Structure Development Studies of SrBi₂(Ta_1−xNb_x)₂O₉ Thin Films

Abstract

In this research, two tasks were pursued: (1) determination of the onset temperature of ferroelectric hysteresis properties of SrBi₂Ta₂O₉ thin films by structure development study, and (2) low temperature processing for thin film fabrication. For task (1), a nondestructive optical method using spectroscopic ellipsometry was utilized for characterizing the structure development of SrBi₂Ta₂O₉ thin films. The optical constants and film thickness were measured as a function of annealing temperature by spectroscopic ellipsometry. By observing the changes in refractive indices and film thickness, the temperatures of phase transformation and grain growth were determined. Consistent results were obtained from x-ray diffraction measurements. By comparing the results of the structure development study and ferroelectric hysteresis properties investigation, the onset temperature of the hysteresis curve of SrBi₂Ta₂O₉ with 50% excess Bi was determined to be about 700 °C. The critical factor for the compound to exhibit a well-defined hysteresis curve was found to be the grain size. For task (2), the effects of excess Bi content and Nb/Ta ratio of SrBi₂(Ta_1–x Nbx)₂O₉ on the ferroelectric hysteresis properties were studied. It was found that the onset temperature for obtaining well-defined hysteresis properties can be reduced by adding excess Bi or increasing Nb/Ta ratio. By choosing SrBi₂Nb₂O₉ with 50% excess Bi, the onset temperature of the hysteresis curve was reduced to about 650 °C.