Thickness and Strain Effects on RF/Microwave Properties of BST Thin Films on NdGaO 3 Substrates
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Thickness and Strain Effects on RF/Microwave Properties of BST Thin Films on NdGaO3 Substrates William Kurt Simon1, E. Koray Akdogan1, Jeffery Bellotti2 and Ahmad 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 oriented NdGaO3 substrates by pulsed-laser deposition. Film thickness ranged from 20 nm to 800 nm. Microstructural features, as evaluated with AFM and FESEM, have exhibited high quality thickness dependent topography. X-ray analyses have shown consistently textured films of high crystallinity. Permittivity, Qfactor, and tunability, were investigated using interdigitated capacitors in the 0.1-20 GHz range. Effect of film thickness on dielectric properties and tunability in polycrystalline textured films exhibited strong thickness dependence in their elasto-dielectric properties. Tunability up to 45% was observed at moderate field levels (~ 7 MV/m), while the Q-factors remained ≤40. INTRODUCTION Barium Strontium Titanate (BST) has been investigated for many years because of its field-dependent nonlinear dielectric properties. The most convenient metric for this is the percent tunability, which compares the zero-bias capacitance and the capacitance under an applied electric field (E), i.e. the tunability. This property makes BST a very promising candidate for passive microwave devices, such as phase shifters and filters as tunability up to 70% were achieved in this system [1-5]. The properties of thin films are strongly dependent on processing, stoichiometry, impurities, microstructure, and the state of strain of the film. Much work has been done on growing heteroepitaxial films of BST with various states of strain, and by using many different types of substrates such as LaAlO3 and MgO, among others. These substrates are of cubic symmetry and have a reasonable lattice mismatch, as well as coefficient of thermal expansion [15]. NdGaO3 (NGO) is an orthorhombic crystal (a = 5.431Å, b = 5.499Å, c = 7.71Å), with a sizable difference in lattice parameter to the BST (60/40) film (a=3.965 Å). This substrate will not grow epitaxial films as the other substrates, but instead will result in highly textured polycrystalline films –the subject of interest of the work presented herein. EXPERIMENTAL BST (60/40) thin films were deposited by pulsed-laser deposition (PLD) at 600°C at 100 x10 Torr in flowing oxygen on twin-free NGO substrates (10 x 10 x 0.5 mm) which were polished on both sides to optical quality. The deposition was performed with a laser pulse rate of 2Hz, and with a fluence of 2 J/cm2. The samples were immediately cooled to room temperature in 1 atmosphere of oxygen upon the completion of deposition. The thickness of the samples was varied by changing the deposition time (15 min., 1 hr., 2.5 hrs, 5 hrs, 8 hrs) in according with a linear calibration chart obtained earlier in this study. The films were then characterized for microstruc
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