We report step-flow homoepitaxial growth of Al2O3 on m-plane sapphire at a relatively low substrate temperature of 650 °C using molecular beam epitaxy (MBE). The films exhibit atomically smooth terraces with step heights of 0.58 nm, corresponding to two-thirds of the m-plane lattice spacing. A decrease in growth rate of sapphire with increasing aluminum flux beyond a certain value is observed, a growth regime not previously reported for sapphire homoepitaxy. Oxygen plasma surface pre-treatment is found to produce highly uniform step heights and terrace widths upon MBE, enabling reproducible step-flow growth. The m-plane orientation stabilizes the growth of (110)-oriented TiN, eliminating the twin domains observed in TiN grown on c-plane sapphire. TiN films grown on homoepitaxial m-plane Al2O3 show superconductivity below Tc = 5.4 K, room-temperature resistivity of 31.9 μΩ cm, and residual resistivity ratio (RRR) of 2.7, compared to 18.2 μΩ cm and RRR = 3.3 grown directly on as-received substrates. The findings indicate that for qubit applications, m-plane sapphire homoepitaxial layers/epitaxial TiN superconductor interfaces without air exposure are feasible.
Verma et al. (2026) studied this question.