Strained LiNbo 3 /Sapphire Heterostructures Grown by Pulsed Laser Deposition
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LiNbO3 /SAPPHIRE HETEROSTRUCTUJRES BY PULSED LASER DEPOSITION
GROWN
Yoshihiko Shibata*, Naohiro Kuze*, Masahiro Matsui*~, Masaki Kanai**, TomoJi Kawai ** * Central Laboratory, Asahi Chemical Industry Co., Ltd. Samejima, Fuji, Shizuoka, 416, Japan **The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka, 567, Japan ABSTRACT by
Ar
F pusdlaser
ablation.
The films
are
evaluated
by
X-
ray diffraction analysis at various temperatures, as well as high-resolution transmission electron microscopy (TEM) . The deposited films are highly epitaxial but that are strained, that is, the a-axis is longer and the c-axis is shorter than those of LiNbO3 single crystals. X-ray diffraction analysis at deposition temperature, as well as TEM show that the strain is caused by the difference in thermal expansion coefficients between LiNbO3 and sapphire substrates. INTRODUCTION Recently, there has been increasing interest in the utilization of epitaxial ferroelectric films, such as LiNb0 3 and BaTi0 3 , to fabricate various devices. LiNb0 3 is one of the most fascinating materials because of its excellent piezoelectric properties. Bulk LiNb08 single crystals have been used for surface acoustic wave (SAW) devices, such as filters or convolvers. High-quality LiNbO3 films on sapphire substrates
enable high-performance
surface acoustic
wave
(SAW)
devices
because the results of theoretical calculation showed that SAW properties of LiNb03 and LiTa0 3 films on sapphire substrates are superior to those oT LiNb0 3 and LiTa03 single crystals [1,2,3]. These results were calculated using materials constants of LiNb0 3 , LiTa08 and sapphire single crystals. Therefore, it is very important to form high-quality epitaxial films for achieving the high-performance SAW properties of LiNb0 3 or LiTa0 3 films. Forming high-quality LiNb0 3 films on sapphire substrates is very difficult because differences in lattice parameters and thermal expansion coefficients between LiNb03 and sapphire are very large as shown in Table I. Epitaxial LiNb0 3 and LiTa0 3 films on sapphire substrates have been formed by chemical vapor deposition, sputtering, sol gel and pulsed laser deposition (PLD). However, piezoelectric LiNb08 and LiTaO8 films have been grown only by PLD[4][5]. PLD is one of the best techniques to form epitaxial LiNb08 films. In this work, we formed high-quality LiNb08 films on sapphire substrates by PLD and researched the epitaxial properties, such as crystallinity and strain structure, of the films. 199 Mat. Res. Soc. Syrup. Prec. Vol. 397 ©1996 Materials Research Society
Table I Lattice parameters and thermal expansion coefficients Lattice parameters a (nm) c (nm) LiNbO 3 Sapphire
0.51483 0.4758
Thermal expansion coefficients a (ppm/deg.) c (ppm/deg.)
13.863 1.2991
15.4 7.1
5.3 8.7
EXPERIMENT Figure 1 shows a schematic representation of the apparatus for pulsed laser deposition. Table II summarizes the deposition conditions. The films were deposited on (001) sapphire single crystals. The film
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