tesi R. Miscioscia.pdf - EleA@UniSA

64 State of the Art
figure 32: band diagram for a Au/Pentacene junction [7]
For instance, the gold Fermi level is approximately WF=5.10
eV and the pentacene HOMO level as estimated in [7] has to be
approximately 5.2 eV, the barrier should appear extremely limited (0.1
- 0.2 eV ) and the quality of the contact is theoretically optimal.
However, in practice, the contact resistance is much higher than
expected and exhibits a non-ohmic contact behaviour. This is also
confirmed by PES (Photo-Electron Spectroscopy) measurements [38]
that have shown a shift of more than 1 eV between the two materials,
actually lowering the metal Fermi level as the contact has been
realized (see figure 33). This was attributed to the formation of
interfacial dipoles that modify the energy profile at the border of the
source and drain contacts.
figure 33: the shift in the energy levels between gold and the HOMO and LUMO levels of the
pentacene due to the formation of interface dipoles [7][38]