Christian Rodenbücher

Cluster-like resistive switching of SrTiO3:Nb surface layers

The understanding of the resistive switching mechanisms in perovskites is of particular importance for the development of novel non-volatile memories. Nanoscale investigations recently revealed that in the model material SrTiO3 a filamentary type of switching is present. In this paper, we show that upon donor doping with Nb the switching type changes fundamentally. We report on the observation of conducting clusters that can be switched independently between a high resistance and a low resistance state when applying a voltage. Furthermore, we show that the resistive switching takes place in a semiconducting surface layer on top of the metallic bulk of SrTiO3:Nb single crystals, which can change its properties easily under external gradients.

Quasi-two-dimensional conducting layer on TiO2 (110) introduced by sputtering as a template for resistive switching

The insulator-to-metal transformation in the surface layer of TiO2 (110) induced by the Ar+ ion sputtering process is analyzed on the nanoscale. Local conductivity atomic force microscopy and photoelectron spectroscopy allow the changes in the valence of the Ti ions in the surface layer to be linked to the formation of its grain-like structure. The investigation of the cleavage plane of the crystal allowed us to estimate the thickness of the quasi-two-dimensional conducting layer generated by ion bombardment as 30 nm. The conducting layer is a template where the resistive switching of each single grain can be carried out.

Homogeneity and variation of donor doping in Verneuil-grown SrTiO3:Nb single crystals

The homogeneity of Verneuil-grown SrTiO3:Nb crystals was investigated. Due to the fast crystal growth process, inhomogeneities in the donor dopant distribution and variation in the dislocation density are expected to occur. In fact, for some crystals optical studies show variations in the density of Ti3+ states on the microscale and a cluster-like surface conductivity was reported in tip-induced resistive switching studies. However, our investigations by TEM, EDX mapping, and 3D atom probe reveal that the Nb donors are distributed in a statistically random manner, indicating that there is clearly no inhomogeneity on the macro-, micro-, and nanoscale in high quality Verneuil-grown crystals. In consequence, the electronic transport in the bulk of donor-doped crystals is homogeneous and it is not significantly channelled by extended defects such as dislocations which justifies using this material, for example, as electronically conducting substrate for epitaxial oxide film growth.

Forming-free metal-oxide ReRAM by oxygen ion implantation process

We propose a new method for obtaining forming-free ReRAM devices by oxygen ion implantation (O<inf>2</inf> IIP) in the metal oxide film during the device fabrication process. By tuning the implantation dose, as-fabricated devices can be transformed into the ON state. Subsequent standard RESET and SET switching cycles reveal that the forming-free devices switch in a similar way to reference (formed) devices. The devices also show good R<inf>OFF</inf>/R<inf>ON</inf> ratio (>200), retention (10⁴ sec@125°C) and endurance reliability (10⁶ cycles), showing the absence of any device degradation caused by the O<inf>2</inf> IIP process. This method is applied on both (PVD) Ta<inf>2</inf>O<inf>5</inf> and (ALD) HfO<inf>2</inf> nanoscale ReRAM devices, demonstrating the versatile applications of the technique.

Lowering forming voltage and forming-free behavior of Ta 2 O 5 ReRAM devices

In this paper, we investigate the impact of Ta and Ta₂O₅ thickness and of thermal treatment for the Ta₂O₅ layer on the forming and switching characteristics of Pt/Ta₂O₅/Ta/Pt ReRAM devices. The forming voltage (V_FORM) decreases with increasing Ta and decreasing Ta₂O₅ thickness. However, V_FORM saturates (~ 2 V) for thicker Ta layers. Thinner Ta₂O₅ switching layer can further reduce the forming voltage to <; 1 V for 3 nm-thick Ta₂O₅. However, thinner Ta₂O₅ degrades the R_OFF / R_ON ratio and retention of the ReRAM device. On the other hand, thermal treatment of the Ta₂O₅ results in decrease of initial resistance and V_FORM, and rapid thermal anneal at 600°C in O₂ ambient induces forming-free behavior for 70% of the characterized devices. These forming-free devices show highly reliable switching operation up to 10⁶ cycles with R_OFF / R_ON > 10 and retention time of 10⁴ s at 125 °C.

Electrically controlled transformation of memristive titanates into mesoporous titanium oxides via incongruent sublimation

Perovskites such as SrTiO3, BaTiO3, and CaTiO3 have become key materials for future energy-efficient memristive data storage and logic applications due to their ability to switch their resistance reversibly upon application of an external voltage. This resistance switching effect is based on the evolution of nanoscale conducting filaments with different stoichiometry and structure than the original oxide. In order to design and optimize memristive devices, a fundamental understanding of the interaction between electrochemical stress, stoichiometry changes and phase transformations is needed. Here, we follow the approach of investigating these effects in a macroscopic model system. We show that by applying a DC voltage under reducing conditions on a perovskite slab it is possible to induce stoichiometry polarization allowing for a controlled decomposition related to incongruent sublimation of the alkaline earth metal starting in the surface region. This way, self-formed mesoporous layers can be generated which are fully depleted by Sr (or Ba, Ca) but consist of titanium oxides including TiO and Ti3O with tens of micrometre thickness. This illustrates that phase transformations can be induced easily by electrochemical driving forces.

Electrical nanopatterning of TiO2 single crystal surfaces in situ via local resistance and potential switching

The resistive switching effect in transition metal oxides allows for a dedicated manipulation of the oxide resistance via electrical stimuli. Here, we perform local-conductivity atomic force microscopy simultaneously with the Kelvin probe force microscopy under ultra-high vacuum conditions using the very same tip investigating the very same sample area to monitor the surface conductivity and surface potential of thermally reduced TiO2 single crystals. We show that the resistance of confined surface areas can be switched by applying a voltage of several volts to the tip during scanning in the contact mode. By conducting in situ oxidation experiments, we present that this surface switching is related to a local redox reaction, which can be controlled electrically allowing for surface nanopatterning and illustrates the capability of transition metal oxides for multilevel resistive switching being a prerequisite for neuromorphic computing. We discuss that the features of the electrically engraved nanopattern can be scaled down to a lower boundary at several tens of nanometers. The observed limit around 25 nm is determined by the presence of intrinsic local variations in electrical surface properties appearing as a common phenomenon of slightly reduced metal oxide surfaces.The resistive switching effect in transition metal oxides allows for a dedicated manipulation of the oxide resistance via electrical stimuli. Here, we perform local-conductivity atomic force microscopy simultaneously with the Kelvin probe force microscopy under ultra-high vacuum conditions using the very same tip investigating the very same sample area to monitor the surface conductivity and surface potential of thermally reduced TiO2 single crystals. We show that the resistance of confined surface areas can be switched by applying a voltage of several volts to the tip during scanning in the contact mode. By conducting in situ oxidation experiments, we present that this surface switching is related to a local redox reaction, which can be controlled electrically allowing for surface nanopatterning and illustrates the capability of transition metal oxides for multilevel resistive switching being a p...