Electron-phonon coupling in nitride superconductors from first principles: The effect of epitaxial strain and nitrogen concentration on superconducting properties*

The recent all-epitaxial integration of NbN superconductors with the III-Nitride family of semiconductors by molecular beam epitaxy has created an opportunity for scalable, integrated semiconductor/superconductor devices with access to industrial fabrication processes [Nature 555, 183–189 (2018)]. NbN has a large superconducting critical temperature (T_c) that makes it a useful superconducting component at liquid helium temperature, but its structural, metallic, and superconducting properties are extremely sensitive to growth conditions and nitrogen concentration. Group IV-B Nitrides (Ti, Zr, Hf) are chemically more stable than NbN, but their T_c’s are markedly smaller. Can the recent demonstration of epitaxial integration of transition metal nitrides with III-Nitrides be used as a strategy for tailoring their superconducting properties?

Using first-principles approaches we explore the effect of epitaxial strain and nitrogen concentration on Group IV-B Nitride superconducting properties. We find that both epitaxial strain and nitrogen concentration can drastically alter the electron-phonon coupling and potentially increase T_c to temperature ranges sensible for superconducting devices.