Nb Underlayer Research Articles

Improvement in the Magnetic Properties of Ni–Fe Thin Films on Thick Nb Electrodes Using Oxidation and Low-Energy Ar Ion Milling

We developed a method to engineer the surface topography of Nb underlayers using surface oxidation followed by low-energy Ar ion milling to improve the properties of subsequently deposited magnetic Ni80Fe 20 (Permalloy) thin films. They had reduced coercivity $H_{c}$ , increased remanent squareness, and improved magnetic anisotropy due to the reduced roughness of the Nb underlayer, especially at high spatial frequency ( $> 25\,\mu {\text{m}}^{- 1}$ ). Typical results for 2.4 nm thick Ni80Fe20 films deposited on 100 nm thick Nb were an easy-axis coercivity $H_{{\rm{ce}}}= 3.7$ Oe (compared to 6 Oe without underlayer smoothing), a hard-axis $H_{{\rm{ch}}}= 1.5$ Oe (5.5 Oe without smoothing), an easy-axis remanent squareness (ratio of remanent and saturation magnetizations) $Sq_{e}= 0.92$ , a hard-axis remanent squareness $Sq_{h}= 0.25$ , and a uniaxial anisotropy $H_{k}= 6.0$ Oe, all measured at a temperature of 10 K. This ion-smoothing technique could potentially be used to improve the properties of magnetic layers in superconducting memory and other magnetoelectronic devices that utilize a thick underlayer that serves as an electrical contact.

Fast Response Hydrogen Microsensor Based on Semiconductor Niobium-oxide Nanostructures via Smart Anodizing of Al/Nb Metal Layers

Nanostructured niobium oxide semiconductor is gaining increasing attention as electro-optic and gas sensing material. However, the preparation of stable niobium oxide nanofilm with reproducible morphology and behaviour remains a challenge. Here we describe a rapid and well-controlled approach to synthesize a niobium oxide film with the columnlike nanostructured morphology via anodic processing of Al/Nb metal layers sputtered onto an oxide-coated Si wafer. The film is developed due to the growth of a nanoporous anodic alumina layer followed by pore-directed oxidation of the Nb underlayer. The post-anodizing treatment results in the controlled formation of Nb2O5 crystal phase, which causes the transformation from dielectric to n-type semiconductor behavior of the film. A laboratory gas sensor fabricated by uniting the anodizing approach developed here with standard micromachining technologies shows superior characteristics for hydrogen gas detection, the response-recovery time being among best ever reported.

Formation–structure–properties of niobium-oxide nanocolumn arrays via self-organized anodization of sputter-deposited aluminum-on-niobium layers

Nanostructured niobium oxide (NO) semiconductors are gaining increasing attention as electronic, optical, and electro-optic materials. However, the preparation of stable NO nanofilms with reproducible morphology and behavior remains a challenge. Here we show a rapid, well-controlled, and efficient way to synthesize NO films with self-organized columnlike nanostructured morphologies and advanced functional properties. The films are developed via the growth of a nanoporous anodic alumina layer, followed by the pore-directed anodization of the Nb underlayer. The columns may grow 30–150 nm wide, up to 900 nm long, with an aspect ratio of up to 20, being anchored to a thin continuous oxide layer that separates the columns from the substrate. The as-anodized films have a graded chemical composition changing from amorphous Nb2O5 mixed with Al2O3, Si-, and P-containing species in the surface region to NbO2 in the lower film layer. The post-anodization treatments result in the controlled formation of Nb2O5, NbO2, and NbO crystal phases, accompanied by transformation from nearly perfect dielectric to n-type semiconductor behavior of the films. The approach allows for the smooth film growth without early dielectric breakdown, stress-generated defects, or destructive dissolution at the respective interfaces, which is a unique situation in the oxide films on niobium. The functional properties of the NO films, revealed to date, allow for potential applications as nanocomposite capacitor dielectrics and active layers for semiconductor gas microsensors with the sensitivity to ethanol and the response to hydrogen being among best ever reported.

Structure analysis of Ni thin films epitaxially grown on bcc metal underlayers formed on MgO(100) substrates

Ni thin films are prepared on Cr, V, and Nb underlayers with bcc structure formed on MgO(100) single-crystal substrates by molecular beam epitaxy. The growth behavior and the crystallographic properties are investigated by in-situ reflection high-energy electron diffraction and pole-figure X-ray diffraction. Cr(100) and V(100) single-crystal underlayers grow epitaxially on the substrates, whereas an Nb epitaxial_underlayer consisting of two bcc(110) variants is formed on the MgO(100) substrate. Metastable crystals nucleate on the Cr and the V underlayers, where the metastable hcp structure is stabilized through heteroepitaxial growth. With increasing the film thickness, the hcp structure starts to transform into more stable fcc structure by atomic displacement parallel to the hcp(0001) close-packed plane. The resulting films are consisting of mixtures of hcp and fcc crystals. On the other hand, only the formation of fcc crystal is recognized for the Ni film grown on Nb(110) underlayer.

Effects of surface morphologies of base electrodes on the qualities of Nb/AlOx/Nb Josephson junctions with high critical densities

We have investigated the effects of surface morphologies of base Nb electrodes on quality parameter, Vm, of high critical current density (Jc) Nb/AlOx/Nb junctions. The results showed that the Vm depended significantly on the surface flatness parameter, root-mean-square (rms), of the sputtered Nb base double layer which consisted of a 200nm-thick Nb layer on a 100nm-thick Nb underlayer. The surface flatness was controlled by changing the Ar gas pressures between 0.1Pa and 1.5Pa during the dc sputter-deposition process. Reducing the Ar gas pressure improved the rms of the base Nb-double layer from 5nm to 1nm. When junctions with underlayers were fabricated at reduced Ar gas pressure, an improvement in Vm was realized. The results indicated that smaller rms values were required for obtaining larger Vm values. At an rms=1.7nm, a maximum Vm of 16.5mV was obtained for junctions with a Jc=14kA/cm2 and a Vm of 3.0mV was realized for a junction with a Jc of 52kA/cm2.

High-Bs Fe–Co–Al–O soft magnetic films

Fe–Co and Fe–Co–Al–O films were prepared by rf magnetron sputtering in an Ar-gas atmosphere. The coercivity (Hc) of the Fe–Co films was greatly decreased by a small amount of Al2O3 addition. However, a film thickness of more than 500 nm is necessary to obtain minimum the Hch (Hc in the hard-axis direction) of 5 Oe. Hch was greatly decreased by using a Ni–Fe underlayer of 0.5–12 nm, and was less than 1 Oe with film thicknesses ranging from 50 to 600 nm for Fe–Co–Al–O films. An amorphous Co–Zr–Nb soft magnetic underlayer was also examined to investigate influence of underlayer crystal structure. A Hch less than 1 Oe was realized for the samples with Co–Zr–Nb underlayer thickness of about 2 nm. Amorphous underlayer films were also effective in improving the soft magnetic properties of Fe–Co–Al–O films. It is finally concluded that combination of a small amount of Al2O3 addition and a soft magnetic underlayer film is essential to obtain small Hch with high saturation magnetic flux density of 2.4 T in a wide range of film thicknesses.

Ta-based tunnel junctions for energy resolving X-ray detection

We report the results of a programme to develop an energy-resolving superconducting tunnel junction based on a Ta X-ray absorber. The design is based on a vertical heterostructure consisting of a Nb underlayer to act as a quasiparticle reflector, a thick Ta absorber layer and an Al trap layer. The upper surface of this trap layer is oxidised to form the tunnel barrier. The counterelectrode consists of an Al and thin Nb layer with thicknesses controlled to give approximately equal energy gaps in both electrodes and minimal absorption in the Nb counterelectrode. The effect of electro-magnetic barrier resonances on detection performance has been evaluated. Very high quality junctions have been fabricated, tested and evaluated for X-ray energy resolution.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Selective trilayer deposition process for fabricating Nb/Al-AlOx/Nb Josephson tunnel junctions

High quality Nb/Al-AlOx/Nb Josephson tunnel junctions have been fabricated using a novel process named the selective trilayer deposition process. The junction is formed on a patterned Nb underlayer through the lift-off deposition of a Nb/Al-AlOx/Nb trilayer. Anodization then provides the insulation between the underlayer and the wiring layer. We show that this easy and simple process is particularly advantageous for the fabrication of many vertically stacked tunnel junctions.

Reactively sputtered niobium nitride thin films for Josephson integrated circuit application

The properties of sputter-deposited NbN thin films were studied under changes in various deposition conditions including nitrogen flow, substrate heating, the addition of carbon impurities, and the use of an Nb underlayer. Without the underlayer, a semiconductorlike resistive behavior above the superconducting transition temperature and a wide superconducting transition were observed in the niobium nitride films, including those with transition temperatures above 15 K. With the underlayer, metallic behavior and a sharp superconducting transition temperature depended strongly on nitrogen flow and substrate heating but weakly on carbon impurities. The authors present details on the preparation and analysis of niobium nitride thin films with and without a niobium underlayer as well as the measured film characteristics. >

Fabrication process for Josephson computer ETL-JC1 using Nb tunnel junctions

The fabrication process that was used to develop a multichip Josephson computer named ETL-JC1 is described. The ETL-JC1 consists of four Josephson LSI chips: a register arithmetic logic unit chip (RALU), a sequence control unit chip (SQCU), a 1280-b read-only memory chip (IROU), and a 1-kb random access memory chip (DRAU). The fabrication process, based on a 3- mu m Nb/AlO/sub x//Nb junction technology, has been developed to make a complete set of the Josephson LSI chips. The present fabrication process includes a trilayer tunnel junction formation, a Nb underlayer method, a self-aligned insulation method, a reactive ion etching (RIE) process, an etching stopper layer formation, and a superconducting contact formation. The Josephson critical current density was controlled by the oxidation time within the fluctuation of +or-20% in the LSI fabrication runs. The resistors were made of palladium metal film on the LSI chips. The sheet resistance was controlled within the fluctuation between -12.5% and +19% in the LSI runs. It was found that the Josephson LSI chips fabricated by this process showed a high reliability for a long-term storage at room temperature and thermal cyclings between 4.2 K and room temperature without any passivation layers on the LSI surface.

Nucleation of body-centered-cubic tantalum films with a thin niobium underlayer

We discuss the structural and electrical properties of high-quality Ta films prepared by ion beam sputter deposition. The Ta films grow in two different crystal structures, body-centered-cubic (bcc) or tetragonal (β-Ta), depending on the substrate preparation and sputtering conditions. Ta films deposited on a thin (&amp;gt;0.3 nm) Nb underlayer grow in the bcc crystal structure with properties approaching those of clean bulk polycrystalline material. The bcc-Ta films have a superconducting transition temperature of 4.3 K and a low-temperature (10 K) resistivity ρ∼6 μΩ cm. Ta films deposited without a Nb underlayer on Si substrates always grow in the tetragonal (β-Ta) structure. The β-Ta films do not superconduct above 1 K and have a high resistivity ρ∼150 μΩ cm. X-ray diffraction and transmission electron microscope studies of both Ta structures are presented. Both bcc-Ta and β-Ta films are deposited on room-temperature substrates. This allows either type of film to be easily patterned by standard photoresist liftoff methods.

Self‐positioned thin Pb‐alloy base electrode Josephson junction

A self‐positioned thin (SPOT) Pb‐alloy base electrode Josephson junction is developed. In this junction, a 50‐nm thick Pb‐alloy base electrode is restricted within the junction region on an Nb underlayer using a self‐alignment technique. The grain size reduction and the base electrode area restriction greatly improve thermal cycling stability, where the thermal cycling tests of 4000 proposed junctions (5×5 μm2) showed no failures after 4000 cycles. In addition, the elimination of insulator layer stress on the Pb‐alloy base electrode rectifies the problem of size effect on current density. The Nb underlayers also serve to isolate the Pb‐alloy base electrodes from the resistors.

Ion-beam deposition of Nb and Ta refractory superconducting films

A Kaufman ion-beam source has been used to sputter deposit high quality superconducting Nb (Tc=9.1 K) and Ta (Tc=4.3 K) films. Superconducting and electrical properties were studied as a function of deposition conditions and an optimum set of conditions was found. Nb films always had the bulk bcc crystal structure. The Nb film deposition rates ranged from 1 to 5 Å/s with a Xe or Ar sputtering gas pressure of ∼1 to 4×10−4 Torr and a beam current density of 5–20 mA/cm2. Values of Tc above 8.3 K for Nb films were obtained with the use of Xe rather than Ar gas. In the case of Ta films, the deposition of a thin (≥3 Å) Nb underlayer was required for the nucleation and growth of Ta in the bulk bcc crystal structure. Ta films deposited without a Nb underlayer always had high resistivity, ∼150 μΩ cm, and a tetragonal β-Ta crystal structure. The Nb and Ta targets and Si substrates all remained below 70 °C during deposition and the films were easily patterned by standard photoresist liftoff.