Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy

The local electronic properties of tantalum oxide (TaO[subscript x], 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO[subscript 3]) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO[subscript x] films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO[subscript 3], we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO[subscript x] and SrRuO[subscript 3] differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-1419807

Redox processes and ionic transport in resistive switching binary metal oxides

The growing demand for non-volatile memories requires new concepts in data storage and mobile computing, as the existing flash technology runs into a physical scaling limit. One of the promising candidates for future non-volatile memories is Redox-based Resistive Random Access Memory (ReRAM). This memory technology is based on a non-volatile and reversible switching of device resistance with an external stimulus. The reversible resistance change in the memristive device can be attributed to an electro reduction, which takes place in the metal oxide and/or at the metal oxide electrode interface. The formation of ionic defects and their motion are involved in the electrically induced redox processes, which for most metal oxides is correlated to a valence change of metal ions. Within this thesis the microscopic processes in memristive devices are investigated in order to understand the defect configuration and defect motion in metal oxide thin films. Furthermore, the redox processes localized in nanoscale filaments induced by field-driven defect motion has been elucidated. The resistive switching characteristic of Ta$_{2}$O$_{5-x}$ single layers as well as Nb$_{2}$O$_{5-x}$/ Ta$_{2}$O$_{5-x}$ heterostructures has been investigated. It could be demonstrated that the forming voltage of the memristive devices can be modified by different reactive sputtering conditions. During electrical biasing a morphological change with a dendrite-like shape occurs at the top electrode. The formation of this dendrite-like structure at the metal oxide/metal interface, has been correlated to an avalanche discharge induced redox process in the metal oxide. The dendrite-like structure can be assigned to an oxygen deficient amorphous phase of the metal oxide. The predominant defects in TiO$_{2-x}$ are determined by ionic charge compensation at intermediate oxygen partial pressure and dominated by electronic charge compensation at reducing conditions. For Ta$_{2}$O$_{5-x}$ ceramics as well as thin films the electrical conductivity is dominated by ionic charge compensation over the entire investigated oxygen partial pressure range. The formation energy for oxygen vacancies of single crystal/ceramics is comparable to the values of the thin films. At oxidizing conditions the thin films show an enhanced ionic conduction due to an increased concentration of defects at the grain boundaries and an accumulation of the space charges. The equilibration process in TiO$_{2-x}$ single crystals as well as in thin films is dominated by a surface exchange process. The velocity of the equilibration process is influenced by the surface orientation of single crystals. In TiO$_{2-x}$ thin films a two-fold surface exchange process has been identified. As origin of this two-fold process the incorporation or release of a second species such as water was discussed as well as the influence of the texturing of the thin film and the influence of the substrate

Reduction of the forming voltage through tailored oxygen non-stoichiometry in tantalum oxide ReRAM devices

In this study, we investigated the influence of oxygen non-stoichiometry on the resistive switching performance of tantalum oxide based memristive devices. Thin-films of tantalum oxide were deposited with varying sputter power and oxygen partial pressure. The electroforming voltage was found to decrease with increasing power density or decreased oxygen partial pressure, while the endurance remained stable and the resistance window ROFF/RON was found to increase. In-depth XPS analysis connects these observations to a controllable oxygen sub-stoichiometry in the sputter-deposited films. Our analysis shows that the decrease of the forming voltage results from an increase in carrier density in the as-prepared thin-films, which is induced by the presence of oxygen vacancies