NbN/MgO/NbN Josephson tunnel junctions fabricated on thin underlayers of MgO

Underlayers of MgO as thin as 8.0 nm have been used in the fabrication of NbN/MgO/NbN Josephson tunnel junctions. NbN/MgO/NbN trilayers with and without MgO underlayers were deposited on thermally oxidized Si substrates at 100 degrees C using RF magnetron sputtering in a semi-UHV (ultrahigh vacuum) load lock vacuum system. Sputtering parameters were first optimized to produce NbN with T/sub c/=14.7 K on SiO/sub 2//Si substrates, and then thin MgO underlayers were used to enhance the T/sub c/ of the trilayers to 15.7 K. X-ray diffraction of NbN films indicates that thin MgO underlayers of 8.0 nm are capable of almost completely removing the NbN [111] diffraction peak found in lower T/sub c/ films and enhancing the NbN [200] peak. MgO underlayers were found to be oriented in the [100] direction when sputtered in an atmosphere of Ar and N/sub 2/ and randomly oriented when sputtered in Ar alone. The authors present details for the preparation and analysis of NbN and MgO films as well as the fabrication and electrical performance of tunnel junctions with and without MgO underlayers.>