Bachelors Defense: Victoria Sosnovtseva – University of Copenhagen

Bachelors Defense: Victoria Sosnovtseva

Fabrication and characterization of superconducting coplanar waveguide resonator on silicon and silicon germanium heterostructures

This thesis describes the theoretical framework, fabrication and measurements of a superconducting NbTiN resonator on silicon and silicon-germanium heterostructures. The idea is to create a circuit quantum electrodynamic architecture with spin qubits in semiconducting quantum dots coupled to a coplanar waveguide resonator, where the high quality allows the system to be in the strong coupling regime; this provides a scalable solution towards long-range entanglement of spin qubits. The thesis suggests a fabrication recipe developed for a liftoff process for making half wavelength 20 nm NbTiN resonators. Identical resonators were deposited on three different materials: silicon, silicon germanium with an etched quantum well and an implanted silicon germanium with a quantum well intact. On the implanted material the design also included accumulation gates and ohmics in order to test the accumulation of the 2DEG and its effect on the resonator. Resonators were tested in a Oxford Instruments Triton TM 200 cryogenic free dilution refrigerator at base temperature; the characterization consisted of S21 transmission measurements with input powers ranging from -40 dBm to -10 dBm, and parallel and perpendicular magnetic field sweeps in order to confirm the location of the resonance peak. Data were analyzed in order to fit a Lorentzian to the resonance peak and extract the quality factor. The quality factor for resonators on silicon and implanted Si/SiGe heterostructures was of the order 103, while the resonator on Si/SiGe with an etched away quantum well showed no resonance. The dependence of the quality factor and resonance frequency on magnetic field were also studied: the quality factor deteriorates with magnetic field while the resonance frequency decreases. Improvements to the fabrication recipe and design are discussed, which can lead to higher quality factors in the future.