# Novel Molecular Non-Volatile Memory: Application of Redox-Active Molecules
## 4 Solid-State Molecular Memory  
The conventional polysilicon floating-gate flash memory has benefited the semiconductor memory technology for decades, due to its scalability, compatibility with CMOS process, and so forth. It relies on hot electron injection from the channel into the floating gate through a tunneling oxide layer. By attaching redox-active molecules to Si as the floating gate in the CMOS structure, one can further enhance the cell density and reduce the cell variations. The combination of top-down lithography and  
Figure 5: Cyclic voltammetry of electrolyte-molecule-oxide-Si (EMOS) structures with (**a**) the Fc-BzOH and (**b**) the Por-BzOH redox molecule attached to SiO\({}_{2}\) at different voltage scan rates; capacitance-voltage (C-V) and conductance-voltage (G-V) curves of (**c**) Fc-BzOH and (**d**) Por-BzOH EMOS capacitors at 100 Hz. Reproduced with permission from [35;37], © AIP Publishing LLC, 2003, 2004.  
bottom-up molecule self-assembly processes renders a uniform charge density, and the monodisperse nature of the molecular orbitals leading to distinct energy levels can enable precise controlling of charged states [39]. Moreover, with a solid-state insulating barrier deposited on both sides of the molecules, the possibility of orbital hybridization from the gate will be further lessened. Such an integration of redox-active molecules in a solid-state memory provides an efficient platform to study the molecules and devices with microelectronic characterization metrologies, which is unique in comparison with the liquid electrolyte-based electrochemical method. The self-assembly process of the redox-active molecules to form a solid-state molecular memory enables not only high-quality attachment of molecules to semiconductor surfaces, but also uniform and high-density fabrication of low-cost devices and circuits. The disadvantages of molecular flash memory mainly lie in its reliability in volatile environments. Some redox-active molecules are instable or even do not function at temperatures higher than 200 \({}^{\circ}\)C due to evaporation or redox center reaction. Under volatile environments with oxygen or water, the molecule's bond might be broken, leading to poor redox behavior. As a result, some specific conditions and environments need to be taken into account when integrating such redox molecules in CMOS devices and circuits. Future molecular technology requires advancement in both molecule properties and device integration processes. The researchers from North Carolina State University reported a study on preventing contact metal penetration into the molecular layer in the metal-insulator-molecule-metal structure [40]. Similar work demonstrating that redox properties of molecules can be preserved after thin film encapsulation, such as atomic layer deposition (ALD), initiated the investigation on high-performance solid-state molecular memory [41]. The Cornell University research group reported a study on a metal-oxide-semiconductor (MOS) structure with molecules encapsulated in a high-k dielectric and functioning as the charge storage medium [42; 43]. As shown in Figure 6, molecules with different redox centers, which can show one and two charged states, have been studied for multi-bit memory. With the electrostatics determined by the alignment between the highest occupied or the lowest unoccupied molecular orbital (HOMO or LUMO, respectively) energy levels and the charge neutrality level of the dielectric, the asymmetric charge injection behavior in flat-band voltage shift (\(\Delta\)V\({}_{\text{FB}}\)) _vs._ programming voltage measurement (Figure 7) is due to the Fermi-level pinning between the molecules and the high-k dielectric [44; 45; 46]. The three programmable molecular orbital states of the 5-(4-Carboxyphenyl)-10,15,20,-triphenyl-porphyrin-Co(II) (CoP) molecule have been experimentally observed and are attractive for low-variation multi-bit memory applications.