Composition Graded, Epitaxial Oxide Nanostructures: Fabrication and Properties

1
Composition Graded, Epitaxial Oxide
Nanostructures Fabrication and Properties
(NSF NIRT Grant 0709293)Efstathios I.
Meletis1, Jiechao Jiang1, Chonglin Chen2, Amar S.
Bhalla2, and Gemunu Gunaratne3 1 University
of Texas at Arlington, Arlington, Texas 2
University of Texas at San Antonio, San Antonio,
Texas 3 University of Houston, Houston, Texas
(II) Double-layered Nanostructure of Ba(Zr,Ti)O3
Epilayer and twincoupled domain structures on
MgO substrate
(III) Two-dimensional Interfacial Structure of
the Epitaxial Oxide Films on MgO
BACKGROUND
The nature of the interfacial structure is very
important in understanding the growth mechanism
of epitaxial films and nanopillars. Cross-section
TEM has been widely used to study the interfacial
structure of heteroepitaxial films and has been
turned out to be a very effective technique for
such studies. The lattice misfit induced strain
energy can be partially or fully released at the
interface between the epitaxial film and
substrate by edge dislocation formation which can
be periodically distributed along the interface.
However, the interfacial structure information
obtained using cross-section TEM is limited in
one-dimensional space. More local information is
needed in order to completely understand the
influence of the substrate surface
characteristics and film/substrate interface on
the microstructure of epitaxial films. As a part
of this project, we recently developed a method
using plan-view TEM to study the interface
structure in 2D space, which is able to provide
critical and valuable information that is lacking
from the cross-section TEM analysis 4. We have
fabricated and studied epitaxial (La,Ca)MnO3 and
(Pb,Sr)TiO3 films on MgO substrate. The lattice
mismatch near the interface regions obtained
using the new method was found to be -8.0 for
(La,Ca)MnO3/MgO and -7.14 for PbTiO3/MgO. Both
values are larger than those obtained using
cross-section TEM (-6.4 for (La,Ca)MnO3/MgO and
-6.2 for PbTiO3/MgO). The (Pb,Sr)TiO3 film is
well commensurate with the substrate over large
areas, whereas (La,Ca)MnO3 film is only locally
commensurate with the substrate 5.
Perovskite oxides are of enormous fundamental
interest and technological importance due to
their intriguing properties. These properties can
be tailored for a wide range of applications in
magnetic, magneto-electronic, photonic, and
spintronic technology. Many perovskite - type
oxides have been synthesized in the past in bulk
form or as thin films. It is expected that
nanostructures of these oxides may offer enormous
opportunities to explore intriguing physics and
applications. However, synthesis of
one-dimensional perovskite -type oxides is a
challenge and has seen very little success due to
their complex composition. Recently, we achieved
fabrication of self-organized, ordered arrays of
coherent, orthogonal epitaxial (La, Sr)MnO3
nanopillars on (001) LaAlO3 by pulsed-laser
deposition (PLD) 1, which to the best of our
knowledge, has been the first report on the
fabrication by self-organization of such
epitaxial oxide nanopillars. The formation of the
nanopillars depends strongly on the processing
temperature and oxide composition 2. Such
nanopillars exhibit novel magnetic properties
different from those of their bulk, thin film, or
nanoparticle counterparts 3. Furthermore,
ferroelectric, compositionally gradient thin
films have been shown to tremendously enhance
piezoelectric response due to the build-in strain
gradient. The coexistence of different properties
that can be coupled in nanocomposite thin films
has recently stimulated much scientific and
technological interest since the coupling can
provide new property tenability. However, major
challenges exist in extending these compositional
variations from thin films to nanopillars since
the fabrication of compositionally graded and
modulated composite nanopillars by
self-organization has not yet been attempted.
.
We recently identified non-lead ferroelectric
material, BaTiO3 and its modified materials (Ba,
Sr)TiO3 and Ba(Zr,Ti)O3 as another counterpart
for the composition graded nanostructures. These
films exhibit high dielectric constant, low
dielectric loss tangent and large electric field
tunability that have attracted considerable
attention for bypass capacitors, IR detectors,
and tunable microwave applications. We fabricated
structure graded Ba(Zr,Ti)O3 films on (001)
MgO. Ba(Zr,Ti)O3 epilayer was first epitaxially
grown on the substrate followed by a layer of
multi-oriented twin domain structures by sharing
their 111 planes with the epilayer. Such
structure graded thin films show interesting
abnormal ferroelectric properties that do not
exist in their bulk counterpart.
OBJECTIVES

• Investigate the principles of formation of
  self-assembled, epitaxial nanopillars of
  ferromagnetic (La,Sr)MnO3 and (La,Ca)MnO3, and
  ferroelectric (Ba,Sr)TiO3 and Ba(Ti,Zr)O3
  perovskite-oxides
• Fabricate compositionally graded and modulated
  composite (La,Sr)MnO3 and (La,Ca)MnO3, and
  ferroelectric (Ba,Sr)TiO3 and Ba(Ti,Zr)O3
  nanopillars
• Characterize and study the mechanism of
  morphological evolution, structure and physical
  properties
• Theoretically identify relationships between
  nanostructure characteristics and materials
  properties
• Develop a tool for designing and exploring 1-D
  nanostructures of interest and other new
  materials.

Fig. 2 XTEM image of twin-coupled structure on
epitaxial Ba(Zr,Ti)O3 film on (001) MgO. (a)
Bright-field image, (b), (c) and (d) dark-field
images showing presence of BZT epilayer and two
twins.
The specifications and long term vision have been
discussed with the project team members during
the kickoff meeting (Oct. 1st, 2007, UTA). These
specifications were updated during the 2nd (May
26, 2008, UTSA) and 3rd (Oct. 16, 2008, UH)
project meeting according to the project feedback
mechanism.
Fig. 3 Schematic illustration of the epilayer
structure and the four possible oriented twin
domains (up) and their crystallographic
orientation relationships between the epilayer
and the twin domains (down). Black blocks
(squares, triangles and ellipses) represent the
zone axes of the epilayer, while those filled
with coarse slope, horizontal, fine slope and
vertical lines represent the zone axes of
Twin-1, Twin-2, Twin-3 and Twin-4, respectively.
Fig. 8 (a) EDP and (b) HRTEM of plan-view
PSTO/MgO interface (c) EDP and (c) HRTEM of
plan-view LCMO/MgO interface.
Fig. 7 Cross-section TEM (a) bright-field image
and (b) EDP of the PSTO/MgO interface (c)
bright-field image and (d) EDP of the LCMO/MgO
interface.
(IV) Theory and Modeling of Self-assembling of
Nanostructured Films
Fig. 4 Hysteresis loop measurement of BZT film
exhibiting interesting abnormal properties due to
the formation of the twins.
Knowledge obtained from these series of
investigations is used for theoretical modeling
for further precisely controlling the formation
of the nanopillar structures. Structures formed
during the growth of an epilayer on a substrate
are determined by minimizing the energy of the
configuration, which consists of (1) elastic
energy of the epilayer, due to the requirement
that it be commensurate with the substrate, (2)
the surface energy of the epilayer, and (3) the
wetting potential 6. We assume that the
substrate lattice is unchanged, and hence that
there is no associated energy. The
spatio-temporal dynamics of the epilayer is
typically described using the evolution of its
height h(x,y) via
Project flow chart for interaction between team
members and overall contribution to Design of new
materials.
(I) Epitaxial (La,Sr)MnO3 Layer and Nanopillar
Structures
We have systematically investigated the effects
of temperature, pressure, laser energy and
frequency and post-annealing on the
microstructure formation of epitaxial (La,Sr)MnO3
thin films. We are able to fabricate (La,Sr)MnO3
continuous epilayer (Fig. 1a) and discrete
epitaxial nanopillars (Figs. 1b and c) by
manipulating the experimental conditions and
parameters and confirmed the repeatability for
achieving a variety of designed nanostructures. A
roadmap for fabricating various distinct
epitaxial nanostructures has been established.
where the diffusion is along the surface h(x,y)
1. Unfortunately, the expressions for the terms
on the right (the free energy density, curvature,
wetting energy etc.) in terms of h(x,y) and its
derivatives are very complicated. The analysis
can be simplified by using the small slope
expansion, which will be valid close to the
Stransky-Krastonow instability, where the
homogeneous solution destabilizes to a patterned
array 7. Under these conditions, the previous
equation reduces to
Fig. 5 Plan-view TEM (a) bright-field and
dark-field images (b), (c), (d) and (e) showing
presence of twin domain Twin-1 (T1), Twin-2
(T2), Twin-3 (T3), and Twin-4 (T4), respectively.
Figure 1. XTEM image of epitaxial (La,Sr)MnO3
continuous film (a) and nanopillars (b) on (001)
LaAlO3 substrate. (c) Plan-view TEM of epitaxial
(La,Sr)MnO3 nanopillars.
Fig. 6 HRTEM image of a plan-view TEM sample
showing coexistence of epilayer and twins.
Educational Outreach UTA established a working
relationship with the Society of Hispanic
Professional Engineers (SHPE) sponsoring six (6)
Hispanic students to participate in Pre-college
Symposia and developing a Latino Summer Camp on
UTA campus during the Summer of 2008.
In deriving this equation, we have scaled h(x,y)
by the height L at which the homogeneous layer
destabilizes. The control parameters L, g, p, and
q can be evaluated n terms of the mechanical
parameters of the substrate and the epilayer.
Under the model dynamics, a uniform (but noisy)
deposition of atoms on a substrate gives
self-assembled quantum-dot arrays. In order to
form large-scale perfect arrays, we use a
technique that we proposed previously namely
masking of the deposition 3. Properties of the
mask can be determined from the spatio-temporal
dynamics of the formation of a disordered pattern
in the absence of the mask 8.
References
1 J.C. Jiang, E.I. Meletis and K.I. Gnanasekar,
Self-organized, ordered array of coherent
orthogonal column nanostructures in epitaxial
La0.8Sr0.2MnO3 thin films, Appl. Phys. Lett. vol
80, 4831-4833, 2002. 2 J.C. Jiang, K.I.
Gnanasekar and E.I. Meletis, Composition and
Growth Temperature Effect on the Microstructure
of Epitaxial La1-xSrxMnO3 Thin Films on (100)
LaAlO3, J. Mater. Res., vol. 18, 2556-2561,
2003. 3 J.C. Jiang, L.L. Henry, K.I.
Gnanasekar, C.L. Chen and E.I. Meletis,
Self-Assembly of Highly Epitaxial (La,Sr)MnO3
Nanorods on (001) LaAlO3, Nano Letters, vol. 14,
741-745, 2004. 4 J.C. Jiang, Z. Yuan, C.L.
Chen and E.I. Meletis, Interface Modulated
Structure of Highly Epitaxial (Pb,Sr)TiO3 Thin
Films on (001) MgO Appl. Phys. Lett., vol. 90,
Art. No. 051904 (2007). 5 J. C. Jiang, J. He,
E.I. Meletis, J. Liu, Z. Yuan, and C. L. Chen,
Two-dimensional Modulated Interfacial Structures
of Highly Epitaxial Ferromagnetic (La,Ca)MnO3 and
Ferroelectric (Pb,Sr)TiO3 Thin Films on (001)
MgO Journal of Nano Research, vol. 3, 59-66
(2008). 6 B. J. Spencer, P. W. Voohees, and S.
H. Davis, Morphological Instability in
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Films Linear Stability Theory, J. Appl.
Phys. 73, 4955 (1993). 7 A. A. Golovin, M. S.
Levine, T. V. Savina, and S. H. Davis, Faceting
Instability in the Presence of Wetting
Interactions A Mechanism for the Formation
of Quantum Dots, Phys. Rev. B 70, 235342
(2004). 8 F. Shi, P. Sharma, D J. Kouri, F.
Hussain, and G. H. Gunaratne, Nanostructures
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Self-Assembly, Phys. Rev. E 78, 025203 (2008).
Fig. 10 2008 Latino Summer Camp on UTA campus
demonstration of temperature effects on the
mechanical behavior of engineering materials.