Resistive random access memory (RRAM) devices

In this thesis, we have investigated the metal oxide based memristor devices. Poor cycle to cycle and device to device redundancy and low on/off ratio are the critical problems with the memristor devices. We fabricated the devices on FTO substrate with zinc oxide sandwich between the two electrodes. Zinc oxide thin films were deposited on substrate using sputtering under the various ambiance conditions. Those conditions included change in thickness and oxygen partial pressure. Top contact was made using thermal evaporation of Al metal through the metal shadow mask onto the substrate. The effects of change of thickness, oxygen partial pressure, doping in ZnO and intermediate layer in switching oxide were studied. All the devices fabricated in this thesis show bipolar switching. The variation of thickness through devices (S1-S3) shows an improvement in on/off ratio. With increase in thickness from 300nm to 700nm the switching mechanism change from filament type switching to interface type switching. Similarly, the oxygen partial pressure increase for the devices (S4-S6) shows a change in resistive devices switching properties. Although with an increase in oxygen pressure the resistance ratio has improved but on the other hand variation in high resistance state (HRS) has also increased. The highest on/off ratio of ⁓82x read at 0.1 V was obtained for sample S5. In order to investigate the effect of doping Al/ZnO:Co/FTO devices were fabricated using 3% doped ZnO sputter target. A poor switching performance was observed due to higher concentration of doping. To improve the switching performance of the doped samples sputtering conditions were changed for devices (S7-S12) and their I-V characteristics were measured. The retention graph shows satisfactory performance and the device can hold the data for 1000s. Overall highest on/off ratio of ⁓102x read at -0.5 V was found for the sample S11. In addition, the effect of intermediate layer inside the switching oxide was investigated. Three different devices embedded with layer of (Cu, Al, GO) with thickness of 3nm half way between the 300nm zinc oxide layer were fabricated. The device with Cu interfacial layer shows a poor switching behavior whereas the device with graphene oxide (GO) shows an improved and stable bipolar switching. Apart from these, different switching models, current conduction mechanisms such as schottky, poole-frenkel and space charge limited conduction mechanisms were investigated. Furthermore, the model for conversion of filamentary conduction into homogenous interface type conduction was discussed in detail. Keywords: Memristor, Zinc Oxide, Graphene Oxide, Titanium Dioxide, Sputtering