Electrical characterization of redox-active molecular monolayers on SiO2 for memory applications

Abstract

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.

Integration of Self-Assembled Redox Molecules in Flash Memory Devices

“…With these assumptions, the surface density N SAM d SAM (in molecules per square centimeter) of FcCOOH and CoP can then be calculated as 1.58 × 10 14 and 3.6 × 10 13 molecules/cm 2 , respectively, for SAMs undiluted with dummy molecules, which agree well with previous estimation by CyV [6]. Considering the assumptions that were made with the estimation method and the background noise, the absolute accuracy is approximately ±35%.…”

“…We further confirm that porphyrin molecules have the thermal budget that can withstand postmetal gate annealing [11]-an important requirement for CMOS compatibility. Compared with the previous studies [6]- [9], we were able to confirm an interaction with multiple states of redox-active molecules, instead of interface traps. …”

“…The native SiO 2 possibly formed during DI water rinse/ N 2 dry after etching by HF and the initial cycles of thermal Al 2 O 3 deposition. The surface density has been estimated to be around 1 × 10 14 and 0.45 × 10 14 cm −2 for FcCOOH and CoP SAMs, respectively [6], assuming a full surface coverage. Furthermore, the thickness values of FcCOOH and CoP SAMs measured by ellipsometry were in the range of 0.9-1.2 and 1.5-1.6 nm, respectively, which is in good agreement with a previous estimate where the thickness is estimated to be 0.8 and 1.5 nm by impedance spectroscopy [8].…”

“…Compared with the CyV measurements, where the insulating barrier was only deposited on one side [6], [9], the new structure provides insulating barriers above and below the molecules, which further lessen the possibility of orbital hybridization from the gate. A strong coupling between the electrode and the molecule may lead to molecule-independent switching behavior and modification of the intrinsic spacing of the energy levels [10], which makes it difficult to study the moleculespecific behaviors.…”

“…The monodisperse nature of the molecular orbitals (MOs) can potentially reduce cell variations, whereas the distinct energy levels may enable stepwise charging for precise control of each memory state. By attaching reduction-oxidation (redox) molecules as a floating gate in the complementary metal-oxide-semiconductor (CMOS) structure, one also creates a high-impedance nonamperometric electrode for probing the molecular redox states, in contrast to the low-impedance connections used in cyclic voltammetry (CyV) [6]- [9]. Previous studies have shown capacitance and conductance peaks associated with the charging and the discharging of the carriers stored at the MOs [7] and CyV hysteresis owing to the redox of molecules attached to the SiO 2 surface [6], [8], [9].…”