Nano-meter bridge with epitaxially deposited NbN on MgO film

Nano-meter(nm)-bridges with high-Tcmaterials hold great technological interest because of their smaller capacitance and expected higher IoRn-product. They are promising especially for applications such as high speed logic and high frequency radiation detectors. Also they can operate over a wide temperature range and are stable against thermal cycles as are incorporating the refractory high-Tcsuperconductors. One of the essential ingredient for high quality bridges is to have NbN films. Ultra-thin NbN films have been prepared by rf reactive-sputtering. NbN films deposited epitaxially on rf sputtered MgO films have high superconducting transition temperature Tc. Tcvalue of the film with thickness of about 5nm was about 14K, and is much higher than those deposited on Al2O3films and Si substrates. Two types of nm-bridges were reproducibly fabricated with NbN films deposited epitaxially on MgO films. The obtained IoRn-products were in a range of 0.5 to 3.6mV. The microwave-induced voltage steps were observed up to the voltage comparable to IoRn- product. The dc and ac quantum effects of the dc SQUIDs were observed in quite wide range of temperature, 4.2 to 11.4K.