Improvement of the superconducting properties of NbN thin film on single-crystal silicon substrate by using a TiN buffer layer

Thin superconducting NbN film on silicon substrate is a promising material for use in some ultrasensitive sensors and detectors, such as the hot-electron bolometer mixer and the superconducting nanowire single-photon detector. Due to the large mismatch between NbN film and Si substrate, the NbN film has a lower critical temperature than NbN film on MgO substrate. TiN has been experimentally verified as a good buffer layer due to having the same face-centered-cubic crystallite structure as and a lattice constant close to that of the NbN film, i.e. the critical temperatures of NbN films (5-20 nm thickness) on TiN buffered silicon are about 1-3 K higher than those of NbN/Si. Such improvement can be ascribed not only to the reduction of the interface thickness in the NbN film, but also to the enlargement of the NbN grains, inherited from the TiN template layer.