FOR 2093

A2: Vertical and horizontal memristive nanocomposite devices without filament formation

In project A2, memristive devices based on nanocomposites with silver nanoparticle gradients in dielectric organic and inorganic matrices, located between two metal electrodes, will be examined in vertical and horizontal geometry. Following a recently introduced concept [Jo et al., Nano Lett. 10 (2010) 1297-301], an electrical field between two metal electrodes will introduce a reversible shift of silver ions from areas of higher to areas of lower nanoparticle concentration, which will lead to changes in the electronic hopping conductivity. Due to the expected homogeneous ion transport, filament formation and the resulting need of an electroforming process, which excludes the design of complex neuromorphic circuits, can be avoided. Especially, filament induced bistability will be avoided and an analog behavior will be achieved that causes a high synaptic plasticity, as known from the multitude of states between the lowest and the highest conductivity in biological systems. By tailoring the nanostructure, high resistance states can be achieved which are required for low loss neuromorphic circuits. Furthermore, a threshold voltage can be established, which is connected with the activation energy of ion transport and can be tuned by the matrix properties. The occurrence of a threshold voltage is, amongst others, important for a non-destructive readout of the resistive state. For the achievement of high cycle numbers, several approaches are planned. By device fabrication in horizontal as well as in vertical geometry, a wide variation of the conductive channel lengths, widths and heights can be achieved, which allows a detailed study of the effect of channel properties on the mode of operation. In addition to classical structuring methods like e-beam lithography, an approach involving nano shadow masks will be used that enables the fabrication of controlled gradients on the nano scale. Furthermore, the horizontal orientation allows the introduction of an additional electrode for the construction of a three terminal device for additional functionality. An electrical characterization can be realized by a third contact, e.g., with a conductive AFM tip. The horizontal orientation allows in situ experiments in the transmission electron microscope. A further key aspect in characterization is the FFT-impedance spectroscopy, which can monitor the electrical and ionic conductivity as well as their correlation separately and will be used for horizontal as well as vertical device geometry.