Evolution of Anisotropie Elastic Strains, and rf/Microwave Dielectric Properties of <110> Textured BST 60/40 Thin
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Evolution of Anisotropic Elastic Strains, and rf/Microwave Dielectric Properties of Textured BST 60/40 Thin Films on NdGaO3 Substrates W. K. Simon1, E. K. Akdogan1, J. A. Bellotti2 and A. Safari1 1 Ceramic and Materials Engineering, Rutgers University, Piscataway, New Jersey 08854, 2 Navy Research Laboratory, Washington D.C., District of Columbia 20375.
ABSTRACT Ba0.60Sr0.40TiO3 thin films were deposited on orthorhombic oriented NdGaO3 substrates by pulsed-laser deposition. Film thickness ranged from 25 nm to 1200 nm. X-ray pole figures have shown consistent textured films with good alignement to the substrate. X-ray strain analysis indicates up to 0.5% compressive strain along the (001) direction, and weaker tensile strain along (-110). Dislocation densities, as computed from strain data, were found to be in the range 5-6x105 cm-1 along both directions. The critical thickness for dislocation formation along (001) and (-110) were found to be 5 and 7 nm, respectively. Permittivity and tunability were investigated using interdigitated capacitors in the 45 MHz-20 GHz range. Dielectric properties and tunability in the oriented films exhibited strong strain and directional properties. Tunability up to 54% was observed at moderate field levels (~ 5 kV/mm).
INTRODUCTION Barium Strontium Titanate (BST) has been intensively investigated because of its field-dependent nonlinear dielectric properties. This non-linear behavior can be quantified by the percent tunability of the material. Tunability is a measure of the difference between the zero-bias relative permittivity and the relative permittivity under an applied electric field (E). Tunabilities up to 70% have been achieved in this system, and make BST a very promising candidate for passive microwave devices, such as phase shifters and filters [1-8]. The electrical properties of BST thin films have been shown to strongly depend on intrinsic parameters such as stoichiometry, especially near the paraelectric (m3m) to ferroelectric (4mm) phase transition. Extrinsic factors, such as processing, dopants, impurities, microstructure, defects, and the state of strain of the film all have shown equal importance as composition when it comes to the properties of BST thin films [1-8]. The effects of the residual epitaxial strains on the electrical properties of BST have been investigated by growing the films on different types of substrates; such as LaAlO3 and MgO, among others. These substrates are of cubic symmetry and have a reasonable lattice mismatch and coefficients of thermal expansion [6,7], and keep the BST film under an equal biaxial state of stress. NdGaO3 (NGO) is an orthorhombic (mm2) crystal (a = 5.431Å, b = 5.499Å, c = 7.71Å) [9], with a sizable difference in lattice parameter to the BST (60/40) film (a=3.965 Å) and different in-plane thermal expansion coefficients (αNGOb = 5.6 ppm/K and αNGOc = 11.6 ppm/K compared to αBST = 10.5 ppm) [9-10]. This substrate will not grow epitaxial films under equal biaxial stress as the other substrates, but instead will result
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