Fabrication and Investigation of Rare-Earth Oxide Thin Films Sandwiched Complexes

Abstract

This dissertation reports the fabrication and characterization of CeO2 based nonvolatile memory devices as metal/insulator/metal structures. The RRAM devices have been deposited under various deposition conditions using different electrodes, with different stack morphology and various thicknesses of active CeO2 layers. In the case of Zr/CeOx/Pt devices a forming-free bipolar resistive switching has been observed. HRTEM and EDX studies have indicated the formation of a ZrOy layer at the Zr/CeOx interface. The observed resistive switching has been suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies. The presence of oxygen vacancies has been noticed through XRD and confirmed by EDX in the nano-polycrystalline CeOx film and in the nonstoichiometric ZrOy interfacial layer. Similar results have been observed for the resistive switching characteristics of Pt/CeOx/TiN devices as confirmed by XPS, but in this case, the observed resistive switching behavior can be attributed to an interfacial layer TiON, as determined by HRTEM image. That is why this device exhibits low operation current (100 μA), high ON/OFF resistance ratio (>105) and good retention both at room temperature and at 85 °C. More interestingly, the TaN/CeOx/Pt based devices exhibited bipolar resistive switching even without any requirement of electroforming step. For these devices, TaON interlayer, as verified by HRTEM and XRD, has been suggested to play the main role in the resistive switching mechanism which stems from connection and disconnection of filamentary paths made of oxygen vacancies. On inserting an ultrathin metallic layer in Ti/CeO2/Al/CeO2/Pt stack, the resultant device has demonstrated dual resistive switching behaviour. These devices could switch between the two operating modes merely by choosing the polarity of RESET voltage. In addition, the requirement of identical current compliance during the SET process of both xxvii modes provides an additional advantage of simplicity in device operation. On the basis of analyses of current–voltage characteristics and temperature dependence of resistance, resistive switching mechanism has been proposed to be originated from a combined effect of field induced diffusion of oxygen and Al ions in the sandwiched ceria matrix. Without metallic insertion, bilayer Ti/CeO2-x:CeO2/ITO memory stacks have demonstrated stable bipolar resistive switching behavior with low-voltage operation and good endurance. In addition, the narrow cycle-to-cycle and device-to-device distributions of resistance switching parameters have been proposed to originate from the electric field induced drift of defects preferably along grain boundaries in the bilayer structure of ceria. On using both electrodes of conducting oxide (instead of metals), fully transparent (with >80% optical transmission) RRAM devices in ITO/CeO2/ITO format (with weak polycrystalline CeO2 phase) have been found to exhibit reliable bipolar switching behavior. The dual role of ITO polycrystalline films as defects reservoir as well as source of O2- ions in these devices becomes the cause of good data retention (over 104 s) and reliable performance both at room temperature and 85 oC. In almost all the devices studied in this PhD work, Ohmic and Poole Frenkel conduction mechanisms are found to be responsible for charge transport in the low- and high-resistance states respectively. The observed RS characteristics and performance of various CeO2-based devices have shown their potential as candidates for future non-volatile memory applications.