PROPERTIES OF SOLID STATE DEVICES BASED ON MIXED IONIC ELECTRONIC CONDUCTORS

There are many experimental observations showing that devices of type metal₁|oxide|metal₂ exhibit unusual electronic current-voltage (I_el-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 I_el-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 I_el-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. Optical properties are also modified by the applied voltage. The I_el-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 I_el-V relations is discussed. 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 equations 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 and thus the filament persists when the voltage is switched off.