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Abstract
Conventional semiconductor memories have encountered their limitations for sustained advance in data processing and storage technologies. One emerging memory, the resistive random access memory (RRAM), has attracted extensive attention for its application in the next-generation non-volatile memories, due to the excellent characteristics of high-density, high-speed, low-power-consumption and good reliability. Cobalt oxide (CoO), a p-type binary transition metal oxide, has shown good potential as a resistive switching (RS) material but has not been extensively studied yet. In this dissertation, the RS behaviours in the oxide molecular beam epitaxy (OMBE) fabricated CoO thin films and CoO-TiO2 p-n heterostructures were investigated by cyclic voltammetry measurement, conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM) methods, trying to reveal the RS mechanism in the CoO material system and to find new material structures as good RRAM candidates.
RS performance and mechanism in the OMBE fabricated CoO thin film were studied. It shows bipolar RS characteristic with good endurance and retention performance, showing good potential for RRAM application. A CAFM writing/reading study reveals the conductive filamentary switching nature in the CoO/Pt memory device.
Growth conditions play a key role in determining the properties of thin films. The impact of oxygen partial pressure during the deposition on the RS behaviour of CoO thin film was studied. A combination of CAFM and KPFM study shows that higher growth oxygen pressure leads to higher conducting filament density in the CoO film, which is attributed to higher intrinsic defects concentration in this p-type oxide. The RS mechanism in this tip/CoO/Pt memory device can be explained by a novel charge-injection/conductive-filamentary model.
RRAMs with multilayered structure may exhibit different RS behaviour from those with single-layered structure. A CoO/TiO2 p-n heterostructure memory cell was fabricated and unique nonpolar RS behaviour has been found. Effect of growth temperatures as well as film thickness on the stability of switching parameters, such as cycle-to-cycle ret and reset voltages and resistance at HRS and LRS, have been explored. Growth conditions that result in better RS performance have been suggested.
In summary, the results in this dissertation reveal cobalt oxide’s great potential in RRAMs application and provide more evince in understanding the RS mechanisms in this material. In addition, a p-n junction structured RRAM device with unique RS properties is developed.