GROWTH OF NANOSCALE BaTiO3/SrTiO3 SUPERLATTICES BY MOLECULAR-BEAM EPITAXY

Open Access
Author:
Soukiassian, Arsen
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 14, 2007
Committee Members:
  • Xiaoxing Xi, Committee Chair
  • Darrell G Schlom, Committee Chair
  • Mark William Horn, Committee Member
  • Long Qing Chen, Committee Member
  • Venkatraman Gopalan, Committee Member
  • James Patrick Runt, Committee Member
Keywords:
  • Ferroelectric thin films
  • ferroelectric superlattices
  • epitaxial growth
  • oxide MBE
  • ferroelectric phase transitions
  • UV Raman spectroscopy
Abstract:
The well known ferroelectric BaTiO3 was confined within nanoscale BaTiO3/SrTiO3 superlattices to investigate the importance of finite size and strain on its ferroelectric properties, especially the paraelectric-to-ferroelectric transition temperature (TC). The BaTiO3/SrTiO3 superlattices were grown by reactive molecular-beam epitaxy (MBE) on three different substrates: TiO2-terminated (001) SrTiO3, (101) DyScO3, and (101) GdScO3. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create commensurate BaTiO3/SrTiO3 superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions for the targeted superlattices. XRD rocking curve measurements of the BaTiO3/SrTiO3 superlattices grown on (101) DyScO3 and (101) GdScO3 substrates exhibit full width at half maximum (FWHM) of 9 and 7 arc sec, respectively, the narrowest ever reported for any oxide superlattices grown by any technique. High-resolution transmission electron microscopy (HRTEM) reveals nearly atomically abrupt BaTiO3/SrTiO3 interfaces. Temperature-dependent ultraviolet (UV) Raman and XRD reveal the TC in these superlattices. Ferroelectricity was observed in BaTiO3/SrTiO3 superlattices containing as few as one BaTiO3 layer in the repeated superlattice structural unit, i.e., a BaTiO3 layer just 4 Å thick. The combination of finite size and strain effects was seen to shift the TC over a 500 K range. Unstrained SrTiO3 layers in commensurate BaTiO3/SrTiO3 superlattices grown on SrTiO3 substrate are poled by the neighboring ferroelectric BaTiO3 layers, while strained SrTiO3 layers in BaTiO3/SrTiO3 superlattices grown on DyScO3 and GdScO3 substrates are not only polar, but also exhibit strain-induced ferroelectricity. In addition to probing finite size and strain effects, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control. The concept and design of acoustic Bragg mirrors and cavities made of BaTiO3/SrTiO3 superlattices with superior acoustic performance and potential applications in electronic and optical THz modulators are described. We have observed folded acoustic phonons at the expected frequencies using UV Raman spectroscopy. Our results demonstrate the feasibility to design, fabricate, and characterize oxide acoustic devices and may be considered as a first step towards a phonon “laser.”