S. Ono et al.: Processing of highly oriented lithium tantalate films by chemical solution deposition
Figure 10 shows SEM photographs of the surfaces of
LiTaO films prepared on sapphire (001) substrates at
examined. However, reliable data were not obtained for
the films prepared around 500 °C. The TG-DTA data
[Fig. 3(b)] suggest that carbon is retained in the
film prepared at 500–550 °C using the precursor solution
with acetic acid; this may inhibit the ellipsometric
measurements.
3
450, 550, and 1000 °C, using the precursor solution with
formic acid. The 450 °C film is free from cracks and
pores and has a smooth surface [Fig. 10(a)]. Grain
growth has taken place in the film heat-treated at 550 °C
[
Fig. 10(b)]. On the surface of the LiTaO film heat-
3
treated at 1000 °C, grains of size about 250 nm are
clearly observed [Fig. 10(c)]. The thickness of the
IV. CONCLUSIONS
Modification of a lithium and tantalum double alkox-
ide with formic acid is highly effective in preparing
transparent films by spin coating. Highly oriented crys-
5
50 and the 1000 °C films according to SEM is
about 100 nm.
Figure 11 shows XPS spectra of LiTaO films pre-
3
talline LiTaO films were successfully prepared on sap-
3
pared on sapphire (001) substrates heat-treated at 350,
5
mic acid. Differences are seen in C 1s and O 1s peaks of
the XPS spectra of the film heat-treated at 350 °C, but
there is no significant difference in the other Li 1s
phire substrates by heat treatment at temperatures above
00, and 1000 °C using the precursor solution with for-
4
50 °C. These crystalline films had a smooth surface
with no cracks or pores. Heat treatment of precursor
films at 550 °C is recommended to prepare high-quality
LiTaO films.
3
(
55 eV) and Ta 4d (230, 242 eV) spectra. In C 1s spectra
of the film heat-treated at 350 °C, the main peak is lo-
cated at 284 eV, with a further peak at 288 eV. The
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2
88 eV peak was still present in the spectra of film heat-
1
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treated at 450 °C but was absent at 500 °C (data not
shown). This C 1s (288 eV) peak is attributed to C in
the Oס
C–O group, which derives from the carboxylic
groups in the precursor molecules. Similarly, in the
O 1s spectrum of the film heat-treated at 350 °C, a
broad peak at 531 eV adjacent to the main peak (530 eV)
is attributed to O in the Oס
C–O group. Table II
summarizes the results of composition analysis by
XPS performed on the surface of the films. Films
prepared at 350 °C had an excess of lithium and re-
tained an Oס
C–O group at their surface. The films
approached the stoichiometric composition on further
heat treatment, since the composition must be stoichio-
metric when crystallization in the film is complete.
The carbon content in the 550 °C film (7.1%) is low
compared with that in the 1000 °C film (10%). Chemical
analysis verifies that a crystallized film free from
carboxylic residues can be prepared by heat treat-
ment at 550 °C.
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Table III shows refractive indices of LiTaO films pre-
3
5. S.D. Cheng, Y. Zhou, C.H. Kam, X.Q. Han, W.X. Que, Y.L. Lam,
Y.C. Chan, J.T. Oh, and W.S. Gan, Mater. Lett. 44, 125
pared on sapphire substrates using the precursor solution
with formic acid. The refractive index of the film in-
creases with heat-treatment temperature. The films pre-
pared at 550 and 1000 °C have refractive indices 2.049
and 2.097, respectively, close to the value for Z-cut
(
2000).
16. S. Ono and S. Hirano, J. Mater. Res. 16, 1155 (2001).
1
1
1
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6
12
lithium tantalate single crystal (n : 2.176). Nashimoto
reports that the refractive index of LiTaO films pre-
0
3
20. K. Kato, Doctoral Dissertation, Nagoya University, Nagoya, Ja-
pared on sapphire (001) substrates by annealing at
pan (1989), p. 81.
2
1. N. Ikeo, Y. Iijima, N. Niimura, M. Sigematsu, T. Tazawa,
S. Matsumoto, K. Kojima, and Y. Nagasawa, Handbook of X-ray
Photoelectron Spectroscopy (JEOL, Tokyo, Japan, 1991), pp. 158,
159, 202, 203, 205.
700 °C is 1.982. Our 550 °C film has a higher refractive
index, indicating that heat treatment of precursor films
at 550 °C is adequate to process high-quality LiTaO3
films. Refractive indices of LiTaO films prepared using
3
22. J.A. Dean, Lange’s Handbook of Chemistry, 15th ed. (McGraw-
the precursor solution with acetic acid were also
Hill, New York, 1998), pp. 7.45, 7.49.
J. Mater. Res., Vol. 17, No. 10, Oct 2002
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