For many years molecular beam epitaxy (MBE) has been used to build layered semiconductor nanostructures atom-by-atom to investigate and improve our understanding of semiconductor physics and create new devices. These devices (which include laser diodes, high-performance transistors, and magnetic field sensors) have advanced healthcare, national security, communications, entertainment, and transportation-resulting in significant improvements in the quality of life for all Americans. Recent progress in research has demonstrated that this same atom-by-atom synthesis technique can be used to build nanostructures of oxides, including ferroelectrics, with comparable nanometer-scale layering control. Since ferroelectric materials exhibit a wide variety of electrical, optical, and electromechanical properties, they are extensively used in healthcare (e.g., medical ultrasound), national defense (e.g., night vision and sonar systems), and communications (e.g., miniature capacitors for cell phones and computers). The ability to customize the layering of ferroelectric materials at the atomic-layer level and strain them opens exciting possibilities to dramatically enhance their properties. The improved understanding gained via this research will be applied to the development of improved optical and acoustic devices. Future scientists in a highly interdisciplinary research environment in a technologically significant area of national importance will be trained and educated within this program. Professors from Pennsylvania State University, University of Wisconsin, University of Michigan and Rutgers University will run hands-on workshops during the summers at each of the campuses involved in this research team to expose K-12 students to the thrill of science.
TECHNICAL DETAILS: The technical objective is to understand the fundamental science underlying the electric, magnetic, and optical responses of strained nanoscale ferroelectrics and multiferroics. An integrated theoretical and experimental effort will be taken. Specifically, "first-principles effective Hamiltonian" approaches based on lattice Wannier functions and Landau-Ginzburg type phenomenological methods will be used to identify ferroelectric and multiferroic materials and heterostructures in which large enhancements in properties are expected with strain. Films will be grown by MBE and laser-MBE, patterned by focused ion beams, and characterized using a combination of x ray diffraction, analytical and transmission electron microscopy, Raman spectroscopy, second harmonic generation, and ferroelectric measurements, all as a function of temperature. Strain is utilized in many semiconductor device structures to improve the transport properties of thin semiconductor layers. Within this project, it will be used to enhance the properties of ferroelectrics. Ferroelectrics are very sensitive to strain and a distinct advantage of thin ferroelectric materials over their bulk counterparts is that they may be strained well beyond where their bulk counterparts would crack. For nanoscale ferroelectrics, huge strains become accessible. This feature combined with the ability to precisely integrate and engineer oxides at the atomic level provides a means to investigate, develop, and exploit the properties of oxides for optical modulators, two-dimensional photonic bandgap structures, and phonon-confining piezoelectric structures relevant to the long-term realization of a phonon laser.
