Effect of substrates on the growth and properties of LiNbO 3 films by the sol-gel method
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Dong H. Won and Kwangsoo No Department of Ceramic Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, Republic of Korea (Received 27 September 1993; accepted 27 October 1993)
Thin films of LiNbO3 were fabricated on sapphire(012), MgO(OOl), and S i ( l l l ) substrates by the sol-gel process. Under optimized conditions, films deposited onto sapphire(012) were epitaxially grown. Preferred orientations, however, were not observed in the films on MgO(OOl) and S i ( l l l ) by x-ray diffraction measurements. Morphology of the epitaxial films on sapphire(012) was examined by scanning electron microscopy, which indicated that the films were smooth and had a pore-free surface. Electrical and optical measurements on the epitaxial films revealed that the properties of the films were very similar to those of the single crystalline LiNbO 3 , while films deposited onto S i ( l l l ) did not show any orientational behaviors. The highest quality films with epitaxy were obtained only on sapphire(012). The remaining substrates appeared to be not suitable for growing epitaxial LiNbO3 films by the sol-gel method.
I. INTRODUCTION
II. EXPERIMENTAL SECTION
During the past few decades there has been great interest in the crystalline lithium niobate (LiNbO3) compound, which is due to its application in numerous electro-optical devices such as optical modulators, surface acoustic wave devices, and so on.1"3 Usually, large size and pure single crystalline materials are necessary to make the devices. Single crystals of LiNbO3 with high optical quality, however, are not easily grown because the compound has a large solid solution range and tends to grow with variable composition.4 Fabrication of LiNbO3 thin films is readily obtained by rf sputtering,5 sol-gel,6'7 molecular beam epitaxy,8 laser ablation,9 and liquid phase epitaxy methods.10 These films can be utilized as alternative materials for the optoelectronic devices. The thin film devices are of interest since they can directly integrate onto semiconductors and largely reduce the difficulty associated with optical interconnection. LiNbO 3 films investigated in this work were produced by a sol-gel method. Major advantages of the sol-gel process are ease of stoichiometry control and low sintering temperature.11 These two factors are especially important in the fabrication of LiNbO3 thin films. Since lithium is very volatile, reducing the reaction temperature is essential to prevent evaporation of lithium and control the stoichiometry.
Lithium isopropoxide (99.9%) and niobium pentaisopropoxide (99.99%) were purchased from Kojundo Chemicals and handled in an argon-filled dry box or under inert atmosphere to avoid contact with moisture. Methanol (Mallinckrodt) was distilled from CaH2 and stored over dried molecular sieves (Linde) which were activated by heating at 350 °C for 20 h under vacuum prior to use. The preparation of a mixed metal alkoxide precursor solution is briefly illustrated in Scheme 1. A 0.5 M stock solution was prepared by mixing two alk
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