Reduced-pressure MOCVD of highly crystalline BaTiO 3 thin films
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Steven Nutt Division of Engineering, Brown University, Providence, Rhode Island 02912 (Received 25 April 1991; accepted 30 October 1991)
Epitaxial BaTiO3 films have been grown on NdGaO3[100] substrates by reduced pressure MOCVD for the first time. The substrate temperature was 1000 °C and the total pressure was 4 Torr. Electron and x-ray diffraction measurements indicate highly textured, single phase films on the NdGaO 3 substrate which are predominantly [100], with [110] also present. TEM and selected area electron diffraction (SAED) indicate two specific orientational relationships between the [110] and the [001] diffraction patterns.
BaTiO3 possesses a variety of technologically useful properties such as the r42 = r51 elements of its linear electro-optic tensor which are exceptionally large, 820 pm/V. Therefore, the intrinsic magnitude of barium titanate's linear electro-optic effect is 25 times that of LiNbOs,1 suggesting its use in integrated optics for phase modulators, couplers, and other compact optical communication elements. Other potential applications of BaTiO3 thin films include digital memory devices and capacitors which would exploit the ferroelectric effect and high dielectric constant known for this material, respectively. Although use of BaTiO3 films in these applications is highly desirable,2 realization of bulk properties and low-loss light propagation requires single crystal films. A variety of techniques have been investigated for growth of BaTiO3 films, including thermal evaporation,3-4 activated reactive evaporation,5 RF sputtering,6-7 and laser ablation8"10; however, single crystal film growth has not been achieved. MOCVD has the potential for epitaxial film growth over large areas at throughputs favorable for device manufacturing. Kwak et al. recently reported the first use of metal organic chemical vapor deposition (MOCVD) for growth of BaTiO3 films on fused silica, A12O3, Si, NaCl, and Pt.11 Preferred orientation of the film perpendicular to the surface (texturing) was observed, although none of the substrates used were lattice matched to BaTiO3. In contrast, the work reported here has concentrated on the growth of BaTiO3 on SrTiO3 [100] and NdGaO 3 [100] substrates whose lattice constants are well matched to BaTiO3, permitting the possibility of epitaxy and ultimately single crystal film growth. The BaTiO3 films were grown in an inverted vertical reactor described previously.12 The quartz reactor tube had a cylindrical configuration and the gas inlet, substrate mounting, and gas outlet were coaxial with the walls of the reactor. In addition, the operating conditions were optimized to give stagnation point flow at the 542 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 3, Mar 1992 Downloaded: 19 Mar 2015
substrate,13'14 resulting in uniform films (±5%) over the central 2/3 of the two-inch diameter susceptor. Titanium isopropoxide and Ba(thd)2-tetraglyme adduct were used as the source reagents and entered the reactor at the bottom. The titanium and barium precursor manifolds
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