S.N. Polyakov

Optimization of RF- and DC-sputtered NbTiN films for integration with Nb-based SIS junctions

NbTiN is one of the most promising materials for use in the tuning circuits of Nb-based SIS mixers for operating frequencies above the gap frequency of Nb (/spl ap/700 GHz). We examine the properties of NbTiN films obtained using an unbalanced sputtering source in both RF and DC operating regimes. It is found that the properties of NbTiN films are strongly affected by the total pressure of the sputtering process. Films obtained under lower pressures have higher compressive stresses and lower resistivities. The best NbTiN films are obtained by DC sputtering and have a transition temperature of 14.4 K, a resistivity of 90 /spl mu//spl Omega//spl middot/cm at 20 K, and a compressive stress of -1 GPa. Films with a resistivity of 110 /spl mu//spl Omega//spl middot/cm at 20 K and a compressive stress of -0.5 GPa have been successfully used as a stripline material for Nb/Al-AlO/sub x//Nb SIS junctions on fused quartz substrates.

Parabolic single-crystal diamond lenses for coherent x-ray imaging

We demonstrate parabolic single-crystal diamond compound refractive lenses designed for coherent x-ray imaging resilient to extreme thermal and radiation loading expected from next generation light sources. To ensure the preservation of coherence and resilience, the lenses are manufactured from the highest-quality single-crystalline synthetic diamond material grown by a high-pressure high-temperature technique. Picosecond laser milling is applied to machine lenses to parabolic shapes with a ≃1 μm precision and surface roughness. A compound refractive lens comprised of six lenses with a radius of curvature R=200 μm at the vertex of the parabola and a geometrical aperture A=900 μm focuses 10 keV x-ray photons from an undulator source at the Advanced Photon Source facility to a focal spot size of ≃20×90 μm2 with a gain factor of ≃50−100.

All-diamond optical assemblies for a beam-multiplexing X-ray monochromator at the Linac Coherent Light Source

All-diamond optical assemblies holding state-of-the-art type IIa diamond crystals enable the construction of a beam-multiplexing X-ray double-crystal monochromator for hard X-ray free-electron lasers. Details on the design, fabrication and X-ray diffraction characterization of the assemblies are reported.

Superconducting transition metal nitride films for THz SIS mixers

The development of sensitive THz SIS mixers requires a low-loss superconducting strip-line material with a transition temperature above 15 K. In this paper we examine the properties of (Nb,Ti)N, NbN, and (Nb,Zr)N thin films sputtered at ambient substrate temperature on glass wafers. The best properties are observed for the (Nb,Ti)N films, sputtered from an NbTi alloy target with 30 at.% Ti. A similar picture is observed for the epitaxial films deposited on the MgO wafers. We have also examined the homogeneity of the (Nb,Ti)N films versus film thickness and in plane since this factor is clearly important for the microwave behavior of the strip-line. We observe that (Nb,Ti)N films deposited on silicon, sapphire, and glass wafers have much worse homogeneity compared to the films deposited on the MgO wafers.

Diamond film deposition by downstream d.c. glow discharge plasma chemical vapour deposition

Abstract A new d.c. glow discharge chemical vapour deposition (CVD) method was developed for diamond film (DF) deposition onto insulator substrates. This method uses ordinary d.c. plasma CVD equipment, but the anode system was modified both to insulate the substrate holder from the anode and to set the substrate downstream of the plasma. This is achieved by two methods. First, the anode was made from the tungsten grid and the insulated substrate was placed under the grid and out of the discharge area. Second, the anode was made from the thick-wall molibden tube and the substrates were placed on the insulator inside the tube. The experiments were carried out under a hydrogen pressure of 50–150 Torr and methane concentration of 1%–2%. The substrates (Mo or Si) were set on the silica substrate holder. The temperature was approximately 1000 °C. Diamond films were grown by both methods. Scanning electron microscopy, cathodoluminescence microscopy and spectroscopy, Raman scattering and X-ray diffractometry were used to study and compare the diamond films.

Properties of DC magnetron sputtered Nb and NbN films for different source conditions

We have compared the quality of Nb and NbN films obtained by DC magnetron sputtering from a new and a fully eroded Nb target. Since current superconducting electronic devices such as SIS mixers, RSFQ digital circuits and hot electron bolometers are produced by reactive sputtering, we are interested in optimum source operating conditions over the target lifetime. We find that stress-free Nb films can, at any state of the target, be obtained under the same Ar pressure and DC power applied to the sputtering source. We show that this approach also works for NbN reactive sputtering if the nitrogen flow rate is maintained proportional to the deposition rate of Nb. In both cases the zero-stress point shifts to lower cathode DC voltage as the target erodes. Additionally we find that the effectiveness of the magnetic trap of the magnetron influences the normal state resistivity of the NbN films.

An improvement of surface smoothness and lattice match of CeO2 buffer layers on R-sapphire processed by MOCVD

Abstract Epitaxial thin films of CeO 2 and Ce 1− x R x O 2− y (R=Y, Nd; x =0–0.32) on R-sapphire were prepared by band flash evaporation MOCVD. It was demonstrated that the smoothness of the films can be improved by a decrease of deposition rate and by post deposition annealing at 1000 °C. The lattice match of buffer layers with R-Al 2 O 3 and YBa 2 Cu 3 O 7−δ was improved by doping of ceria yttrium and neodymium correspondingly. A bilayer structure Ce 0.7 Nd 0.3 O 2− y /Ce 0.68 Y 0.32 O 2 − y /R-Al 2 O 3 is proposed as potential substrate material for YBa 2 Cu 3 O 7 − 6 deposition.

Structure and properties of Langmuir-Blodgett films containing cluster molecules

Abstract An approach based on the use of mixed Langmuir–Blodgett (LB) films containing various chemically synthesized nanocluster molecules has been proposed in order to create single-electron transfer model systems functioning at room temperatures. The nanoclusters are shown to form ordered structures (quasilinear chains, two-dimensional arrays, etc.) in such monolayers deposited onto the solid substrates. The tunnel current–bias voltage ( I – V ) characteristics of isolated cluster molecules Pt 5 (CO) 6 (PPh 3 ) 4 taken at 300 K exhibit rich structures resulting from the interplay between Coulomb charging effects and discreteness of an electronic spectrum of the cluster. The platinum Pt 5 (CO) 6 (PPh 3 ) 4 clusters appear to posses surface-active properties and can form stable homogeneous Langmuir monolayers on the water surface. Such monolayers can be effectively transferred onto the solid substrates so that corresponding multilayer LB films are obtained. X-ray difractograms recorded for those films show a number of Bragg peaks caused by the ordered layered structure of the films. Close values of intensity of the second harmonic generation measured for the mono- and 11-layer LB films of the platinum clusters give evidence of the centrosymmetrical Y-type packing of the cluster molecules in multilayer LB films. The pertinent theoretical model of cluster molecule packing in such films has been proposed.

Radicals as EPR probes of magnetization of gadolinium stearate Langmuir–Blodgett film

Abstract In the present work we have applied the method of the EPR spin probes which allows performing simultaneously EPR and magnetization measurements to the investigation of magnetism of the Gd stearate Langmuir–Blodgett (LB) films. For this purpose we have prepared and studied by the EPR technique the Gd and Y stearate LB films. Placing the small BDPA crystal on the film surface we have found that for the Gd LB sample the effective g -value of the radicals resonance depends on the film orientation in respect to the external magnetic field direction. The relative shift of the EPR signal corresponded to the magnetization of the film along the field direction. Such effect has not been observed for the Y stearate LB film. The data obtained give an experimental proof for the room temperature magnetic ordering in the gadolinium stearate LB film.

Synthesis of Ni-containing nanoparticles in Langmuiur–Blodgett films

Abstract Synthesis of Ni-containing anisotropic flatten nanoparticles was carried out in a mixed Ni stearate/stearic acid and Ni arachidate/arachidic acid LB films by borohydrid reduction under different reaction conditions. The structure and composition of the films were characterized by Fourier transform infrared spectroscopy, X-ray diffraction and atomic force microscopy. Ordered lamellar structure of LB films was not disrupted by nickel reduction procedure and the presence of oblate nanoparticles was observed by AFM.