Ph.D Thesis


Ph.D StudentDima Kalaev
SubjectProperties of Solid State Devices Based on Mixed Ionic
Electronic Conductors
DepartmentDepartment of Physics
Supervisors Professor Emeritus Riess Ilan
Professor Rothschild Avner
Full Thesis textFull thesis text - English Version


Abstract

There are many experimental observations showing that devices of type metal1|oxide|metal2 exhibit unusual electronic current-voltage (Iel-V) relations, including hysteresis and resistive switching, which do not fit the classical theory of solid state devices. These effects can be exploited to design new types of memristive devices, in particular nonvolatile memory, where the "0" and "1" states are represented by low and high resistance states of the device.

The unusual Iel-V relations are observed in a diversity of oxides. This implies that there is a common physical root cause underlying the unusual patterns. We show that some types of Iel-V relations in metal1|oxide|metal2 devices can be explained by taking into account the motion of ionic defects, such as oxygen vacancies in metal oxides. Thus, the oxides are assumed to be mixed-ionic-electronic-conductors (MIECs), able to change their electronic properties by changing the amount and distribution of the mobile ionic defects and their valence state under an applied driving force .

The Iel-V relations in metal1|MIEC|metal2 devices are calculated by solving the current-density equations for the mobile donors and the conduction electrons taking the Poisson equation and the relevant continuity equations into consideration. The influence of asymmetry in the device structure and in the operating conditions on the shape of the Iel-V relations is discussed. The effects of the boundary conditions in the form of contact potentials of the metal electrodes with the oxide and the impedance of the electrode to material transfer are examined.

In the framework of our model, we propose an explanation for several experimental observations in metal1|MIEC|metal2 devices. In particular we show that the direction of growth of the conducting filament (represented by a high concentration of ionic defects) depends not only on the polarity of the applied voltage but also on the type of the electrodes, e.g., their reactivity and permeability for oxygen exchange with the ambient. To model the transport of the charge carriers on a nano scale, we use the high driving force nonlinear current density equation for both the ionic and electronic carriers. The nonlinearity leads to an exponential decrease in growth time of the conducting filament with an applied voltage.