Ein numerisches Simulationsmodell für valenzwechselbasiertes resistives Schalten (2017)

Due to their superior scalability and performance, nanoscale resistive switches based on the valence-change mechanism are considered promising candidates for future nonvolatile memory and logic applications. These devices are metal-oxide-metal structures that can be reversibly switched between different resistance states by electrical signals. Typically, they contain one Schottky-like and one ohmic-like metal-oxide contact and exhibit bipolar switching. The switching mechanism and the initial electroforming process are widely acknowledged to be based on temperature-accelerated migration of donors, such as oxygen vacancies, inside the oxide layer and a concomitant change in the electronic barrier at the Schottky-like metal-oxide interface. On the way towards a sound understanding of the physics of resistive switching, the development of comprehensive simulation models is still a key challenge. In this work, a numerical model of valence-change-based resistive switching is developed and implemented using a finite-volume formulation of the governing partial differential equations. It encompasses the solution of the nonisothermal drift-diffusion equations for electronic and ionic charge carriers, including incomplete ionization of dopants, Joule heating, oxygen exchange at the electrodes, and an advanced model of electron transport across the metal-oxide boundary. The latter involves thermionic emission across and tunnelling through the contact potential barrier. It allows for a unified treatment of Schottky and ohmic contacts, thus capturing the transition between both upon barrier modification due to changes in donor concentration near the interface. The simulation model is employed to investigate a variety of resistive-switching phenomena. One-dimensional simulations assuming static donor profiles are performed to explore the influence of various physical parameters on the current-density-voltage relationship. Based on the results, the switching mechanism of Schottky-barrier modification induced by changes in donor density is evaluated. Dynamic simulations involving donor migration are demonstrated to result in bipolar switching with an abrupt set and a gradual reset transition for devices with asymmetric barriers. It is shown that a transition to complementary switching occurs when reducing the asymmetry of the barriers. The nonlinear set kinetics is evaluated for different parameter settings, confirming the experimentally observed universal relationship between the dissipated power and the set times.Two-dimensional axisymmetric simulations are employed to investigate the reset process in TaOx-based memory cells, identifying the gradual nature to rely on the interaction between ionic drift and diffusion processes that approach equilibrium. Furthermore, filament evolution during quasi-static current-voltage sweeps is studied. Simulations of electroforming reveal a dependence of the filament growth direction and the post-forming resistance state on the rate of oxygen exchange vs. the rate of ionic transport inside the oxide layer. Finally, simulations of potential profiles of pristine Pt/Fe:SrTiO3/Nb:SrTiO3 structures suggest that acceptor-type dopants at the Nb:SrTiO3 bottom-electrode interface significantly impact the local field distribution.