Molecular control of the threshold voltage of an NMOS inversion layer

We have developed a hybrid molecular-MOSFET structure that is sensitive to the presence of a molecular monolayer attached to its surface. The device is fabricated from a silicon-on-insulator wafer and we use a substrate voltage, V_sub, to invert the buried Si:SiO₂ interface. This allows the top surface of the silicon to be free of any insulating layers, apart from a thin native oxide that forms on exposure to air. The buried inversion layer is only a few hundred angstroms away from the exposed surface, and the threshold voltage of the device, V_th, is strongly influenced by the surface potential. A spiropyran monolayer is attached to the surface of the device by means of carboxylic-SiO₂ bonding. After attachment of the monolayer, the threshold voltage of the device shifts to more negative values. We explain this result in terms of an increase in the concentration of fixed positive charge at the upper Si:SiO₂ interface due to protonation of the surface by the molecular monolayer.