CURRENT RESEARCH PROGRAMS

Ferroelectric Multilayers, Superlattices, and Functionally Graded Films

This project focuses on the engineering of artificially layered ferroelectric superlattices and compositionally graded ferroelectric films with enhanced properties through spatial variations in internal stresses, film composition, and microstructure. Making use of the unique intrinsic characteristics of ferroelectric materials and introducing 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 ongoing work is a combined experimental and theoretical effort.

This program is in collaboration with Dr. Joe Mantese (Delphi Research Labs) and supported by the National Science Foundation under grant #: DMR-0132918, and University of Connecticut Research Foundation.

Dislocations in ferroic films

Ferroics such as ferroelectrics, ferro- or ferrimagnets, and ferroelastics are high energy-density materials that store, convert, and release energy (electrical, magnetic, and mechanical) in a well-controlled manner, making them highly useful as sensors, actuators, and non-volatile memory elements. To exploit these unique properties in integrated circuits, ferroics are deposited as thin films on suitable substrates that promote lattice-matched growth. Internal stresses due to epitaxy can be relaxed to a certain extent via the formation of arrays of misfit dislocations at the interlayer interface. Our preliminary results indicate that these dislocations can severely impact all physical properties of thin film ferroelectrics, rendering them practically unusable.

This program is supported by the American Chemical Society under grant number PRF#43122-AC5.


Graded ferroelectrics for tunable microwave device applications

This research endeavor, focused on the development of temperature insensitive active thin films for tunable devices, will also feed into the "Software Defined Radio (SDR) Components for Joint Tactical Radio System (JTRS) Cluster 5 Manufacturing Technology Objective (MTO)" program. Alignment with both the near term MTO and the long term JTRS programs, and will the research to be accomplished to allow the realization of high performance temperature stable preselectors which will enable the development of embedded network communications and reuse across JTRS. JTRS man-portable radios will support multiple key programs, including Land Warrior (LW) and the Army's Future Force (AFF). To date, there is no known "Working Solution" for realization of affordable, high performance, temperature stable phase shifters and/or preselectors that are commensurate with the Army's size, weight, cost, and performance specifications.

A comprehensive theoretical analysis based on established models is being improved and employed to synthesize ferroelectric films with high tunability (>70%), low loss tangent (<0.01), and temperature insensitive dielectric constant over a 100C temperature range. The theoretical formalism is used to design tunability and the dielectric response as a function of multiple sources of internal stresses, film thickness, compositional grading, deposition temperature, and film texture.

This program is in cooperation with the US Army Labs and supported by the US Army Office.

Strain controlled polydomain heterostructures

Strain controlled polydomain heterostructures can be obtained as a result of all solid-solid phase transformations: ordering, decomposition into isomorphic or polymorphic solid solutions as well as polymorphic martensitic, ferroelastic, ferroelectric, and ferromagnetic transformations. Such polydomain structures are of potential practical importance, since films and heterostructures with internal stresses and numerous interfaces can possess unique and desirable physical properties. A heterophase structure induces structural modulation at film-substrate interfaces and can considerably alter the interface properties. In addition, multiphase epitaxial structures may contain new and unusual phases that are not stable in the respective bulk material or in homogeneous epitaxial structures.

Microstructure-properties relations in ferroelectric thin films

Formation of polydomain (polytwin) structures in epitaxial films undergoing a phase transformation is a mechanism that relaxes internal stresses that are a result of the lattice misfit due to the structural phase transformation and the difference in the thermal expansion coefficients of the film and the substrate. The polydomain structure of tetragonal epitaxial ferroelectric films consists of the three possible orientational domains of the tetragonal phase separated from each other by elastically compatible 90o domain walls. Figure 2 shows the domain structure of a 300 nm thick PbZr0.2Ti0.8O3 (PZT) film on a SrTiO3 (STO) substrate with and without conducting oxide electrode layers. In Figure 3, a more complex domain structure of a 500 nm thick PZT film on a STO substrate is shown. Our fundamental goal is to design special domain architectures with enhanced electrical and electromechanical properties.

Effect of internal stresses on the structural and physical properties of constrained films

We have shown that the physical properties of epitaxial ferroelectric or dielectric thin films strongly depend on the misfit between the film and the substrate if the temperature of measurement is close to the phase transition temperature or if the internal stresses are sufficiently large to shift the temperature of transformation to the temperature of measurement. Experimental and theoretical studies are underway to design multilayer ferroelectric/dielectric thin films which have high dielectric and piezoelectric constants. The pyroelectric response of ferroelectric thin films on various substrates have been shown to be strong function of the misfit and film thickness. Click here for the theoretical plots.

Miscellaneous

Martensitic transformation and the shape memory effect in constrained thin films; Micro-electromechanical systems (MEMS) with ferroelectric, ferromagnetic, or ferroelastic active components.