Electrical Characterization of Silicon Nanogaps

Doctoral thesis, 2005

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.Before the etching the only current component detected is the tunneling current through the thin oxide, but after the etching large current instabilities are seen. This parasitic current can change rapidly up and down more than one order of magnitude. Measurements of the low-frequency noise show a power spectrum close to 1/f, and the current power spectrum density scales as the current squared.Stressing the etched devices by applying a constant voltage can reduce the current levels. Heating the devices in nitrogen reduces the noise levels. Storing or heating the chips in air results in oxidized electrode surfaces, and reduced currents. Comparisons to soft breakdown (a degradation mechanism for thin oxides) are made and a model based on percolation theory is presented. Another important issue addressed is the surface leakage currents after using chemical methods to deposit nanoscale objects to the electrodes. These surface leakage currents are time dependent diffusion currents, and the magnitudes are highly dependent on the chemical treatments performed. Chemical treatments leaving more conductive residuals cause time independent surface leakage currents. Simulations show that molecular resonant tunneling diodes (RTDs) are promising candidates for reducing the standby power of DRAM cells, due to the high peak-to-valley ratio displayed in the current-voltage characteristics of 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.