Epitaxial Ferroelectric BaTiO 3 Thin Films for Microphotonic Applications
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EPITAXIAL FERROELECTRIC BaTiO3 THIN FILMS FOR MICROPHOTONIC APPLICATIONS F.Niu, A.R.Teren, B.H.Hoerman and B.W.Wessels Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston, Illinois 60208 ABSTRACT Epitaxial ferroelectric BaTiO3 thin films have been developed as a material for microphotonics. Efforts have been directed toward developing these materials for thin film electro-optic modulators. Films were deposited by metalorganic chemical vapor deposition (MOCVD) on both MgO and silicon substrates. The electro-optic properties of the thin films were measured. For BaTiO3 thin films grown on (100) MgO substrates, the effective electrooptic coefficient, reff depended on the magnitude and direction of the electric field. Coefficients as high as 260 pm/V have been measured. Investigation of BaTiO3 films on silicon has been undertaken. Epitaxial BaTiO3 thin films were deposited by MOCVD on (100) MgO layers grown on silicon (100) substrates by metal-organic molecular beam epitaxy (MOMBE). The MgO serves as the low index optical cladding layer as well as an insulating layer. X-ray diffraction and transmission electron microscopy (TEM) indicated that BaTiO3 was epitaxial with an orientational relation given by BaTiO3 (100)//Si (100) and BaTiO3 [011]//Si [011]. Polarization measurements indicated that the BaTiO3 epitaxial films on Si were in the ferroelectric state. INTRODUCTION Future optical systems will require optical devices that can process information at bandwidths exceeding 100 GHz. Furthermore the need for integration of these devices is anticipated. One approach to integration is to utilize ferroelectrics as thin film waveguides. By using a thin film on a low index substrate, highly confining planar waveguides can be fabricated. Using epitaxial BaTiO3 (BTO) as the ferroelectric we have demonstrated low-loss waveguides and thin film electro-optic modulators [1-2]. These thin film modulators exhibited V-π products less than 2 V. To date these waveguides and electro-optic (EO) modulators have been fabricated on single crystal MgO substrates. For many applications, integration of both electronic and opto-electronic functions on the same substrate is desired. Thus utilization of silicon as the substrate for active and passive optical devices is preferred. However in order to make low-loss, high bandwidth integrated modulators, epitaxial ferroelectrics are required. The EO properties are very dependent on the detailed microstructure and domain structure of the ferroelectric layers. Thus the challenge is to fabricate low loss, epitaxial ferroelectrics on silicon that have bulk-like EO properties. In this paper, we report on the properties of epitaxial ferroelectric BaTiO3 deposited on MgO substrates and on epitaxial MgO on Si. EXPERIMENTAL Two ferroelectric thin film structures were investigated in this study. The first structure was formed by depositing epitaxial BaTiO3 by metalorganic chemical vapor deposition directly onto (100) oriented MgO substrates. The second
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