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| Dissertation / PhD Thesis/Book | PreJuSER-15673 |
2011
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-707-8
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Please use a persistent id in citations: http://hdl.handle.net/2128/28184
Abstract: The continuing improved performance of the digital electronic devices requires new memory technologies which should be inexpensively fabricated for higher integration capacity, faster operation, and low power consumption. Resistive random access memory has great potential to become the front runner as the non volatile memory technology. The resistance states stored in such cell can remain for long time and can be read out none-destructively by a very small electrical pulse. In this work the typically two terminal memory cells containing a thin TiO$_{2}$ layer are studied. Polycrystalline TiO$_{2}$ thin films are deposited with atomic layer deposition and magnetron reactive sputtering processes, which are both physically and electrically characterized. The resistive switching cells are constructed in a metal/TiO$_{2}$/metal structure. Electroforming process initiate the cell from the beginning good insulator to a real memory cell to program the resistive states. Multilevel resistive bipolar switching controlled by current compliance is the common characteristic observed in these cells, which is potentially to be used as so called multi-bit memory cells to improve the memory capacity. With different top electrodes of Pt, Cu, Ag the resistive switching behaviors are studied. The switching behaviors are different depending on the top metal such as the minimum current compliance, the endurance of the programmed resistance states and the morphology change during the switching. The temperature dependence of different resistance states are investigated. A reduction of the activation energy and their possible conduction mechanisms is discussed on the base of the basic current conduction models. It is found that the resistance state transfers from semiconductor to metallic property with the reducing resistances. The calculated temperature coefficients of their metallic states on the Cu/TiO$_{2}$/Pt and Ag/TiO$_{2}$/Pt are very close to the reported literature data, which implying a Cu and Ag filament formation. With the help of conductive atomic force microspectroscopy the local conductivity of the initial TiO$_{2}$ thin films and the TiO$_{2}$ in ON state after removing the top Cu electrode are investigated. The results from both cases confirm the local filaments formation in nanometer scale under the proper electrical voltages contributing to the observed switching behaviors. The time of flight secondary ion mass spectroscopy analysis shows the interdiffusion of Cu into the TiO$_{2}$ layer. Therefore the resistive switching in the TiO$_{2}$ with Cu electrodes could involve the electrochemical metallization process which forms and dissolves the Cu filament with respect to the applied voltage.
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