Scheme of a RTD network with a common silicon bottom electrode and separate carbon nanotube top electrodes
Molecular electronics aims at using individual molecules or small groups of organized molecules as the active part of electronic devices. It takes advantage of the size, the diversity, the quantum properties and the self-organization properties of organic molecules.
In this framework, our goal is to develop circuits based on molecular resonant tunneling diodes (RTDs) on silicon. One of the electrodes is deoxygenated silicon, in order to improve the charge injection within the molecular system. The other electrode is a carbon nanotube, in direct contact with the organic monolayer that constitutes the “active” part of the diode. The whole device will be built thanks to lithographic techniques, both optical and electronic (Figure 1). The organic monolayer is sigma-pi-sigma type, so that the charge transfer from silicon to the carbon nanotube behaves non linearly with respect to the applied bias. That resonant tunnelling effect has already been observed on similar organic systems sandwiched between two metallic electrodes, or with silicon under high vacuum.
The organic monolayer is grafted on hydrogenated silicon by thermal grafting from alkenes. Infrared and photoelectron spectroscopies are used to characterize the coating, and more particularly the reoxidation of the substrate. Indeed, by comparing the XPS spectra recorded after grafting of undecenoic and hexenoic acids, with or without protection of the acid group by formation of an activated ester (Figure 2), we have shown that the reoxidation of the silicon surface after the thermal grafting depends strongly on the thickness of the grafted monolayer and on the nature of the chemical groups borne by the molecules within the monolayer.
The complete sigma-pi-sigma monolayer was formed in two steps: covalent grafting of an acid-terminated monolayer on hydrogenated silicon, and then chemical coupling under soft conditions of the terminal pi-sigma part. Once again, the reoxidation of the silicon surface was thoroughly followed and found difficult to prevent. (Figure 3)
The measuring device was realized by classical lithographic techniques from a double trench structure in thick silicon oxide. Using vertical silicon/monolayer/nanotube cross junctions makes it possible to foresee the parallel fabrication of RTD networks. The double trenches are built via two successive alignments of electronic lithography. The first one allows the formation of the first (large) trench, which is etched by buffered fluorhydric acid (BHF). The second step defines the central (narrow) trench that is made by reactive ion etching. Both etching steps have to be calibrated finely to reach the bottom silicon and leave enough silicon oxide at the bottom of the first trench to isolate the connecting electrodes from the silicon. Once the nanotubes are deposited, a final lithographic step allows the formation of the gold pads used for connection to the measurement apparatus. Figure 4 shows an AFM image of a single wall carbon nanotube across the double trench, and a profile of the double trench