Daesung Park

Scaling Potential of Local Redox Processes in Memristive SrTiO

In this work, we address the following question: Where do the resistive switching processes take place in memristive thin-film devices of the single crystalline model material Fe-doped SrTiO3? We compare resistive switching induced by the tip of the atomic force microscope on the bare thin-film surface with the switching properties observed in memristive devices with Pt top electrode. In order to close the gap between these two approaches, we combine conductive-tip atomic force microscopy with a delamination technique to remove the top electrode of Fe-doped SrTiO3 metal-insulator-metal (MIM) structures to gain insight into the active switching interface with nanoscale lateral resolution. This enables us to prove the coexistence of a filamentary and area-dependent switching process with opposite switching polarities in the same sample. The spatially resolved analysis by transmission electron microscopy and photoelectron spectromicroscopy gives us some hints that the two switching types take place in device regions with different defect density and significant stoichiometry difference.

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We deliberately fabricated SrTiO3 thin films deviating from ideal stoichiometry and from two-dimensional layer-by-layer growth mode, in order to study the impact of well pronounced defect arrangements on the nanoscale electrical properties. By combining transmission electron microscopy with conductive-tip atomic force microscopy we succeeded to elucidate the microstructure of thin films grown by pulsed laser deposition under kinetically limited growth conditions and to correlate it with the local electrical properties. SrTiO3 thin films, grown in a layer-by-layer growth mode, exhibit a defect structure and conductivity pattern close to single crystals, containing irregularly distributed, resistive switching spots. In contrast to this, Ti-rich films exhibit short-range-ordered, well-conducting resistive switching units. For Ti-rich films grown in a kinetically more restricted island growth mode, we succeeded to identify defective island boundaries with the location of tip-induced resistive switching. The o...

Thin-Film Devices

Adhesive organs like arolia of insects allow these animals to climb on different substrates by creating high adhesion forces. According to the Dahlquist criterion, adhesive organs must be very soft, exhibiting an effective Youngs modulus of below 100 kPa to adhere well to substrates. Such a low effective Youngs modulus allows the adhesive organs to make almost direct contact with the substrate and results in van der Waals forces along with capillary forces. In previous studies, the effective Youngs moduli of adhesive organs were determined using indentation tests, revealing their structure to be very soft. However, adhesive organs show a layered structure, thus the measured values comprise the effective Youngs moduli of several layers of the adhesive organs. In this study, a new approach is illustrated to measure the Youngs modulus of the outermost layer of the arolium, i.e. of the epicuticle, of the stick insect Carausius morosus. As a result of the epicuticle being supported by upright fibres, tensile tests allow the determination of the Youngs modulus of the epicuticle with hardly influence from subjacent layers. In our tensile tests, arolia of stick insects adhering on a latex membrane were stretched by stretching the membrane while the elongation of the contact area between an arolium and the membrane was recorded. For analysis, mathematical models of the mechanical system were developed. When fed with the observed elongations, these models yield estimates for the Youngs modulus of the epicuticle of approximately 100 MPa. Thus, in arolia, a very thin layer (~225 nm) of a rather stiff material, which is less susceptible to abrasion, makes contact with the substrates, whereas the inner fibrous structure of arolia is responsible for their softness.

Correlation between growth kinetics and nanoscale resistive switching properties of SrTiO3 thin films

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 eg peak were observed in the BaTiO3 film. These results verify the presence of a ferroelectric polarization in the ultrathin BaTiO3 film.

Determination of the Young's modulus of the epicuticle of the smooth adhesive organs of Carausius morosus using tensile testing

New multilayers of boron carbide/cerium dioxide (B4C/CeO2) combination on silicon (Si) substrate are manufactured to represent reflective-optics candidates for future lithography at 6.x nm wavelength. This is one of only a few attempts to make multilayers of this kind. Combination of several innovative experiments enables detailed study of optical properties, structural properties, and interface profiles of the multilayers in order to open up a room for further optimization of the manufacturing process. The interface profile is visualized by high-angle annular dark-field imaging which provides highly sensitive contrast to atomic number. Synchrotron based at-wavelength extreme ultraviolet (EUV) reflectance measurements near the boron (B) absorption edge allow derivation of optical parameters with high sensitivity to local atom interactions. X-ray reflectivity measurements at Cu-Kalpha (8 keV) determine the period of multilayers with high in-depth resolution. By combining these measurements and choosing rob...

Studies of local structural distortions in strained ultrathin BaTiO3 films using scanning transmission electron microscopy.

Ferroelectric materials can be used for the ferroelectric random access memory (FeRAM) For the miniaturization the thin film structures are used in general. However, there is the critical thickness of ferroelectric materials, below which ferroelectricity abruptly disappears. Besides, the depolarization field, the epitaxial strain, the termination of the ferroelectric film, and the electric filed affect the ferroelectric properties.

Deposition and characterization of B4C/CeO2 multilayers at 6.x nm extreme ultraviolet wavelengths

Over the past decade, electron energy-loss spectroscopy (EELS) has become an increasingly important tool for characterising the chemical and electronic structure of materials on the nanometre scale [1]. In particular the characteristic fine structure beyond the onset of the core-loss ionization edges (the Energy-loss Near Edge Structure, ELNES) provides detailed insight into the coordination environment around the excited atom. Hence this feature can also be exploited by comparison with spectra of known structures to identify the material under investigation [2]. However, in practise one faces often the problem that spectra of the relevant reference materials are not readily available or cannot be obtained due to the lack of suitable samples. A promising way out of this quandary is provided by artificial core-loss spectra that can be calculated by quantum mechanical calculations from crystallographic data as input.