Homoepitaxial growth of sapphire for superconducting circuits by molecular-beam epitaxy

C-plane sapphire with low dielectric loss is used to fabricate superconducting microwave circuits. Dielectric losses in the substrate can be potentially reduced by using a thick homoepitaxial sapphire layer. Towards that goal, we test various conditions to achieve homoepitaxial C-plane sapphire by molecular-beam epitaxy (MBE). Thin films deposited by MBE on C-plane and C-M miscut sapphire substrates were found to remain polycrystalline for growth temperatures ranging from 400 C to 1000 C. Hence, further growths on C-sapphire substrates of different miscut directions and angles at higher temperatures would be needed. Moreover, the dielectric loss in superconducting resonators fabricated on such epitaxial crystalline C-plane sapphire can be measured, and compared with that in resonators on substrates without homoepitaxy.