B. A. Davidson

Superconducting Y‐Ba‐Cu‐O oxide films by sputtering

We have prepared superconducting thin films of Y‐Ba‐Cu‐O with Tc onsets above 95 K by both diode and magnetron sputtering. Films with full superconductivity (R=0) at 85 K have been produced by dc magnetron sputtering. The compositions of the films are fairly uniform across an area 50 mm in diameter and through the film thickness. Structural properties of the films were studied by x‐ray diffraction. Critical current densities in the range of 3000 to 104 A/cm2 have been measured at 4.2 K.We have prepared superconducting thin films of Y‐Ba‐Cu‐O with Tc onsets above 95 K by both diode and magnetron sputtering. Films with full superconductivity (R=0) at 85 K have been produced by dc magnetron sputtering. The compositions of the films are fairly uniform across an area 50 mm in diameter and through the film thickness. Structural properties of the films were studied by x‐ray diffraction. Critical current densities in the range of 3000 to 104 A/cm2 have been measured at 4.2 K.

Ge-Si layered structures : artificial crystals and complex cell ordered superlattices

We report the first successful synthesis of ordered GeSi superlattices grown on (001) Si substrates by molecular beam epitaxy. Two types of structures were prepared and characterized: superlattices with one‐dimensional periodicity of one unit cell (GeGeSiSi...) and complex cell superlattices made up of sublayers of pure Si and alternating bilayers of Ge and Si. In the first case, the artificial stacking in the [001] direction results in a vertical array of alternating Ge and Si monolayers parallel to the (110) or (110) planes. In spite of the lattice mismatch of 4.2%, Rutherford backscattering and channeling experiments indicate high quality crystallinity in both types of structures. Long‐range order is deduced from the electron diffraction patterns that exhibit characteristic superlattice reflections and from high resolution lattice imaging. The precise deposition control on the scale of a fraction of a monolayer should allow band structure engineering in this and in other related systems and in turn ta...

Improvements in the heteroepitaxy of GaAs on Si

Successful application of GaAs on Si heteroepitaxy to majority‐carrier device fabrication has recently been demonstrated. However, the quality of the GaAs heteroepitaxial films is considerably below that routinely achieved for films grown on GaAs substrates. We have investigated the initial growth stages of GaAs on Si using ion channeling and double‐crystal ‐ray diffraction, and report improvements in the growth technique leading to higher quality GaAs films. The crystalline perfection of the films was found to be critically dependent on the growth parameters of the initial GaAs buffer layer. Growth interruption after deposition of this layer, followed by an in situ annealing step (10 min at 750 °C) prior to final GaAs growth, improved both the structural and optical properties of the films.

Structure and optical properties of Ge‐Si ordered superlattices

We report the synthesis, structural characterization, and optical studies of ultrathin Ge‐Si superlattices, grown by molecular beam epitaxy, on (001) silicon substrates. Structures consist of alternating layers of pure Ge and Si, with layer thicknesses of 1, 2, 4, and 6 monolayers. Using high‐resolution transmission electron microscopy, we provide direct observation of order in these pseudomorphic layered films. Systematic study of optical transitions by means of Schottky barrier electroreflectance reveals that each of the ordered structures displays a unique set of optical transitions. Of particular interest is the 4×4 structure which shows new, well defined optical transitions at 0.76, 1.25, and 2.31 eV. These transitions constitute the first observation of structurally induced optical transitions in Ge‐Si and may make the 4×4 structure suitable for optoelectronic devices.

Superconducting Tl‐Ba‐Ca‐Cu‐O films by sputtering

Superconducting Tl‐Ba‐Ca‐Cu‐O thin films 0.2–1.0 μm thick have been prepared on MgO(100) and SrTiO3(100) substrates by dc diode sputtering using a single oxide target. Films containing primarily the Tl2Ba2Ca1Cu2O8 phase were obtained with a Tc (R=0) at 102 K and a transport Jc of 104 A/cm2 at 90 K. For the nearly pure phase Tl2Ba2Ca2Cu3O10 films, the Tc’s (R=0) are higher at 116 K and the transport Jc’s at 100 K are of 105 A/cm2. Both types of films show a strong preferred orientation with the c axis perpendicular to the film plane. The rocking curve of the Tl2Ba2Ca2Cu3O10 films is 0.32° wide and the typical grain size is over 10 μm.

Preparation of high Tc and Jc films of Ba2YCu3O7 using laser evaporation of a composite target containing BaF2

High Tc and Jc superconducting films of Ba2YCu3O7 are prepared on SrTiO3 〈100〉 substrates using pulsed excimer laser evaporation of a composite target containing BaF2, Y2O3, and CuO, followed by annealing in wet oxygen. High transition temperatures (R=0 from 89.5 to 91 K) and high critical current densities (Jc≥7×105 A cm−2) are obtained. The electrical transport properties of these films are significantly better than films previously grown via laser ablation of Ba‐Y‐Cu‐O targets.

Observation of a halide (F/Cl) stabilized, new perovskite phase in superconducting Y2Ba5Cu7Ox films

A new, ordered perovskite structure was stabilized as a majority phase in superconducting Y2Ba5Cu7O20−δ’(F/Cl)δ‘ films epitaxially grown on SrTiO3(100). The new phase was formed via fluorination or chlorination introduced during the post‐deposition O2 furnace anneal for recrystallization. The structure is determined by x‐ray diffraction to be an orthorhombic unit cell [a∼b=3.86(1) A, c=27.24(6) A] with reflection conditions consistent with a fully twinned A‐centered cell. Films containing primarily the new phase show a lower resistivity ρ(300 K)∼177 μΩ cm, a comparable ratio of ρ(300 K)/ρ(100 K)∼3.0, and a lower superconducting temperature of 78–74 K when compared to Y1Ba2Cu3O7.

Y-BA-CU-O FILMS BY RF MAGNETRON SPUTTERING USING SINGLE COMPOSITE TARGETS - SUPERCONDUCTING AND STRUCTURAL-PROPERTIES

High Tc superconducting Y‐Ba‐Cu‐O films have been produced by rf magnetron sputtering using single unreacted composite targets of Y2O3, BaF2, and CuO powders. Transport measurements of the films showed a sharp resistive Tc (R=0) at 89 K and a metallic behavior of resistivity versus temperature. The films show preferred orientations when deposited on single‐crystal SrTiO3 substrates (100) or (110) and followed by a post O2 annealing at 825 °C.

Statistical equilibrium in particle channeling

We have used a unique sample configuration consisting of thin, epitaxial films of Ge sandwiched within a Si(100) lattice to explore the conditions for statistical equilibrium in particle channeling. Indications of nonequilibrium came about from a strong asymmetry in the off‐normal angular scans. We find the depth necessary to achieve statistical equilibrium to be ∼2200 A for 1.8 MeV He particle channeling along the Si〈110〉 direction. This measurement provides a useful guide for the interpretation of channeling experiments in a variety of near‐surface applications.

Single Crystal Superconducting Y1Ba2Cu3O7−x Oxide Films by Molecular Beam Epitaxy

Recent breakthrough of superconductivity over 90 K in the Cu oxide based perovskite-type materials is of immense importance in both science and technology.1,2 The superconducting oxide crystals prepared in thin film form are vital for fundamental physical studies as well as for device applications.3,4 We report here the structural and superconducting properties of single crystal Y1Ba2Cu3O7−x films5 prepared by molecular beam epitaxy (MBE) developed for thin film semiconductor technology. The films are epitaxial crystals grown on SrTiO3 (100) face with an orientation of a axis perpendicular to the film plane. Sharp resistive superconducting transitions with Tc (R=0) above 77 K have been reproducibly achieved. The critical current densities are 103 A/cm2 at 77 K, 5 × 104 A/cm2 at 70 K, and ~ 106 A/cm2 at 4.2 K.