nBn Structure Research Articles

Studies on energy band structure of NbC and NbN using DFT

Energy band structure of NbC and NbN are calculated using generalized gradient approximation (GGA) within density functional theory (DFT) including five high symmetry points W, L, Γ, X and K. The lowest band corresponds to 2s band of non metal (C and N) atoms and the next lowest band is formed by 2p nonmetal. The decomposing points of t 2g states (Γ 25), e g states (Γ 12) and C or N 2p states (Γ 15) show interesting behavior different from earlier reports.

Hetero-structure coherent epitaxial growth in AlN/NbN nano-structured multilayers

Monolithic AlN，NbN films and AlN/NbN multilayers with different modulation periods were prepared by reactive magnetic sputtering. The films were characterized by X-ray diffraction, X-ray reflectivity and high-resolution transmission electron microscopy. The results showed that the crystal structure of monolithic AlN and NbN films is close-packed hexagonal (hcp) and face-centered cubic (fcc), respectively. The crystal structure of AlN and NbN is hcp and fcc, respectively, in AlN/NbN multilayers. The interfaces between AlN layers and NbN layers are coherent， i.e., c-NbN (111)∥h-AlN(0002). The lattice mismatch of AlN/NbN multilayers is 013%. The thermodynamic calculation revealed that no matter how thickness of AlN or NbN layer is, the AlN layer does not form nonequilibrium structure of fcc, but the equilibrium structure of hcp. The AlN layers grow in the way of hetero-epitaxial coherent growth with NbN layers.

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

Abstract The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived x Nb 2 O 5 −(100− x )TiO 2 coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.

Use of nBn structures to suppress surface leakage currents in unpassivated InAs infrared photodetectors

Surface leakage current is a significant source of dark current in conventional narrow-gap p-n junction photodetectors. It is shown that the nBn photodetector, which was originally designed to eliminate dark current arising from generation-recombination mechanisms, also effectively eliminates surface leakage currents. The result is a measured dark current lower by over six orders of magnitude than that of the InAs p-n photodetector for device temperatures of 140 K.

Atomic Layer Deposition of NbN and Nb(Si)N for Metal Electrodes

Niobium nitride NbN and niobium silicon nitride Nb(Si)N were deposited by Atomic Layer Deposition (ALD) from niobium pentachloride NbCl5, silicon tetrachloride SiCl4, and ammonia NH3. It was shown that low resistivity NbN can be more readily processed by ALD than low resistivity TaN using just NH3 as reducing agent. Silicon could be uniformly added into the NbN structure, and increase in silicon content increased the resistivity of the films due to silicon incorporation as silicon nitride. A successful scale-up onto 200 mm wafer was performed for both processes.

Structure-dependent ferroelectricity of niobium clusters(NbN,N=2–52)

The ground-state structures and ferroelectric properties of NbN (N=2-52) have been investigated by a combination of density-functional theory (DFT) in the generalized gradient approximation (GGA) and an unbiased global search with the guided simulated annealing. It is found that the electric dipole moment (EDM) exists in the most of NbN and varies considerably with their sizes. And the larger NbN (N>=25) prefer the amorphous packing. Most importantly, our numerical EDM values of NbN (N>=38) exhibit an extraordinary even-odd oscillation, which is well consistent with the experimental observation, showing a close relationship with the geometrical structures of NbN. Finally, an inverse coordination number (ICN) function is proposed to account for the structural relation of the EDM values, especially their even-odd oscillations starting from Nb38.

Time delay of resistive-state formation in superconducting stripes excited by single optical photons

We have observed a 65(\ifmmode\pm\else\textpm\fi{}5)-ps time delay in the onset of a resistive-state formation in 10-nm-thick, 130-nm-wide NbN superconducting stripes exposed to single photons. The delay in the photoresponse decreased to zero when the stripe was irradiated by multi-photon (classical) optical pulses. Our NbN structures were kept at 4.2 K, well below the material's critical temperature, and were illuminated by 100-fs-wide optical pulses. The time-delay phenomenon has been explained within the framework of a model based on photon-induced generation of a hotspot in the superconducting stripe and subsequent, supercurrent-assisted, resistive-state formation across the entire stripe cross section. The measured time delays in both the single-photon and two-photon detection regimes agree well with theoretical predictions of the resistive-state dynamics in one-dimensional superconducting stripes.

Superhardness effects of heterostructure NbN/TaN nanostructured multilayers

Although superhardness effects have been extensively investigated for epitaxial ceramic nanomultilayer films with the same crystal structures in the last decade, those for multilayers with different crystal structures have been seldom studied. In this article, NbN/TaN nanomultilayers have been designed and deposited by reactive magnetron sputtering. The results showed that the crystal structures of NbN and TaN are face-centered cubic and hexagonal in superlattice films, respectively, and the lattice plane (111) of NbN is coherent with the (110) of TaN, i.e., {111}fcc-NbN∥{110}h-TaN. The results of microhardness measurement showed that the superhardness effects of NbN/TaN multilayers exist in a wide range of modulation period from 2.3 to 17.0 nm. This phenomenon is different from that of epitaxial ceramic multilayers where the maximum hardness usually takes place at a modulation period of 5.0–10.0 nm. It is proposed that the coherent stresses and the structural barriers (fcc/hexagonal) to dislocation motion between NbN and TaN layers are the main reasons for the high-hardness value in a wide range of modulation periods.

Structure and properties of NbN and TaN films prepared by ion beam assisted deposition

Niobium nitride NbN x and tantalum nitride TaN x films were deposited on 316L austenitic stainless steel and silicon wafer substrates by evaporation of niobium and tantalum under simultaneous nitrogen ion irradiation, applying an acceleration voltage between 2 and 20 kV. In order to study the influence of the nitrogen content, the atom-to-ion transport ratio Nb, Ta/N was varied. The compositional and structural characterization of the films were carried out using Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. It was observed that the surface morphology strongly depends on the deposition conditions. X-ray diffraction patterns showed that the crystal structure of the films was changed from nitride to solid solution when the transport ratio was increased from 1 to 10, respectively. The crystal structure of the films deposited at Nb, Ta N = 6 and 10 had the feature of a nitrogen supersaturated bcc solid solution. Electron diffraction revealed the coexistence of an amorphous phase and a fine structured nitride phase in these films. The chemical bonding state of metal atoms observed by XPS changed with increasing the transport ratio, whereas the bonding state of nitrogen atoms was independent of the transport ratio. The corrosion behavior was evaluated in an oxygen saturated 5% sulfuric acid solution, using multisweep cyclic voltammetry measurements. The best corrosion protection was observed for the NbN film at an transport ratio of Nb N = 2 and for the TaN solid solution at Ta N = 6 .

Metastability of NbN in the ordered vacancy NbO phase.

A metastable phase of NbN with superconducting Tc=16.4 K was reported recently by Treece and collaborators. The reported structure of the thin film sample deviates from the rocksalt (B1) NbN structure with 25% ordered vacancies on each sublattice (space group Pm3m) and a lattice constant of 4.442 Angstroms. Using full potential electronic structure methods, we contrast the electronic structure with that of B1 NbN. The calculated energy, 1.00 eV/molecule higher than B1 NbN, and calculated lattice constant of 4.214 Angstroms indicate that the new phase must be something other than the ordered stoichiometric Pm3m phase.

Electronic Structure and Superconductivity of NbN under High Pressure

AbstractThe band structure and superconductivity of niobium nitride in the NaCl structure under pressure is theoretically calculated. The effect of pressure on the band structure is obtained by means of the self‐consistent linear muffin‐tin orbital method. The calculated equation of state is in good agreement with experimental results. The calculated value of the cell parameter is 0.4475 nm which is in agreement with the experimental value. McMillan's formula is used to calculate the values of the superconducting transition temperature (Tc). The parameters necessary to calculate Tc are taken from the band structure outputs. The theoretically calculated value of Tc under ambient condition is 14.594 K which compares well with the experimental value. Further increase in pressure increases the Tc values which may be due to sp → d electron transfer from nonmetal to metal atom.

Non-stoichiometry and twinning in NbN 1− x: A study of synthesis and structure in a defect NaCl-type solid

Samples of high-purity NbN 1− x were prepared by a high temperature technique. Single-crystal studies revealed the twinning of the tetragonal low temperature phase which forms when cubic NbN 1− x is cooled from a high temperature. The X-ray diffraction results confirm the previously found structure of tetragonal NbN 1− x .

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.

Effect of Quenching Immediately after Sputtering on Superconducting NbN Films

In order to stabilize the metastable NaCl structure (the δ phase) of NbN at low temperatures, NbN films sputtered on sapphire substrates were thermally quenched immediately after deposition by thermal conduction from a water-or liq. N2-cooled anode container, or by direct spraying of liq. N2 onto the as-sputtered film. When the cooling rate Qr was increased, the superconducting transition temperature Tc and the resistance ratio β(=R300K/R20K) changed from a slightly downward trend to an upward one near Qr=104°C/min. Above this value of Qr, X-ray analysis indicated that the growth of the δ phase was enhanced, while that of the ε phase was depressed. The broad transition width ΔTc which appeared at a value of Qr between 2×103 and 104°C/min is considered to be closely related to the coexistence of the δ and ε phases.

The electronic structure of NbC and NbN

A self-consistent band-structure calculation has been performed for niobium carbide using the non-relativistic APW method and the X alpha exchange approximation. An analogous calculation already exists for NbN. Both band structures are fitted with a Slater-Koster LCAO scheme including overlap contributions. From a comparison of the two compounds a pronounced change in the non-metal p and niobium d interaction is found, which is discussed in connection with the binding mechanism, ESCA shifts and superconductivity. The X-ray emission spectra of NbC as computed from the present APW results agree well with experiments.

Electronic structure of Nb, NbC, and NbN studied by X-ray emission

New high-resolution X-ray fluorescence measurements of the Nb L3 emission band and Nb and NbN are presented. The Nb L3 emission of Nb, NbC and NbN are compared with available density-of-states calculations. It is shown that only the NbC band can be described satisfactorily by an APW model. A complex satellite structure appears on the high-energy side of the parent Nb L alpha 1,2 emission lines, and it is interpreted as due to L3M-M4,5M double-vacancy distribution. No Nb L alpha 1,2 line shift is observed, in agreement with earlier results on NbC.

Transition temperatures and crystal structures of single-crystal and polycrystalline NbNx films

Single-crystal cubic NbNx films with thicknesses of a few thousand angstroms were epitaxially grown on cleaved (100) planes of single-crystal MgO plates by the vapor phase growth technique. By heat treatment of some of these films in NH3+H2 or in H2 alone, films having the superlattice structures of Nb4N3, NbN, or Nb4N5 were prepared. The maximum Tc was observed in cubic NbNx. Both Nb4N3 and Nb4N5, the tetragonal phases with long-range-ordered arrangement of vacancies, exhibited superconductivity. NbN1.0 and Nb5N6, both with hexagonal structure, did not exhibit superconductivity down to 1.77 K. As the composition of cubic NbNx became close to stoichiometric, its Tc increased. However, the maximum Tc of cubic NbNx with nearly stoichiometric composition was limited by the lattice instability of cubic NbNx and by the resulting cubic NbNx to hexagonal NbN transformation. It might be expected that a higher Tc for compounds of the NbN family could be obtained if a NaCl-type crystal with less vacancies could exist as a stable or metastable phase at ordinary temperatures.

The Preparation of Thin Films of B-1 Structure Superconducting Ternary Compounds

NbN, a binary B-1 structure superconducting compound, has been successfully prepared in thin film form by a high-purity sputtering process [J. R. Gavaler, J. K. Hulm, M. A. Janocko, and C. K. Jones, J. Vac. Sci. Technol. 6, 177 (1969)]. In this paper, we report on the deposition of ternary compounds, based on NbN, using a similar technique. The inclusion of a third element, such as carbon, titanium, or zirconium into the NbN structure was accomplished by sputtering in an argon-nitrogen atmosphere from a two-element target. These targets were made either by hot pressing a powder mixture of the elements into a proper form or by welding together two metal sheets into a single unit. Superconducting transition temperatures of ≳ 17 K have been obtained in both Nb–C–N and Nb–TiN thin films by depositing these films at very high temperatures. However, critical current and field data measured at 4.2 K showed that the use of very high substrate temperatures also caused a deleterious effect on these properties.

Structure des Nitrures de Niobium

The crystal structure studies of niobium nitrides by electron and X-ray diffraction technique led to the identification of four phases: NbN (cubic, δ-phase), Nb2N (hexagonal, β-phase), NbN (hexagonal, ε-phase) and Nb4N3 (tetragonal, γ-phase). Various methods were used to prepare thin films of Nb nitrides for electron diffraction studies. Firstly, three kinds of oxynitrides, Nb(N,O)I, Nb(N,O)II and Nb(N,O)III were obtained by nitriding the evaporated thin films of Nb in an atmosphere of ammonia. NbN (cub) was formed on heating in vacuo of thin films of Nb(N,O)I, and it was further decomposed into Nb2N (hex) at a higher temperature. Covering with Nb fine powders makes possible the nitriding of thin films of Nb into NbN (hex). Successive structure transformations NbN (hex) → NbN (cub) → Nb4N3 (tet) → Nb2N (hex) were observed by heating in vacuo. Electron diffraction diagrams show the superlattice structures of Nb2N (hex) and Nb4N3 (tet). The crystal structure of Nb2N (hex) is the same type as that of Nb2C. The structure of Nb4N3 (tet), however, is derived from the cubic NaCl type structure of NbN (cub) by the loss of one quarter of nitrogen atoms.