Epitaxial growth of ultra-thin NbN films on AlxGa1-xN buffer-layers

The suitability of AlxGa1-xN epilayers to deposit onto ultra-thin NbN films has been demonstrated for the first time. High quality single-crystal films with 5 nm thickness confirmed by high resolution transmission electron microscopy (HRTEM) have been deposited in a reproducible manner by means of reactive DC magnetron sputtering at elevated temperatures and exhibit critical temperatures (T-c) as high as 13.2 K and residual resistivity ratio (RRR) similar to 1 on hexagonal GaN epilayers. On increasing the Al content x in the AlxGa1-xN epilayer above 20%, a gradual deterioration of T-c to 10 K was observed. Deposition of NbN on bare silicon substrates served as a reference and comparison. Excellent spatial homogeneity of the fabricated films was confirmed by R(T) measurements of patterned micro-bridges across the entire film area. The superconducting properties of these films were further characterized by critical magnetic field and critical current measurements. It is expected that the employment of GaN material as a buffer-layer for the deposition of ultra-thin NbN films will prospectively benefit terahertz electronics, particularly hot electron bolometer (HEB) mixers.