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| Home > Publications database > Operando X-ray photoemission electronmicroscopy (XPEEM) investigations of resistive switching metal-insulator-metal devices |
| Book/Dissertation / PhD Thesis | FZJ-2018-02614 |
2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-283-2
Please use a persistent id in citations: http://hdl.handle.net/2128/18794
Abstract: Resistive switching materials - including resistive oxides - raise significant scientific and industrial interest due to their potential applications in next generation non-volatile data storage devices and as building blocks for novel logic elements. Even though numerous resistive switching materials already found their way into application, the underlying physical mechanisms still remain highly elusive. While the electronic response of these systems is well-documented throughout literature, experimental data on microscopic and chemical origin of resistive switching is rare. Scope of the present work is to gain a deeper physical understanding of chemical and electronic changes taking place during the switching process by means of chemically-sensitive and spatially resolving X-ray photoelectron emission microscopy (XPEEM). This technique is used to identify chemical and electronic changes between the ON and OFF states of non-functional delaminated memristive SrTiO$_{3}$ devices. For the low resistive (ON) state, sub-micron filamentary regions are observed showing significant contributions of trivalent Ti, whereas the high resistive (OFF) statelacks these states and structures. The experimentally derived chemical and spatial fingerprints provide evidence for oxygen vacancy accumulation respectively depletion being predicted by the valence change model. Further analysis of the observed filament substructure indicates that the observed macroscopic filaments consist of an inhomogeneous matrix of nanoscale islands. These findings contribute to the fundamental understanding of the switching mechanism being essential to improve device simulations and to identify scaling limits. Beyond the conventional static characterization of the ON and OFF states of non-functional devices, the focus of the thesis is to evaluate how the highly surface-sensitive PEEM approach can be improved to allow monitoring the anticipated chemical changes also during operation of a device (’operando’). In this context different experimental approaches and device geometries are discussed and evaluated, which have the potential to circumvent the top electrode surface sensitivity dilemma in photoemission-based techniques. Ultra-thin graphene top electrodes are demonstrated to be sufficiently transparent for electrons and thus allow to image chemical signals originating from the buried active interface of a metal-insulator-metal structure by means of X-ray absorption spectroscopy. The novel strategies and concepts are realized into a set of functional devices. Additional technical modifications are implemented into a PEEM instrument operated at a synchrotron facility. Using this new setup chemical and electronic characterization of a working device are simultaneously performed in a single experiment providing a direct correlation between current-voltage (I-V) response and chemical state for the first time. Results from the operando and in-situ experiments are obtained from single devices and thus exclude typically observed device-to-device variations and experimental artifacts typically hampering the analysis. The instrumental advances and improved methods documented in this thesis enable operando characterization of functional devices using PEEM. They are not limited to resistive switching and they present a significant step towards the long-termgoal of visualizing the switching dynamics on a nanosecond timescale with sub-micron spatial resolution.
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