Silicon nanogaps: An electronic bridge to the molecular world

Licentiate thesis, 2003

We have designed and manufactured silicon nanogaps aimed at electrically interfacing molecules, utilizing the precision thickness control in an oxidation process. An insulating layer was formed on a silicon wafer, and then polysilicon was deposited on top. By using a selective etch, a vertical nanometer-sized gap was formed between the silicon electrodes.Electrical measurements reveal significant surface currents after chemical treatments. These surface leakage currents are time dependent diffusion currents, and the magnitudes are highly dependent on the chemical treatments performed. For conducting polymers large time independent surface currents are observed.Currents arising from the nanogaps after etching the devices show severe instabilities. These instabilities are reduced after applying constant voltage stress to the devices, and thereafter the currents are proportional to the device edge length. The same behavior is observed also after the deposition of an insulating polymer.Application of nanocrystals gives no clearly distinguishable impact on the electrical characteristics. Silicidation with nickel has been tested, and electrical measurements showed no signs of short-circuit for the nanogaps. A nickel oxide layer was formed on the field oxide, and it was found to be insulating.Simulations show that molecular resonant tunneling diodes are promising candidates for reducing the standby power of DRAM cells, due to the high peak-to-valley ratio showed by some molecules. Still molecules with resonance for lower applied voltages and with lower current levels in general must be developed if molecular RTDs are to be used in this application.