Fully transparent thin-film varactors: fabrication and performance

Abstract

We report the fabrication of fully transparent thin-film varactors using a dielectric material, barium strontium titanate (Ba_0.6Sr_0.4TiO₃, BST), as an electric field tunable layer, transparent conductive oxide (Sb doped SnO₂, ATO) as a bottom electrode and ATO/Ag tri-layer film as a top electrode. This kind of varactor using ATO/Ag/BST/ATO capacitors shows a transmittance of about 80% (including the substrate) in the visible region, a medium capacitance of about 463 pF, a low loss tangent of about 0.015, and superior tunable dielectric properties at room temperature. Calculations of dielectric tunability and figure of merit (FOM) display a maximum value of ∼53% at 560 kV cm⁻¹ and ∼66, respectively. We believe that the transparent thin-film varactors using ATO/Ag/BST/ATO capacitors presented in this paper could be a milestone for future see-through electronic devices.