Fabrication and characterization of memory devices based on nanoparticles

摘要

The objective of this study is to understand the electrical properties of non-volatile memories based on metal oxide nanoparticles embedded into an insulating polymer matrix. These memories are classified as resistive random access memories (RRAM), as they undergo resistive switching between well-defined conductance states when submitted to a voltage pulse. A number of memory devices were fabricated and studied using electrical techniques. Current-voltage characteristics were studied as a function of the ambient atmosphere and temperature. The dynamic electrical behaviour was probed using triangular voltage profiles with different scan rates, transient techniques and electrical noise techniques. Electrical measurements were complemented with morphological characterization. Important outcomes of this thesis are the following: It was shown that adsorbed moisture on the surface of the devices causes resistive switching. This type of resistive switching can lead to very high on/off ratios, and therefore it is not reliable. Silver oxide nanoparticles undergo an electroforming process similar to a soft-breakdown mechanism as reported for binary oxides. A model that explains the basic features of the electroforming mechanism was proposed. After the electroforming, the devices show resistance switching properties with a high on/off ratio (> 104), good retention time, and programming endurance. A resistive switching mechanism was proposed. The model assumes that during electroforming a percolation network of micro conducting paths (filaments) is established between the electrodes. The creation and rupture of these micro-paths is responsible for the changes in conductance. Results from this study indicate that nanostructured thin films made of silver oxide nanoparticles embedded in an insulating polymer show an electrical behaviour like the bulk oxide based memory structures. The planar structures present the advantage of being programmed in multi-resistance levels suggesting a very interesting finding that may pave the way to achieve a multi-bit memory device.