Defect interactions and diffusion in resistive switching oxides (2019)

Resistive switching is the superordinate concept for a wide variety of physicochemical mechanisms for which, upon external electrical stimulation, a material can undergo reversible changes in its resistance between, at least, two distinct states. In the last decade, resistive switching has been in the spotlight of academic and industrial research because of its potential for fabricating novel non-volatile memory devices referred to as resistive (switching) random-access memories (ReRAMs). One particular resistive switching mechanism is the valence change mechanism (VCM). Its working principle is based on the existence of point defects in high concentrations. It is found that either oxygen vacancies or cation interstitials migrate through the oxide layer upon application of an electric field and modify the oxide's resistance. Although intensely studied, there are still many open questions, especially concerning point defect physics and the electronic conductivity mechanisms arising from it. In this thesis, simulations on particular questions concerning the defect formation, interactions and diffusion in the resistive switching oxides, STO3 and HfOx are investigated. Although the defect chemistry in these materials and, for STO3, the standard diffusion behavior is well-known, the effects of defect interactions on, for example, the ionic conductivity, the generation of defects under non-equilibrium conditions like an electric field or the diffusion properties for an amorphous oxide call for a more in-depth analysis and became the main topics of this thesis. The research work in this thesis can be divided into three parts. In the first part the effect of cation impurities trapping oxygen vacancies in crystalline STO3 is studied. It is shown that the trapping is a plausible explanation for contradicting results for the ionic conductivity in the literature. Atomistic simulations revealed that the trapping process is not only a nearest neighbor effect but also extends up to 5th nearest neighbor positions. The results are condensed into an analytical model for predicting the activation energy of oxygen vacancies depending on the dopant concentration, its type and the temperature. In the second part, the widespread idea of field-enhanced creation of anti-Frenkel pairs in Hfox is discussed. By means of a thermodynamic model in analogy to electron-hole creation, supporting molecular dynamics simulations on the formation and recombination of anti-Frenkel pairs and by a critique on the proposed bond-breaking mechanism, it is shown that this route of defect generation is highly implausible. Consequently, it is deduced that the model of oxygen vacancy generation near an interface to an oxygen reservoir remains the most convincing option. In the third part, molecular dynamics simulations are employed to study the oxygen migration in amorphous HfOx and to compare it to the migration behavior in the cubic and monoclinic crystalline phases. Anomalous sub-diffusive behavior is found for the amorphous phase, interestingly for both the oxygen-deficient and the stoichiometric cells. Based on the simulations, a model describing the diffusion in the amorphous is proposed exhibiting regimes of sub-diffusive behavior for short timescales (&lt;1 ns) and normal diffusion behavior for longer timescales. Additionally, it is shown that the standard definitions of a point defect relatively to a lattice site of the crystal or as a coordination defect do not hold for am-HfOx.