Investigations on Sol-Gel Derived Ba0.5Sr0.5Ti1-δMnδO3 (δ = 0.0 to 5.0 at%) Thin Films for Phase Shifter Applications

Abstract

AbstractHighly (100) oriented Ba0.5Sr0.5Ti1-δMnδO3 thin films were deposited on (100) LaAlO3 (LAO) substrate by sol-gel technique. We have studied systematically the effect of Mn doping on the degree of texturing, surface morphology, dielectric properties and phase transition behavior of barium strontium titanate (BST) thin films. Up to 3 at % Mn doping the degree of (100) texturing and grain size of BST (50/50) thin films were markedly improved, which led to an increased tunability from 29% (undoped) to 39% (3 at % Mn doped); measured at 1 MHz and 2.34V/mm bias field. The transition and Curie-Weiss temperatures of BST (50/50) thin films were found to be about 266 K and 185 K respectively, which confirmed the first order phase transition in the films. The variation of transition temperatures as a function of Mn doping contents in BST (50/50) thin films were influenced by the variation of stress state and surface morphology modifications induced by Mn doping. The bias field dependence of the dielectric constant and loss tangents of undoped and Mn doped films were analyzed in terms of a model based on Devonshire theory. Phase shift measurements showed that the degree of phase shift increases from 239° to 337° with 0 to 3 at% Mn doping. The insertion loss also increases from 5.4 dB (undoped) to 9.9 dB (3 at % Mn doped) with doping content so that there is no effective improvement in the k factor, which remains in the range of 33 - 44°/dB. Modification in surface morphology and film stoichiometry induced by Mn doping is thought to play significant role in observed phase shifter characteristics.