K. Szot

TiO2—a prototypical memristive material

Redox-based memristive switching has been observed in many binary transition metal oxides and related compounds. Since, on the one hand, many recent reports utilize TiO(2) for their studies of the memristive phenomenon and, on the other hand, there is a long history of the electronic structure and the crystallographic structure of TiO(2) under the impact of reduction and oxidation processes, we selected this material as a prototypical material to provide deeper insight into the mechanisms behind memristive switching. In part I, we briefly outline the results of the historical and recent studies of electroforming and resistive switching of TiO(2)-based cells. We describe the (tiny) stoichiometrical range for TiO(2 - x) as a homogeneous compound, the aggregation of point defects (oxygen vacancies) into extended defects, and the formation of the various Magnéli phases. Furthermore, we discuss the driving forces for these solid-state reactions from the thermodynamical point of view. In part II, we provide new experimental details about the hierarchical transformation of TiO(2) single crystals into Magnéli phases, and vice versa, under the influence of chemical, electrical and thermal gradients, on the basis of the macroscopic and nanoscopic measurements. Those include thermogravimetry, high-temperature x-ray diffraction (XRD), high-temperature conductivity measurements, as well as low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and LC-AFM (atomic force microscope equipped with a conducting tip) studies. Conclusions are drawn concerning the relevant parameters that need to be controlled in order to tailor the memristive properties.

Piezoresponse force microscopy of lead titanate nanograins possibly reaching the limit of ferroelectricity

Single ferroelectric lead titanate (PTO) grains down to 15 nm were fabricated by chemical solution deposition. Varying the dilution of the precursor solution leads to different grain sizes between 15 and 200 nm. The grain-size-dependent domain configuration was studied using three-dimensional piezoresponse force microscopy (PFM). It is found that the PTO grains in a dense film contain laminar 90° domain walls, whereas separated PTO grains show more complicated structures of mainly 180° domain walls. For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the superparaelectric phase which has no spontaneous polarization. Recent calculations showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4–14 nm.

Formation of micro-crystals on the (100) surface of SrTiO3 at elevated temperatures

Abstract Droplet-like features and regularly shaped micro-crystals appear on the (100) surface of single-crystals of stoichiometric and doped SrTiO 3 as a result of heat treatment around 1000–1100°C under ambient pressure. Secondary ion mass spectrometry, atomic force microscopy and X-ray photoemission are employed to characterize the morphology of the modified surface. The results provide evidence of an accumulation of SrO x on the surface in a liquid form and subsequent recrystallization as SrO on prolonged annealing. The phenomena are discussed in relation to the restructuring in the near-surface region and the loss of material at the temperatures employed, as evident from thermogravimetrical measurements.

Separation of bulk and interface contributions to electroforming and resistive switching behavior of epitaxial Fe-doped SrTiO3

We succeeded in the separation of bulk and interface contributions to the electroforming and resistive switching behavior of Pt/STO(Fe)/Nb:STO devices by performing impedance spectroscopy. Two distinctive features observed in the impedance spectra could be assigned to the STO(Fe) bulk and to the depletion layer of the Pt/STO(Fe) Schottky contact. We attribute the resistance change during the dc forming process to a local bypassing of the depletion layer caused by oxygen effusion to the environment. By comparing the impedance spectra in the resistive “on” and “off” states, we propose that the resistance of the STO(Fe)/Nb:STO interface locally changes during the switching process.

The influence of copper top electrodes on the resistive switching effect in TiO2 thin films studied by conductive atomic force microscopy

Titanium dioxide thin films (30 nm) are deposited on platinized substrates by atomic layer deposition and locally studied by conductive atomic force microscopy showing repetitive bipolar resistive switching. Experiments using macroscopic copper top electrodes, which are electroformed, bipolar switched, and removed again from the TiO2–Pt stack, prove the formation of local conductive filaments with bipolar switching properties. The localized filaments can be switched repetitively with a resistance ratio of 30. Our findings underline that Cu diffusion and the formation of filaments are the major mechanism for the resistive switching in Cu/TiO2/Pt cells.

Towards the limit of ferroelectric nanosized grains

Ferroelectric random access memories are non-volatile, low voltage, high read/write speed devices which have been introduced into the market in recent years and which show the clear potential of future gigabit scale universal non-volatile memories. The ultimate limit of this concept will depend on the ferroelectric limit (synonymous superparaelectric limit), i.e. the size limit below which the ferroelectricity is quenched. While there are clear indications that 2D ferroelectric oxide films may sustain their ferroelectric polarization below 4 nm in thickness (Tybell T, Ahn C H and Triscone J M 1999 Appl. Phys. Lett. 75 856), the limit will be quite different for isolated 3D nanostructures (nanograins, nanoclusters). To investigate scaling effects of ferroelectric nanograins on Si wafers, we studied PbTiO3 (PTO) and Pb(ZrxTi1−x)O3 grown by a self-assembly chemical solution deposition method. Preparing highly diluted precursor solutions we achieved single separated ferroelectric grains with grain sizes ranging from 200 nm down to less than 20 nm. For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the paraelectric phase which has no spontaneous polarization. Recent calculations (Zhong W L, Wang Y G, Zhang P L and Qu B D 1994 Phys. Rev. B 50 698) and experiments (Jiang B, Peng J L, Zhong W L and Bursill L A 2000 J. Appl. Phys. 87 3462) showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4.2–20 nm.

Piezoresponse in the light of surface adsorbates: Relevance of defined surface conditions for perovskite materials

We report on the influence of a surface layer prevailing on perovskites on the piezoelectricity measured by piezoresponse force microscopy. Surface sensitive measurements show that this layer consists of chemisorbates and physisorbates. The surface layer can be removed to a large extent by heating the sample under ultrahigh vacuum conditions. It is shown that the effect of this treatment on the piezoresponse of the material is significant as the potential difference applied to the sample is no longer reduced by a voltage drop across the adsorbate layer. As a consequence the internal electric field is higher in comparison to the presence of a layer and so the piezoresponse is higher as well.

Impact of the electroforming process on the device stability of epitaxial Fe-doped SrTiO3 resistive switching cells

In this work, the results of our detailed investigations on the electroforming procedure in Pt/SrTi0.99Fe0.01O3/SrTi0.99Nb0.01O3 [Pt/STO(Fe)/Nb:STO] metal-insulator-metal (MIM)-devices and its impact on the performance of resistive switching memory devices are presented. Questions about the exact location of the modifications triggered by the electroforming procedure within the investigated MIM-devices will be addressed. From a technological point of view, the thermal stability of formed devices becomes important. An increase in the device resistances during retention measurements has been observed indicating the presence of internal redistribution effects. These may result from an oxygen vacancy gradient induced by the forming process. However, these internal relaxation effects will not end up in the unformed state. Annealing experiments under defined atmospheric conditions allowed distinguishing between internal and external rediffusion effects. We found that SrTiO3 starts to interact with the surroundi...

Realization of regular arrays of nanoscale resistive switching blocks in thin films of Nb-doped SrTiO3

We report on the realization of short-range-ordered arrays of nanoscale resistive switching blocks in epitaxial Nb-doped SrTiO3 thin films. These blocks can be individually addressed by the tip of a conductive tip atomic force microscope and reversibly switched between a high and a low resistance state reaching an Roff to Ron ratio of up to 50. Scanning micrometer-scale areas with an appropriately biased tip, all blocks within the scanned area can be switched between the two resistive states. We suggest a connection between these nanoscale switching blocks and defect-rich nanoclusters which were detected with high resolution transmission electron microscopy.

Microscopic nature of the metal to insulator phase transition induced through electroreduction in single-crystal KNbO3

Electroreduction of the crystalline perovskite KNbO3 results in a change of the electrical resistivity by about 12–14 orders of magnitude from insulating to semimetallic at room temperature. This change is completely reversible through oxidation of the material. The semimetallic phase is characterized by a three‐dimensional network of planar defects extending throughout the crystal. We have characterized the microscopic nature of this semimetallic phase by photoelectron spectroscopy after preparing the surface of the crystal to the same state. We also present evidence that at temperatures below 100 K these electroreduced materials may even become superconducting.