R. H. Hammond

Deposition of in-plane textured MgO on amorphous Si3N4 substrates by ion-beam-assisted deposition and comparisons with ion-beam-assisted deposited yttria-stabilized-zirconia

We report the growth of in-plane textured (100) MgO on amorphous Si3N4 substrates by ion beam assisted deposition (IBAD). The textured MgO can be used as a structural template for subsequent epitaxial thin film deposition. The results are compared with IBAD of (100) and (111) yttria-stabilized-zirconia (YSZ). Based on transmission electron microscopy (TEM) and in situ reflection high energy-electron diffraction (RHEED), we find that MgO texturing is a nucleation-controlled process and the alignment is a function of nuclei size and density. This differs greatly from the evolutionary-type texturing process observed for IBAD (100) YSZ. Consequently, we are able to make 100 A thick MgO films with 7° in-plane alignment, whereas IBAD (100) YSZ films need to be thicker than 5000 A to achieve in-plane alignment better than 13°. This has important implications for the economical application of IBAD induced alignment in real manufacturing processes, including high Tc superconductor (i.e., YBCO) coated tapes, photov...

Critical current densities and transport in superconducting YBa2Cu3O7−δ films made by electron beam coevaporation

Thin films of the high Tc superconductor YBa2Cu3O7−δ were prepared and characterized. The films made on SrTiO3 showed epitaxial growth and high critical current densities in excess of 9×104 at 78 K and 2×106 at 4.2 K. Also, surface pinning in the parallel direction was found to be very high. The measurements put a lower limit on the depairing critical current density of 5×107 A/cm2.

Correlation between the in situ growth conditions of YBCO thin films and the thermodynamic stability criteria

Abstract Data from the literature on the successful in situ growth of YBCO thin films in terms of the growth temperature and the oxygen pressure are compared with the thermodynamic stability as determined by an electrochemical method. The correlation over a wide range of growth temperatures and oxygen pressures, and for most methods of deposition, suggests that proximity to the equilibrium decomposition line may be favorable for in situ growth.

Thin-film synthesis of the high- T c oxide superconductor YBa 2 Cu 3 O 7 by electron-beam codeposition

The successful synthesis of high- T c YBa 2 Cu 3 O 7 films by means of electron-beam codeposition are reported. Several important growth parameters have been surveyed in a preliminary way. The substrates investigated include Al 2 O 3 , ZrO 2 , MgO, and SrTiO 3 , The films were characterized by resistivity measurements, x-ray diffraction, microprobe, and Rutherford backscattering analysis. Some TEM and critical current density studies were also carried out. The best results to date have been obtained on SrTiO 3 substrates with which polycrystalline epitaxial growth has been achieved. Resistive superconducting transitions with zero resistance at 89.5 K and a 2 K width have been observed in these films.

Feasibility of the high Tc superconducting bolometer

A design analysis is given for a bolometric infrared detector that uses the resistive transition of a high‐temperature superconductor as the temperature sensing element, and liquid nitrogen (LN) as the coolant. It is shown that for highly oriented c‐axis films, the measured low‐frequency noise causes little or no degradation of the performance. With the incoming radiation chopped at 10 Hz, noise equivalent powers (NEP) in the range (1–20)×10−12 W Hz−1/2 should be achievable. These values compare favorably with the NEP of other detectors operating at or above LN temperatures for wavelengths greater than 20 μm.

Magnetic flux noise in thin-film rings of YBa2Cu3O7−δ

The low‐frequency magnetic flux noise in thin‐film rings of YBa2Cu3O7−δ (YBCO) is measured over the temperature range 1.3–125 K by means of a dc superconducting quantum interference device (SQUID) maintained at liquid‐helium temperatures. Below the transition temperature Tc of the YBCO, the spectral density of the noise scales as 1/f, where f is the frequency, and generally increases with increasing temperature. The magnitude of the noise depends strongly on the microstructure of the film, and is lowest for a sample which is predominantly oriented with its c axis perpendicular to the substrate. These results imply that SQUID’s and flux transformers of YBCO must be fabricated from highly oriented films to produce good resolution at low frequencies.

Electron beam evaporation synthesis of A15 superconducting compounds: Accomplishments and prospects

The accomplishments and the prospects of forming A15 superconductors using electron beam evaporation are reviewed. The techniques of monitoring and controlling the evaporation are discussed. The special advantages to the synthesis of A15 materials are pointed out, followed by a number of examples. The prospect of the large scale production of A15 superconductors using electron beam processing is also considered.

In situ growth of superconducting MgB2 thin films with preferential orientation by molecular-beam epitaxy

We report the synthesis of superconducting MgB2 thin films grown in situ by molecular-beam epitaxy. Mg-rich fluxes are deposited with B flux by electron-beam evaporation onto c-plane sapphire substrates. The films exhibit c-axis oriented peaks of MgB2, and a full width at half maximum of 3° in their rocking curves. In-plane alignment of MgB2 shows 12-fold symmetry, which is observed by the selected area diffraction pattern in transmission electron microscopy. The MgB2 films show a superconducting transition at 34.5 K with ΔTc<1 K. Even though the residual resistivity of the films is quite high (∼60 μΩ cm), the normal-state resistivity has a very similar temperature dependence but is five times larger than that of a single crystal, implying that conduction through the whole sample is imperfectly connected or inhomogeneous. Upper critical fields are obtained from measurement of the field dependence of the resistivity. It is estimated that the upper critical field at 20 K is more than 15 T, which is one of t...

Ferroelectric field effect in ultrathin SrRuO3 films

We report the observation of a ferroelectric field effect in the conducting oxide SrRuO3 using Pb(Zr0.52Ti0.48)O3/SrRuO3 epitaxial heterostructures. Upon reversing the polarization of the ferroelectric Pb(Zr0.52Ti0.48)O3 layer, we measured a 9% change in the resistance of a nominally 30 A SrRuO3 film at room temperature. This change was nonvolatile for a period of several days. Conductivity measurements taken between 4.2 and 300 K are consistent with n-type conduction throughout this temperature range. Hall effect measurements also yield n-type conduction, with n≈2×1022 electrons/cm3, and furthermore allow us to understand quantitatively the magnitude of the observed resistivity change.

Atomic oxygen detection by a silver‐coated quartz deposition monitor

A method for measuring the flux of atomic oxygen utilizing a silver film on a quartz‐crystal deposition rate monitor is described. Measuring the initial oxidation rate of the silver, which is proportional to the atomic oxygen flux, determines a lower limit on the atomic oxygen flux. This method is more direct than measuring the conductance of the silver film, has an intrinsic flux detection range of 1013–1017 atoms/cm2 s, and is reversible by exposing the sensor to an atomic hydrogen flux.