L.A. Bendersky et al.: Microstructure and microstructural evolution in BaTiO3 films fabricated using the precursor method
dislocations. The dislocations are periodically spaced,
and the spacing is about 20 unit cells, which is consis-
tent with near-complete relaxation of the lattice mis-
match between LAO and BaTiO3. The spacing was
estimated using expression D␦ ס
(af + as)2/[4(af − as)],10
where D␦ is the repeat distance between the misfit
dislocations fully compensating misfit between the
high-temperature LAO substrate (as ס
0.3818 nm) and
the BTO film (af ס
.4046 nm). Energy filtering trans-
mission electron microscopy (EFTEM) maps of Ba and
Ti of the sample (Fig. 11) show a homogeneous distri-
bution, suggesting complete interdiffusion of these
elements.
(4) At up to 900 °C, interdiffusion is completed, and
epitaxial BaTiO3 grains continue to nucleate and grow.
Growth and impingement of BaTiO3 result in consump-
tion of polycrystalline, now metastable regions, and in
the formation of a continuous epitaxial film.
The presence of off-stoichiometry polycrystalline re-
gions in fully annealed films can be explained as (a)
residual nonstoichiometric regions, not yet consumed by
the recrystallization growth of BaTiO3, and (b) nonstoi-
chiometric regions resulting from the inhomogeneous
deposition of BaF2. The observations here further con-
firm the validity of the combinatorial precursor synthesis
technique of creating compounds from amorphous pre-
cursors as applied to this material system. On the basis of
the present results, we also believe that many other metal
oxide systems (in particular perovskites) follow a similar
evolution process starting from precursor layers.
The present findings are consistent with an earlier in-
vestigation on similarly processed samples using Ruth-
erford backscattering spectroscopy and x-ray diffraction
that most significant diffusion and the compound crys-
tallization take place at ജ700 °C.11
ACKNOWLEDGMENTS
The authors acknowledge the assistance of R.L. Parke
in TEM specimen preparation. We acknowledge support
from the NSF [DMR 0094265 and Materials Research
Science and Engineering Center (MRSEC)] at the Uni-
versity of Maryland.
IV. CONCLUSIONS
PLD deposition of TiO2 and BaF2 layers and subse-
quent annealing were used to form homogeneous epit-
axial BaTiO3 films. In this paper we have investigated
the microstructures of the films at different stages of the
annealing using cross-sectional TEM, high-resolution
imaging, energy-filtered imaging, and EELS spectros-
copy. It was shown that large epitaxial BaTiO3 grains
could be formed on a LaAlO3 substrate, and their forma-
tion process consisted of the following stages:
(1) At 200 °C, the microstructure of the precursors is
believed to be very similar to the as-deposited structure.
The TiO2 layer is amorphous, as was previously sug-
gested, and the BaF2 layer is polycrystalline, with grains
having a Ba fluorite structure and being partially
oxidized.
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