Transferable Freestanding Varactor Based on a Membrane Stack for Microwave Application

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.5}$</tex-math></inline-formula> Sr<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{0.5}$</tex-math></inline-formula>TiO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> dielectric layer and a SrMoO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> conductive layer was grown using pulsed laser deposition on a water-soluble sacrificial layer Sr<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub>. 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 (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i>=1.7) at 120 V/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m, a quality factor exceeding 100 at 1 MHz, and a low leakage current density well below 5 A/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>. 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.