Epitaxially grown niobium nitride on Sapphire (001) and MgO (001) substrates by nitrogen plasma assisted molecular beam epitaxy

NbN with rock-salt structure (space group: F m -3 m) has attracted significant attention for Josephson electronic applications such as rapid-single-flux-quantum (RSFQ) technology due to its relatively high superconducting transition temperature (T_c ~ 17 K), ease of processing, chemical, mechanical, and thermal stability. However, previous attempts at developing NbN-based Josephson junctions have faced limitations such as poor surface/interface roughness and structural defects due to twinning particularly on c-plane (001) Sapphire (space group: R -3 c) due to lattice mismatch (~18%) and symmetry mismatch.

In this work, we aim to improve the surface morphology and crystal structure of NbN films by utilizing the low lattice mismatch (4.2%) and symmetry matching with MgO (space group: F m -3 m) substrates. The epitaxial NbN films grown using a Veeco GenXplor MBE system, were characterized by atomic force microscopy (AFM) revealing ~ 1 nm root-mean-square roughness, with crystal structure analyzed via x-ray diffraction (XRD). The T_c of ~ 12 K was measured in these films using a Quantum Design Physical Property Measurement System (PPMS). In this talk, we will compare the NbN films grown on MgO at different substrate temperatures ranging from 600^oC to 1000^oC with the NbN films grown on Sapphire. We will also compare the NbN films grown on MgO substrates with and without high temperature in situ annealing (at 1000^oC) before growth to understand the importance of surface preparation of the hygroscopic MgO substrates. Finally, by optimizing the growth conditions, we would be able to obtain high-quality, single-crystal NbN films with excellent surface morphology, providing an important foundation for the growth of Josephson junctions for advanced superconducting circuits.