Integration of Self-Assembled Redox Molecules in Flash Memory Devices

Self-assembled monolayers (SAMs) of either ferrocenecarboxylic acid or 5-(4-Carboxyphenyl)-10,15,20-triphenylporphyrin-Co(II) (CoP) with a high-kappa dielectric were integrated into the Flash memory gate stack. The molecular reduction-oxidation (redox) states are used as charge storage nodes to reduce charging energy and memory window variations. Through the program/erase operations over tunneling barriers, the device structure also provides a unique capability to measure the redox energy without strong orbital hybridization of metal electrodes in direct contact. Asymmetric charge injection behavior was observed, which can be attributed to the Fermi-level pinning between the molecules and the high-kappa dielectric. With increasing redox molecule density in the SAM, the memory window exhibits a saturation trend. Three programmable molecular orbital states, i.e., CoP(0), CoP(1-), and CoP(2-), can be experimentally observed through a charge-based nonvolatile memory structure at room temperature. The electrostatics is 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 surrounding dielectric. Engineering the HOMO-LUMO gap with different redox molecules can potentially realize a multibit memory cell with less variation.