Low temperature deposition of superconducting yellow aluminum thin films for quantum information applications

We explore the synthesis of superconducting aluminum films on c-plane sapphire and silicon (111) at room temperature and cryogenic temperatures of < 20 K during deposition. The films are grown in state-of-the-art low-temperature molecular beam epitaxy equipment with in-situ reflection high energy electron diffraction (RHEED) characterization and base pressure of < 5e-11 mbar. We study the structure, crystallinity and composition of the films using X-ray diffraction, RHEED, atomic force microscopy and X-ray photoelectron spectroscopy. We measure electrical DC transport under perpendicular magnetic fields and temperatures as low as 70 mK and correlate the superconducting properties of aluminum with its structure. We find that low temperature deposition increases the degree of structural disorder which changes the optical properties of the film giving it a yellow color, and also enhances the critical temperature and critical field of aluminum from Tc = 1.2 K and Hc = 100 Oe to Tc = 1.5 K and Hc = 690 Oe. We then fabricate superconducting distributed element resonators operating between 4 to 8 GHz from these films and study the microwave loss and kinetic inductance in aluminum grown at cryogenic temperatures.