Charge and the unexpected : solid state processes involving charged species

Abstract

We explore links between performance under electrical stress and the way in which charge is localised in two contrasted systems. Both systems are arnorphous. In one case inhomogeneity which plays a role; for the other, it is topology. For silicon dioxide, the gate dielectric of microelectronics, we show that the charge state of the oxidising species (mobile atomic or molecular oxygen) is critical for ultra-thin oxides. The surprises are firstly that the differences between crystalline and amorphous oxides are substantial, and secondly the way that the important charge states should vary systematically through the oxide thickness. We identify ways in which the oxide performance might be improved. For an amorphous polymer.. we establish that the mesostructure (spaghetti structure) of the polymer can have significant effects on its performance, both for insulating and conducting polymers. In the case of conducting polymers, the surprise is that modelling shows that, even for “trap-free” cases, electrons can be trapped for times long compared lo average transit times because the local configuration other carriers obstructs the direct routes lo the anode. In neither case is a simple continuum space charge description valid. We also present: results from self-consistent molecular dynamics which show that electronic excitation can cause local rebonding in ways which appear to resemble the initiation electrical breakdown.