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Aluminum plays a central role in the world of electronic oxide materials. Yet, aluminum sources are very difficult to handle during oxide molecular-beam epitaxy, the main reason for which is the high oxidization potential of aluminum. In this work, we present a thorough study of the behavior of aluminum sources during oxide thermal laser epitaxy. We identify two distinct operating regimes. At high laser-beam fluences, the source emanates reproducible fluxes independent of an applied oxygen pressure of

We demonstrate the epitaxial growth of the first two members, and the [Formula: see text] member of the homologous Ruddlesden–Popper series of [Formula: see text] of which the [Formula: see text] 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 [[Formula: see text]O (g)]. To facilitate ex situ characterization of the highly hygroscopic barium indate films, a capping layer of amorphous [Formula: see text] 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.

Nanoscale advances (2021). doi:10.1039/d1na00190f, Published by Royal Society of Chemistry, Cambridge

Solid-state diffusion experiments were used to probe the behavior of cation vacancies in the perovskite oxide ${\mathrm{SrTiO}}_{3}$. Two types of nominally undoped (effectively acceptor-doped) ${\mathrm{SrTiO}}_{3}$ systems were studied: (1) single crystals and (2) epitaxial thin films with different Sr/Ti stoichiometries produced by pulsed laser deposition. As diffusion sources, thin films of the perovskite oxide ${\mathrm{BaZrO}}_{3}$ were employed, and diffusion anneals were carried out in air at $1323\ensuremath{\le}T/\mathrm{K}\ensuremath{\le}1523$ for single crystals and at $1073\ensuremath{\le}T/\mathrm{K}\ensuremath{\le}1223$ for thin films. Sample analysis by means of time-of flight secondary ion mass spectrometry (ToF-SIMS) yielded diffusion coefficients of Ba and Zr in ${\mathrm{SrTiO}}_{3}\phantom{\rule{4pt}{0ex}}({D}_{\mathrm{Ba}}$ and ${D}_{\mathrm{Zr}})$. Diffusion profiles in single-crystal samples showed the expected error-function form and yielded ${D}_{\mathrm{Ba}}\ensuremath{\approx}{D}_{\mathrm{Zr}}$ at each temperature, and hence, activation enthalpies of diffusion that are approximately the same, at $(3.0\ifmmode\pm\else\textpm\fi{}0.4)$ eV and $(2.8\ifmmode\pm\else\textpm\fi{}0.4)$ eV. Diffusion profiles in the thin-film samples were unexpectedly complex, showing multiple error-function features. They also yielded ${D}_{\mathrm{Ba}}\ensuremath{\approx}{D}_{\mathrm{Zr}}$ at each temperature, however, but no clear trend was found as a function of Sr/Ti ratio. Comparing results for the two systems, we conclude that the concentration of cation vacancies is orders of magnitude higher in our thin-film samples than in the single crystals. Our results also provide experimental evidence that oxygen vacancies, ${\mathrm{v}}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}}$, can decrease the activation enthalpy of strontium-vacancy migration by forming ${({\mathrm{v}}_{\mathrm{O}}{\mathrm{v}}_{\mathrm{Sr}})}^{\ifmmode\times\else\texttimes\fi{}}$ defect associates, and we derive an analytical model for the cation diffusivity as a function of temperature and defect concentrations.

Redox-based resistive switching is one of the most-promising concepts in the focus of research to meet the ever-growing demand for faster and smaller non-volatile memory devices. In this work we present detailed studies of the impact of cation stoichiometry and surface segregation effects on the performance of the valence change memory model material SrTiO3. In order to clarify if the enhanced switching performance of Sr-rich SrTiO3 devices can be attributed to SrO segregation or to the formation of Sr-rich extended defects, we artificially engineered the formation of SrO islands by depositing additional SrO on top of stoichiometric SrTiO3. We thereby unravel that the enhanced switching performance is solely accounted for by the formation of SrO islands and not influenced by extended defects. Consequently following our findings, we design devices with a further improved retention by tailoring the amount of SrO on the surface.

This paper introduces a growth method---suboxide molecular-beam epitaxy (S-MBE)---which enables the growth of Ga2O3 and related materials at growth rates exceeding 1 micrometer per hours with excellent crystallinity in an adsorptioncontrolled regime. Using a Ga + Ga2O3 mixture with an oxygen mole fraction of x(O) = 0.4 as an MBE source, we overcome kinetic limits that had previously hampered the adsorption-controlled growth of Ga2O3 by MBE. We present growth rates up to 1.6 micrometer per hour for Ga2O3--Al2O3 heterostructures with unprecedented crystalline quality and also at unparalleled low growth temperature for this level of perfection. We combine thermodynamic knowledge of how to create molecular-beams of targeted suboxides with a kinetic model developed for the S-MBE of III-VI compounds to identify appropriate growth conditions. Using S-MBE we demonstrate the growth of phase-pure, smooth, and high-purity homoepitaxial Ga2O3 films that are thicker than 4 micrometer. With the high growth rate of S-MBE we anticipate a significant improvement to vertical Ga2O3-based devices. We describe and demonstrate how this growth method can be applied to a wide-range of oxides. S-MBE rivals leading synthesis methods currently used for the production of Ga2O3-based devices., Comment: 15 pages, 12 figures

One major obstacle for the implementation of redox-based memristive devices in non-volatile memory or neuromorphic computing is the large cycle-to-cycle and device-to-device variability [1]. Here, we combine different complementary analysis methods such as atomic force microscopy (AFM), thermography and operando photoelectron emission spectroscopy (PEEM) [2] to monitor the filament shape and position in memristive SrTiO3 cells and correlate it to the observed variability of the resistance values. Our operando PEEM analysis revealed that some devices exhibit cycle-dependent variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament [3]. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub-)filaments observed by transmission electron microscopy [4] and that the active, current-carrying filament may change from cycle to cycle. Besides this, thermal imaging enabled us to monitor the position of the filament during repeated cycling. During cycling of some devices, we observed a movement of the filament towards the edge of the device that goes along with a drift of the low resistive state. As soon as the filament reaches the boarder, the resistance value stabilizes and remains constant during the following cycles. By modifying the interface at the top electrode, we could engineer the position of the filament and its stability against cycling. Our studies thereby represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices. References [1] R. Dittmann and J. P. Strachan, Perspective article, APL Mater. 7, 110903, 2019 [2] C. Bäumer et al., Nature Commun. 7, 12398 2016 [3] C. Bäumer et al., ACS Nano 11 (7), 6921, 2017 [4] H. Du et al., Chem. Mat. 29, 7, 3173, 2017

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities. References [1] D. Cooper et al. Adv. Mat.. 29, 1700212 (2017) [2] T. Heisig et al., ,Adv. Mater. 30, 1800957 (2018) [3] F.V.E. Hensling, et al., Solid State Ionics 325, 247 (2018) [4] Bäumer et al., Nature Commun. 6, 8610 (2015) Figure 1

We have studied the bonding and elastic properties of amorphous AlYB 14 using theoretical and experimental means. Based on pair distribution functions and Voronoi tessellation, the icosahedral bonding is expected. A rather large Young's modulus of 365 GPa is predicted for amorphous AlYB 14 . To verify these predictions, we have measured density, pair distribution functions, binding energy and elastic properties of Al–Y–B thin films synthesized by magnetron sputtering. The calculated and measured densities are with a deviation of 3.5% in good agreement. The measured binding energy and pair distribution functions are also consistent with icosahedral bonding. The measured Young's modulus is 305±19 GPa, which is 16% smaller than the theoretical value and hence in good agreement. Overall consistency between theory and experiments was obtained indicating that the computational strategy employed here is useful to describe correlations between bonding, elasticity, density as well as (chemical) short range order and may hence enable future knowledge-based design of these ternary borides which show great potential for surface protection applications.

Orbital ordering has been less investigated in epitaxial thin films, due to the difficulty to evidence directly the occurrence of this phenomenon in thin film samples. Atomic resolution electron microscopy enabled us to observe the structural details of the ultrathin LaVO3 films. The transition to orbital ordering of epitaxial layers as thin as ≈4 nm was probed by temperature-dependent Raman scattering spectroscopy of multilayer samples. From the occurrence and temperature dependence of the 700 cm–1 Raman active mode it can be inferred that the structural phase transition associated with orbital ordering takes place in ultrathin LaVO3 films at about 130 K.

Resistive 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.