Over the last several decades, one of the most important technological trends has been the continued scaling of semiconductor devices to smaller dimensions. While silicon has been the material of choice so far, it is clear that the next generations of scaled devices (having dimensions on the order of nanometers) will be extremely difficult to fabricate on silicon. A promising alternative approach to fabricating nanometer-scale devices is self-assembly of organic molecules. Our goal is to develop and study electronic devices in which small numbers of organic molecules define critical device dimensions.

Self-assembly of organic layers is critical to this goal. We typically grow single layers of molecules by employing a thiol-Au interaction. In this well-studied process, a gold substrate is immersed into a solution that contains organic molecules having a thiol (S-H) group on one end. The thiol group bonds to the gold, and over a period of hours a single, well-ordered layer of molecules attaches to the gold. We also study multilayer devices, in which controlled numbers of self-assembled layers are grown sequentially following the process described by Evans et al*. In addition to self-assembly, a wide range of conventional semiconductor fabrication processes, such as photolithography, silicon etching, and electron-beam evaporation, are used to define device structures and form electrical contacts to the organic layers.

Three-terminal (transistor) devices are necessary if molecular devices are to be used in conventional logic circuits. However, such devices, in which in the electric field from a third, gating terminal controls current through the organic layer, are difficult to fabricate. Another challenge is that the metals that form the electrical contacts to the organic layers can often penetrate through the nanometer-thick organic layer, leading to electrical defects. We have developed several approaches to fabricating two-terminal devices that minimize the latter problem, allowing us to achieve high yields (>90%) of defect-free devices. These same structures, with minimal modification, could be used to make three-terminal devices.

Currently we are focused on two areas. The first area consists of improvement of device structure. Projects include modification of current device structures to allow three-terminal electrical measurements and the development of new structures to achieve the same. The second area of focus consists of monolayer and multilayer growth and characterization. Examples of work in this area include the study of alternative self-assembly chemistries (eg Si/silane interactions) as well as characterization of organic layers by analytical techniques such as Rutherford Backscattering (RBS) and Fourier Transform Infrared Spectroscopy (FTIR).

*Evans, SD et al. J Am Chem Soc 113, 5866-6868 (1991).