Vertically aligned, untangled planarized arrays of multiwall carbon nanotubes (MWNTs) with Ohmic back contacts were grown in nanopore templates on arbitrary substrates. The templates were prepared by sputter depositing Nd-doped Al films onto W-coated substrates, followed by anodization to form an aluminum oxide nanopore array. The W underlayer helps eliminate the aluminum oxide barrier that typically occurs at the nanopore bottoms by instead forming a thin WO3 layer. The WO3 can be selectively etched to enable electrodeposition of Co catalysts with control over the Co site density. This led to control of the site density of MWNTs grown by thermal chemical vapor deposition, with W also serving as a back electrical contact. Ohmic contact to MWNTs was confirmed, even following ultrasonic cutting of the entire array to a uniform height.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Chemical solution deposition (CSD) is used to grow high-quality (100)-oriented films of SrTiO3 (STO) on CSD Ba0.2 Ca0.8TiO3(100) (BCT) templates on textured W-doped Ni(100) (Ni:W) tape substrates. The BCT template films form a thin layer or “skin” that bridges its significant porosity. STO films grown at 1000 °C appear optimized for heteroepitaxial orientation, surface coverage, and film smoothness. Both interfaces in the STO(100)/BCT(100)/Ni:W(100) stack demonstrate excellent atomic registry and compositional abruptness. Doping STO with a few atomic percent of Nb reduces oxygen diffusion into the film by an order of magnitude and provides greater protection to the Ni interfacial surface from oxidation during the growth of additional functional oxides requiring relatively higher p(O2) high-temperature processing, such as superconducting YBa2Cu3O7−δ. CSD growth of BCT and STO also planarizes pre-existing grooves in the Ni:W(100) tapes while maintaining a high degree of orientation by forming facets at the interfaces.

Systematic studies have been performed on 1000 Å YBa2Cu3O7−δ films produced by the BaF2 process and annealed in an oxygen partial pressure (Po2) range from 740 Torr to 10 mTorr as well as a temperature range from 600 to 1050 °C. The results show that while high quality films can be annealed in a wide range of oxygen partial pressure, they have different characteristics. In general, crystalline quality and Tc are optimized at high Po2 and high annealing temperature, while strong flux pinning and low normal state resistivity are achieved at lower values of both variables. Under optimized low Po2 conditions, an ion channeling Xmin of 6% is obtained on films as thick as 5000 Å. This study will serve as a useful guide to tailoring film properties to the application at hand.

We performed a series of glancing angle and reflection x-ray diffraction experiments to study both the in-plane and out-of-plane structure of epitaxial YSi2−x films grown on Si(111), with thicknesses ranging from 85 Å to 510 Å. These measurements allowed us to characterize the mean film lattice constants, the position correlation lengths of the film, and the presence and extent of strain as a function of film thickness. We find that the strain along the basal plane increases as a function of increasing thickness to approximately 1% in the 510 Å film; the corresponding out-of-plane strain is such that the film unit cell volume increases as a function of thickness. The corresponding in-plane microscopic strain varies from 0.5% for the 85 Å film to 0.3% for the 510 Å film. We relate our results to the mode of film growth and the presence of pinholes in the films.

The development of passive and active electronics from high-temperature superconducting thin films depends on the development of process technology capable of producing appropriate feature sizes without degrading the key superconducting properties. We present a new class of chelating etches based on di- and tri-carboxylic acids that are compatible with positive photoresists and can produce submicron feature sizes while typically producing increases in the microwave surface resistance at 94 GHz by less than 10%. This simple etching process works well for both the Y-Ba-Cu-O and Tl-Ba-Ca-Cu-O systems. In addition, we demonstrate that the use of chelating etches with an activator such as HF allows the etching of related oxides such as LaAlO3, which is a key substrate material, and Pb(Zr0.53Ti0.47)O3 (PZT) which is a key ferroelectric material for HTS and other applications such as nonvolatile memories.

A parametric study has been performed of amorphous tetrahedral carbon (a-tC) films produced by ion- assisted pulsed laser deposition (IAPLD). The ion voltage, current density, and feed gas composition (nitrogen in argon) have been varied. The resultant films were characterized by thickness, residual stress, Raman spectroscopy, and electrical resistivity. The Raman spectra have been fit to two gaussian peaks, the so called graphitic (“G”) peak and the disorder (“D”) peak. It has been found that the magnitude of the D peak and the residual compressive stress are inversely correlated. At low beam voltages and currents, the magnitude of the D peak is low, increasing as the ion beam voltage and current are raised. The ion beam voltage has the most dramatic effect on the magnitude of the D peak. At low voltages (200–500 V) the magnitude of the D peak is greater for ion beams with high percentages of nitrogen possibly indicative of C-N bonding in the films. At higher voltages (500 - 1500V) the D peak intensity is less sensitive to the nitrogen content of the beam.

Epitaxial films of Ba2YCu3O7−δ (BYCO), as thin as 250 Å and with Jc's approaching those for the best in situ grown films, can be formed by coevaporating BaF2, Y, and Cu followed by a two-stage anneal. These results extend the work of R. Feenstra et al. [J. Appl. Phys. 69, 6569 (1991)] for film thicknesses >2000 Å. This involves using low oxygen partial pressure [p(O2) = 4.3 Torr] during the high temperature portion of the anneal, which we vary from Ta = 600 to 950 °C. The BYCO melt line is seen to be the upper limit for Ta. The use of lower p(O2) shifts the window for stable BYCO film growth to lower temperature. The lower growth temperature required for the low p(O2) process allows the formation of smooth films with greater microstructural disorder than for films grown in p(O2) = 740 Torr at higher Ta, resulting in higher Jc values by a factor of four. The relationship between the Ta required to grow films with the strongest pinning force and p(O2) is log [p(O2)] ∝ Ta−1, independent of growth method (in situ or ex situ) over a range of five orders of magnitude in p(O2).

We have compared the quality of Ba2YCu3O7−δ (BYCO) films grown by the BaF2 process on (100) substrates of the perovskites LaAlO3, LaGaO3, NdAlO3, NdGaO3, LiBaF3, and SrTiO3. The films were grown by coevaporation of Y, Cu, and BaF2 followed by a two-stage anneal. The high temperature stage of the anneal, the part of the process during which the BYCO crystal structure and morphology develop, has been varied. LaAlO3 and SrTiO3 support much better films, both electrically and structurally, than the other substrates. Of the oxides, NdGaO3 supports the worst films, while films on LiBaF3 are nonconducting. These results emphasize the overriding importance of chemical compatibility in determining the suitability of a potential substrate material. Unless this criterion is satisfied, the issue of lattice matching is unimportant.