CAREER: Ferroelectric Multilayers, Superlattices, and Compositionally Graded Films

University Of Connecticut, Storrs CT

Investigators

Abstract

This CAREER project focuses on the study of artificially layered ferroelectric superlattices and compositionally graded ferroelectric films for applications in microelectronics. In the paraelectric state ferroelectric materials possess some of the largest dielectric constants attainable, a property particularly relevant for applications where transient charge storage is required, such as capacitors in dyanmic random access memories(DRAM). Enhanced properties are sought through spatial variations in internal stresses, film composition, and microstructure. It is anticipated that through intrinsic characteristics of ferroelectric materials and the introduction of compositional and internal stress gradients, exceptional and unusual electrical and electromechanical properties can be obtained which are not possible for bulk ferroelectrics and ferroelectric thin films. The proposed work is a combined experimental and theoretical effort. The theoretical part will be based on an extended mean field Landau-Ginzburg-Devonshire phenomenology. Elastic and electrical interactions between individual layers will be modeled and physical properties of ferroelectric multilayers and compositionally graded films will be determined. The model developed will serve as a basis to design ferroelectric stacks with enhanced physical properties via artificially created non-uniformities such as compositional variations, different stress levels, and interfacial defect structures. Ferroelectric and/or paraelectric layers with systematic variations in composition, thickness, and misfit with respect to the underlying substrate will be deposited by rf-sputtering or pulsed lased deposition techniques. Initially, the focus will be on the deposition of the prototypical BaTiO3/SrTiO3 system. Based on the information gathered, multilayers of other perovskite systems such as PbTiO3/CaTiO3, PbTiO3/SrTiO3, and KNbO3/KTaO3, will be grown. Ferroelectric stacks and compositionally graded films will be characterized crystallographically and microstructurally via x-ray diffraction and transmission electron microscopy. Local compositional distribution of elements will be determined by energy-dispersive x-ray spectroscopy (EDXS) using an electron energy filter. Physical properties such as polarization hysteresis, dielectric, piezoelectric, and pyroelectric properties will be measured. The unconventional ferroelectric properties of these multilayers due to chemical and structural non-uniformity will be the basis for designing novel devices and circuits. This part of the research will involve an established industrial collaboration. %%% The project addresses fundamental research issues in a topical area of materials science having high technological relevance. The research is accompanied by an intensive educational plan. To promote comprehensive and active learning, advanced interactive course web sites will be developed for undergraduate and graduate level courses. Also, a "popular science" interactive web site on thin film science and technology aimed at a general, non-technical audience will be designed. The target audience for this web site is high school teachers, juniors, and seniors. Two new interdisciplinary graduate level classes will be designed and implemented. Another educational benefit of the proposed project sought is summer internships for graduate and undergraduate students involved in this research through industrial collaboration. Overall, the project provides students with new challenges and research approaches in materials synthesis, processing, and characterization, and provides new tools and approaches to education and training. Thus, an important feature of the project is the strong emphasis on education, and the integration of research and education. ***