MBE Growth and Properties of Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper Phases

Open Access
Author:
Lee, Che-hui
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 14, 2012
Committee Members:
  • Long Qing Chen, Dissertation Advisor
  • Long Qing Chen, Committee Chair
  • Zi Kui Liu, Committee Member
  • Venkatraman Gopalan, Committee Member
  • Renee Denise Diehl, Committee Member
  • Darrell Schlom, Special Member
  • Xiaoxing Xi, Special Member
Keywords:
  • MBE
  • oxide
  • Ruddlesden-Popper
  • strain
  • tunable dielectrics
Abstract:
In this work epitaxial films of the Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper homologous series with n = 1, 2, 3, 4, 5, 6, 10 and ∞ were grown and their structural, dielectric, and optical properties were characterized. These phases were grown by reactive molecular-beam epitaxy (MBE) on different substrates: (001) (LaAlO3)0.29(SrAl0.5Ta0.5O3)0.71 (LSAT), (001) SrTiO3, (110) DyScO3, and (110) GdScO3 to establish the effect of biaxial strain and dimensionality on the dielectric properties of these layered relatives of SrTiO3. With the aid of shuttered reflection high-energy electron diffraction (RHEED) intensity oscillations, the strontium-to-titanium ratio can be precisely calibrated to within ~1% of stoichiometry. Such accurate compositional control is required for the growth of Sr(n+1)Ti(n)O(3n+1) phases that are non-equilibrium phases, i.e., with 3 < n < ∞. To form these phases, precise monolayer doses of SrO and TiO2 are sequentially deposited onto a substrate to create Sr(n+1)Ti(n)O(3n+1) phases with n as high as 10. X-ray diffraction (XRD) θ-2θ scans reveal that each sample contains all 002l peaks at the expected positions, corresponding to phase-pure single crystalline Sr(n+1)Ti(n)O(3n+1). XRD rocking curves confirm the high structural perfection of the films; the full width at half maximum (FWHM) of the Sr(n+1)Ti(n)O(3n+1) films is <23 arc sec (<0.007°), comparable to the rocking curve FWHM of the underlying substrates. Nonetheless, annular-dark-field scanning transmission electron microscope (STEM) images of selected films revealed the existence of stacking faults and vertically running Ruddlesden-Popper defect layers (double SrO layers). Such defects are not resolved in the θ-2θ scans and most likely stem from inexact supply of constituent species in the shuttered doses, i.e., imperfect composition control, during film growth. The density of defects was seen to increase with n. A histogram analysis by bright-field STEM of the layering disorder in an n = 6 Sr(n+1)Ti(n)O(3n+1) film grown on DyScO3 reveals that 65% of the layers in the growth direction have the desired 6 perovskite layers. The remaining layers have spacings that are harmonics of n = 6, i.e., locally n = 12, 18, and 24 which are well lattice matched to the surrounding n = 6 matrix. These harmonic n layer spacings and the vertically running Ruddlesden-Popper defect layers likely form to accommodate local stoichiometry variations encountered during growth. For the Sr(n+1)Ti(n)O(3n+1) phases deposited on LSAT (n = 1–5 and 10), we investigated the dependence of the optical band gap on dimensionality [n of the Sr(n+1)Ti(n)O(3n+1)] in the series measured by spectroscopic ellipsometry, optical transmission, and cathodoluminescence. First-principles calculations gave insight into the origin of this change in band gap. The thermal conductivity along the c axis of the same phases were measured by time-domain thermoreflectance and compared to atomic-level simulations. For the Sr(n+1)Ti(n)O(3n+1) phases deposited on DyScO3 and GdScO3 (n = 1–6), we studied strain-induced ferroelectricity. Density function theory predicts the emergence of a ferroic ground state for n ≥ 3 on DyScO3, which is in agreement with experimental findings. Dielectric properties of these strained thin films were measured over the frequency range from 1 kHz–125 GHz with a broadband, on-wafer technique. The n = 6 Sr(n+1)Ti(n)O(3n+1) film grown on DyScO3 showed a dielectric tunability around 20% with an exceptionally high figure of merit (∆K/(K tan δ)) at room temperature (~50 at 10 GHz and ~100 at 5 GHz) that rivals all known tunable microwave dielectrics. Finally, with meticulous control of the film stoichiometry we establish the intrinsic dielectric properties of SrTiO3 thin film commensurately strained to DyScO3. Direct comparisons of the structural and dielectric properties of stoichiometric and non-stoichiometric films grown on (110) DyScO3 were made. In contrast to non-stoichiometric samples, the stoichiometric film shows the shortest out-of-plane lattice constant, narrow rocking curves, a dispersion-free low-frequency permittivity, a sharp permittivity maximum at the ferroelectric phase transition temperature, and pronounced soft mode behavior.