Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi level pinning at the molecule-metal interface

Stéphane Lenfant (1) , David Guérin (1) , François Tran-Van (2) , Claude Chevrot (2) , Serge Palacin (3) , Jean-Philippe Bourgoin (4) , Othman Bouloussa (5) , Francis Rondelez (5) , Dominique Vuillaume (1)

1 IEMN - Institut d’Électronique, de Microélectronique et de Nanotechnologie - UMR 8520
2 LPPI - Laboratoire de Physico-chimie des Polymères et des Interfaces
3 LCSI - Laboratoire de Chimie des Surfaces et Interfaces
4 LEM - Laboratoire d'Electronique Moléculaire
5 PCC - Physico-Chimie-Curie

We report the synthesis and characterization of molecular rectifying diodes on silicon using sequential grafting of self-assembled monolayers of alkyl chains bearing a group at their outer end (Si/-/metal junctions). We investigate the structure-performance relationships of these molecular devices, and we examine the extent to which the nature of the end group (change in the energy position of their molecular orbitals) drives the properties of these molecular diodes. Self-assembled monolayers of alkyl chains (different chain lengths from 6 to 15 methylene groups) functionalized by phenyl, anthracene, pyrene, ethylene dioxythiophene, ethylene dioxyphenyl, thiophene, terthiophene, and quaterthiophene were synthesized and characterized by contact angle measurements, ellipsometry, Fourier transform infrared spectroscopy, and atomic force microscopy. We demonstrate that reasonably well-packed monolayers are obtained in all cases. Their electrical properties were assessed by dc current-voltage characteristics and high-frequency (1-MHz) capacitance measurements. For all of the groups investigated here, we observed rectification behavior. These results extend our preliminary work using phenyl and thiophene groups (Lenfant et al., Nano Lett. 2003, 3, 741). The experimental current-voltage curves were analyzed with a simple analytical model, from which we extracted the energy position of the molecular orbital of the group in resonance with the Fermi energy of the electrodes. We report experimental studies of the band lineup in these silicon/alkyl -conjugated molecule/metal junctions. We conclude that Fermi-level pinning at the group/metal interface is mainly responsible for the observed absence of a dependence of the rectification effect on the nature of the groups, even though the groups examined were selected to have significant variations in their electronic molecular orbitals.

Domaines: Matériaux

Stéphane Lenfant, David Guérin, François Tran-Van, Claude Chevrot, Serge Palacin, et al.. Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi level pinning at the molecule-metal interface. Journal of Physical Chemistry B, 2006, 110, pp.13947-13958. ⟨10.1021/jp053510u⟩. ⟨cea-01056613⟩