Positron annihilation studies of ferroelectrics and related perovskite oxides.

摘要

Advances in the fabrication of ferroelectric thin films have resulted in the use of ferroelectrics in non-volatile memories, integrated pyroelectric detectors and electro-optic switches. Although the material degradation and fatigue properties have improved considerably, a clear understanding of the mechanisms is still lacking. The two main problems that currently plague the ferroelectric devices are failure due to imprint and/or fatigue. Some of the current models indicate that point defects may play a significant role in these mechanisms.; Positron annihilation techniques have been applied to characterize vacancy related defects in ferroelectric thin film structures. Both, bulk ferroelectric material and layered capacitor structures, were studied. Lifetime studies on bulk ceramic plate Pb{dollar}sb{lcub}rm x{rcub}{dollar}La{dollar}sb{lcub}rm 1-x{rcub}{dollar}Zr{dollar}sb{lcub}rm y{rcub}{dollar}Ti{dollar}sb{lcub}rm 1-y{rcub}{dollar} samples indicate that there might be a lead-oxygen vacancy complex assisted pinning associated with the fatigue mechanisms. Lifetime studies on bulk ceramic plate samples subjected to various anneals indicate that there might be some vacancy complex formation or clustering of defects at temperatures above 600{dollar}spcirc{dollar}C. The Variable Energy Positron Beam (VEPB) technique was used to study sol-gel and laser ablated layered capacitor structures. We've looked at the effect of processing conditions, donor doping and post growth anneals on the vacancy related defects in both sol-gel and laser ablated ferroelectric thin films. The concentration of open volume or negatively charged defects is effected by the type of surface electrodes as well as by donor doping. In the case of the sol-gel grown samples, the capacitors with the oxide electrode (RuO{dollar}sb2{dollar}) have a lower concentration of defects as compared to the samples with metal electrodes (Pt). VEPB studies on the laser ablated structures indicate that the electrode may have the more dominant effect on the ferroelectric capacitors than the ferroelectric material itself.