The behaviors of organic memory devices based on carbon nanotubes in two-terminal structures are reported. The memory structures were fabricated in the forms of metal-insulator-semiconductor (MIS) and metal-insulator-metal (MIM) structures. The devices utilize AlOx or layer-by-layer (LbL) deposited single walled carbon nanotubes (SWCNTs) in the memory stack. For MIS-based memories, SWCNTs were embedded, between SU8 and polymethylmethacrylate (PMMA) to achieve an efficient encapsulation. The devices produced a clear clockwise hysteresis (with a 6V memory window) centered closed to 0V. Hysteresis in these devices attributed to the charging and discharging of SWCNTs from metal gate electrode. The devices exhibited high charge storage densities and demonstrated 94% charge retention due to virtue of the superior encapsulation. CNTs is also used for the MIM memory structures as charge traps embedded between two PMMA insulating layers. The stack was sandwiched between two aluminium electrodes to form an Al/PMMA/SWCNTs/PMMA/Al structure. The current-voltage (I-V) characteristics of this type of memory devices exhibit electrical bistability and non-volatile memory characteristics in terms of switching between high conductive (ON) and low conductive (OFF) states. The two conductive states were programmed by applying a positive and negative voltage pulses for the ON and OFF states, respectively.
Another filamentary based electrochemical metallization (ECM) memory structure based on AlOx was also fabricated and characterized. Copper/AlOx/tungsten (Cu/AlOx/W) ECM memory cells show reproducible resistive switching with an ON/OFF ratio of about 5x102 at a reading voltage of 0.1 V and reliable retention characteristics. The conduction of the devices was explained through back-to-back Schottky contacts in the OFF state, while it exhibits ohmic behavior in the ON state. Thermionic emission model was used to calculate the barrier heights of the Schottky contacts. The rupture of the Cu filament proved to occur at the weakest point of the filament inside the AlOx. Using Ohms Law, the slope of the linear I-V characteristics in the ON state was used to extract the Cu filament resistance and its diameter was estimated to be between 6 and 23 nm.