Epitaxial growth of Mo/Al$_{2}$O$_{3}$/Mo trilayers for Josephson junction qubits

The growth of ultrathin epitaxial Al$_{2}$O$_{3}$ tunnel barriers on Re has been proven to reduce the number of spurious resonators in Josephson phase qubits when compared to qubits fabricated with amorphous tunnel barriers. Other refractory metals might also be used as base layers for epitaxial Al$_{2}$O$_{3}$ growth. In this work, Mo films were deposited onto Al$_{2}$O$_{3}\left( {11\bar {2}0} \right)$ substrates by UHV magnetron sputtering. To achieve epitaxy, the substrate was held at elevated temperature during the deposition. Post-deposition high temperature anneals increase the terrace size and occurrence of step-bunching as observed by STM. Ultrathin films of Al$_{2}$O$_{3}$ were then deposited at room temperature onto the Mo base layer by reactive evaporation of Al in a controlled oxygen background. A high temperature anneal in an oxygen background crystallizes the amorphous Al$_{2}$O$_{3}$ layer. Finally, the structure is capped with polycrystalline Mo by sputter deposition at room temperature. Dielectric loss measurements on LC oscillators fabricated from these structures will be reported.