Enhanced dielectric properties from barium strontium titanate films with strontium titanate buffer layers

摘要

In order to enhance the permittivity and tunability of the dielectric component, a thin film dielectric composite consisting of a radio frequency sputtered SrTiO_3 (STO) buffer layer and metalorganic solution deposited Mg-doped Ba_xSr_(1-x)TiO_3 (Mg-BST) thin film overgrowth was developed using affordable industry standard processes and materials. The effect of the STO buffer layer thickness on the dielectric response of the heterostructure was investigated. Our results demonstrate that the composite film heterostructure, evaluated in the metal-insulator-metal configuration Pt/STO/Mg-BST/Pt on sapphire substrate, with the thinner (9-17 nm) STO buffer layers possessed enhanced permittivity (ε_r～491) with respect to the thicker 41 nm buffer layer (ε_r～360) and that of a control Mg-BST film without a STO buffer layer (ε_r～380). Additionally, the composite film with the thinner buffer layers were shown to have low losses (tan δ～0.02), low leakage characteristics (7 = 7.0 ×l0~(-9) A/cm~2), high breakdown voltage (V_(BR)＞10V), a large grain microstructure (～125nm), and smooth pin-hole free surfaces. The enhanced permittivity of the composite dielectric film resulted from three major factors: (i) the template-effect of the thin STO buffer layer on the thicker Mg-BST over-layer film to achieve a large grain microstructure, (ii) the low viscosity of the metallo-organic solution deposition (MOSD) solution, which ensured heterogeneous nucleation of the Mg-BST overgrowth film on the surface of the STO buffer layer, and (iii) minimization of the low permittivity grain boundary phase (TiO_(2-x) phase). The dielectric response of the BST can be explained using a thermodynamic model taking into account interlayer electrostatic and electromechanical interactions. Additionally, Mg doping of the BST enabled low loss and low leakage characteristics of the heterostructure. The large permittivity, low loss, low leakage characteristics, and defect free surfaces of the composite dielectric heterostructure promote tunable device miniaturization and hold the potential to enable enhanced electromagnetic coupling in ferromagnetic/high permittivity dielectric heterostructures, which in turn would facilitate the realization of integrated charge mediated voltage controlled magnetic radio frequency/microwave communication devices.