(213c) Chemical Vapor Deposition Route to Multifunctional Multiferroics

Multiferroic materials exhibit at least two of the ?ferroic? properties?ferroelectricity, ferromagnetism, ferroelasticity and ferrotoroidicity?in the same phase. By combining these different physical properties in the same material, multiferroics offer a promising route to create multifunctional devices. The current decade is witnessing a flurry of research activity on these materials and related phenomena. Magnetoelectric multiferroics have drawn the most interest. They have both ferroelectric and ferromagnetic domains in the same phase with a coupling between them, and hence hold the promise to achieve the long sought-after combination of electronic and magnetic functionalities in a single device component. Realization of such devices would further offer advancements towards device miniaturization and better technology. High-speed low-power electrically controlled magnetic memory elements, electrically tunable microwave devices and highly sensitive magneto sensors are only a few of the vast array of aspired applications.

Single-phase multiferroics thin films, particularly BiFeO₃ and rare-earth manganites, have received the most attention. Although through doping newer phases have been explored, the ordering temperatures are very low and the coupling effect in single-phase multiferroics is still too weak for device applications. Heterostructures are more promising for device applications since the coupling in such structures is many orders of magnitude stronger. These heterostructures also offer more degrees of freedom such as composition, microstructure and orientation for optimized coupling. Concerted efforts need to be directed towards the study of bilayered and multilayered epitaxial composites synthesized using techniques such as chemical vapor deposition that offer precise control over stoichiometry and microstructure along with the flexibility of using a wide variety of precursors and processing conditions. Through wide choice of materials constituting different layers, such composites provide pathways to achieve strong coupling between ferroic properties above room temperature.

Recent research activities, including results from our own work on BiFeO₃ and ferrite-piezolectric composites, based on metalorganic chemical vapor deposition (MOCVD) technique for synthesizing single-phase and composite multiferroic materials will be discussed. MOCVD is one of the most flexible and versatile yet simple and robust process that can deposit conformal films over large areas with high purity and abrupt interfaces. In-situ thin film probing capabilities offer another advantage for research purposes. The desired properties of organometallic precursors suitable for use in MOCVD will also be discussed. For many metals, only solid precursors are available. In order to overcome the associated difficulties in delivering the solid precursors to the reactor, ingenious non-conventional delivery approaches have been developed which will also be presented and discussed.