Niobium Nitride Thin Films Research Articles

Room temperature deposition of superconducting NbN for superconductor–insulator–superconductor junctions

Reactive dc magnetron sputtering is used to deposit hard, refractory, superconducting thin films of niobium nitride on amorphous (glass) as well as crystalline (sapphire) substrates held at room temperature. Stoichiometric NbN films so obtained possess the desired B1 crystal structure with a distinct (111) texture, and exhibit high (∼16 K) superconducting transition temperature. Room temperature deposition of the high Tc films is achieved primarily by optimizing the reactant fluxes of niobium and nitrogen reaching the substrate, through a systematic study of the nitrogen consumption versus injection characteristics which are found to be an absolute indicator of the quality of NbN formed. Superconductor–insulator–superconductor (SIS) junctions are fabricated by using high Tc NbN as the base electrode, a thin layer of native oxide grown in oxygen plasma or in room air as the insulator, and vacuum deposited Pb as the counterelectrode. The current versus voltage (I–V) characteristics of the junctions exhibit high (∼110) nonlinearity parameter and the superconducting gap parameter for NbN, Δ∼2.8 meV. The transition temperature, superconducting gap parameter, and nonlinearity parameter of the junctions are identical for the films deposited at room temperature with predominant (111) texture as well as the films deposited at elevated temperatures possessing (200) texture.

High T c superconducting NbN films deposited at room temperature

Thin films of niobium nitride with superconducting transition temperature (Tc ) of 15.7 K have been deposited on a variety of amorphous as well as crystalline substrates including glass, glazed ceramic, fused quartz, and sapphire, maintained at room temperature, by dc reactive magnetron sputtering in a mixture of Ar and N2 gases. The effects of the deposition conditions, particularly the carrier gas pressure and composition, on the film crystal structure, orientation, and resistivity have been studied in an effort to maximize the superconducting transition temperature. A study of the variation of nitrogen consumption with nitrogen injection pressures for constant background argon pressures is conducted and found to be an absolute indicator of the NbN formation systematics. Initially, the consumption increases linearly with the injection pressure but beyond a certain threshold, it shows a distinct drop. The desired high Tc NbN with B1 crystal structure is formed in the vicinity of this turning point of the reactive gas consumption-injection characteristic. High Tc films possess B1 (fcc, NaCl-type) crystal structure as revealed by their x-ray diffraction patterns. An initial increase in the injection pressure of the reactive gas (N2) results in a remarkable increase in the (111) diffraction line intensity along with an increase in the film Tc. This trend continues up to the turning point of consumption-injection characteristic, beyond which the crystal structure distorts into the substoichiometric tetragonal phase with a consequent reduction in the transition temperature. A general protocol for studying the formation systematics of transition metal nitrides has thus emerged.

Niobium nitride thin films for use in Josephson junctions

The properties of rf diode and magnetron reactively sputtered NbN films have been studied under a variety of preparation conditions. The aim of this investigation is to achieve high transition temperature, low resistivity films under conditions suitable for use in all refractory tunnel junction fabrication. We have systematically varied the relative amounts of Ar, N 2 and CH 4 gases, and the substrate temperature used during film growth. The transition temperature, resistivity, lattice parameter and crystal structure have been studied and correlated with the partial pressure of methane used during sputtering. The crystal structure was investigated using diffractometer and Read camera photographic X-ray techniques. We have prepared NbN films using both rf diode and magnetron sputtering with resistivities less than 70 μΩ-cm and transition temperatures greater than 16 K. The lattice parameter for our NbN films ranges between 4.39 A and 4.45 A and is dependent upon the amount of nitrogen and carbon used in the film preparation. We are currently investigating all refractory tunnel junctions with artificial barriers using these films as base electrodes and niobium counter electrodes.

Elaboration et caractérisation en hyperfréquences de films minces de NbN

Thin films of niobium nitride have been deposited onto niobium substrates by dc-magneton reactive sputtering and their surface resistance and penetration depth measured in the high frequency domain (10 GHz) as a function of temperature and film thickness. The experimental device allows the study of various samples during the same run. Results obtained confirm the superiority of this sputtering technique over classical ones, and show very good agreement with theory and with results provided by other measurement methods Preparation des couches minces par pulverisation reactive a cathode magnition en courant continu sur des supports de Nb et caracterisation a 10 GHz: resistance de surface et profondeur de penetration des champs en fonction de la temperature et de l'epaisseur de la couche

Properties of niobium nitride-based Josephson tunnel junctions

High T c (16.8 K) niobium nitride thin films have been prepared by rf diode sputtering in N 2/Ar atmosphere and subsequent high temperature annealing. Josephson tunnel junctions have been made by thermal oxidation of the films. The geometry is defined by high resolution photolithography. The Josephson junctions have been characterized by magnetic field diffraction measurements and other techniques and are shown to be particularly suitable for applications especially in superconducting microwave integrated devices.

The effects of methane in the deposition of superconducting niobium nitride thin films at ambient substrate temperature

Thin films of the niobium-carbon-nitrogen system have been prepared at ambient substrate temperature by rf diode reactive sputtering in an argon-nitrogen atmosphere with controlled amounts of methane added to the sputter gas. Superconducting transition temperatures ranged from approximately 11 to 15.85 °K. Auger and x-ray diffraction analysis indicate that all films were of the single phase B1 structure with a small amount of β-phase hexagonal structure in the very low carbon containing films. A correlation of the superconducting properties, room temperature sheet resistance, preferred crystallite orientation, and film composition was observed. The results of this investigation show that high Tc niobium nitride/niobium carbonitride thin films can be prepared at ambient substrate temperatures with the proper amount of nitrogen and methane partial pressures during film deposition. These films have potential application for the fabrication of high Tc Josephson tunnel junctions.

Superconductive tunneling into NbN deposited near room temperature

We have developed the ability to deposit niobium nitride thin films routinely having a superconducting transition temperature of 14 K by dc reactive magnetron sputtering onto substrates held near room temperature (Ts <90 °C). This allows the use of conventional photoresist liftoff techniques for patterning. Pb-counterelectrode tunnel junctions formed on these films show excellent V-I characteristics, suitable for device applications and offering potential advantages over conventional Pb-alloy/Pb-alloy and Nb/Pb-alloy junctions. These high quality junctions are also suitable for tunneling investigations of superconductivity in the Nb-N system.

3957. Niobium nitride thin film SQUID's biased at 20 MHz and 9.2 GHz. (USA)

3957. Niobium nitride thin film SQUID's biased at 20 MHz and 9.2 GHz. (USA)

Niobium nitride thin-film SQUID’s biased at 20 MHz and 9.2 GHz

Niobium nitride thin films have been reactively sputtered onto cylindrical quartz substrates. Films 500 Å or thicker, exhibited superconducting transition temperatures in the range 15–16 K, thinner films had lower Tc’s. Good 20-MHz SQUID behavior was noted in samples incorporating variable thickness weak links in which the ’’local’’ critical temperatures are depressed to about 12 K. Some of these SQUID’s exhibited nearly intrinsic magnetic flux noise. Correlations were obtained between the ratio of quasiparticle diffusion distance to the zero-temperature coherence length and the quality of the SQUID behavior.

1298. Structure and composition of thin films of niobium nitride and carbonitride obtained from reactive cathodic sputtering: A Aubert and J Spitz, Vide, 30 (175), 1975, 1–7 ( in French)

1298. Structure and composition of thin films of niobium nitride and carbonitride obtained from reactive cathodic sputtering: A Aubert and J Spitz, Vide, 30 (175), 1975, 1–7 ( in French)

Some superconducting and normal state properties of niobium nitride thin films

We have measured the temperature dependence of the resistance, the superconducting transition temperature, and the upper critical field of niobium nitride thin films. Our samples were made by sputtering in a nitrogen-argon ion beam. The pressure during deposition is typically one to two orders of magnitude lower than that of standard sputtering techniques.

Propriétés et structure des couches minces de nitrure de niobium élaborées par pulvérisation cathodique réactive I. Élaboration et propriétés

The formation of thin films of niobium nitride by reactive sputtering is described. The influence of the various parameters employed in the preparation on the composition, structure and electrical properties (resistivity, temperature coefficient and critical temperature) of these films has been investigated. The maximum observed critical temperature (15.85 K) corresponds to the formation of the stoichiometric compound niobium nitride. The existence of a material of composition Nb 1 − xN (0 < x < 0.34) with the same structure as δ-NbN has also been observed.

Notes on the negative temperature coefficient of resistance and superconductivity of niobium nitride thin films

NbN thin films have been studied which exhibit both a negative TCR in the normal state and superconductivity. The disappearance of superconductivity along these films appears to be caused by the onset of strong localization of the electronic states in the interislandic regions.

Relationship between Substrate Temperature, Structure, and Superconducting Properties of Reactively Sputtered Niobium Nitride Thin Films

Niobium nitride thin films have been reactively sputtered at substrate temperatures of 300 ° and 700 °C. The background pressure was in the low 10−7-Torr range, total sputtering pressure was kept at 2×10−3 Torr and partial pressures of argon and nitrogen were monitored and controlled with a residual gas analyzer. The structure was examined by reflection electron diffraction and Jc-H characteristics were measured using a pulsed field and pulsed current technique. Substrate temperature and Ar/N2 partial-pressure ratio strongly influenced the crystallographic structure which in turn determined the superconducting properties of the films. Consequently, a clear relationship between deposition parameters, particular nitride phases and their mixtures and superconducting properties has been established. The highest Tc of 14.9 K has been observed for a film deposited at 700 °C which exhibited the δ NbN phase.

Very High Critical Current and Field Characteristics of Niobium Nitride Thin Films

Previous results indicated that NbN thin films possess critical current and field characteristics significantly superior to that of bulk NbN having a similar Tc. Further measurements made on NbN films, with thicknesses between 50 Å and 8 μ, now show that, at 4.2°K, certain of these films exhibit the highest current densities of any presently known superconductor in all fields from zero up to the limit of our measurement capability (210 kOe). In addition, anomalously high current and field values have been measured in very thin (&amp;lt;300 Å) films. These thinner films show no depressions in Jc (measured at 4.2°K) or in Hc2 (O) values despite decreases in Tc from almost 16°K (in the thicker films) down to 11°K.

Characteristics of NbN Dayem Bridges

Dayem-bridge weak links have been fabricated by sputter etching niobium nitride thin films having Tc's of ∼15 K. These junctions exhibit a negative resistance region extending to 45 mV at 3.2 K, in which are seen self-induced subharmonic current steps and structure near the energy gap voltage. Temperature dependence of these features and effects of applied microwave radiation are discussed, and possible explanations of the negative resistance region and of the self-induced step structure are given.

High Field Properties of Pure Niobium Nitride Thin Films

Thin films of the rocksalt structure (B1) compounds of the transition metals can be prepared by the method of reactive sputtering. By a refinement of this technique we have produced, for the first time, thin films of niobium nitride (NbN) with transition temperatures similar to those of the bulk materials (≳ 15°K). Films have been prepared on both hastelloy and fused silica substrates. Measurements of the normal state resistivity ρn yield values in excess of 200 μΩ·cm. Preliminary studies of the resistive transitions in these materials near Tc indicate that Hr(J), the field at which ρ/ρn=½, is independent of current density below 200 A/cm2 for films 1000–4000 Å thick. Identifying Hr(J) (J&amp;lt;50 A/cm2) with the upper critical field Hc2(t), the measurements indicate that Hc2(0) is significantly in excess of 200 kOe, and thus is substantially higher than the values obtained for the bulk material. These results are discussed in terms of the current theories of high field superconductivity with particular reference to the importance of Pauli spin paramagnetism and spin-orbit scattering in these materials.