E. J. Cukauskas

Formation of high Tc superconducting films by organometallic chemical vapor deposition

The first growth of superconducting YBaCuO films by organometallic chemical vapor deposition is described. Metal β‐diketonates were decomposed thermally on MgO substrates in an oxygen‐rich atmosphere to produce amorphous brown films. Subsequent annealing in oxygen yielded dull gray films whose thickness corresponded to deposition rates of approximately 8 nm min−1. These films showed semiconductor‐like behavior at higher temperatures, followed by a broad resistive transition from 80 to 36 K with the resistance becoming zero at ∼20 K. Analysis of x‐ray data indicated the presence of the orthorhombic superconducting phase and various other metal oxides. Profilometer measurements yielded film thicknesses up to 950 nm, and scanning electron microscopy revealed faceted grains from 0.5 to 1.0 μm in size.

Niobium nitride‐niobium Josephson tunnel junctions with sputtered amorphous silicon barriers

Niobium nitride‐niobium Josephson tunnel junctions with sputtered amorphous silicon barriers (NbN‐αSi‐Nb) have been prepared using processing that is fully compatible with integrated circuit fabrication. These junctions are of suitable quality and uniformity for digital circuit and S‐I‐S detector applications. The junction quality depends critically upon the properties of the NbN surface, and seems to correlate well with the UV/visible reflectivity of this surface.

Growth of superconducting thin films of bismuth-strontium-calcium-copper oxide by organometallic chemical vapor deposition

Abstract The first growth of superconducting Bi-Sr-Ca-Cu-O films by organometallic chemical vapor deposition is described. Triphenyl bismuth and beta-diketonates of strontium, calcium, and copper were decomposed thermally at 500°C on MgO substrates in an oxygen-rich atmosphere to produce fine grained or amorphous, dark brown to black films. Subsequent annealing in oxygen yielded gray-black films with thicknesses of 0.95 to 1.25 μm. X-ray analysis indicated the presence of a tetragonal superconducting phase along with other metal oxides. The sheet resistance versus temperature for the sample with the highest T c had a resistive transition with a midpoint at 78.8 K, a width of 7.7 K, and R = 0 at 73.2 K.

Ba(1−x)SrxTiO3 thin films by off-axis cosputtering BaTiO3 and SrTiO3

Thin films of Ba(1−x)SrxTiO3 (BST) have been deposited by off-axis cosputtering targets of SrTiO3 (STO) and BaTiO3 (BTO). The films were grown on (100) MgO and LaAlO3 substrates at 550 °C at an argon/oxygen gas pressure of 20 Pa. Reaction of the sputtered species in the plasma resulted in the growth of BST films. The film composition was set by the relative power levels of the STO and BTO targets. X-ray diffractometry indicated the as-deposited films were predominantly (h00) oriented. The calculated lattice parameter was 3.955±0.01 A, slightly larger than bulk Ba0.5Sr0.5TiO3. Postannealing the films in flowing O2 at 780 °C resulted in greater peak heights and no change in orientation. Capacitance–voltage characteristics measured at 10 GHz yielded Q’s of 65 with nearly 14% tuning for BST on LAO and Q>1000 with a 2% tuning for films on MgO.

Barium–strontium–titanate thin films for application in radio-frequency-microelectromechanical capacitive switches

In this letter we report the application of barium–strontium–titanate (BST) thin film oxides as the dielectric layer in radio-frequency-microelectromechanical system (rf-MEMS) capacitive switches. BST thin films deposited at ambient temperature by off-axis sputtering have been employed for application in rf-MEMS switches. Their dielectric properties have been characterized in the frequency range from 1 to 20 GHz both on magnesium oxide and on gold metal films. Switches have been fabricated which demonstrate promising on-state capacitance and good dielectric breakdown properties. Dielectric breakdown in excess of 400 kV/cm has been measured on switches cycled in excess of 2000 times during testing.

Low-loss Ba0.5Sr0.5TiO3 thin films by inverted cylindrical magnetron sputtering

The structural and electrical characteristics of Ba0.5Sr0.5TiO3 (BST) thin films deposited by inverted cylindrical magnetron rf sputtering have been investigated. This unconventional sputter deposition technique consisting of a hollow cylindrical composite target of BST, high argon/oxygen gas pressure 53.2 Pa (400 μm), and 750 °C substrate temperature was employed for depositing low-loss BST thin films. The films were postannealed in a tube furnace at 780 °C for 8 h in flowing oxygen. Atomic-force microscopy revealed anisotropic grain growth with a columnar grain structure protruding from the surface with a 0.25 μm grain size. X-ray diffractometry shows the films to be purely (h00) oriented for certain deposition parameters. The lattice parameter of the best film was slightly larger than that for bulk BST. Other deposition conditions yielded films having many of the BST powder peaks. Capacitance versus voltage characteristics have been measured from 50 MHz to 20 GHz. Device Q values >600, beyond the resol...

Paraconductivity in YBa2Cu3O7 and YBa2Cu3O7 thin film composites with cerium oxide and gold

The paraconductivity for thin films of YBa2Cu3O7 (YBCO) and composites of YBCO with CeO2 and gold was investigated and analyzed using the Aslamazov and Larkin theory. Plain YBCO thin films sputter deposited on MgO, SrTiO3, and yttrium stabilized zirconium typically showed no dimensional crossover, were described by three dimensional (3D) fluctuation conductivity, and had a critical slope just above the critical region in agreement with the prediction of theory. With few exceptions, the CeO2 and gold composite films showed a dimensional crossover and a reduced critical exponent. The change in dimensionality was typically from 3D to two dimensional (2D) fluctuation conductivity. These composite films all had reduced values for the critical exponent which we attribute to an enlarged or extended critical region. This is in agreement with the critical current response of these composites to small magnetic fields. Additionally, the composites showed a two step transition in the paraconductivity and a minimum be...

Thin film composites of Au and YBa2Cu3O7−δ

Using a novel bilayer deposition process, we have grown composite films of Au with YBa2Cu3O7−δ on various substrates. Our composites have well‐segregated regions of Au and YBa2Cu3O7−δ, and we have not observed structural degradation of the grains or grain boundaries in the films. Transition temperatures and critical current density measurements have behaviors consistent with regions of weakened grain boundary coupling in the films. Photoresponse measurements are also reported. We believe that Au is diffusing into the grain boundaries and weakening the coupling between YBa2Cu3O7−δ grains. By varying the amount of Au in the films, the coupling can be adjusted. These Au/YBa2Cu3O7−δ composite films are promising materials for device applications relying on vortex motion.

Niobium nitride thin films for use in Josephson junctions

The properties of rf diode and magnetron reactively sputtered NbN films have been studied under a variety of preparation conditions. The aim of this investigation is to achieve high transition temperature, low resistivity films under conditions suitable for use in all refractory tunnel junction fabrication. We have systematically varied the relative amounts of Ar, N 2 and CH 4 gases, and the substrate temperature used during film growth. The transition temperature, resistivity, lattice parameter and crystal structure have been studied and correlated with the partial pressure of methane used during sputtering. The crystal structure was investigated using diffractometer and Read camera photographic X-ray techniques. We have prepared NbN films using both rf diode and magnetron sputtering with resistivities less than 70 μΩ-cm and transition temperatures greater than 16 K. The lattice parameter for our NbN films ranges between 4.39 A and 4.45 A and is dependent upon the amount of nitrogen and carbon used in the film preparation. We are currently investigating all refractory tunnel junctions with artificial barriers using these films as base electrodes and niobium counter electrodes.

Microwave amplification to 2.5 GHz in a quantum state transfer device

Reflection amplification has been demonstrated from 40 MHz to 2.5 GHz using a bilevel superlattice structure known as a quantum state transfer device. Measurements of the magnitude and phase of the voltage reflection coefficient were made at 300 K using a specialized microwave probe. The maximum normalized gain, at 40 MHz, approaches the gain expected from dc measurements of the negative differential resistance.