Anupama B. Kaul

Internally shunted sputtered NbN Josephson junctions with a TaNx barrier for nonlatching logic applications

We report on the growth, fabrication, and device characterization of NbN internally shunted Josephson junctions with a TaNx barrier. The resistivity of TaNx films could be varied from a few hundred micro-ohms to a few hundred milliohms by increasing the N2 pressure during reactive sputtering. The temperature dependence of IcRn of the junctions with ∼13 mΩ cm barrier resistivity was measured for various barrier thicknesses. The coherence length of the barrier was determined to be 5 nm. By adjusting the barrier thickness, IcRn values >500 μV were observed up to 8.3 K, with Ic and Rn of magnitudes that are suitable for single-flux-quantum digital circuits.

Ultra‐High Optical Absorption Efficiency from the Ultraviolet to the Infrared Using Multi‐Walled Carbon Nanotube Ensembles

The optical absorption efficiencies of vertically aligned multi-walled (MW)-carbon nanotube (CNT) ensembles are characterized in the 350-7000 nm wavelength range where CNT site densities > 1 × 10(11) /cm(2) are achieved directly on metallic substrates. The site density directly impacts the optical absorption characteristics, and while high-density arrays of CNTs on electrically insulating and non-metallic substrates have been commonly reported, achieving high site-densities on metals has been challenging and remains an area of active research. These absorber ensembles are ultra-thin (<10 μm) and yet they still exhibit a reflectance as low as ∼0.02%, which is 100 times lower than the reference; these characteristics make them potentially attractive for high-sensitivity and high-speed thermal detectors. In addition, the use of a plasma-enhanced chemical vapor deposition process for the synthesis of the absorbers increases the portfolio of materials that can be integrated with such absorbers due to the potential for reduced synthesis temperatures. The remarkable ruggedness of the absorbers is also demonstrated as they are exposed to high temperatures in an oxidizing ambient environment, making them well-suited for extreme thermal environments encountered in the field, potentially for solar cell applications. Finally, a phenomenological model enables the determinatiom of the extinction coefficients in these nanostructures and the results compare well with experiment.

Geometrical dependence of the low-frequency noise in superconducting flux qubits

A general method for directly measuring the low-frequency flux noise (below 10 Hz) in compound Josephson-junction superconducting flux qubits has been used to study a series of 85 devices of varying design. The variation in flux noise across sets of qubits with identical designs was observed to be small. However, the levels of flux noise systematically varied between qubit designs with strong dependence upon qubit wiring length and wiring width. Furthermore, qubits fabricated above a superconducting ground plane yielded lower noise than qubits without such a layer. These results support the hypothesis that local impurities in the vicinity of the qubit wiring are a key source of low-frequency flux noise in superconducting devices.

Probing noise in flux qubits via macroscopic resonant tunneling.

Macroscopic resonant tunneling between the two lowest lying states of a bistable rf SQUID is used to characterize noise in a flux qubit. Measurements of the incoherent decay rate as a function of flux bias revealed a Gaussian-shaped profile that is not peaked at the resonance point but is shifted to a bias at which the initial well is higher than the target well. The rms amplitude of the noise, which is proportional to the dephasing rate 1/tauphi, was observed to be weakly dependent on temperature below 70 mK. Analysis of these results indicates that the dominant source of low energy flux noise in this device is a quantum mechanical environment in thermal equilibrium.

In situ characterization of vertically oriented carbon nanofibers for three-dimensional nano-electro-mechanical device applications

We have performed mechanical and electrical characterization of individual as-grown, vertically oriented carbon nanofibers (CNFs) using in situ techniques, where such high-aspect-ratio, nanoscale structures are of interest for three-dimensional (3D) electronics, in particular 3D nano-electro-mechanical-systems (NEMS). Nanoindentation and uniaxial compression tests conducted in an in situ nanomechanical instrument, SEMentor, suggest that the CNFs undergo severe bending prior to fracture, which always occurs close to the bottom rather than at the substrate-tube interface, suggesting that the CNFs are well adhered to the substrate. This is also consistent with bending tests on individual tubes which indicated that bending angles as large as approximately 70 degrees could be accommodated elastically. In situ electrical transport measurements revealed that the CNFs grown on refractory metallic nitride buffer layers were conducting via the sidewalls, whereas those synthesized directly on Si were electrically unsuitable for low-voltage dc NEMS applications. Electrostatic actuation was also demonstrated with a nanoprobe in close proximity to a single CNF and suggests that such structures are attractive for nonvolatile memory applications. Since the magnitude of the actuation voltage is intimately dictated by the physical characteristics of the CNFs, such as diameter and length, we also addressed the ability to tune these parameters, to some extent, by adjusting the plasma-enhanced chemical vapor deposition growth parameters with this bottom-up synthesis approach.

Fabrication of wide-IF 200-300 GHz superconductor-insulator-superconductor mixers with suspended metal beam leads formed on silicon-on-insulator

We report on a fabrication process that uses SOI substrates and micromachining techniques to form wide-IF SIS mixer devices that have suspended metal beam leads for rf grounding. The mixers are formed on thin 25 µm membranes of Si, and are designed to operate in the 200–300 GHz band. Potential applications are in tropospheric chemistry, where increased sensitivity detectors and wide-IF bandwidth receivers are desired. They will also be useful in astrophysics to monitor absorption lines for CO at 230 GHz to study distant, highly redshifted galaxies by reducing scan times. Aside from a description of the fabrication process, electrical measurements of these Nb/Al–AlNx/Nb trilayer devices will also be presented. Since device quality is sensitive to thermal excursions, the new beam lead process appears to be compatible with conventional SIS device fabrication technology.

Single, aligned carbon nanotubes in 3D nanoscale architectures enabled by top-down and bottom-up manufacturable processes

We have developed manufacturable approaches for forming single, vertically aligned carbon nanotubes, where the tubes are centered precisely, and placed within a few hundred nm of 1-1.5 microm deep trenches. These wafer-scale approaches were enabled by using chemically amplified resists and high density, low pressure plasma etching techniques to form the 3D nanoscale architectures. The tube growth was performed using dc plasma-enhanced chemical vapor deposition (PECVD), and the materials used in the pre-fabricated 3D architectures were chemically and structurally compatible with the high temperature (700 degrees C) PECVD synthesis of our tubes, in an ammonia and acetylene ambient. Such scalable, high throughput top-down fabrication processes, when integrated with the bottom-up tube synthesis techniques, should accelerate the development of plasma grown tubes for a wide variety of applications in electronics, such as nanoelectromechanical systems, interconnects, field emitters and sensors. Tube characteristics were also engineered to some extent, by adjusting the Ni catalyst thickness, as well as the pressure and plasma power during growth.

Engineering chemically exfoliated dispersions of two-dimensional graphite and molybdenum disulphide for ink-jet printing

Stable ink dispersions of two-dimensional-layered-materials (2DLMs) MoS2 and graphite are successfully obtained in organic solvents exhibiting a wide range of polarities and surface energies. The role of sonication time, ink viscosity and surface tension is explored in the context of dispersion stability using these solvents, which include N-methyl-2-pyrrolidone (NMP), N,N-Dimethylacetamide (DMA), dimethylformamide (DMF), Cyclohexanone (C), as well as less-toxic and more environmentally friendly Isopropanol (IPA) and Terpineol (T). The ink viscosity is engineered through the addition of Ethyl-Cellulose (EC) which has been shown to optimize the jettability of the dispersions. In contrast to prior work, the addition of EC after sonication-instead of prior to it-is noted to be effective in generating a high-density dispersion, yielding a uniform film morphology. High-quality inks are obtained using C/T and NMP as solvents for MoS2 and graphite, respectively, as gauged through optical absorption spectroscopy. Electronic transport data on the solution-cast inks is gathered at room temperature. Arrays of 2D graphite-rod based inks are printed on rigid Si, as well as flexible and transparent polyethylene terephthalate (PET) substrates. The results clearly show the promise of ink-jet printing for casting 2DLMs into hierarchically assembled structures over a range of substrates for flexible and printed-electronics applications.

NbN/TaN/sub x//NbN SNS Josephson junctions by pulsed laser deposition

We have made NbN/TaN/sub x//NbN Josephson junctions with nonhysteretic I-V characteristics that may have potential application for single-flux-quantum logic or SQUID magnetometers. The trilayer growth was done in-situ using pulsed laser deposition. The resistivity of the TaN/sub x/ was varied by the N/sub 2/ pressure during growth from a few hundred micro-ohm-cm at low pressure to a few hundred milli-ohm-cm at pressures up to 300 mTorr. The films were characterized using Rutherford backscattering, X-ray diffraction, and atomic force microscopy. Junction processing was done in the usual trilayer technology. The fabricated Josephson junctions show I/sub c/R/sub n/ values >0.3 mV at 4.2 K. We also observed that at a high laser fluence, NbN films with low a real particulate density and T/sub c//spl sim/16.2 K could be grown on 25/spl deg/C substrates.

Interrogating vertically oriented carbon nanofibers with nanomanipulation for nanoelectromechanical switching applications

We have demonstrated electrostatic switching in vertically oriented carbon nanofibers synthesized on refractory metallic nitride substrates, where pull-in voltages Vpi ranged from 10 to 40 V. A nanoprobe was used as the actuating electrode inside a scanning-electron microscope and van der Waals interactions at these length scales appeared significant, suggesting such structures are promising for nonvolatile memory applications. A finite element model was also developed to determine a theoretical Vpi and results were compared to experiment. Nanomanipulation tests also revealed tubes synthesized directly on Si by dc plasma-enhanced chemical-vapor deposition with ammonia and acetylene were electrically unsuitable for dc nanoelectromechanical switching applications.