Dynamics of write and erase mechanisms in a novel organic memory with extremely low ON resistance

Organic nonvolatile memory devices, which combine desirable electrical characteristics with the potential for low-cost fabrication, are a promising technology [1], However, most organic devices have high resistances, which may limit their usefulness in high-speed applications that require large currents for fast signal processing and to minimize resistance-capacitance (RC) delays [2]. We have recently reported a planar organic nonvolatile memory device with a very high conductance/area of 10³ S/cm² at 1 V [3]. In this work, we describe edge-structure devices with even higher conductance/area near 10⁶ S/cm² and examine in detail the dynamics of the write and erase mechanisms in these devices. Devices consisting of thin (10-nm) self-assembled films of the insulating molecule 11-mercapto-undecanoic acid (MUA) contacted by Au electrodes were grown in minimal-area edge-type structures developed to study transport in thin (monolayer) organic films (Fig. 1) [4]. Devices could be switched into a high-conductance ("ON") state with a 4-V "write" pulse and returned to a low-conductance ("OFF") state with a 10-V "erase" pulse; reading of the stored state was conducted with a 1-V pulse after the programming pulse (Fig. 2). Memory states remained stable for many months in an inert atmosphere, and devices could endure more than 10⁴ write/erase cycles without degrading (Fig. 3). Three critical results are derived from a detailed study of the write and erase mechanisms: (i). The programming effect has a sharp turn-on for voltages > 2.5 V, so that a write voltage of 3 to 5 V programs the device into the ON state and higher voltages erase to the OFF state. The dependence of the memory-state conductance on programming voltage was measured by programming the device with voltage pulses and then measuring the resulting device conductance at 1 V (voltages below 2.5 V did not change the conductance) (Fig. 4). A conductance ratio of 10³ between OFF and ON states was obtained, with remarkably high ON conductances of up to 10⁶ S/cm². This ON conductance/area is a factor of 10⁴ higher than previously seen in switchable organic memory [1,2,5], and is critical to achieving low readout and programming times in matrix arrays due to the capacitance charging times that must be overcome. (ii). The delay when writing the device (from OFF to ON) decreases exponentially as the write voltage is increased, reaching 1 μs at 5 V (vs. up to 1 ms write times in FLASH memory [6]) (Fig. 5, 6). (iii). The erase time (for programming the device from ON to OFF) is under 100 ns, independent of applied voltage (within the limits of our experimental apparatus) (Fig. 7). These results support a model for the programming mechanism based on the formation and destruction of conduction paths in the MUA film (Fig. 8) [7]. In this model, mobile gold ions injected into the film form conducting paths at voltages between 3-5 V, leading to a high-conductance state; these paths are destroyed by joule heating at higher voltages to return the device to a low-conductance state. In conclusion, an investigation of writing and erasing mechanisms in novel highly-conductive organic memory devices indicates that the devices can be written with delays of 1 μs and erased in less than 100 ns. The short delay times and large current densities achieved in these devices make them attractive candidates for high-speed nonvolatile memory arrays. This work was supported by ONR contract N00014-02-1-075 and the New Jersey Center for Organic Optoelectronics.