During the past decade there has been intense interest in developing molecular-based devices for applications in electronics, including memory.[1±4] Among the various approaches towards building devices that incorporate molecules, the hybrid silicon/molecular approach is attractive as a transition technology because it leverages certain advantages afforded by a molecule-based active medium with the vast infrastructure of traditional metal±oxide±semiconductor (MOS) technology. Recently, we demonstrated that self-assembled monolayers (SAMs) of redox-active molecules on silicon are excellent candidates for hybrid memory devices. The SAMs were prepared using either a benzyl alcohol-tethered ferrocene (4-ferrocenylbenzyl alcohol, Fc-BzOH) or a benzyl alcohol-tethered porphyrin (5-(4-hydroxymethylphenyl)-10,15,20-trimesitylporphinatozinc(II), Por-BzOH). In addition to the neutral state, the Fc-BzOH provides one state (monopositive) while the Por-BzOH provides two states (monopositive, dipositive). The availability of charged states at distinct voltages is highly advantageous for applications in charge-storage memory devices such as dynamic random access memory (DRAM) and FLASH memory. In addition, the molecularbased devices exhibit very low write and erase voltages [5,6] and long charge-retention times. [7,8] One strategy to increase memory density entails a multibit approach wherein the charge-storage element contains molecules with multiple redox states. We have previously demonstrated this approach using a variety of porphyrinic molecules, including a ferrocene±porphyrin conjugate bearing a single thiol tether. [9] In this design, the characteristic oxidation potentials of the ferrocene and the porphyrin components are maintained, thereby affording four states (neutral and the three cationic oxidation states from the ferrocene and porphyrin). This approach is limited only by the requirements for synthesis of the covalently linked multiredox molecule. An alternative and perhaps simpler strategy for achieving multibit functionality is afforded by mixing different redoxactive molecules whose potentials are well separated. In this paper, we demonstrate this approach using mixed SAMs of Fc-BzOH and Por-BzOH on silicon surfaces to achieve a fourstate (2-bit) memory element. The four states include the neutral state and three distinct cationic states obtained upon oxidation of the Fc-BzOH (monopositive) and the Por-BzOH (monopositive, dipositive) molecules. Although mixed SAMs have been previously investigated, these studies typically involved mixtures of redox-active and non-redox-active molecules.[10±14] In our study, cyclic voltammetry (CV) has been used to measure the coverage and redox potentials of the mixed SAMs. Since each redox state represents the transfer of a single electron per molecule, the total measured charge per unit area can be directly converted to molecules per unit area. Conventional capacitance±voltage and conductance±voltage (C±V and G±V) methods have been used to further characterize the mix...

Hybrid silicon capacitors have been successfully fabricated by attaching monolayers of redox-active molecules via self-assembly to ultrathin silicon dioxide layers. Capacitance, conductance, and cyclic voltammetric measurements have been used to characterize these capacitors. The presence of distinct capacitance and conductance peaks associated with oxidation and reduction of the monolayers at low gate voltages indicates discrete electron storage states for these capacitors, suggesting their feasibility in memory devices. The inherent molecular scalability and low-power operation coupled with existing silicon technology support the approach of hybrid molecule-silicon devices as a strong candidate for next generation electronic devices.

Hybrid molecule-silicon capacitors have been fabricated by the self-assembly of a monolayer of porphyrin molecules on a silicon oxide surface. The porphyrin employed [5-(4-dihydroxyphosphorylphenyl)-10,15,20-trimesitylporphinatozinc(II)] attaches to silicon oxide via a phosphonate linkage. Cyclic voltammetry current and capacitance/conductance measurements have been used to characterize the capacitors. The presence of multiple distinct peaks in current density and capacitance/conductance measurements are associated with oxidation and reduction of the molecular monolayer. The charge-storage states of the capacitor indicate applicability for use in multiple-bit memory devices.

In this work, monolayers of the redox-active molecules, with cationic- accessible states, were incorporated on p- and n-type silicons of varying doping concentrations. The redox voltages and kinetics were found to be strongly dependent on the silicon doping concentrations, and ambient light in case of n-Si substrate, while there was no significant impact of substrate doping concentration or ambient light in case of p-Si substrate. These results suggest the redox energy states in the molecule align within the valence band of the silicon substrate. Based on this, a model for electronic coupling and charge transfer at the molecule-semiconductor interfaces is proposed.

Uniform arrays of nano-scale electrolyte-molecule-silicon capacitors have been successfully fabricated. This was done by a combination of reactive ion etch and a selective wet etch through an anodic aluminium oxide mask to form nano-holes in silicon oxide/silicon nitride insulator layers on silicon. Self-assembled monolayers of 4-ferrocenylbenzyl alcohol were then attached to the exposed silicon surfaces at the bottom of the nano-holes. Characterization by conventional capacitance and conductance techniques showed very high capacitance and conductance peaks near -0.6 V, that were attributed to the charging and discharging of electrons into and from discrete levels in the monolayer owing to the presence of the redox-active ferrocenes.

Self-assembled monolayers of redox-active molecules were formed on varying thickness of silicon dioxide (SiO 2 ). Cyclic voltammetry (CyV) and impedance spectroscopy (capacitance-voltage and conductance-voltage) techniques were used to characterize these structures. The charge retention properties of these molecule-oxide-silicon capacitor structures were studied by applying oxidizing voltages in two successive CyV scans without applying a reducing voltage in between the two scans. A variation of this technique, wherein a reducing voltage is applied in the second scan, was also employed. The wait time between the two scans was varied from 0 to 300 s. The number of molecules oxidized (or reduced) in the second scan increased (or decreased) with increasing wait time, which is attributed to increasing charge leakage with increasing time. The retention properties of these structures were studied and correlated to increasing oxide thickness. It was observed that the retention times increased with increasing oxide thickness if the voltage applied during the wait time was in between the oxidation and reduction peak voltages. The molecular scalability and ability to tune the retention times by varying the oxide thickness make these Si/molecular hybrid devices attractive candidates for next-generation memory applications.

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