Laser Deposited Epitaxial Oxide Heterostructures as Prototype Ferroelectric Optical Waveguides
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LASER DEPOSITED EPITAXIAL OXIDE HETEROSTRUCTURES AS PROTOTYPE FERROELECTRIC OPTICAL WAVEGUIDES D. K. Fork and G. B. Anderson Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT The pulsed laser deposition process is a powerful tool for investigating prototype epitaxial structures. This report outlines recent developments in epitaxial structures which may usefully serve as ferroelectric optical waveguides. Emphasis is given to structures on semiconductor substrates, motivated by hybrid optical/semiconductor integration. Earlier pulsed laser deposited structures, such as BaTiO 3 /MgO/GaAs (100) are discussed in conjunction with current results on Z-lithium niobate on GaAs (111)A and GaAs (111)B. BaTiO3/MgO/GaAs (100) grows with cube-on-cube crystallography. The epitaxial system z-lithium niobate on GaAs (111)A and GaAs (111)B has been demonstrated both with and without intermediate MgO (111) layers. The in-plane epitaxial relationships are LiNbO 3 [110] // GaAs [211] and [211] indicating the existence of 1800 boundaries in the LiNbO 3 with and without the MgO layer, which grows cube-on-cube with the GaAs. Out-of-plane texture is typically 1.00 and 1.20 for the MgO and LiNb0 3 layers respectively. In-plane texture is typically 2.80 and 4.5° for MgO and LiNbO 3 layers respectively. These and similar epitaxial systems may be useful for monolithic electro-optic or frequency doubling applications in conjunction with semiconductor laser sources. INTRODUCTION Considerable technology hurdles must be crossed before semiconductor devices monolithically integrated with oxide ferroelectrics can become a practical reality. The mainstay of research on ferroelectric thin films on semiconductors has been nonvolatile memory applications, however, noteworthy efforts toward optical applications continue to appear.l, 2 Optical applications add several requirements in addition to those needed for memory structures. Typically, confined optical modes are needed, hence a cladding layer of low index must be provided to separate the semiconductor (na- 3.8) from the ferroelectric (n - 2.2). The uniaxial or biaxial nature of ferroelectrics (LiNbO 3 , BaTiO 3 , PbTiO3 , KNbO 3 etc.) implicitly produce the requirement for either highly oriented or epitaxial films to permit usage of tensor properties and to minimize light scattering. Surface roughness and uniformity is more constrained for optical applications, where thickness variations cause losses and/or propagation constant variations depending on their spatial frequency. 3 Certain themes are common to ferroelectric memory and optical applications. For example, integration with Si or GaAs impose limits on the processing temperature, the passivity of the materials, thermal expansion effects, and in most cases, the dielectric and ferroelectric characteristics which are intimately tied to film crystallinity and composition. Mat. Res. Soc. Symp. Proc. Vol. 285. @1993 Materials Research Society
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Pulsed laser deposition (PLD) is simultaneously a powerful yet li
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