Hybrid CMOS/molecular memories using redox-active self-assembled monolayers

Self-assembled monolayers of redox-active molecules with phosphonate linkers have been attached to silicon dioxide (SiO₂) surfaces. These redox-active molecules exhibit charge states at distinct voltages. Conventional cyclic voltammetry techniques and impedance spectroscopy have been used to characterize capacitor structures formed using these monolayers, with electrolyte as top electrode. Distinct capacitance and conductance peaks have been observed, which confirm the processes of charging and discharging of the molecules. Attachments of these redox-active molecules have been achieved on varying thickness of SiO₂, ranging from 1 to 3 nm. Measurements have revealed the dependence of the tunneling rate on the oxide thickness and frequency of measurement. The oxidation and reduction voltages changed with the varying oxide thickness and hysteresis was also observed. These voltage shifts and hysteresis of the molecular capacitors were found to increase with increasing SiO₂ thickness. With increasing frequency, decrease in capacitance peaks and increase in conductance peaks were observed until the frequency at which both the peaks disappear. This cut-off frequency was seen to decrease with increasing thickness of the oxide. These results verify the presence of reversible charge trapping, a key requirement for memory applications.