NbN Layers Research Articles

Oxidation and nitridation of niobium in air above 1150�C

When niobium is heated in static air, layers of oxides and nitrides form on the surface, depending on the temperature. At 1150–1500°C, a layer of solid Nb2O5 forms with linear growth kinetics. At 1500–1900°C, a layer of Nb2N forms under a two-phase layer of liquid oxide+solid NbO2. The two-phase oxide layer has linear growth kinetics, while the Nb2N layer has parabolic kinetics. Above 1900°C, successive layers of Nb2N, NbN, and liquid oxide form, with both nitrides exhibiting parabolic growth kinetics. The activation energies of layer growth are as follows: Q=105 kJ/mol for Nb2O5, 120 kJ/mol for the two-phase oxide layer, and 220 kJ/mol for Nb2N (Q for liquid oxide and NbN layer formation was not determined). Above 1500°C, the nitride layers are somewhat protective, in that they slow the oxide formation rate and prevent ignition.

The interfaces of NbN-MgO-NbN tunnel junctions

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.

Formation of polyhedral voids at surface cusps during growth of epitaxial TiN/NbN superlattice and alloy films

Epitaxial TiN/NbN(001) superlattice thin films with periods λ between 0 (alloy) and 9.6 nm have been grown by ultrahigh vacuum reactive magnetron sputter deposition on MgO(001) substrates in mixed Ar/N2 discharges. Cross-sectional transmission electron microscopy was used for characterizing layer and defect structure. Coherency strain relaxation close to the film/substrate interface resulted in nonplanar growth with surface cusps and the creation of apparent column boundaries at which TiN and NbN layers curved towards the substrate, but across which the basic lattice was ordered. Also, a fraction of the boundaries were associated with threading defects along the growth direction. The column boundaries and threading defects were decorated by voids that exhibited an orthorhombohedral shape, were elongated in the growth direction, and typically had flat surfaces on {100} planes. The origin of voids is suggested to be through self-shadowing at surface cusps in combination with limited adatom mobility. The voids subsequently attained equilibrium shape as the result of surface and bulk diffusion. The void density varied with λ and the void length in the growth direction was generally an integer number of periods, indicating that the void formation was influenced by the superlattice layers. The formation of limited voids is suggested to be characteristic of thin film growth close to the epitaxial temperature.

Role of nitrogen ions in ion‐beam reactive sputtering of NbN

Using ion‐beam reactive sputtering of a niobium target, we have studied the effects of energetic‐nitrogen‐ion bombardment on the target reaction and on the resulting NbN film properties. Nitrogen is either added into the ion source with the noble gas to obtain a beam of nitrogen and argon ions, or injected directly into the chamber as neutral molecules so the ion beam is composed of essentially all argon. The target reaction rate is seen to be controlled by the adsorbed thermal nitrogen, and only minimally affected by the presence of ionized nitrogen. Thus, argon‐ion bombardment of the target is responsible for stimulating the reaction between the adsorbed nitrogen and the metal target producing the NbN layer. However, the film properties are affected by the presence of nitrogen ions. Films grown with N2 added in the ion source have a higher resistivity and lower superconducting transition temperature than films grown with N2 injected directly into the chamber. These differences increase with N2 flow; the differences are attributed to damage of the growing film by energetic nitrogen reflecting from the target, as measured by an energetic‐neutral‐particle detector.

Irradiation-induced texture variations and atomic displacements in Au and NbN layers

Irradiation of Au and NbN sputtered layers with 600 keV Ar2 + ions leads to a reduced orientation distribution of the [111] direction of the Au layers from a full width at half maximum (FWHM) of the orientation distribution of 12° to a FWHM of 6° and to only very small FWHM changes in the texture distributions of the NbN layers. However, the FWHM of the distribution of the reflections of the irradiated NbN layers is increased significantly from Δθ = 0.65 to 1.17° for one sample position. This is caused by small atomic displacements in the NbN lattice. The FWHM of the reflection distribution of the Au layers increased from Δθ = 0.60 to 0.65° after irradiation. Oblique incidence of Ar2 + ions causes, by absence of channeling, stronger distortions than perpendicular incidence.

Improvements in the properties of NbN films through the presence of small amounts of aluminum during sputtering

Pure phase B1 NbN layers could be sputtered in our laboratory only with a maximum T c of 15.9 K and a residual resistance ratio RRR of <0.9. Samples with a T c of up to 16.7 K invariably had a large fraction of hexagonal foreign phase. If under similar conditions one sputters layers in the presence of small amounts of Al, one obtains pure B1 phase NbN layers with a maximum T c of 16.9 K. The RRR rises to values of up to 1.16. The specific resistance drops from typical values of around 400 μωcm to values as low as 73 μωcm. The width of the X-ray lines drops, indicating increased grain size. Al acts like a catalyst, being substituted with less than 0.03 at% for the high T c layer. When Al is substituted for Nb in detectable quantities both T c and the lattice parameter drop. To verify the above results the experiment was repeated in a magnetron rather than a RF sputter system with similar outcome.

Rf characterization of thermally diffused superconducting niobium nitride

We have prepared niobium nitride by a thermal diffusion technique of nitrogen into bulk niobium at 800 °C. The resulting NbN layer has been characterized at both low and high frequencies. We obtained transition temperature Tc=11.5 K, ‘‘penetration depth’’ λ0=85 nm at 9 GHz, and surface resistance Rs evaluated at 5.5 K and 9 GHz of 2.8×10−5 Ω. Moreover, this high-frequency Rs measured for thermally diffused NbN is better than the Rs of Nb, and is comparable to the Rs of sputtered NbN. Our diffusion technique is well adapted to all kinds of shapes. In addition, improved diffusion conditions allow Tc as high as 16 K.

Critical current enhancement in NbN/AlN multilayers

Measurements are presented of the critical current density, Jc, and parallel upper critical field, Hc2∥, for the refractory superconductor/insulator multilayer system NbN/AlN. A dramatic increase in Jc, together with an increase of Hc2∥, is observed with decreasing NbN layer thickness. The Jc enhancement arises from flux pinning in the AlN or at the NbN-AlN interface, not at defects in the NbN structure. The results indicate that the development of superconductor/insulator composites is a viable strategy to increase the critical current. Thermal stabilization which does not degrade the very high Jc values was also accomplished in a novel structure by addition of copper layers within the superlattice.