Resonant Tunneling Through Individual Molecules on Silicon: Implications for Electronics and Chemical Spectroscopy

Mark Hersam

Materials Science & Engineering Dept.

Northwestern University

In recent years, substantial progress has been made in the emerging field of molecular electronics. In particular, metal-molecule-metal junctions have been widely studied. In this talk, charge transport through molecule-semiconductor junctions is considered. The presence of the energy band gap in semiconductors provides opportunities for resonant tunneling through individual molecules, leading to interesting effects such as room temperature negative differential resistance (NDR). Furthermore, by doping the substrate, the majority charge carrier can be tailored, thus allowing asymmetry to be intentionally designed into the current-voltage characteristic. Through judicious choice of the molecular species, the bias voltage of the NDR can also be controlled. By demonstrating these effects on the Si(100) surface, semiconductor-based molecular electronic devices have the potential of being directly interfaced to conventional silicon integrated circuit technology.

As an introduction, this talk will summarize recent theoretical [1] and experimental [2] work on silicon-based molecular resonant tunneling diodes. In particular, the ultra-high vacuum scanning tunneling microscope (STM) has allowed individual molecules to be imaged, addressed, and manipulated on the Si(100)-2×1 surface with atomic resolution at room temperature. While previous work has focused on the characterization of individual styrene and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) molecules, this talk will also detail previously unpublished results for isolated cyclopentene molecules and full monolayers of TEMPO and cyclopentene. In all cases, STM current-voltage characteristics on individual molecules mounted on degenerately n-type Si(100) show multiple NDR events at negative sample bias. On the other hand, at positive sample bias, the current-voltage characteristics do not show NDR, although discontinuities in the differential conductance are observed. When the Si(100) substrate is changed to degenerate p-type doping, multiple NDR events are observed at positive sample bias while the discontinuities in the differential conductance occur at negative sample bias. These empirical observations can be qualitatively explained by considering the energy band diagram for a semiconductor-molecule-metal junction.

For full organic monolayers on degenerately doped Si(100), similar behavior is observed except that the degree of NDR is significantly attenuated compared to charge transport measurements on isolated molecules. Possible explanations for this behavior include broadening of molecular energy levels through intermolecular interactions or concurrent tunneling paths through multiple molecules. This talk will conclude by describing recent efforts to quantify and understand the critical parameters that dictate charge transport through individual molecules and organic monolayers on Si(100) surfaces.

[1] T. Rakshit, G.-C. Liang, A. W. Ghosh, and S. Datta, arXiv:Cond-Mat, 0305695v1 (2003).

[2] N. P. Guisinger, M. E. Greene, R. Basu, A. S. Baluch, and M. C. Hersam, Nano Letters, 4, 55 (2004).