Sol-gel법에 의한 박막광도파로용 LiNbO_3 박막제조에 관한 연구

다국어 초록 (Multilingual Abstract)

LiNbO_3 thin films for optical waveguides have been epitaxially grown on Al_2O_3 substrate by sol-gel process using lithium ethoxide, niobium pentaethoxide and 2-methoxyethanol. The epitaxial growth of LiNbO_3 thin films was oobtained with non-hydrolytic conditions in the sol-gel process. In order to improve the quality of LiNbO_3 thin films for optical waveguide applications. the growth of MgTiO_3 thin films as a buffer layer was investigated by sol-gel method. Epitaxial growth of MgTiO_3 thin films was also obtained using magnesium acetylacetonate and titanium isopropoxide in the sol-gel process. At non-hydrolytic conditions, MgTiO_3 this films were epitaxially grown at crystallization temperatures of 700 - 800 ℃. Subsequently, highly oriented LiNbO_3 thin films were grown on MgTiO_3/Al_2O_3. Epitaxial nature of LiNbO_3/Al_2O_3, MgTiO_3/Al_2O_3 and NiNbO_3/MgTiO_3/Al_2O_3 was examined by XRD normal scan and pole figure measurements. It was found that thin buffer layer of MgTiO_3 was required to obtain highly oriented LiNbO_3/MgTiO_3/Al_2O_3.
LiNbO_3 have been extensively studied and used for optical devices due to its various properties such as non-linearity[1], electro-optic[2], acousto-optic[3] and piezoelectric properties[4]. These devices include an optical waveguide, light modulator[5], optical switch and optical filter for optical communication as well as SAW (surface acoustic wave) devices.[6] The integrated-optic devices have been constructed on single crystal LiNbO_3 by Ti-indiffusion or proton-exchange method.[7] Recently, the fabrication and optical properties of LiNbO_3 thin films have been studied as the integration of the passive optical devices with laser sources becomes attractive in opto-electronic integrated circuits (OEIC).[8] In order to apply LiNbO_3 thin films for the optical devices, high quality LiNbO_3 thin films should be fabricated. Primary requirement for optical applications is to obtain thin films with low propagation loss, which subsequently requires high quality epitaxial LiNbO_3 thin films to reduce light scattering at grainboundaries and defects in the films.
Various methods such as rf sputtering, pulsed laser deposition[9], chemical vapor deposition and sol-gel method[10,11] have been applied to fabricate epitaxial LiNbO_3 thin films. Among them, sol-gel method is widely known to provide not only precise composition control but also solid phase epitaxial growth. Recently, epitaxial LiNbO_3 thin films have been prepared by sol-gel process using non-prehydrolyzed metal alkoxide solutions. However, the high quality of epitaxial LiNbO_3 thin films have not been obtained for optical waveguide applications due to lattice mismatch between LiNbO_3 thin films and sapphire substrate. Various efforts to improve the quality of LiNbO_3 thin films have been made. In order words, ultra-thin LiNbO_3 seed layer[10] or buffer layer(F_e2O_3)[12] has been used in the growth of epitaxial LiNbO_3 thin films.
MgTiO_3 has been known for dielectric material which has good thermal stability as well as good dielectric properties at high frequencies. MgTiO_3 has been widely used in microwave ceramic resonators.[13] To date, MgTiO_3 ahs been fabricated and studied in the form of bulk ceramic. There has been few reports on MgTiO_3 thin films fabrication and properties. In this study, we have used MgTiO_3 as a buffer layer since MgTiO_3 is ilmenite structure the same as LiNbO_3 crystal structure. Moreover, the lattice parameter of a-axis of MgTiO_3 lies between those of LiNbO_3 and Al_2O_3. Compared to the lattice mismatch between LiNbO_3 and Al_2O_3, the reduced lattice mismatch between MgTiO_3 and Al_2O_3 may lead to enhanced quality of LiNbO_3 thin films grown on Al_2O_3 with the MgTiO_3 buffer layer. This paper reports the epitaxial growth of LiNbO_3 and MgTiO_3. further LiNbO_3/ MgTiO_3 on Al_2OS by sol-gel process.
For the synthesis of LiNbOS thin films, Lithium ethoxied (Kojundo Chemical Laboratory Co., Ltd), niobium pentaethoxide(Aldrich Chemical Co., Inc) and 2-methoxyethanol distilled at nitrogen atmosphere were used. LiNbO_3 precursor solution was made by dissolving lithium ethoxide in the 2-methoxyethanol. followed by refluxing at nitrogen atmosphere for 3 hours for ligands exchange reaction. Subsequently, Niobium pentaethoxide was added to the solution, and further reacted vigorously for 24 hours. resulting in 0.25 M LiNbO_3 precursor solution. MgTiO_3 precursor solution was made using magnesium acetylacetonate and titanium isopropoxide. Instead of using metal alkoxide (magnesium ethoxide), magnesium acetylacetonate was used for the presursor solution since magnesium acetylacetonate is more stable than magnesium ethoxide at various atmospheres. Magnesium acetylacetonate was first dissolved in 2-methoxyethanol and reacted for 3 hour. Then titanium isopropoxide was added to the solution and refluxed vigorously for 24 hours. The resulting precursor solutions were made at 0.3 M. The precursor solutions in non-hydrolyzed state were spin cast at 3500 rpm on the Al_2O_3 substrates, followed by drying and further heat treatment for crystallization. The heat treatments following drying were carried out at 600 - 800℃ for 1 hour. The resulting structures of the MgTiO_3 and LiNbO_3 thin films were characterized by X-ray diffraction (e.g., normal -2 and scans). For thermal analysis. the precursor solutions were dried at 80℃ for 10 hours. Then the dried powders were heated with a ramp rate of 10℃/min Thermogravimetry analysis(TGA) of the dried powders obtained from the LiNbO_3 and MgTiO_3 precursor solution respectively revealed two step thermal decomposition, i. e., rapid decrease in the weight below 400℃ and gradual decrease above 400℃. Thermal decompositions of organic components were completed at 700℃ for LiNbO_3 powder and 650℃ for MgTiO_3 powder
Figure 1 shows X-ray diffraction patterns of LiNbO_3 thin films spin coated on Al_2O_3 substrates (c-plane) and heat treated at various temperatures. Only (006) peaks of LiNbO_3 and LiNbO_3 were observed above 450℃, indicating that the LiNbO_3 thin films grown on Al_2O_3 were highly oriented. As the crystallization temperature increases, the peak intensity of the (006) plane increases due to grain growth and the increase in the crystallinity of the films. However, the X-ray diffraction of the LiNbO_3 thin film crystallized at 750℃ exhibits a small diffraction peak around 23˚, as shown in figure 1. primarily due to Li deficient phase caused by Li evaporation. In-plane texture of the LiNbO_3 thin film was investigated by pole figure measurement. ￠ scan was carried out on the LiNbO_3 thin film heat treated at 650℃. LiNbO_3 (012) plane was used as diffraction plane. Figure 2 illustrates the result of the scan. 3 diffraction peaks were observed with a spacing of 120o and no other additional peak was observed, indicating that the LiNbO_3 thin film was grown epitaxially on Al_2O_3 (c-plane) with 3 fold axis symmetry.
Figure 3 shows X-ray diffraction patterns of MgTiO_3 thin films grown on Al_2O_3 (c-plane) at various crystallization temperatures. Only (003) peak of MgTiO_3 thin films was observed at crystallization temperatures of 650℃ to 800℃ indicating that the highly oriented ilmenite structure developed in MgTiO_3 began to develop at 650℃, no diffraction peak from the ilmenite structure was observed. It is well known that double alkoxide of LiNb(OR)_6 forms when LiNbO_3 thin films are prepared in alcohol based solution. In 2-methosyethanol solution. lithium, oxygen and niobium form networks in the form of lithium methoxyethoxide and niobium methoxyethoxide through refluxing As a result, LiNbO_3 phase can form below 650℃ at which thermal decomposition of organic groups is not completed. On the other hand. when MgTiO_3 thin films are prepared, the formation of the networks between Mg and Ti is not likely to occur primarily due to blocking effect of acetylacetone ligands in solution. In other words, acetylacetone ligands in Mg-acetylacetonate may prevent oligomerization since they behave as chelating ligands.[14] Therefore, ilmenite MgTiO_3 phase could not form below at 650℃ crystallization temperature.
Although LiNbO_3 thin films were epitaxially grown on Al_2O_3, high quality of LiNbO_3 thin films were not obtained on Al2O3 primary due to relatively large lattice mismatch (LiNbO_3 a=5.1494 Å. Al_2O_3 A=4.758 Å). MgTiO_3 has a lattice parameter (a=5.054Å) which lies between those of LiNbO_3 and Al_2O_3. Thus it is expected that the MgTiO_3 can be used as a buffer layer for the growth of LiNbO_3 thin films on Al_2O_3. In order to use the MgTiO_3 was investigated. Figure 4 shows the result of scan carried out on MgTiO_3 thin films on Al_2O_3. The MgTiO_3 thin film was heat treated at 800℃. The scan result shows the diffraction peaks obtained from (012) planes are spaced at 120˚, indicating that the MgTiO_3 thin film on Al_2O_3 also has a 3 fold axis symmetry. Thus both LiNbO_3 and MgTiO_3 thin films are epitaxially grown on Al_2O_3 (c-plane).
Further LiNbO_3/MgTiO_3/Al_2O_3 structures are fabricated in order to improve the quality of LiNbO_3 thin films grown on Al_2O_3 substrate. First, MgTiO_3 was spin coated on subatrate and heat treated at 800℃. Subsequently, LiNbO_3 was spin coated and heat treated at 650℃. The thickness of the MgTiO3 buffer layer was varied by multicoating. Figure 5 illustrates X-ray diffraction pattern of LiNbO_3/MgTiO_3/Al_2O_3 structures. in which the MgTiO_3 buffer layer was made by single spin coating. The X-ray diffraction pattern shows only (006) LiNbO_3, (003) MgTiO_3 and (006) Al_2O_3 peaks. This result indicates that highly oriented LiNbO_3/MgTiO_3/Al_2O_3 structures were obtained with thin MgTiO_3 buffer layer. When the thickness of the MgTiO_3 buffer layer increases by multicoating, polycrystalline LiNbO_3 thin films on MgTiO_3/Al_2O_3 were obtained. As the thickness of the MgTiO_3 buffer layer increases, the quality and crystallinity of the MgTiO_3 thin films will deteriorate. further influencing or degrading the quality of LiNbO_3 thin film grown on the MgTiO_3/Al_2O_3. The study on the optimum thickness of MgTiO_3 thin film and the concentration of MgTiO_3 precursor solution is in progress.
In summary, LiNbO_3 and MgTiO_3 thin films were epitaxially grown on Al_2O_3 substrate by sol-gel method. It is required to have high quality of LiNbO_3 thin films for optical waveguide applications. The MgTiO_3 thin films were used as buffer layers in order to improve the quality of subsequently deposited LiNbO_3 thin films. Consequently, highly oriented LiNbO_3/MgTiO_3/Al_2O_3 structures were obtained, in which case thin layer of MgTiO_3 was requried to obtain the highly oriented structures.
This study is support by the academic research fund of Ministry of Education, Republic of Korea.

목차 (Table of Contents)

• 1. 서론
• 2. 실험재료 및 방법
• 3. 연구결과 및 논의
• 4. 결론
• 5. 참고문헌
• 6. 발표논문 및 초록
• 7. 연구결과 활용방안
• 8. 참고 및 건의사항
• 9. 배출 인력 양성