacking behavior
Gradient sublimation

14. Crystal growth Ultra high vacuum Growth rates Impurity adsorption rate
P ~ torr
Growth rates
Å/s (0.7 ML/h - 30 ML/s)
Impurity adsorption rate
Substrate temperature
Turbomolec pumps
Impurities accum at low growth rates
Recommended: ML/s (similar to MBE semicond)
Ex: if residual gas is nitrogen, P = 5e-9 torr -> tau ~ 3 min per ML; P = 5e-10 torr -> tau ~ 30 min
Cannot deposit too many layers as flux drops off quickly due to cooling of rod
~ 3-30 min/ML

15. Depo rate

16. Growth modes Inorganic Organic
Strong interatomic covalent bonding (cohesion)
meV/atom
Close lattice matching required
1-1 commensurate relationship
Organic
Weak van der Waals (vdW) interaction
1-10 meV/atom; but ~1000 meV/molecule
Lattice matching requirement relaxed
Higher tolerance for interface-adlayer strain
Growth determined by substrate rather than bulk
Organic/inorganic lattice constants usually don’t match anyway

17. Long range order is possible without strict lattice matching
PbPc
Molecules rotate to minimize vdW binding
Rotational degree of freedom
Electrostatic interactions between metal atom with anion (-) in substrate lattice; e charge localized on Pb atom
n < 5 ML possible until disordered bulk
Alignment at high
symmetry axes
is preferred

18. Growth modes (cont.) Conventional: chemisorption
usually equilibrium growth
Quasi-epitaxy:
- physisorption (vdW)
non-equilibrium growth
azimuthal order
higher strain tolerance for ordered growth
QE - layer by layer, layer plus island; long range order although incommensurate
Unstrained vdWE - layer plus island

19. STM imaging Commensurate growth, layer-plus-island mode
Competition of vdW binding energies may lead to occasional defects
minimize energy among molecules within layer
vs.
minimize energy between layer and substrate
With large number of d.o.f., calculating energy minimization is cumbersome

20. Passivated substrates
Very weak MPc-substrate interaction
No adlayer wetting
Standing up configuration
High degree of ordering observed in AFM and LEED

21. Superlattices No deposition Deposit B on A Growth of each
Adhesive bond energy
DNB - benzene derivative
MLD - molecular layer deposition
Substrate temperature control
Differential sublimation temperature
Chosen due to considerably different sublimation temperatures
Slow process, difficult to control - but proof of concept
Deposit B on A
Growth of each
layer self terminates
n ~ 15, d = 100 Å

22. Organic/inorganic hybrid multilayer
Strong, sharp Bragg peaks indicate low disorder

23. Charge transport Ohm’s law for small E Mobility Temperature dependence
Effective mass growth, band narrowing
Polaron-hopping transport
Thermally activated diffusion between trap states
Highly anisotropic
Electron mobilities change by 15 x along different directions for anthracene 

24. Mobilities

25. Polaron-hopping
Charges move by thermal hopping between localized sites
Tunneling from metal band state to localized HOMO/LUMO of organic molecule
Random walk, incoherent transport
At high T, polaron hopping dominates mobility
When bandwidth and kT are similar, mean free path ~ lattice constant
Structural properties of solid affect electrical mobility
Driven by electron-phonon interaction

26. Conclusion
Organic semiconductors