Microstructure and optical loss in epitaxial (Pb, La)TiO 3 thin films on (001) MgO
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Microstructure and optical loss in epitaxial (Pb, La)TiO3 thin films on (001) MgO Young Min Kang and Sunggi Baik Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea (Received 3 June 1997; accepted 20 August 1997)
Surface morphologies and microstructures of epitaxial (Pb12x Lax )TiO3 (PLT, x 0.00, 0.08, 0.16, and 0.24) thin films grown on (001) MgO have been investigated using SEM, AFM, and TEM. Surface roughness of PLT films varies severely with La concentration. For 0.08La-PLT film, very smooth surface has been achieved with ˚ of RMS roughness. 0.00La- and 0.08La-PLT films show 90± domain structure, 9.3 A and periodic dimension of the domain is reduced by La substitution. 0.16La- and 0.24La-PLT films show the presence of triangular grains, causing rough surface and poor crystal quality. However, they are distributed uniformly in 0.16La-PLT film while agglomerated in 0.24La-PLT film. Optical propagation losses of PLT films measured by prism coupling technique were 22.3, 6.0, 11.4, and 20.7 dBycm for x 0.00, 0.08, 0.16, and 0.24, respectively. Such a variation in optical losses seemed to be due to the surface morphology and abundance of domain boundaries that change continuously as a function of La concentration in epitaxial PLT thin films.
I. INTRODUCTION
Thin films of ferroelectric materials are potentially applicable to the fabrication of various functional devices such as switch or modulator which control optical signal in an optical integrated circuit. Its motivation is their excellent electro-optic properties. Optical signals must survive after passing through such devices. However, they are often weakened by various reasons, including absorption, scattering, or leakage. The optical propagation loss has been the most serious barrier for practical application of ferroelectric thin films to waveguide devices. Hence it must be reduced below a level of about 1 dBycm for practical application.1,2 Optical loss in waveguide devices originates from various structural defects, such as point defects, grain boundaries, misorientation, surface roughness, cladding, and so on.2 Hence fabrication of structural defect-free thin films, epitaxial or single crystalline thin film, is required for optical waveguide devices. (Pb12x Lax )TiO3 (PLT) is a candidate material for optical waveguide devices because it is a transparent ferroelectric material which, in thin film form, shows excellent electro-optic effects as a function of La concentration, x.3 Epitaxial thin films have been successfully grown on various single crystal substrates.4,5 Epitaxial thin films of PbTiO3 which is the basic component of PLT have the mixture of c-domains and a-domains forming 90±-domain boundaries,6–8 that could be one of the major internal scattering centers causing optical loss.2 We have shown that such a domain structure could be eliminated by substituting La for Pb in PLT films, J. Mater. Res., Vol. 13, No. 4, Apr 1998
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