Electrical transport of semiconductor surfaces and nanostructures are strongly influenced by interfacial processes. Adsorption and reaction events which result in charge re-distribution can modulate near surface conductivity through long-range electric field effects. These effects are being exploited to tailor the electronic properties of nanomaterials and devices as well as in the development of electrically-based chemical and biological sensors. Silicon-on-insulator (SOI) substrates are particularly well-suited for investigating the effects of surface processes on electrical transport. SOI substrates also facilitate formation of point contact pseudo-MOSFETs, allowing transistor characteristics to be obtained without the need for device fabrication. This method is demonstrated to be a simple and straightforward way to monitor the effect of adsorption and reaction events on the electronic properties of the silicon substrate. Adsorption of certain polar molecules (water, pyridine and ammonia) on hydrogen terminated H-SOI substrates (with both (100) and (111) orientations), results in large reversible increases in conductivity. These conductivity changes are attributed to charge transfer effects that induce accumulation of majority carriers on n-type and formation of minority carrier channels (inversion) on p-type substrates. Adsorption of the prototypical electron acceptor tetracyanoethylene (TCNE) results in a strong decrease in conductivity on n-type substrates due to depletion of carriers. This effect is not fully reversible due to reactions of TCNE with the H-terminated surface. Gas phase photochemical reaction of alkenes has been used to chemically passivate these surfaces while maintaining a low density of electrically active defects (<1x1011 cm-2). Alkyl monolayer passivated SOI surfaces show a large reversible response to TCNE, suggesting that these monolayers function as good ultrathin gate dielectrics for the fabrication of molecular sensors. Application of chemically modified SOI to the electrical detection of other high electron affinity compounds will also be shown.

Molecular modulation of conductivity on chemically modified silicon-on-insulator substrates

247th National Meeting (2014)

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