Pure strontium titanate exhibits a cubic perovskite‐type structure at room temperature, but many approaches to break this high degree of symmetry are accessible. The present review summarizes some possible methods by discussing the effects of stoichiometry variation, ion implantation, temperature treatment and external electric fields. Since oxygen vacancies are the most prominent and most mobile defect species in strontium titanate, they play a crucial role also for structural changes due to external influences. Possible functionalities range from tuning of permittivity, band gap and conductivity to field‐induced structure changes, which lead to applications as solid‐state battery or switchable pyroelectric.

Oxygen migration in perovskites is well known to occur via vacancies along the TiO6 octahedron edges. Ionic conduction depends further on the orientation of the crystal in the electric field. To study the anisotropy in cubic SrTiO3 single crystals, temperature-dependent electroformation measurements ranging from 11 °C to 50 °C have been conducted for representative crystallographic directions within the crystal system. Electroformation of pure SrTiO3 follows an Arrhenius behavior, implying an ionic migration process of intrinsic oxygen defects. Activation energies E A for oxygen vacancy migration have been determined to 0.70 eV for [Formula: see text] and [Formula: see text] directions in contrast to 0.77 eV for [Formula: see text]. Mobility of oxygen vacancies is enhanced in [Formula: see text] compared to [Formula: see text] and [Formula: see text] by up to half an order of magnitude. A migration model based on atomistic migration paths and their multiplicities accounts for these experimental variations in mobility.

Successive crystallization of amorphous Cr-Zr-O thin films, formation of the (Cr,Zr)2O3/(Zr,Cr)O2 nanocomposites and the thermally induced changes in the hexagonal crystal structure of metastable (Cr,Zr)2O3 were investigated by means of in situ high-temperature synchrotron diffraction experiments up to 1100°C. The thin films were deposited at room temperature by using reactive ion beam sputtering, and contained 3–15at.% Zr. At low Zr concentrations, chromium-rich (Cr,Zr)2O3 crystallized first, while the crystallization of zirconium-rich (Zr,Cr)O2 was retarded. Increasing amount of zirconium shifted the onset of crystallization in both phases to higher temperatures. For 3at.% of zirconium in amorphous Cr-Zr-O, (Cr,Zr)2O3 crystallized at 600°C. At 8at.% Zr in the films, the crystallization of (Cr,Zr)2O3 started at 700°C. At 15at.% Zr, the Cr-Zr-O films remained amorphous up to the annealing temperature of 1000°C. Metastable hexagonal (Cr,Zr)2O3 accommodated up to ~3at.% Zr. Excess of zirconium formed tetragonal zirconia, which was stabilized by chromium.

Subjecting strontium titanate single crystals to an electric field in the order of 10(6) Vm(-1) is accompanied by a distortion of the cubic crystal structure, so that inversion symmetry vanishes and a polar phase is established. Since the polar nature of the migration-induced field-stabilized polar (MFP) phase is still unclear, the present work investigates and confirms the pyroelectric structure. We present measurements of thermally stimulated and pyroelectric currents that reveal a pyroelectric coefficient p(MFP) in the order of 30 mu C K(-1)m(-2). Therefore, a dielectric to pyroelectric phase transition in an originally centrosymmetric crystal structure with an inherent dipole moment is found, which is induced by defect migration. From symmetry considerations, we derive space group P4mm for the MFP phase of SrTiO3. The entire electroformation cycle yields additional information about the directed movement and defect chemistry of oxygen vacancies.

Different physical vapor deposition methods have been used to fabricate strontium titanate thin films. Within the binary phase diagram of SrO and TiO 2 the stoichiometry ranges from Ti rich to Sr rich, respectively. The crystallization of these amorphous SrTiO 3 layers is investigated by in situ grazing-incidence X-ray diffraction using synchrotron radiation. The crystallization dynamics and evolution of the lattice constants as well as crystallite sizes of the SrTiO 3 layers were determined for temperatures up to 1223 K under atmospheric conditions applying different heating rates. At approximately 473 K, crystallization of perovskite-type SrTiO 3 is initiated for Sr-rich electron beam evaporated layers, whereas Sr-depleted sputter-deposited thin films crystallize at 739 K. During annealing, a significant diffusion of Si from the substrate into the SrTiO 3 layers occurs in the case of Sr-rich composition. This leads to the formation of secondary silicate phases which are observed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.

The oxygen concentration profiles, which develop at the interfaces between niobium pentoxide and the Al or Pt electrode in a metal insulator metal stack, were investigated by means of the X-ray and electron energy loss spectroscopies in a scanning transmission electron microscope with high resolution. The contact between Al and Nb2O5 was found to facilitate diffusion of oxygen from Nb2O5 to the Al electrode and to support the formation of a thin aluminum oxide layer at the Nb2O5/Al interface. In contrast, almost no diffusion of oxygen from Nb2O5 was observed at the Nb2O5/Pt interface. Different extent of the oxygen diffusion correlates with the observed differences in the resistive switching of the Pt/Nb2O5/Al and Pt/Nb2O5/Pt stacks. (C) 2015 Elsevier B.V. All rights reserved.

Resistive switching effects in metal–insulator–metal (MIM) structures are strongly influenced by the electrode materials. In this work a platinum-free symmetric Al/Nb2O5/Al device is compared to a device with platinum bottom electrode. For the device with the platinum bottom electrode, filamentary based resistive switching with good data retention was observed up to 125 °C. For the Al/Nb2O5/Al device, an area dependent pure electronic based resistive switching was observed. Electron trapping at the bottom electrode interface is responsible for the observed analog switching behavior which makes an Al/Nb2O5/Al device suitable for neuromorphic applications.

Redistribution of oxygen vacancies in a strontium titanate single crystal is caused by an external electric field. We present electrical measurements during and directly after electroformation, showing that intrinsic defect separation establishes a non-equilibrium state in the transition metal oxide accompanied by an electromotive force. A comprehensive thermodynamic deduction in terms of theoretical energy and entropy calculations indicate an exergonic electrochemical reaction after the electric field is switched off. Based on that driving force the experimental and theoretical proof of concept of an all-in-one rechargeable SrTiO3 single crystal energy storage is reported here.

Stoichiometric perovskite-type strontium titanate acts as an insulator because of its wide electronic band gap and has therefore great potential as high-k dielectric and storage material in memory applications. Degradation phenomena of insulating properties of transition metal oxides occur during long time voltage application. From the defect chemistry point of view the question arises how mobile species react on an external electric field and which impact the redistribution has on the stability of the crystal structure. Here, we discuss near-surface reversible structural changes in SrTiO3 single crystals caused by oxygen vacancy redistribution in an external electric field. We present in-situ X-ray diffraction during and after electroformation. Several reflections are monitored and show a tetragonal elongation of the cubic unit cell. Raman investigations were carried out to verify that the expansion involves a transition from the centrosymmetric to a less symmetric structure. Regarding a whole formation cycle, two different time scales occur: a slow one during the increase of the lattice constant and a fast one after switching off the electric field. Based on the experimental data we suggest a model containing the formation of a polar SrTiO3 unit cell stabilized by the electric field, which is referred to as migration-induced field-stabilized polar phase [1] at room temperature. As expected by a non-centrosymmetric crystal structure, pyroelectric properties will be presented in conjunction with temperature modulated electroformation cycles. Furthermore, we show that intrinsic defect separation establishes a non-equilibrium accompanied by an electromotive force. A comprehensive thermodynamic deduction in terms of theoretical energy and entropy calculations indicates an exergonic electrochemical reaction after the electric field is switched off. Based on that driving force the experimental and theoretical proof of concept of a solid-state SrTiO3 battery is reported.

Nitrogen ion implantation into strontium titanate single crystals causes a slight shift of the Ti-K edge position compared to pristine SrTiO3 and a strong increase of the second pre-edge peak in X-ray absorption near-edge spectroscopy (XANES) using grazing incidence geometry. Calculations by a finite difference method demonstrate that the strong increase of the second pre-edge feature in the defect distorted phase can be attributed to a static displacement of the Ti atom relative to the surrounding oxygen octahedron. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Pyroelectric materials have a broad spectrum for practical application. Apart from the established infrared sensor technology, recently the pyroelectric effect has been employed unconventionally in waste heat recovery, X-ray generation or water disinfection. This coupling phenomenon is the temperature dependence of a ferroelectric's spontaneous polarisation. A crystal structure that allows pyroelectricity cannot have an inversion centre, if it had a polar axis would not exist. Hence the well-known perovskite strontium titanate, crystallizing in the space group Pm-3m, is known to be dielectric. Nonetheless, under an external electric field of 1MV/m charged defects like oxygen vacancies redistribute in a strontium titanate single crystal, leading to a distortion of the unit cell and subsequently to the formation of a defect structure called the migration-induced field-stabilized polar (MFP) phase [1]. Raman scattering shows that the MFP phase of strontium titanate may exhibit broken centrosymmetry, suggesting the existence of a polar axis. Here, we investigate the pyroelectric properties of strontium titanate single crystals at room temperature during these electroformation cycles with a modified Sharp-Garn method [2]. Our frequency and field dependent measurements indicate the pyroelectricity of the MFP phase. Additionally the measurement method elucidates the kinetics of the oxygen vacancy migration as well as electric properties during electroformation. Inducing pyroelectricity in a centro-symmetric crystal structure opens the scope for a new class of pyroelectric materials.

Strontium titanate is a promising insulator material in resistance switching random access memories. Strontium titanate thin films are prepared by atomic layer deposition from bis(tri-isopropylcyclopentadienyl)-strontium (Sr(iPr3Cp)2), Tetrakis-(dimethylamido)titanium(IV) (Ti[N(CH3)2]4) and water at a substrate temperature of 300°C. The layer stoichiometry is analyzed by X-ray fluorescence spectroscopy for the main element composition and by X-ray photoelectron spectroscopy to detect light element contamination. A significant carbon contamination is found whereas nitrogen is not detected. These results are discussed with possible decomposition reactions of the Sr(iPr3Cp)2 molecule at the given deposition temperature. The film microstructure is characterized by grazing incidence X-ray diffraction. Optical and electrical characterizations show that the strontium titanate layers are transparent up to an optical gap of 3.85eV and insulating.

Resistive switching in MIM (metal-insulator-metal) stacks is an effect that enables a promising data storage technology which is able to overcome the size limitations of conventional non-volatile memories. The resistive switching effect was already demonstrated for several binary as well as ternary transition metal oxides (TiO2, NiO, SrTiO3, Nb2O5) [1,2]. The current models of the switching mechanisms suggest the important role of defects like oxygen vacancies [3]. Here, we report on the local structural and electronic properties of transition metal oxides embedded in MIM stacks that were obtained by using transmission electron microscopy and electron spectroscopy. We focus on the development of the stoichiometry across the MIM stack for amorphous and partial crystalline niobium oxides. Therefore, electron energy loss spectra (EELS) as well as the energy dispersive X-ray spectra (EDS) were collected on the atomic scale utilizing a nanometer probe in the scanning transmission electron microscope (STEM). The differences in the oxygen content among the electrodes and the concentration profiles at the metal/oxide interfaces in particular were investigated in dependence on the preparation method and on the electrode material. Besides, focusing on the electron loss near edge structure (ELNES) of the oxygen K edge we employed simulations using FEFF9 to describe the modifications of the electronic structure with variations in the oxygen content.

Local reversible structural changes in SrTiO3 single crystals in an external electric field are induced by oxygen redistribution. We present in situ x-ray diffraction measurements during and immediately after electroformation. Several reflections are monitored and show an elongation of the cubic unit cell of strontium titanate. Raman investigations verify that the expansion of the unit cell involves a transition from the centrosymmetric to a lower symmetry phase. During a complete formation cycle, including the hold time of the electric field and relaxation time without field, two different dynamics are observed for the reversible transitions from cubic symmetry to tetragonal distortion: a slow one during the increase of the lattice constant in field direction and a fast one after switching off the electric field. Based on the experimental data, we propose the formation of a polar strontium titanate unit cell at room temperature stabilized by the electric field, which is referred to as migration-induced field-stabilized polar phase.

In this chapter, VE- positron annihilation lifetime spectroscopy (PALS) measurements are reported on a series of Fe-doped SrTiO3 (STO) thin films grown by pulsed laser deposition (PLD). Fe-doped SrTiO3 thin films provide an insightful model system for the study of resistive switching memory device mechanisms. The chapter presents the ab initio calculations to address the defect structure of β-Ga2O3. Homoepitaxial STO thin films are a suitable oxide model system to investigate the effect of defects on the electrical properties. The initial conductivity of STO can be enhanced by introduction of donor or acceptor dopants or intrinsic point defects by increasing the oxygen vacancy concentration. Bulk polycrystalline thermistor ceramics on BaTiO3-basis with positive temperature coefficient (PTC) posses an increasing current limiting electric insulation resistance when they are heated above the Curie temperature Tc of the ferroelectric compound BaTiO3. Acceptor doped LaMO3 materials have attractive magnetic and electrical properties and possess ferroelasticity. Controlled Vocabulary Terms ceramics; ferroelectric materials; oxygen; switching; thin films

Vacuum annealing is a widely used method to increase the electric conductivity of SrTiO3 single crystals. The induced oxygen vacancies act as intrinsic donors and lead to n-type conductivity. Apart from the changed electronic structure, however, also crystal structure modifications arise from this treatment. Hence, electronic properties are determined by the interplay between point defects and line defects. The present paper provides a survey of the real structure of commercially available SrTiO3 single crystals and the changes induced by reducing vacuum heat-treatment. Therefore, all investigations were performed ex situ, i.e., after the annealing process. Used characterization methods include atomic force microscopy, transmission electron microscopy, spectroscopic ellipsometry, infrared spectroscopy, and photoluminescence spectroscopy. Besides the expected variation of bulk properties, especially surface modifications have been detected. The intrinsic number of near-surface dislocations in the samples was reduced by vacuum annealing. X-ray photoelectron spectroscopy proves the existence of a layer of adsorbed molecules, which influences the SrTiO3 work function. Also, the interaction between adsorbates and surface point defects as well as laser annealing due to local oxygen absorption are discussed.

Resistance switching in metal – insulator - metal (MIM) structures with transition metal oxides as the insulator material is a promising concept for upcoming non-volatile memories. The electronic properties of transition metal oxides can be tailored in a wide range by doping and external fields. In this study SrTiO 3 single crystals are subjected to high temperature vacuum annealing. The vacuum annealing introduces oxygen vacancies, which act as donor centers. MIM stacks are produced by physical vapor deposition of Au and Ti contacts on the front and rear face of the SrTiO 3 crystal. The time dependent forming of the MIM stacks under an external voltage is investigated for crystals with varying bulk conductivities. For continued formation, the resistivity increases up to failure of the system where no current can be measured anymore and switching becomes impossible.

Perovskite-type transition metal oxides have great potential as storage material in resistive random-access memory (RRAM) devices. Typical non-volatile memory cells are realized in metal-insulator-metal (MIM) stacks with insulator thicknesses of few nanometers. We report on the investigation of single-crystal SrTiO 3 to understand the role of volume and interface real structure for the electrical conductivity in such materials. Conductivity in SrTiO 3 single crystals was established by a reducing high vacuum (HV) annealing introducing charged oxygen vacancies acting as donor centers. Titanium electrodes are evaporated on both crystal faces to obtain an MIM element.