Binghamton University
Department of Physics, Applied Physics and Astronomy
PHYSICS COLLOQUIUM
“Evidence of rhombohedral structure within hetero-epitaxial BiFeO3 thin films”
Dr. In Tae Bae
S3IP Analytical and Diagnostics Laboratory, Binghamton University
Abstract:
BiFeO3 (BFO) is a multi ferroic material that shows ferroelectricity and antiferromagnetism. It was reported that
when BFO grows thin film, its spontaneous polarization becomes ~60mC/cm2, which is one order of magnitude
higher than its bulk form.i In order to understand the mechanism of the substantial increase in spontaneous
polarization, the knowledge about detailed crystal structure of the hetero-epitaxially grown BFO film so
important that a number of studies have attempted to reveal its crystal structure. Some studies reported thin film
BFO has cubic structure with monoclinic tilting,ii,iii while others reported monoclinically distorted
rhombohedral,iv and monoclinic structures with domains.v,vi Despite these previous efforts, the crystal structure
of thin film BFO remains debated. It may be worth noting that most of the previous studies have implemented
x-ray scattering based techniques. While x-ray scattering has strength for investigating subtle changes in single
crystal structure by focusing on a local reciprocal area, transmission electron microscopy (TEM) technique has
advantage to investigate overall crystal structure in that it readily yields information on two-dimensional
reciprocal lattices up to a scattering vector of ~220 nm-1.vii
In this work, cross-sectional TEM study was performed on BFO layers grown on SrTiO3 (STO) substrates from
multiple BFO zone axes to acquire multiple cross-sections of three-dimensional reciprocal lattice of BFO to
precisely evaluate its crystal structure and epitaxial growth mechanism.
Nano-beam electron diffraction (NBED) patterns combined with structure factor calculation and high-resolution
TEM images unambiguously revealed that BFO thin layer grew with a rhombohedral structure identical to its
bulk form. To the best of my knowledge, this is the first to provide evidence of rhombohedral structure within
hetero-epitaxially grown BFO film. No evidence of monoclinic and/or tetragonal distortion was found. The
rhombohedral BFO thin layer was found to grow onto STO substrate by maintaining an epitaxial relationship in
a manner that minimizes the lattice mismatch at the BFO/STO interface. High resolution TEM images
compared with multi-slice simulation also proved rhombohedral structure within the BFO films. In addition,
electron energy loss spectrum obtained from O K-edge of BFO thin layer shows highly similar characteristics
with its bulk form, providing additional evidence of rhombohedral structure of BFO thin layer. NBED
combined with SF calculation and high-resolution TEM analysis combined with multi-slice simulation turned
out highly effective to properly interpret crystal structure of BFO thin films and to study its growth mechanism.
J. Wang et al., Science 299, 1719 (2003).
M. K. Singh, S. Ryu and H. M. Jang, Phys. Rev. B 72, 132101 (2005).
H. Naganuma, M. Ooagne, and Y. Ando, J. Appl. Phys. 109, 07D736 (2011).
J. Li et al., Appl. Phys. Lett. 84 5261 (2004).
G. Xu, J. Li, and D. Viehland, Appl. Phys. Lett. 89, 222901 (2006).
H. Liu et al., Appl. Phys. Lett. 98, 102902 (2011).
I.-T. Bae and H. Naganuma, Appl. Phys. Express 8, 031501 (2015).
Monday, September 21, 2015
Science Library Room 212
PRESENTATION 11:00 PM– 12:00PM
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