Christoph Bäumer

Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy.

The local electronic properties of tantalum oxide (TaOx, 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO3) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaOx films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO3, we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaOx and SrRuO3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.

Disentanglement of growth dynamic and thermodynamic effects in LaAlO3/SrTiO3 heterostructures.

The influence of non-equilibrium and equilibrium processes during growth of LaAlO3/SrTiO3 (LAO/STO) heterostructures is analyzed. We investigate the electronic properties of LAO/STO heterostructures obtained at constant growth conditions after annealing in different oxygen atmospheres within the typical growth window (1 × 10−4 mbar –1 × 10−2 mbar). The variation of annealing conditions is found to cause a similar change of electronic properties as observed for samples grown in different oxygen pressure. The results indicate that equilibrium defect formation is the dominant process for establishing the properties of the two-dimensional electron gas (2DEG), while growth dynamics play a minor role in the typical LAO/STO growth regime. Furthermore, the effects of non-equilibrium processes occurring during growth are investigated in detail by quenching just-grown LAO/STO heterostructures directly after growth. We show that during growth the sample is pushed into a non-equilibrium state. After growth, the sample then relaxes towards equilibrium, while the relaxation rate strongly depends on the ambient pressure. The observed relaxation behavior is mainly associated with a reoxidation of the STO bulk, while the 2DEG is formed immediately after the growth.

Pulsed laser deposition of SrRuO3 thin-films: The role of the pulse repetition rate

SrRuO3 thin-films were deposited with different pulse repetition rates, fdep, epitaxially on vicinal SrTiO3 substrates by means of pulsed laser deposition. The measurement of several physical properties (e.g., composition by means of X-ray photoelectron spectroscopy, the out-of-plane lattice parameter, the electric conductivity, and the Curie temperature) consistently reveals that an increase in laser repetition rate results in an increase in ruthenium deficiency in the films. By the same token, it is shown that when using low repetition rates, approaching a nearly stoichiometric cation ratio in SrRuO3 becomes feasible. Based on these results, we propose a mechanism to explain the widely observed Ru deficiency of SrRuO3 thin-films. Our findings demand these theoretical considerations to be based on kinetic rather than widely employed thermodynamic arguments.

Spectroscopic characterization of local valence change processes in resistively switching complex oxides

An increasingly interconnected world creates a high demand for high-density and lowcost data storage. Redox-based memristive devices, which allow switching between high and low electrical resistances through the application of voltages, are highly attractive candidates for next-generation non-volatile memory. But their control and rational design is complicated by poorly understood switching and failure mechanisms. The complex nanoscale redox processes that are suspected to drive so-called resistive switching in these devices remain inadequately characterized. Especially, quantitative information about these processes, which is essential for further advances in the educated design, has been experimentally inaccessible so far. Therefore, spectroscopic tools with high spatial resolution are employed in this work to elucidate both switching and failure mechanism of memristive devices based on the model material SrTiO3. After thorough electrical characterization, two alternative photoelectron emission microscopy approaches are used. As photoemission is a surface sensitive process, the top electrodes of the devices are removed before investigation in the rst approach. In the second step, thin graphene electrodes are employed, enabling in operando characterization. In combination with cross sectional, in operando transmission electron microscopy and spectroscopy, a clear evidence of a reversible, localized redox reaction is identi ed. In the low resistance state, a nanoscale lament in the SrTiO3 is oxygen-de cient, while it is nearly stoichiometric in the high resistance state, resulting in a valence change between Ti and Ti. The carrier concentration modulation resulting from this valence change is quanti ed through comparison with calibration spectra. A carrier concentration change by a factor of two causes two orders of magnitude change in device resistance through a modulation of the e ective Schottky barrier at the electrode/oxide interface. The microscopic origin of the polarity of the resistance hysteresis in these devices has long been debated, as it cannot be explained by the typically involved purely internal redistribution of oxygen vacancies. The spectroscopic results of this work reveal that instead, oxygen evolution and reincorporation reactions at the electrode/oxide interface are responsible for the valence change in the SrTiO3. Regarding the failure mechanism, it is found that fast reoxidation frequently results in retention failure in SrTiO3 devices, which can be inhibited by incidental, local phase separations. Mimicking this phase separation by intentionally introducing retentionstabilization layers with slow oxygen transport is therefore derived as a design rule for retention-failure-resistant devices.

Direct Observation of Redox Switching in Resistive Memory Devices Operated In-situ in a Transmission Electron Microscope by Electron Energy Loss Spectroscopy and Off-Axis Electron Holography

The Pt/SrTiO3/Nb:SrTiO3 system is considered to be a model system for the study of resistive memories [1]. It is thought that changes in the oxygen concentration underneath the top electrode are responsible for the switching. During the past few years, a bewildering array of characterisation studies have appeared in the literature presenting contradictory views about the operation of these resistive memories. It is desirable to improve these devices by correct determination of the switching mechanism rather than using empirical approaches. Transmission electron microscopy (TEM) is a suitable characterisation technique and the use of modern aberration corrected TEMs allow a range of complimentary characterisation techniques to be performed during one experiment on the same specimen. In this presentation we will present experimental results that have been obtained using aberration-corrected scanning (S)TEM, electron energy-loss spectroscopy (EELS) for composition and bonding measurements and off-axis electron holography for measurements of electrostatic potentials that have been obtained on different types of SrTiO3 based memory devices that are switched in-situ in the TEM.