Homoepitaxial sapphire for superconducting qubits

Superconducting qubits are fundamentally limited by the material loss at the interface between superconductor and dielectrics, and bulk substrate. Semiconductor industry has circumvented these kinds of losses by using a nucleation layer grown above the substrate to suppress the bulk and interface defects. One of the most promising methods of achieving this is by molecular beam epitaxy which promises defect-free material growth and atomically-abrupt heterojunction. As Sapphire is one of the widely used substrates owing to its low dielectric loss, we demonstrate the homoepitaxial MBE growth of the sapphire nucleation layer on M- plane sapphire substrates. Temperature and flux study was performed and an Oxygen-rich growth regime found to be beneficial for the growth of atomically smooth homoepitaxial layer. A growth rate of 45nm/hr hour was observed at 600C substrate temperature. Atomically smooth surface with terrace width of 35nm (~ 30 unit cells) was found using Atomic Force Microscopy. We observed that a high temperature anneal (1050C) in a furnace and an in-situ Oxygen polishing prior to growth were essential for observing atomic steps. XRD and XRR analysis revealed phase pure films. Comparative studies are being performed between superconductors (NbN, TiN etc) growth epitaxially on substrates with and without the sapphire nucleation layer by fabrication of Coplanar- Waveguide resonators to correlate material properties with the quality factor and eventually to qubit lifetimes.