Florian Hanzig

Single crystal strontium titanate surface and bulk modifications due to vacuum annealing

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 w...

Analog resistive switching behavior of Al/Nb2O5/Al device

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.

Dielectric to pyroelectric phase transition induced by defect migration

Subjecting strontium titanate single crystals to an electric field in the order of 106 V m−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 pMFP in the order of 30 μC K−1m−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 for the MFP phase of SrTiO3. The entire electroformation cycle yields additional information about the directed movement and defect chemistry of oxygen vacancies.

The anisotropy of oxygen vacancy migration in SrTiO3.

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.

Crystallization dynamics and interface stability of strontium titanate thin films on silicon

Nonstoichiometric SrTiO3 thin films were fabricated by different thin-film deposition methods. The impact on the oxide/silcon interface stability as well as the crystallization onset temperature is investigated.

Defect separation in strontium titanate: Formation of a polar phase

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 migrationinduced 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.