Robert J. Houlton

Growth Mechanism of Sputtered Films of YBa2Cu3O7 Studied by Scanning Tunneling Microscopy

The surface microstructures of c-axis-oriented films of YBa2Cu3O7, deposited by off-axis magnetron sputtering on MgO and SrTiO3 single crystal (100) substrates, have been investigated with scanning tunneling microscopy and atomic force microscopy. There is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step. This results in columnar grains, each of which contains a screw dislocation at its center. This microstructure may be of significance in determining superconducting properties such as critical current, and represents a significant difference between thin films (especially those grown in situ) and bulk materials.

Dense diffusion barrier limiting current oxygen sensors

Abstract A new type of miniature amperometric oxygen sensor has been developed. The sensors are high temperature micro-electrochemical devices that use a dense diffusion barrier of a metal oxide that readily transports oxygen and conduct electrons. The diffusion barrier is deposited in thin film form on top of a zirconia-based electrochemical pump. When a voltage is applied to the pump, oxygen is depleted from one side of the diffusion barrier and the ionic current is proportional to the flux of oxygen across the thin film layer. If the pumping voltage reaches a high enough value, the transport of oxygen across the membrane and hence the device’s output current, will be limited by the external, oxygen concentration and the transport characteristics of the diffusion barrier. The sensors can be manufactured in a planar design that offers a faster time response, much simpler design and potentially lower cost than existing limiting current oxygen sensors.

Microstructure and growth mechanism of thin sputtered films of YBa2Cu3O7 on MgO substrates

The microstructures of very thin sputtered films of YBa2Cu3O7 deposited on (100) faces of single‐crystal MgO, have been investigated using scanning tunneling and atomic force microscopies. By a thickness of 10 nm, the substrate is completely covered by a fine‐grained layer of the superconductor. The average grain size is about 100 nm. Many of these grains show evidence of a spiral growth mechanism. In somewhat thicker films (20 nm), the grain size has increased considerably (to about 200 nm), and this trend continues as the thickness is increased. The small grain size close to the interface suggests a possible origin for widely observed ion‐beam dechanneling in this region of the film.

Surface resistance of YBa2Cu3O7 films on SrTiO3 and LaGaO3 substrates

Surface resistance measurements of films of YBa2Cu3O7 deposited onto single‐crystal substrates of LaGaO3 and SrTiO3 have been made at a frequency of 22 GHz. The measurements were made in either a copper or niobium cavity by replacing the end wall with the superconducting film. Typical surface resistance at 20 K are 1–2 mΩ for films on LaGaO3 and 6–8 mΩ for films on SrTiO3, as measured in the copper cavity. The LaGaO3 values lie within the sensitivity range of the Cu cavity (∼2 mΩ) and can only be considered upper limits. Similar measurements in a Nb superconducting cavity resulted in a surface resistance value of 0.2±0.1 mΩ at 4 K for the best LaGaO3‐based film. This value is more than an order of magnitude lower than Cu, and suggests that LaGaO3‐based films may offer immediate advantages in a number of applications.

Surface resistance of YBa sub 2 Cu sub 3 O sub 7 films on SrTiO sub 3 and LaGaO sub 3 substrates

Surface resistance measurements of films of YBa2Cu3O7 deposited onto single‐crystal substrates of LaGaO3 and SrTiO3 have been made at a frequency of 22 GHz. The measurements were made in either a copper or niobium cavity by replacing the end wall with the superconducting film. Typical surface resistance at 20 K are 1–2 mΩ for films on LaGaO3 and 6–8 mΩ for films on SrTiO3, as measured in the copper cavity. The LaGaO3 values lie within the sensitivity range of the Cu cavity (∼2 mΩ) and can only be considered upper limits. Similar measurements in a Nb superconducting cavity resulted in a surface resistance value of 0.2±0.1 mΩ at 4 K for the best LaGaO3‐based film. This value is more than an order of magnitude lower than Cu, and suggests that LaGaO3‐based films may offer immediate advantages in a number of applications.

Growth of highly oriented La 0.84 Sr 0.16 MnO 3 perovskite films

We have grown thin films of La{sub 0.84}Sr{sub 0.16}MnO{sub 3} on SrTiO{sub 3} (100), MgO (100), CeO{sub 2} (100)/Al{sub 2}O{sub 3}, and (100) oriented yttria-stabilized zirconia (YSZ) substrates by using a 90{degree} off-axis RF magnetron sputtering deposition. X-ray diffraction analysis and ion beam channeling experiments reveal that the deposited films grow epitaxially on SrTiO{sub 3}, biaxially textured on MgO, and highly textured on YSZ. Scanning tunneling microscopy reveals that the thin films possess extremely smooth surfaces. {copyright} 1995 Materials Research Society

CMR films structure as a function of growth and processing

Abstract Among the family encompassed by the complex metal oxides are the colossal magneto-resistive perovskites with potential applications in advanced high density magnetic data storage and sensor devices based on single or multilayers of these materials fabricated by vapor phase deposition methods. Realization of this potential requires solving basic thin film fabrication problems coupled to understanding the resulting structure-properties relationships. The specific objectives addressed in this study were to determine the dependence of growth and transport properties: (1) reproducibility on the pulsed laser deposition (PLD) target, (2) on sputter deposition chamber gas and substrate, and (3) on a post-deposition anneal in a reducing atmosphere. Toward these ends, we have grown La 0.67 Ca 0.33 MnO 3 films on perovskite substrates by PLD and RF off-axis sputter deposition from stoichiometric targets at temperatures from 400 to 900°C. The PLD films were grown from a new commercial target and post-deposition annealed in O 2 to reproduce, for comparison, the conditions used previously for the films grown with an in-house fabricated target. The sputter deposited films were grown in both ArO 2 mixed and Ar-only atmospheres, some grown simultaneously on different substrates; one ArO 2 grown film was post-annealed in a reducing atmosphere. Microstructures were studied by scanning probe microscopies and temperature-dependent transport properties by 4-point resistance measurements. Previously we reported on characterization of the T substrate ( T s )-dependent microstructure of PLD as-deposited and post-annealed films grown from an in-house fabricated target. Annealed films grown from the new commercial target exhibited improved consistency in T -dependent transport and magnetization properties, independent of growth temperature. None of the as-grown sputter deposited films grown in the ArO 2 mixture exhibited a metallic transition while those grown in Ar-only at T s > 800°C had a range of transition temperatures between 110 and 180 K. The variation in substrate-dependent structure demonstrated again the sensitivity of growth mechanism to template. Finally, the 500°C film grown in the ArO 2 mixture, went from insulating at all T s to exhibiting a 215°C metallic transition after the reducing anneal treatment.

High‐temperature superconductor Josephson junctions with a gradient Pr‐doped Y1−xPrxBa2Cu3O7−δ (x=0.1, 0.3, 0.5) as barriers

High‐temperature superconductor YBa2Cu3O7−δ based superconducting‐normal‐superconducting (SNS) Josephson junctions were fabricated using a unique device design. The normal material included a gradient Pr‐doped Y1−xPrxBa2Cu3O7−δ layer which was composed of a light doping (x=0.1) next to both YBa2Cu3O7−δ electrodes, a slightly higher doping (x=0.3) towards the center, and a doping concentration of x=0.5 in the middle of the N layer. This design graded the lattice mismatch between YBa2Cu3O7−δ and the N layer, thus avoiding the accumulation of all the lattice strain at one interface. It also results in good chemical, thermal, and structural compatibility between adjacent layers for the desired multilayer structures. The SNS junctions fabricated in this way showed resistively shunted junction current‐voltage characteristics under dc bias and Shapiro steps under microwave irradiation at a temperature in the range of 75–87 K. Direct current superconducting quantum interference devices showed a voltage modulation...

Microwave surface resistance of magnetron-sputtered Tl-Ba-Ca-Cu-O films on silver substrates

Surface resistance measurements on Tl‐Ba‐Ca‐Cu‐O thick films (∼15 μm) magnetron sputtered onto BaF2‐buffered, silver‐based (Consil 995) substrates have been made at a microwave frequency of 22 GHz. The relatively large‐area films (∼5 cm2) are characterized by surface resistance values of 6.9±2 mΩ at 11.3 K and 30.2±1 mΩ at 77 K; the corresponding values for Cu are 10 and 22 mΩ, respectively. These results demonstrate that Tl‐Ba‐Ca‐Cu‐O can be deposited onto large‐area, metallic substrates with characteristic surface resistance values lower than Cu at 4 K. Orientation of the film should improve the surface resistance at 77 K, thereby making the fabrication of microwave cavities that are superior to Cu possible.

Growth of highly oriented La{sub 0.84}Sr{sub 0.16}MnO{sub 3} perovskite films

We have grown thin films of La{sub 0.84}Sr{sub 0.16}MnO{sub 3} on SrTiO{sub 3} (100), MgO (100), CeO{sub 2} (100)/Al{sub 2}O{sub 3}, and (100) oriented yttria-stabilized zirconia (YSZ) substrates by using a 90{degree} off-axis RF magnetron sputtering deposition. X-ray diffraction analysis and ion beam channeling experiments reveal that the deposited films grow epitaxially on SrTiO{sub 3}, biaxially textured on MgO, and highly textured on YSZ. Scanning tunneling microscopy reveals that the thin films possess extremely smooth surfaces. {copyright} 1995 Materials Research Society