Resistive switching phenomena of extended defects in Nb-doped SrTiO$_{3}$ under influence of external gradients

Abstract

Redox-based memristive materials have attracted much attention in the last decade owing to their ability to change the resistance upon application of an electric field making them promising candidates for future non-volatile memories. However, a fundamental understanding of the nature of the resistive switching effect, which is indispensable for designing future technological applications,is still lacking. As a prototype material of a memristive oxide, strontium titanate (SrTiO$_{3}$) has been investigated intensively and it was revealed that the valence change of a Ti “d” electron plays an important role during resistive switching related to insulator-to-metal transition. Such a transition can be induced by electrical gradients, by chemical gradients, by a combination of these gradients or by donor doping. Hence, SrTiO$_{3}$ doped with the donor Nb should have metallic properties and is used commonly as a conducting substrate for the growth of functional oxide thin films. Nevertheless, the resistive switching effect has also be observed in Nb-doped SrTiO$_{3}$. This paradoxical situation offers a unique opportunity to gain an insight into the processes during the insulator-to metal transition. In this thesis, a comprehensive study of the influence of external gradients on SrTiO$_{3}$:Nb single crystals is presented. The focus is especially set on the investigation of the crystallographic structure, the chemical composition, the electronic structure, the lattice dynamics and the electronic transport phenomena using surface-sensitive methods on the macro- and nanoscale. On the as-received epi-polished single crystals, the evolution of a surface layer having a slight excess of strontium and – in contrast to the bulk of the material – semiconducting properties are observed. Hence, the key for understanding of the resistive switching effect is the knowledge of the nature of the surface layer. On the basis of systematic studies of the influence of external gradients on the physical and chemical properties of the surface layer it is demonstrated that a transformation between a Sr-rich and a Ti-rich surface layer can be induced easily illustrating the relevance of ionic movements and phase transformations for the resistive switching effect. On the nanoscale, the switching mechanism is investigated through the use of local conductivity atomic force microscopy with atomic resolution revealing the presence of conducting clusters with a size of 20-60 nm that can be switched independently between ON and OFF states. Since distinct inhomogeneities of the donor distribution are detected by various methods, it is assumed that the conducting clusters, which can be regarded as three-dimensional extended defects, are related to Nb segregation on the nanoscale, which already evolved during the crystal growth by the Verneuil method. In order to gain an insight into the processes taking place when switching the clusters, an emulation of switching is performed on the macroscale by applying extremal electrical gradients, resulting in an evolution of a phase transformation from strontium titanate to titanium oxide in the surface layer. At the end of the thesis, a potential phenomenological model for resistive switching of SrTiO$_{3}$:Nb based on the experimental results as well as on ab initio and finite element simulations is presented. This is done after taking into account the formation of substoichiometric titanium oxide phases, which build up a switchable bridge between the conducting clusters, thereby illustrating that the resistive switching effect in oxides is a very complex phenomenon related to many different mechanisms that need to be considered