Redox-active Organic Molecules on Silicon and Silicon Dioxide Surfaces for Hybrid Silicon-molecular Memory Devices.

Abstract

The focus of this dissertation is on creating electronic devices that utilize unique charge storage properties of redox-active organic molecules for memory applications. A hybrid silicon-molecular approach has been adopted to make use of the advantages of the existing silicon technology, as well as to study and exploit the interaction between the organic molecules and the bulk semiconductor. As technology heads into the nano regime, this hybrid approach may prove to be the bridge between the existing Si-only technology and a future molecule-only technology.

Functionalized monolayers of redox-active molecules were formed on silicon surfaces of different doping types and densities. Electrolyte-molecule-silicon test structures were electrically characterized and studied using cyclic voltammetry and impedance spectroscopy techniques. The dependence of the oxidation and reduction processes on the silicon doping type and density were analyzed and explained using voltage balance equations and surface potentials of silicon. The role played by the silicon substrate on the operation of these memory devices was identified. Multiple bits in a single cell were achieved using either molecules exhibiting multiple stable redox states or mixed monolayer of different molecules.

Self-assembled monolayers of redox-active molecules were also incorporated on varying thickness of silicon dioxide on n- and p- silicon substrates in an attempt to create non-volatile memory. The dependences of read/write/erase voltages and retention times of these devices were correlated to the SiO2 thickness by using a combination of Butler-Volmer and semiconductor theories. The region of operation of the silicon surface (accumulation, depletion or inversion) and the extent of tunneling current through the silicon dioxide were found to influence the charging and discharging of the molecules in the monolayer. Increased retention times due to the presence of SiO2 can be useful in realizing non-volatile memories.

Polymeric films of molecules were formed on Si and SiO2 substrates and exhibited very high surface densities. Metal films were deposited directly on these films and the resultant devices were found to exhibit redox-independent behavior. A combination of metal gate and dielectric was deposited on molecules in an attempt to create solid-state hybrid silicon-molecular devices. The metal gate and dielectric can replace the electrolyte and electrolytic double-layer to create an electronic cell instead of an ionic cell. The redox properties of the molecules were retained after the deposition of dielectric and metal, which augurs well for a solid-state device.

FET type structures were fabricated and molecules incorporated on them in order to modulate the characteristics of the FETs by charging and discharging the molecules. Drain current and transfer characteristics of electrolyte-gated "moleFETs" were modulated by oxidizing and reducing molecules on the channel region. Hybrid moleFET devices may be ideal tools for creating non-volatile FLASH type memory devices.

This work has recognized the interaction of organic molecules and bulk silicon and utilized the advantages of current CMOS technology along with the unique properties of molecules, such as discrete quantum states, low voltage operation etc., to create a class of hybrid memory devices. A way to create solid-state molecular devices retaining the inherent properties of molecules has been proposed and demonstrated. This work might be useful in providing a smooth transition from silicon electronics to molecular electronics.