Preparation and characterization of high-quality stoichiometric LiNbO3 thick films prepared by the sol–gel method
Introduction
Lithium niobate (LiNbO3) is one of the most interesting materials extensively studied by many laboratories [1], [2], [3] because of its potential for a wide variety of application, e.g. optical waveguides [4], surface acoustic wave devices [5], optical modulators [6], etc. Crystals on the market are congruent LiNbO3 (CLN) crystals with a composition of Li: Nb=0.94: 1 and those with a stoichiometric composition (Li: Nb=1: 1) are called stoichiometric LiNbO3 (SLN). It is well known that the electro-optic and non-linear optical effects of a SLN crystal are superior to those of a CLN crystal [7]. Although it is very difficult to grow SLN crystals by the Czochralski method, it is easy to prepare SLN films using the sol–gel method. Since Hirano and Kato [8] reported that stoichiometric LiNbO3 films could be synthesized by using the LiNb(OH)6 precursor made by reacting lithium ethoxide, niobium ethoxide and water, many investigations have been conducted for the sol–gel synthesis of LiNbO3 thin films [9], [10].
However, such a synthesis was not yet employed in an industry because the sol–gel synthesis has two major problems:
- 1
The film thickness of a single layer is too small to be of practical use.
- 2
The sol is not stable, since the average precursor particle size and the viscosity of the solution increase continuously with time.
As approaches for improving the film thickness, Kozuka et al. [11], [12] recently reported that crack-free BaTiO3 and PbZrTiO3 films over 1 μm in thickness could be prepared by a single-step dip coating from an alkoxide-acetate solution containing polyvinylpyrrolidone (PVP). This result was explained as follows. Organic polymers such as PVP can be hybridized with metalloxane polymers, and this hybridization inhibits the stress-induced film cracking due to a structural relaxation occurring when the gel films are fired. Innocenzi et al. have also proposed similar ideas [13].
Therefore, we have considered that adding large amounts of protective colloids such as polyvinyl alcohol (PVA), polyethylene glycol (PEG), etc. to the precursor solution (colloidal solution) might well enhance the precursor concentration and the viscosity of the solution, so that the film thickness obtained by a single-step deposition-heat treatment process would be increased. In the dry gel film, the organic polymers such as protective colloids would act as precursor particles that relieve the stress due to structural relaxation when the gel films are fired, allowing crack-free LiNbO3 thick films to be prepared.
In this paper, we demonstrate that high-quality, stoichiometric, thick LiNbO3 films can be easily prepared by adding to the precursor solution generous amounts of polyvinyl alcohol (PVA) that act as protective colloids.
Section snippets
Experimental details
The preparation procedure for LiNbO3 films is shown in Fig. 1. The starting materials were LiOC2H5 (99.9%, Kojundo Chemical Lab. Co., Ltd.), Nb(OC2H5)5 (99.9%, Kojundo), PVA with an average molecular weight of 22 000, absolute ethanol, acetic acid and ultra-pure water. LiOC2H5 and Nb(OC2H5)5 were dissolved in the absolute ethanol, yielding a solution with the molar ratio of LiOC2H5 to Nb(OC2H5)5 being (1.00–1.20):1.00. The solution was refluxed by stirring in N2 at 77 °C for 20 h, producing the
Results and discussion
The dependence of the mole ratio of Li to Nb of the precipitates (Li/Nb) on the mole ratio of Li+ to Nb5+ of the concentration (Li+/Nb5+) in the starting solution is shown in Fig. 2. The ratio (Li/Nb) of the precipitates increases with the ratio ([Li+]/[Nb5+]), a ratio is always smaller than that of the starting solution. When the ratio ([Li+]/[Nb5+]) approaches 1.10, the ratio (Li/Nb) of the precipitates becomes 1.01±0.03. At a ratio ([Li+]/[Nb5+])>1.10 in the starting solution, the ratio
Conclusion
High-quality LiNbO3 thick films have been prepared by the sol–gel method from solutions containing 0.08 M precursor and a large amount of polyvinyl alcohol (PVA). The effect of PVA is summarized as follows:
- 1
PVA acts as a protective colloid inhibiting the growth of the precursor particles, so that the sol is stabilized for an extended period (2 weeks).
- 2
The viscosity of the sol increases with the increase in PVA concentration, so that the film thickness exceeds that for the sol without PVA.
- 3
A large
Acknowledgments
This work was partially supported by a grant from the High-Tech Research Center Establishment Project of the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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2014, Materials Chemistry and PhysicsCitation Excerpt :Though the sol–gel reaction can be performed at a mild condition, it requires caustic catalysts of nitric acid or ammonia, and is difficult to control on the crystalline structure of LN. In some previous studies, soluble organic additive was applied to control the growth of LN [19]. For example, a polymer matrix of poly(4-vinylpyridine) was used as a host matrix to regulate LN growth [9].