"Growth and Microstructural Control of Epitaxial NbN Thin Films for Superconducting Applications" 

Yingkai Jiao, MSE PhD Candidate 

Advisor: Professor Haiyan Wang

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ABSTRACT

Niobium nitride (NbN) is a technologically important superconductor with a relatively high transition temperature, short coherence length, and compatibility with thin-film processing. These features make NbN attractive for a variety of applications including single-photon detectors, superconducting spintronics and quantum circuits. Over the past two decades, research on NbN thin films has expanded significantly. This growth is driven by advances in epitaxial growth techniques such as pulsed laser deposition and molecular beam epitaxy, as well as the need to integrate superconductors with diverse substrates. The polymorphic nature of NbN is crucial as it affects the sensitivity of its superconducting properties to growth windows, stoichiometry, and substrate-induced strain, which in turn impacts its application potential. Challenges remain in achieving reproducible cubic δ-NbN films with optimal performance while minimizing disorder and granularity in ultrathin limits. Recent work has also introduced vertically aligned nanocomposites, such as NbN-Co hybrids, to couple superconductivity with magnetic or plasmonic responses, opening new routes to multifunctionality. This review critically examines the processing-structure-property relationships of NbN thin films, emphasizing epitaxial growth, phase stability, and microstructural control. By summarizing the key advances in epitaxial growth, phase stability, and microstructural control, and identifying existing gaps, the analysis provides a framework for optimizing NbN as a platform for next-generation superconducting devices.