Phase Composition and Microstructure as a Function of Deposition Conditions for Potassium Tantalate Niobate Twin Films G
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PHASE COMPOSITION AND MICROSTRUCTURE AS A FUNCTION OF DEPOSITION CONDITIONS FOR POTASSIUM TANTALATE NIOBATE 'l'llN FILMS GROWN BY PULSED LASER DEPOSITION C.M. COTELLa and R.E. LEUCHTNERb a Surface Modification Branch, Naval Research Laboratory, Washington, D.C. 20375 b ONT/NRL Post-Doctoral Research Associate; present address: Department of Physics, University of New Hampshire, Durham, NH 03824 ABSTRACT Potassium tantalate niobate [K(TalxNbx)O3 or KTNJ films were deposited by pulsed laser deposition (PLD) on (100)MgO substrates from targets of KTN with x=0.45. The effects of substrate temperature (300-700'C) and ambient oxygen pressure (50 and 300 mTorr) on the characteristics of films were investigated. At 500°C and 300 mTorr, films were amorphous with a few isolated, randomly-oriented crystalline grains of perovskite and pyrochlore. At 600'C and 300 mTorr, the films comprised a columnar microstructure consisting of a mixture of amorphous phase with (100) perovskite. At 650'C, films were almost entirely (100) perovskite. At 7000 C, the films were predominantly (100) perovskite, but contained a much higher fraction of pyrochlore. At 50 mTorr, pyrochlore was found in significant fractions up to 650 0 C. At 7000 C, films were predominantly (100) perovskite. Rutherford Backscattering analysis of the chemical composition showed that all the films were potassium-deficient and the tantalum to niobimn ratio exceeded that found in the targets for all deposition conditions. There appeared to be a relationship between oxygen pressure during deposition and the amount of potassium retained in the films. INTRODUCTION The isomorphous compounds KTaO3 and KNbO 3 form a continuous series of solid solutions, K(Tal_.xNbx)O3 (or KTN), with a perovskite structure. The Curie temperature varies linearly with x from about 2K (x-0) to 700K (x=l1). In the vicinity of the Curie temperature, KTN crystals display extremely high dielectric constants and large electro-optic constants,1 Furthermore, bulk crystals of KTN possess outstanding non-linear-optical properties.1 Thin films of KTN, therefore, are of interest for applications in optoelectronic integrated circuits in which active low loss thin film waveguides are required. Such applications require a high degree of structural perfection and homogeneity. Single crystal layers of KTN have been grown by liquid phase epitaxy on (100) KTaO 3 substrates 2 ,3 and by pulsed laser deposition (PLD) on (110) SrTiO 3 using a segmented target consisting of KNO 3 and KTN. 4 The segmented target approach to PLD of KTN was used to compensate for potassium deficiency in films deposited by PLD from pure KTN targets. The present study examines in some detail the relationship between deposition parameters (temperature and pressure) and the characteristics (chemical composition and structure) of KTN films deposited by PLD on (100) MgO substrates. EXPERIMENTAL PROCEDURES The PLD apparatus has been described in detail previously. 5 Briefly, the deposition chamber was a stainless steel vacuum chamber, with a base
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