Title

Stress relaxation and phase transformations in ferroelectric heterostructures

Date of Completion

January 2006

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Ferroelectric materials form a special class of dielectrics that can sustain a remnant polarization in the absence of an externally applied electric field below their Curie point. They have unique piezoelectric, electrostrictive, dielectric and pyroelectric properties. These materials exhibit a giant dielectric and piezoelectric response in the vicinity of a ferroelectric-to-paraelectric phase transformation. Therefore, there is a continued interest to incorporate these materials into embedded DRAMs and non-volatile memories, micro-electromechanical systems (MEMS), tunable capacitors and infrared detectors in thin film form. ^ Ferroelectrics are usually deposited at high temperatures. Upon cooling, the film experiences internal stresses resulting from thermal expansion mismatch and the self strain of the paraelectric-to-ferroelectric phase transformation. In case of epitaxial films, there is an additional internal stress source arising from the lattice mismatch between the film and the substrate. Internal stress due to this mismatch can be relaxed via the formation of a dislocation network at the film-substrate interface which may be proceeded by twinning upon the paraelectric-to-ferroelectric phase transformation. This leads to a rich variety of microstructural features in ferroelectric thin films that directly affect the electrical and electromechanical properties. ^ In this thesis, epitaxial ferroelectric films have been studied both experimentally and theoretically to understand the changes in the structural characteristics of a paraelectric-to-ferroelectric transformation compared to their bulk counterparts. Results indicate that the phase transformation characteristics and the electrical properties of ferroelectric heterostructures are altered in the presence of internal stresses with regards to their bulk counterparts. A thermodynamic analysis has been carried out to investigate the role of different types of dislocations in ferroelectric materials. Due to the coupling of the stress field of the dislocation and the polarization, there is a drastic variation in the polarization near the dislocation. These polarization gradients may result in strong depolarizing fields that suppress the ferroelectricity in a region that extends over several nanometers. Thus, a significant degradation in the dielectric and the piezoelectric responses is expected in the films with a high concentration of dislocations with dislocation lines in the film-substrate interface. ^