Electrical characterization of manganite and titanate heterostructures

As the current memory technology will reach its physical limit within the next decades, innovative concepts have to be developed to ensure future improvements in size, power consumption and costs in data storage. A promising candidate among "Beyond CMOS" technologies is resistive RAM, which is based on the non-volatile and reversible switching of the resistance of a device with the help of an external stimulus. More specific, fast writing and reading operations in resistively switching transition metal oxides seem to be enabled by redox-based mechanisms, which usually go along with the formation of a conducting filament to bridge the insulating matrix. For a few materials, the switching occurs homogeneously over the entire device area, which offers one more degree of freedom in device design. For this switching type, the interface effects between the involved resistively switching oxide and the active electrode play the key role. The main topic of this work is this redox-based interplay between the oxidizable electrode material and a complex oxide in a homogeneously switching ReRAM device, realized for the quaternary transition metal oxide (Pr,Ca)MnO3 as epitaxial model system. In order to obtain high quality epitaxial thin films by pulsed laser deposition, detailed growth studies were performed. In spite of the rather high lattice mismatch of -2.39 percent to the SrTiO3 substrate, efect-poor (Pr,Ca)MnO3 could be successfully grown in various layer stacks. For film thicknesses larger than 40 nm, the elastic strain energy becomes sufficient high for relaxation effects. We identified two concurring processes for relaxation, crack formation and the incorporation of edge dislocations, which result in crucially different structural and electrical properties of the (Pr,Ca)MnO3 thin film. After an initial electroforming step, (Pr,Ca)MnO3 with Ti as active top electrode exhibits stable resistive switching properties, for which we could obtain a resistance ratio of one order of magnitude and retention times of at least 300 days. A redox reaction was found to occur at the interface involving an oxygen transfer from the (Pr,Ca)MnO3 to the Ti. A change of the valence state on the Ti-side was identified during the forming and also the switching processes by X-ray spectroscopy. The current transport in the devices could be described by a polaron hopping contribution and a serial ohmic resistor for the symmetric low resistive states, and with the presence of a trapezoidal tunnel barrier for the high resistive states. Combining both results, a coherent model was derived to explain the switching effect in (Pr,Ca)MnO3/Ti devices by the formation and bridging of a TiO2 tunnel barrier, naturally formed at the (Pr,Ca)MnO3/Ti interface

Competing strain relaxation mechanisms in epitaxially grown Pr0.48_{0.48}Ca0.52_{0.52}MnO3_{3} on SrTiO3_{3}

We investigated the impact of strain relaxation on the current transport of Pr0.48Ca0.52MnO3 (PCMO) thin films grown epitaxially on SrTiO3 single crystals by pulsed laser deposition. The incorporation of misfit dislocations and the formation of cracks are identified as competing mechanisms for the relaxation of the biaxial tensile strain. Crack formation leads to a higher crystal quality within the domains but the cracks disable the macroscopic charge transport through the PCMO layer. Progressive strain relaxation by the incorporation of misfit dislocations, on the other hand, results in a significant decrease of the activation energy for polaron hopping with increasing film thickness

Studies of Local Structural Disortions in Strained Ultrathin BaTiO3 Films sing Scanning Transmission Electron Microscopy

Ultrathin ferroelectric heterostructures (SrTiO3/BaTiO3/BaRuO3/SrRuO3) were studied by scanning transmission electron microscopy (STEM) in terms of structural distortions and atomic displacements. The TiO2-termination at the top interface of the BaTiO3 layer was changed into a BaO-termination by adding an additional BaRuO3 layer. High-angle annular dark-field (HAADF) imaging by aberration-corrected STEM revealed that an artificially introduced BaO-termination can be achieved by this interface engineering. By using fast sequential imaging and frame-by-frame drift correction, the effect of the specimen drift was significantly reduced and the signal-to-noise ratio of the HAADF images was improved. Thus, a quantitative analysis of the HAADF images was feasible, and an in-plane and out-of-plane lattice spacing of the BaTiO3 layer of 3.90 and 4.22 Å were determined. A 25 pm shift of the Ti columns from the center of the unit cell of BaTiO3 along the c-axis was observed. By spatially resolved electron energy-loss spectroscopy studies, a reduction of the crystal field splitting (CFS, ΔL3=1.93 eV) and an asymmetric broadening of the e g peak were observed in the BaTiO3 film. These results verify the presence of a ferroelectric polarization in the ultrathin BaTiO3 film