|M.Sc Student||Amir Leshem|
|Subject||Evaluation of Time Dependent Phenomena in Metal/Mixed-Ionic-|
|Department||Department of Physics||Supervisor||Professor Emeritus Riess Ilan|
|Full Thesis text|
The time dependent electronic properties of a device based on a mixed-ionic-electronic conductor (MIEC) placed between two inert metal electrodes are investigated. The MIEC is a p-type oxide semiconductor (SC) with mobile charged acceptors. Thus MIEC allows for ionic and electronic currents. The electronic current relates to holes current and the ionic current relates to the mobile acceptors current. The MIEC is considered to be one dimensional and under a uniform and constant temperature. A dilute limit of defect concentration is further considered. The hole concentrations are pinned by the metal electrodes on both ends of the MIEC. No acceptors are allowed to go in or out of the MIEC, due to inert electrodes. Non steady states are considered. Three types of time dependent voltage sources are simulated - cyclic voltammetry (CV), high and low ac. The model yields a set of non-linear partial differential equations of the first order. The equations are solved numerically. The solutions yield the defect distribution profiles, the electric field and currents distributions. The model properties are characterized by several parameters, MIEC thickness, ion to hole mobility ratio, acceptor ionization constant, time constant and electrode types. Changing the parameters’ values yields different results. A MIEC is regarded as thick when the space-charge region is confined to small fraction of the sample and thin when the whole MIEC is within the space charge.
For a high voltage amplitude, the current-voltage (I-V) relations present, in most cases, hysteresis loops. The shape and thickness of these loops are found to be mostly affected by MIEC thickness, ions mobility and voltage scan rate. For an intermediate MIEC thickness and low frequency, hysteresis loops with peaks are obtained. For a thick sample, high ion mobility and low frequency, a negative resistance is found. For a low enough frequency, the slope of the I-V relation is discontinues, resembling switching effect. The hysteresis loops are found to be related to ionic motion.
For low ac amplitudes, complex impedance relations present arcs. When the ion mobility is low, the arcs are found to be two semi circles. When the ion mobility is high, both mobile acceptors and holes are swept from one MIEC edge to another and recombine to a neutral acceptor. It is found that for high ion mobility, a linear part is present (similar to a “Warburg” contribution), indicating acceptors diffusion as the dominant effect.