Epitaxial BaTiO3/SrTiO3 Superlattices for Phonon Bragg Mirrors and Cavities

A. SOUKIASSIAN; W. TIAN; D. A. TENNE; X. X. XI; D. G. SCHLOM; N. D. LANZILLOTTI-KIMURA; A. BRUCHHAUSEN; A. FAINSTEIN; H. P. SUN; X. Q. PAN; A. CROS; A. CANTARERO

Pennsylvania, USA

Conferencia; 48th Electronic Materials Conference; 2006

We discuss the design and material parameters of BaTiO3/SrTiO3 heterostructures relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control. The first step to phonon confinement structures is to grow a lambda/4 thick planar periodic stack of two different materials with different acoustic impedance, which will work lambda/4 thick planar periodic stack of  two different materials with different acoustic impedance, which will work as a Bragg reflector with a stop band around ë and functions as a phonon mirror. The second step is the construction of a phonon cavity by enclosing a spacer of thickness d_c = m lambda_c/2 between two phonon mirrors, where lambda_c is the acoustical phonon wavelength at the center of the phonon minigap and m is an integer. The advantages of using these ferroelectric superlattices include that they have an enormous stop band compared to the GaAs/AlAs superlattices previously reported for this application and that due to their high photoelastic coefficient there can be greatly amplified light-sound interaction in these ferroelectric materials. We have used reactive molecular-beam epitaxy (MBE) for growth of BaTiO3/SrTiO3 superlattices on TiO2-terminated (001) SrTiO3, (110) DyScO3, and (110) GdScO3 substrates. With the aid of RHEED, precise one monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create BaTiO3/SrTiO3 superlattices. Structural characterization by XRD and TEM revealed that the samples studied are of high quality with nearly atomically abrupt interfaces.A theta-2theta X-ray diffraction scan showed that nearly all superlattice peaks are present for 2theta < 55° which is an indication of atomically sharp interfaces between the superlattice layers. This was confirmed by high-resolution TEM images. BaTiO3/SrTiO3 superlattices grown on (110) DyScO3 and (110) GdScO3 substrates demonstrate the highest structural perfection ever reported for oxide superlattices with rocking curve full widths at half maximum as narrow as 0.0028°. This is due to the higher structural perfection and smaller lattice mismatch of GdScO3 and DyScO3 substrates with BaTiO3/SrTiO3 superlattices compared to SrTiO3 substrates. UV Raman results (the first ever reported for thin ferroelectric films) show that BaTiO3 is tetragonal and SrTiO3 is polar due to strain in these BaTiO3/SrTiO3 superlattices. Although the UV Raman resolution was insufficient to resolve the acoustic information, we have observed folded acoustic phonons at the expected energy with the expected selection rules. The observed results are in excellent agreement with theoretical predictions.