Microstructural Architecture of (Ba,Sr)TiO 3 Thin Films for Tunable Microwave Applications
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ABSTRACT BaxSrl-xTiO 3 (BST, x=0.5 and 0.6) thin films have been deposited onto (100) MgO single crystal substrates by pulsed laser deposition (PLD). The room temperature capacitance and dielectric quality factor (Q=l/tan8) have been measured as a function of electric field (< 100 kV/cm) at microwave frequencies (I to 20 GHz) using silver interdigitated electrodes deposited on top of the BST film. It has been observed that the dielectric constant of the film and its change with electric field are closely related to film phase (amorphous to crystalline phase) and film strain which affects the ionic polarization of the film. Amorphous BST films show high dielectric Q (> 100) with low dielectric constant (-30-200) and low dielectric tuning (< 1%), presumably due to small ionic polarization. Crystalline films have a higher dielectric constant (-1000-3000) and a higher dielectric tuning (- 65%) but a lower dielectric Q (-20). As an optimal microstructure of the film for tunable microwave applications, strain-relieved largegrained (-5000 A) randomly oriented polycrystalline films were deposited using a thin amorphous buffer layer of BST (-50 A). Very large grains (size up to a few microns) were observed in BST films prepared using a thicker amorphous buffer layer (-500 A). We will present results on how careful control of microstructure can lead to films with optimal dielectric properties for the tunable microwave devices.
INTRODUCTION BaxSrl-xTiO 3 (BST) is a solid solution ferroelectric material that exhibits an electric field dependent dielectric constant and (Ba,Sr) composition dependent Curie temperature [1]. These properties are currently being used to develop high frequency (1-20 GHz) tunable microwave devices for room temperature applications. Since the early stage of this research, the ideal form of ferroelectric thin films for tunable microwave devices was believed to be a highly oriented single crystalline phase. However, we have observed that these materials exhibited either high tuning or high Q but not both at the same time, as in Figure 1(a). The general trend indicates that the epitaxial single crystalline materials may not be the ideal form of BST films for this application. The inverse relationship between dielectric tuning and dielectric Q shown in Figure l(a) is due to the Kramer-Krdnig relationship, film strain, and film crystallinity. In this paper, we will discuss the ideal microstructure of BST thin films for tunable microwave applications.
181 Mat. Res. Soc. Symp. Proc. Vol. 603 0 2000 Materials Research Society
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Figure 1 (a) A general trend in the dielectric properties of highly oriented and single crystalline phase BST thin films and (b) an ideal form of BST thin films providing both high tuning and high Q.
EXPERIMENTAL BST (x=0.5 and 0.6) thin films (-0.5 jim thick) were grown on (100) MgO single crystal substrates at 750 'C in an oxygen ambient gas by pulsed laser deposition (PLD). The output of a shor
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