Transferable Freestanding Varactor Based on a Membrane Stack for Microwave Application

Abstract

This work demonstrates the fabrication and characterization of a freestanding oxide varactor membrane designed for integration onto silicon substrates. An epitaxial varactor heterostructure composed of a 1% Mn-doped Ba$_{0.5}$Sr$_{0.5}$TiO$_{3}$ dielectric layer and a $\rm {SrMoO}_{3}$ conductive layer was grown using pulsed laser deposition on a water-soluble sacrificial layer $\rm {Sr_{3}Al_{2}O_{6}}$. After the lift-off process, the varactor heterostructure was successfully transferred onto a silicon substrate. Structural analysis confirms the high crystallinity and strain relaxation of the heterostructure after transfer. Electrical measurements reveal high tunability (n=1.7) at 120 V/$\mu \rm {m}$, a quality factor exceeding 100 at 1 MHz, and a low leakage current density well below 5 $\text{A/m}^{2}$. This approach overcomes the challenges of direct oxide growth of epitaxial varactor heterostructures on silicon, such as lattice mismatch and chemical incompatibility. These results validate the potential of freestanding varactor membranes for agile microwave and RF applications, offering a scalable route for high-performance, multifunctional devices with low energy consumption in next-generation telecommunications and wireless networks.