Theoretical and experimental invetigation of electronic transport phenomena in oxide based resistive switches

In this work the conduction mechanism of resistive switching devices is investigated, which could be used as novel data storage memory or for the realization of neuromorphic computing architectures. Those memory cells consist of an insulating oxide between two metal contact electrodes. Under an externally applied electric field the migration of oxygen vacancies is initiated. This reconfiguration induces a non-volatile change in the electrical resistance, which is the central state variable. For a better physical understanding, the investigation of the internal conduction mechanism is of fundamental meaning. In several studies, the conduction mechanism is not the main focus. As a result, several conduction mechanism are suggested for comparable current-voltage (I-V) characteristics.A privilege of this work is the opportunity to investigate the current transport experimentally and also apply different simulation methods. Two representative resistive switching cells based on SrTiO3 and HfO2 are considered, which significantly distinguish in their I-V curve. Measurements and simulations of the SrTiO3 based cell show a transport over the conduction band. The simulation in the case of HfO2 show a transport over the defect states. Independent of this difference, the resistive switching mechanism in both cases could be unified to a huge extend. Additionally, a volatile threshold switching based on a pure electronic nature is explored.At the beginning of this work, electrical measurements on the SrTiO3 based cell are applied at different temperatures. A typical method to find the conduction mechanism is the fitting of analytical equations to the I-V curve. Even though the fitting of the Schottky emission equation leads to positive impressions, a detailed review reveal physical contradictions. The same contradictions generally occur for thermal activated current transport equations. For this reason an atomic quantum mechanical approach is used by means of density functional theory combined with the non-equilibrium Greens function formalism (DFT + NEGF). These ab initio simulations reveal a band transport, which is limited by a tunnelling process through the Schottky barrier at the metal electrode. This simulation method reproduce parts of the measurements. In an additional investigation the simulations are applied, to analyse the Schottky barrier lowering effect. The problem of this method is the huge requirement of computational resources, which limits the flexibility. Consequently, not all measurements could be reproduced with DFT + NEGF. Therefore, a continuum single band transport model is developed on the basis of the DFT + NEGF results. This model could predict correctly all trends of the temperature dependent I-V curves. Based on this, a compact model compatible simplification is developed for the detected current transport. This allows a generalized description of the current in the compact model and replaces the previous preselected conduction mechanism. The descriptions so far reference to the SrTiO3 based resistive cell with a strong exponential I-V dependence. Other resistive switching systems distinguishes because of a less strong I-V dependence. For such an example a HfO2 based resistive switching cell is investigated using DFT + NEGF. Here, the current transport is mainly transmitted over defect states and limited by the defect to electrode tunnelling. In both cases, the tunnelling connection to the electrode is the limiting mechanism. This observation allows to generalize the resistive switching mechanism in HfO2 and SrTiO3. In both cases the resistance change is induced by an increasing and decreasing tunnelling distance to the electrode. Besides the ionic non-volatile resistive switching a further effect of volatile resistive switching is investigated. This effect is purely electronic and based on a thermal instability, which leads to a high number of thermal activated charge carriers. This thermal instability occurs at a certain threshold voltage, which can be predicted in continuum model. Out of it, empirical equation are derived for the dependence on material constants and cell geometry. The same thermal instability could be found in the compact model at comparable voltages. Hence, this volatile switching could have a high relevance for the dynamic of the resistive switching

A comprehensive model of electron conduction in oxide-based memristive devices

Memristive devices are two-terminal devices that can change their resistance state upon application of appropriate voltage stimuli. The resistance can be tuned over a wide resistance range enabling applications such as multibit data storage or analog computing-in-memory concepts. One of the most promising classes of memristive devices is based on the valence change mechanism in oxide-based devices. In these devices, a configurational change of oxygen defects, i.e. oxygen vacancies, leads to the change of the device resistance. A microscopic understanding of the conduction is necessary in order to design memristive devices with specific resistance properties. In this paper, we discuss the conduction mechanism proposed in the literature and propose a comprehensive, microscopic model of the conduction mechanism in this class of devices. To develop this microscopic picture of the conduction, ab initio simulation models are developed. These simulations suggest two different types of conduction, which are both limited by a tunneling through the Schottky barrier at the metal electrode contact. The difference between the two conduction mechanisms is the following: for the first type, the electrons tunnel into the conduction band and, in the second type, into the vacancy defect states. These two types of conduction differ in their current voltage relation, which has been detected experimentally. The origin of the resistive switching is identical for the two types of conduction and is based on a modification of the tunneling distance due to the oxygen vacancy induced screening of the Schottky barrier. This understanding may help to design optimized devices in terms of the dynamic resistance range for specific applications