Single crystal nm-thick aluminum superconducting microwave resonators with internal quality factors above one million

Single-crystal thin films with a highly ordered atomic structure can host controlled interfaces to their adjacent layers, as the key to reducing dielectric loss in superconducting quantum circuits. In this work, we have demonstrated single-crystal aluminum (Al) thin films on sapphire substrates with very high crystallinity and high internal quality factors (Qi). Using in-situ oxide deposition instead of a conventional oxidation process, we have achieved a well-controlled interface between oxide and Al film and protected these nm-thick Al films from oxidation in the ambient.

High crystallinity of single-crystal Al films was characterized using synchrotron radiation X-ray diffraction. Clear Pendellosung fringes were observed, indicative of high crystallinity of the Al films and small interface roughness (< 0.5 nm) between the Al films and the adjacent layers. The full-width at half-maximum (FWHM) of θ-rocking curves of the Al films are record-low values of 0.015 degrees (54 arcsec) from samples with thicknesses from 3 nm to 20 nm-thick. The samples exhibited smooth surfaces and interfaces confirmed by X-ray reflectivity (XRR) and atomic force microscope. Microstrip resonators made from such single-crystal Al films reveal internal quality factors over 1 million around 7 GHz in the low power limit at 10 mK. Such single-crystal materials offer excellent potential of low dielectric loss, thus improving the coherence of superconducting quantum circuits.