Alpha-Tantalum Thin Film Growth and Characterization for Qubit Applications

Due to its long coherence times, Tantalum has recently emerged as a material choice for superconducting Qubits. Tantalum crystallizes into two allotropes, the body-centered cubic alpha-phase, and the tetragonal beta-phase. The alpha phase is the allotrope of interest for fabricating superconducting resonators. Utilizing ultra-high vacuum sputter epitaxy we grow thin films of alpha-Ta on low-loss sapphire substrates, studying the nucleation, growth orientations, and native surface oxide as a function of thickness and annealing temperature. Trends in thin film morphology were determined via scanning probe microscopy for terrace size and shape. Utilizing triple-axis X-ray diffraction and density functional theory, we determine the nucleation, orientation, and lowest energy formations on c-plane sapphire. Transition temperatures and q-factors of films varying in phase, orientation, and thicknesses were found using millikelvin microwave transport measurements.