The interfaces of NbN-MgO-NbN tunnel junctions

Abstract

The chemical composition of the surfaces and interfaces of NbN-MgO-NbN trilayers has been studied by x-ray photoelectron spectroscopy during the fabrication of the trilayer without breaking the vacuum. The NbN and MgO layers were prepared by rf magnetron sputtering. The results of the chemical analysis have been correlated to the electrical characteristics of the completed NbN-MgO-NbN tunnel junctions. During the deposition of the MgO barrier layer the presence of a high amount of energetic oxygen ions and atoms in the sputtering plasma results in a strong plasma oxidation of the NbN base electrode and, hence, in mixed Nb2O5-MgO barriers. The oxygen ions and atoms are generated by the dissociation of the target material and the water of the background pressure. Their amount was found to increase with increasing argon pressure during the MgO sputtering process. Also, adsorption layers of hydroxides on the MgO-target result in the formation of an uncontrollable amount of niobium oxide components at the interface between the NbN base electrode and the MgO barrier. The current-voltage characteristics of tunnel junctions with such barriers show large subgap leakage currents. Pure MgO barriers can be prepared by reducing the oxygen bombardment of the NbN films during the MgO deposition. Pure MgO barriers are oxygen deficient and easily adsorb hydroxides. These hydroxides react with the first layers of the NbN top electrode to NbO2 and NbO thereby reducing and broadening the sumgap value of the tunnel junctions. Tunnel junctions with pure MgO barriers of a nominal thickness of less than 2 nm usually have current-voltage characteristics indicative for microshorts. A special annealing procedure of the NbN-MgO bilayers prior to the deposition of the top electrode desorbs the hydroxides, transforms Mg(OH)2 to MgO without forming metallic magnesium and prevents the formation of intermediate layers of niobium suboxides and metallic shorts.