RUI: Collaborative Research: Acoustic Study of Lattice Dynamics and Elastic Properties in Perovskite Dielectrics and Ferroelectrics

Towson University, Towson MD

Investigators

Abstract

Non-technical Summary: Research teams at Gordon College and Towson University are collaborating in this project to carry out a comprehensive study of structural, electrical and elastic/acoustic properties of two technologically important materials, Strontium Titanate and Potassium Tantalum Niobate, with a focus on how their properties change when bulk (large size scales common in everyday uses) material is progressively thinned to layers approaching atomic scale sizes. These materials demonstrate unique and novel properties when grown in microscopic volumes in the form of atomically thin layers. The project enhances the knowledge base for a new generation of novel nano-electromechanical devices that can be integrated on microchips in future electronics, including radiofrequency and optical signal processing circuits useful for numerous technologies serving both civil and defense sectors. This collaboration brings together faculty and students in two predominantly undergraduate institutions to engage in experimental and theoretical research at the frontiers of materials physics. Through the integration of research and education, the proposed effort positively impacts the educational experience of undergraduate physics students at both institutions and graduate students in the Applied Physics Master?s program at Towson University. Students are expected to participate in the planning, developing equipment and theoretical models, performing measurements and computer simulations, presenting results of their work at scientific conferences and publishing in scientific journals. Such educational experiences integrated with research are key to developing a globally competent science, technology, engineering and mathematics (STEM) work force. Technical description. This project investigates spontaneous and electric-field induced phase transitions in perovskites by using a combination of ultrasound pulse-echo probing of acoustic phonons, along with dielectric and Raman spectroscopies. The experimental studies are complemented by theoretical inquiry using first principle density functional theory calculations and Monte Carlo-based simulations of phonon transport. Investigations begin by carefully extending models for single crystals of ferroelectric potassium tantalum niobate to para-electric strontium titanate single crystals. Models are then further developed for reduced-dimensional systems, including strontium titanate films and microbridge devices. The thin film samples for the experiments are grown by pulsed laser deposition which allows for the careful manipulation of a wide range of parameters that can be used to control the films strain state, stoichiometry and defect structure. The structural quality of the films is monitored using X-ray diffraction, while the films morphology and defect structure studies are performed by scanning electron microscopy and atomic force microscopy. The cation stoichiometry is determined using electron energy dispersive spectroscopy. The reduced dimensionality effects are explored in connection with lattice mismatch strain, which is engineered by choosing substrates that cause bi-axial tensile or compressive stresses. Acoustic measurements in the films are performed using specially designed surface acoustic wave devices. In order to exclude the influence of substrate, microbridges of free standing thin films are employed. The results of this research open up new approaches to exploring phase transition phenomena. Systematic investigation of ferroelectricity in thin films is of significance for technological applications in electro-mechanical, electro-optical and acousto-optical devices.