Conducting Polymer Memory Devices Based on Dynamic Doping

Abstract

Molecular electronic junctions consisting of a 20 nm thick layer of polypyrrole (PPy) and 10 nm of TiO2 between conducting layers of carbon and gold were investigated as potential nonvolatile memory devices. By making the polymer layer much thinner than conventional polymer electronic devices, it is possible to dynamically oxidize and reduce the polypyrrole layer by an applied bias. When the electrode in contact with the PPy is biased positive, oxidation of the PPy occurs to yield a conducting polaron state. The junctions exhibit a large increase in conductance in response to the positive bias, which is reversed by a subsequent negatively biased pulse. Switching between the conducting and nonconducting state can occur for pulses at least as short as 10 micros, and the conducting state persists after a positive bias pulse for at least 1 week. The read/write/read/erase cycle may be repeated for at least 1700 cycles, although with an error rate of approximately 3% due mainly to an incomplete "erase" step. The speed and retention of the PPy/TiO2 junctions are far superior to those of the analogous fluorene/TiO2 devices lacking the polymer, and the conductance changes are absent if SiO2 is substituted for TiO2. The observations are consistent with "dynamic doping" of the solid-state polymer layer, with the possible involvement of adventitious mobile ions. Although the speed of the current polymer/TiO2 junctions is slower than commercial dynamic random access memory, their retention is approximately 5 orders of magnitude longer.