Open Access Dissertation
Date of Award
Electrical and Computer Engineering
Dr. Gregory W. Auner
This study demonstrates that a new pyroelectric device can be created from ferroelectrics with gradient in composition, and that the field dependence of the observed hysteresis offset is as predicted from theory. Homogeneous thin films of BaxSr1-xTiO3 (x=0.7, 0.8, 0.9, and 1.0) were prepared by metallorganic decomposition (MOD). The relative permittivity, dissipation, polarization, resistivity, and grain size were studied as a function of composition and temperature. Ferroelectric hysteresis loops were observed for all values of x and were found to be independent of measurement temperature though strongly dependent upon grain size. Ferroelectric BST thin films with compositional gradients normal to the growth surface have been formed by successive deposition and annealing of films having step-variable Ba to Sr ratios. These films, graded ferroelectric devices or GFD's, are the dielectric equivalent of the semiconductor p-n and n-p junctions. Unlike semiconductor diode junctions which have a built-in potential arising from the diffusion of free charge across the junction, the intrinsic potential in graded ferroelectric devices is due to a gradient in bound charge. In their simplest forms, graded ferroelectric devices give rise to a totally new, but controllable hysteresis phenomenon that is not observed in homogenous ferroelectric materials. Slater's empirical model for ferroelectric materials has been also extended to describe the graded ferroelectric devices. This model accounts for several aspects of these structures including: the broadness of the permittivity plots with temperature, the formation of a spontaneous potential upon oscillatory field excitation, offsets in the hysteresis graphs along the displacement axis with directions which are gradient dependent, and the electric field dependence of that offset. The graded ferroelectric devices, as formed by MOD, show very low conventional pyroelectric coefficient. However, the effective pyroelectric coefficients obtained from these active devices were much greater, reaching a maximum of 0.06 µC/cm2-°C, compared to conventional value of 0.045 µC/cm2-°C for homogeneous BST films. This study demonstrates that the method used to fabricate the GFD's is inadequate to produce the desired sensitivity. Suggestions for ways to increase the sensitivity of the GFD's are given.
Mohammed, Majed S., "Graded ferroelectric devices: the dielectric analogues of semiconductor diode junctions" (1998). Wayne State University Dissertations. 1237.