Joseph V. Mantese

Interlayer coupling in ferroelectric bilayer and superlattice heterostructures

Ferroelectric multilayers and superlattices have gained interest for dynamic random access memory (DRAM) applications and as active elements in tunable microwave devices in the telecommunications industry. A number of experimental studies have shown that these materials have many peculiar properties which cannot be described by a simple series connection of the individual layers that make up the heterostructures. A thermodynamic analysis is presented to demonstrate that ferroelectric multilayers interact through internal elastic, electrical, and electromechanical fields and the strength of the coupling can be quantitatively described using Landau theory of phase transformations, theory of elasticity, and principles of electrostatics. The theoretical analysis shows that compositional variations across ferroelectric bilayers result in a broken spatial inversion symmetry that can lead to asymmetric thermodynamic potentials favoring one ferroelectric ground state over the other. Furthermore, the thermodynamic modeling indicates that there is a strong electrostatic coupling between the layers that leads to the suppression of ferroelectricity at a critical paraelectric layer thickness for ferroelectric-paraelectric bilayers. This bilayer is expected to have a gigantic dielectric response similar to the dielectric anomaly near Curie-Weiss temperature in homogeneous ferroelectrics at this critical thickness

Functionally Graded Polar Heterostuctures: New Materials for Multifunctional Devices

Mixing materials of different compositions is an ancient art. As early as 3000 B.C., metallic alloys brass & bronze were used for sculpture work. Over the last century, major strides were made in the art of crystal growth of metals, dielectrics, and semiconductors. An alloy offers an opportunity to exploit physical, electrical, and optical properties of materials which are either intermediate, or absent in its constituent materials. This has been the driving force behind the study and discovery of new generations of alloys. With the advent of epitaxial growth techniques such as Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD), such hybrid materials can now be engineered at the atomic scale. Polar semiconductors of the wurtzite crystal structure possess spontaneous and piezoelectric polarization, the most relevant examples being the III-V Nitride semiconductors GaN, AlN, and InN. The electric dipoles in each unit cell of such materials are ‘frozen’ once the crystal is grown, and can contribute to the formation of bound as well as mobile charges. Closely related to such polar semiconductors are ferroelectric crystals, in which the electric dipoles can be flipped with external electric fields. Some ferroelectric crystals have electronic bandgaps comparable to wide-bandgap semiconductors, and are only recently being investigated in that light; they are referred to as ‘ferroelectric semiconductors’. Finally, ferromagnetic crystals are analogous in that they possess a magnetic moment (spin) in each unit cell. Controlled doping of semiconductors with magnetic atoms allows one to marry the properties of semiconductors and ferromagnets in a new class of materials called Dilute Magnetic Semiconductors (DMS), the alloy Ga1−xMnxAs being the prime example. Thus, by the technique of alloying, the distinction between the seemingly different classes of materials semiconductors, ferroelectrics, and ferromagnets is slowly becoming blurry. The same alloy material can exhibit the functionality of different material classes, and hence such materials are labeled ‘multifunctional’. In this chapter, the science and the applications of a new class of compositionally ‘graded’ alloy materials is presented. A rich range of physical phenomena emerge