Nanoscale Ferroelectric Heterostructures Studied by Ultraviolet Raman Spectroscopy

Dr. Dmitri A. Tenne
Department of Physics
Boise State University

Nanoscale ferroelectrics possess essentially new properties compared to bulk materials and provide an opportunity to manipulate and enhance the ferroelectric properties. Vibrational (Raman and infrared) spectroscopies can provide valuable information for understanding the behavior of nanoscale ferroelectrics, as the lattice dynamics determines the fundamental ferroelectric properties. However, conventional vibrational spectroscopies operating in visible and infrared range fail to measure the phonon spectra of nanoscale ferroelectric structures because of extremely weak signals and the overwhelming substrate contribution. In this talk, application of ultraviolet (UV) Raman spectroscopy for studies of lattice dynamics and ferroelectric phase transitions in nanoscale ferroelectrics will be presented. We demonstrate that UV Raman spectroscopy is an effective technique allowing the observation of phonons and determination of the ferroelectric phase transition temperature (Tc) in nanoscale ferroelectrics, specifically, BaTiO₃/SrTiO₃ superlattices having the ferroelectric BaTiO₃ layers as thin as 1 unit cell, and single BaTiO₃ layers as thin as 3.6 nm. BaTiO₃/SrTiO₃ superlattices and ultrathin BaTiO₃ films studied were grown by molecular beam epitaxy on SrTiO₃ as well as GdScO₃ and DyScO₃ substrates. Excellent epitaxial quality and atomically abrupt interfaces are evidenced by X-ray diffraction and high resolution transmission electron microscopy. UV Raman results show that one-unit-cell thick BaTiO₃ layers in BaTiO₃/SrTiO₃ superlattices are ferroelectric with the Tc as high as 250 K, and induce the polarization in much thicker SrTiO₃ layers adjacent to them. The Tc in superlattices was tuned by hundreds of degrees from ~170 to 650 K by varying the thicknesses of BaTiO₃ and SrTiO₃ layers. Using scandate substrates enables growth of superlattices with systematically changed coherent strain, thus allowing studying the stress effect on the ferroelectric phase transitions. UV Raman data are supported by the thermodynamic calculations of polarization in superlattices as a function of temperature.