Structural and Electrical Properties of Ba 0.5 Sr 0.5 TiO 3 Thin Films for Tunable Microwave Applications
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Structural and Electrical Properties of Ba0.5Sr0.5TiO3 Thin Films for Tunable Microwave Applications Sriraj G. Manavalan1,2 , Ashok Kumar2,3, T. Weller1 and A.K. Sikder2 1
Department of Electrical Engineering Nanomaterials and Nanomanufacturing Research Center, 3Department of Mechanical Engineering, University of South Florida, Tampa, FL 33620 2
ABSTRACT The primary objective of this research is to optimize the different deposition conditions to obtain high tunability and low dielectric loss of Barium Strontium Titanate (BST) thin films at microwave frequencies. Ba0.5Sr0.5TiO3 thin films were deposited on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition technique (PLD). Deposition conditions like temperature, oxygen pressure, substrate to target distance and laser energy are varied to obtain the objective. Deposition of the BST thin films on the Pt/TiO2/SiO2/Si substrates was carried out at temperatures of 450ºC, 550°C, 650ºC and oxygen pressures of 250mTorr and 450mTorr with laser fluence of 250 mJ/cm2 and 450mJ/cm2 at 10 pulses per second. The microstructural and phase analysis of the deposited BST films at different temperatures and different oxygen pressures were performed using X-ray diffraction (XRD) method. The diffraction patterns are attributed to cubic (perovskite) crystal system. Atomic force microscopy (AFM) was used to perform the surface analysis of the films deposited at different substrate to target distances, varied laser energies and oxygen pressures. The BST capacitor was fabricated using the Coplanar Waveguide Structure and the capacitance and dielectric constant were measured using the Vector Network Analyzer (VNA). Tunability of 3.1:1 and loss tangent of 0.0121 was achieved at 0.4 – 0.8 GHz. INTRODUCTION Ferroelectric thin films are being investigated as attractive candidates for applications in tunable microwave devices such as varactors, phase shifters, tunable filters and resonators due to the dependence of dielectric permittivity on the applied electric field [1-4].As the dielectric constant of the ferroelectric thin films change with the electric field, the phase velocity of the films change which allows tuning of the films. For optimum performance at microwave frequencies, the critical parameters needed are high tunability and low dielectric loss. Among the different technologies for tunable microwave applications, BST has been regarded as most promising due to its large electric field dependent dielectric constant, relatively low loss at microwave frequencies, high breakdown voltage, fast tuning speed and low cost. RF sputtering, plasma enhanced MOCVD (metal organic chemical vapor deposition), Sol-gel and PLD (pulsed laser deposition) are some of the deposition methods used to deposit BST [5-7]. PLD has been used extensively as it can reproduce the stoichiometry of the target in the deposited film. The dielectric properties of the BST thin films depend on many factors such as the Ba/Sr ratio, oxygen vacancies, grain size, strain and stress, film thickness, dopants and defe
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