Epitaxial growth of the first two members of the Ban +1InnO2.5 n +1Ruddlesden-Popper homologous series

We demonstrate the epitaxial growth of the first two members, and the n = ∞ member of the homologous Ruddlesden-Popper series of Ba n + 1 In n O 2.5 n + 1 of which the n = 1 member was previously unknown. The films were grown by suboxide molecular-beam epitaxy where the indium is provided by a molecular beam of indium-suboxide [In 2O (g)]. To facilitate ex situ characterization of the highly hygroscopic barium indate films, a capping layer of amorphous SiO 2 was deposited prior to air exposure. The structural quality of the films was assessed by x-ray diffraction, reflective high-energy electron diffraction, and scanning transmission electron microscopy

Antiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO3 Memristive Devices

AbstractResistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure

Growth of PdCoO2 films with controlled termination by molecular-beam epitaxy and determination of their electronic structure by angle-resolved photoemission spectroscopy

Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO2. Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO2 have been made by replacing cobalt with iron in PdCoO2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO2 films with Pd-, CoO2-, and FeO2-termination. Nevertheless, Pd- and CoO2-terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFexCo1-xO2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order

Defect engineering in oxide thin films

Transition metal oxides constitute one of the most interesting material classes due to their wide variety of interesting and unusual properties. Often these properties are closely related to their defect structure. Within the transition metal oxide community SrTiO3 is often referred to as a model material due to its well known defect chemistry. Therefore, in this work the possibilities of defect engineering are considered for this model material and the resulting properties are utilized for a highly interesting application: Resistive switching of SrTiO3 in a metal insulator metal structure, a field of research where defects are key for the basic operation principle. The interest in transition metal oxides has been accompanied by an increased use of pulsed laser deposition, since it is a powerful and versatile method to achieve epitaxial complex metal oxide thin films. Usually pulsed laser deposition is performed in ultra-high vacuum systems with the substrate being heated. Often oxygen is applied for the process as a method to compensate possible oxygen loss. The pressure applied in this manner is referred to as oxygen pressure, not considering the influence of residual gases. This thesis presents evidence that this pressure in reality does not correspond to the oxygen partial pressure. The ionization based measurement devices applied to the vacuum chamber can shift the equilibrium of the residual gases, which, for low pressure, cannot be neglected. The result is a markedly lower oxygen partial pressures than the applied oxygen pressure suggests, resulting in an increased oxygen vacancy formation. Within this thesis further a method to inhibit the formation of oxygen vacancies in the considered temperature and pressure regimes is presented. It is found that the formation of oxygen vacancies in SrTiO3 is dependent on its termination. As the termination of SrTiO3 can easily be controlled, this constitutes a practical possibility to engineer the oxygen vacancy formation. Pulsed laser deposition itself is a non-equilibrium growth technique, thus deviations from the equilibrium defect concentration and types are expected. An influence on this non-equilibrium process that was up to now not considered is the radiation resulting from the plasma plume. In this work it is shown that the plasma plume present during \STO\: depositions emits UV-radiation, which in turn enhances the oxygen vacancy formation. Besides this, a the possibility to control the cation stoichiometry of the film by a change of the laser fluence is investigated. This method is employed to modify the Sr/Ti-ratio and a Sr-surplus is identified to be highly advantageous for the switching performance of SrTiO3 devices. Two main accommodation mechanisms for Sr-excess are identified, namely Ruddlesden-Popper-type anti phase boundaries and SrO surface segregation. This work presents methods to engineer both defect scenarios in nominally stoichiometric thin films. Ruddlesden-Popper-type anti phase boundaries can be achieved by the stabilization of additional SrO on the substrate surface, which acts as a seed for their formation. SrO surface segregation can be achieved by depositing additional SrO on top of the thin film. These defect engineered thin films are subsequently investigated with respect to their switching properties. It is found that Ruddlesden-Popper-type anti phase boundaries in SrTiO3 result in forming free switching, a highly desirable property for resistive switching devices. Additional SrO on top of the film is shown to form SrCO3, which in turn is a main influence factor on all resistive switching properties. Its heat confinement is shown to determine the memory window and the variability; its high diffusion barrier for oxygen is shown to determine the memory's time stability and the forming step. Summarizing, this works elucidates the sources of defect formation for the model material SrTiO3, shows methods to control the formation of these defects and explains their role for the properties of the material. The presented results in principle apply to other resistive switching oxides in the same fashion and provide a guideline on how to improve the device performance by defect engineering

Unraveling the enhanced Oxygen Vacancy Formation in Complex Oxides during Annealing and Growth

The reduction of oxides during annealing and growth in low pressure processes is a widely known problem. We hence investigate the influence of mere annealing and of growth in vacuum systems to shed light on the reasons behind the reduction of perovskites. When comparing the existing literature regarding the reduction of the perovskite model material SrTiO3 it is conspicuous that one finds different oxygen pressures required to achieve reduction for vacuum annealing and for chemically controlled reducing atmospheres. The unraveling of this discrepancy is of high interest for low pressure physical vapor depositions of thin films heterostructures to gain further understanding of the reduction of the SrTiO3. For thermal annealing, our results prove the attached measurement devices (mass spectrometer/ cold cathode gauge) to be primarily responsible for the reduction of SrTiO3 in the deposition chamber by shifting the thermodynamic equilibrium to a more reducing atmosphere. We investigated the impact of our findings on the pulsed laser deposition growth at low pressure for LaAlO3/SrTiO3. During deposition the reduction triggered by the presence of the laser plume dominates and the impact of the measurement devices plays a minor role. During post annealing a complete reoxidization of samples is inhibited by an insufficient supply of oxygen

Trade-off between variability and retention of memristive epitaxial SrTiO3_{3} devices

We present a study of the trade-off between the retention and variability of SrTiO3-based memristive devices. We identified the applied switching current and the device stoichiometry as main influence factors. We show that the SrO formation at the electrode interface, which has been revealed to improve the device retention significantly, is associated with an increased cycle-to-cycle and device-to-device variability. On the other hand, devices with homogeneous, Ti-terminated SrTiO3–Pt interfaces exhibit poor retention but the smallest variability. These results give valuable insights for the application of memristive SrTiO3 devices as non-volatile memory or in neural networks, where the control of variability is of key relevance.We acknowledge funding from the W2/W3 program of the Helmholtz Association. This research was supported by the Deutsche Forschungsgemeinschaft (Grant No. SFB 917 “Nanoswitches”), the Helmholtz Association Initiative and Networking Fund under Project No. SO-092 (Advanced Computing Architectures, ACA), and the Federal Ministry of Education and Research (Project NEUROTEC, Grant No. 16ES1133K)

SrTiO 3 termination control: a method to tailor the oxygen exchange kinetics

We provide insights into the influence of surface termination on the oxygen vacancy incorporation for the perovskite model material SrTiO3 during annealing in reducing gas environments. We present a novel approach to tailor the oxygen vacancy formation by controlling the termination. We prove that a SrO-termination can inhibit the incorporation of oxygen vacancies across the (100)-surface and apply this to control their incorporation during thin film growth. Utilizing the conducting interface between LaAlO3 and SrTiO3, we could tailor the oxygen-vacancy based conductivity contribution by the level of SrO termination at the interface

SrTiO 3 termination control: a method to tailor the oxygen exchange kinetics

We provide insights into the influence of surface termination on the oxygen vacancy incorporation for the perovskite model material SrTiO3 during annealing in reducing gas environments. We present a novel approach to tailor the oxygen vacancy formation by controlling the termination. We prove that a SrO-termination can inhibit the incorporation of oxygen vacancies across the (100)-surface and apply this to control their incorporation during thin film growth. Utilizing the conducting interface between LaAlO3 and SrTiO3, we could tailor the oxygen-vacancy based conductivity contribution by the level of SrO termination at the interface

Engineering antiphase boundaries in epitaxial SrTiO 3 to achieve forming free memristive devices

We here present a method to engineer Ruddlesden-Popper-type antiphase boundaries in stoichiometric homoepitaxial SrTiO3 thin films. This is achieved by using a substrate with an intentionally high miscut, which stabilizes the growth of additional SrO at the bottom interface. We prove the success of this strategy utilizing transmission electron microscopy. We find that these antiphase boundaries significantly influence the resistive switching properties. In particular, devices based on SrTiO3 thin films with intentionally induced antiphase boundaries do not require a forming step, which is ascribed to the existence of preformed filaments