Jonathan H. James

AES, XPS and SIMS characterization of YBa2Cu3O7 superconducting high Tc thin films

Abstract High Tc superconducting YBa2Cu3O7− x (YBCO) films 100–200 nm thick have been grown in situ by ion beam sputter co-deposition using four Kaufman ion beam sources. As-deposited films have Tc (R = 0) at 74K. Postannealing improved Tc (R = 0) to 79 K. The critical current exceeded 5 × 104Acm−2 at 77 K. Films were deposited on SrTiO3, or on Si with an intermediate layer of SiO2 or Y2O3. Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectroscopy (SIMS) were used to depth profile films deposited at 350 and 650°C. For quantification of XPS and AES analysis, standards of BaF2, BaCo3, BaO, BaO2, CuO, Cu2O, and Y2O3, and monocrystals of BaCuO2 and YBa2O3O7 − x were measured. XPS binding energies and AES sensitivity factors were determined. Using the YBa2Cu3O7 − x monocrystal new AES sensitivity factors differing from handbook factors have been established. Static SIMS data of the monocrystal was compared to thin film spectra of the first 1–2 monolayers showing almost identical cracking patterns.

In situ growth of superconducting YBa2Cu3O7-δ thin films on Si with conducting indium-tin-oxide buffer layers

Superconducting YBa2Cu3O7−δ (YBCO) thin films have been grown in situ on Si with conducting indium‐tin‐oxide (ITO) buffer layers. ITO allows YBCO to be electrically connected to the underlying Si substrate. Both the YBCO film and ITO buffer layer, grown by ion beam sputtering, are textured and polycrystalline with a combined room‐ temperature resistivity of about 2 mΩ cm. Superconducting onsets are 92 K with zero resistance at 68 K.

Large area superconducting YBa2Cu3O7-x films grown by single target ion beam sputtering

The authors have demonstrated, by using a simple single YBa2Cu3O7-x target ion beam system that, with a sufficiently low power ion beam, preferential sputtering is avoided and high-quality YBa2Cu3O7-x films are deposited over areas larger than a30 cm2 in a reproducible way. As-deposited films on 〈100〉SrTiO3 are 50-100 nm thick, c-oriented and show the following reproducible electrical properties (within the given variations): Tc0=90±0.5 K, transitions widths less than 1 K, jc(77 K)=1.0-1.2×106 A cm-2, ρ(300 K)=300±50 μΩ cm, ρ(300 K)/ρ(100 K)=2.9±0.1. The extrapolated residual resistivity ρres(0 K) is between 0 and 5% of ρ(300 K)

In-situ superconducting YBa2Cu3O7 thin films grown by ion beam co-deposition

The authors present superconducting YBa2C3O7 (YBCO) thin films grown in-situ by three-ion-beam sputtering. Y, Y2O3, Cu, Cu2O, BaF2 and BaCO3 sputter targets have been investigated. The highest quality films were prepared using a BaCo3 target. Auger analysis of films grown using a BaCO3 target show no carbon content. Y2O3 and Cu2O are more suitable than the native metals as sputter targets for YBCO growth as they are much less prone to sputter rate variations with oxygen partial pressure. They also supply oxygen to the growing film. As-deposited YBCO films are metallic (resistivity 240 μΩ cm at 100 K), reflective, and of highly homogeneous composition with Tco transition temperatures of 73 K and transition widths of 15 K. Post-annealing in flowing oxygen improves TCOs to 82 K. Critical currents are in excess of 105 A cm-2 at 77 K. Films are textured with c-axis orientation perpendicular to the (100) SrTiO3 substrate surface. As-deposited superconducting YBCO films have also been prepared on SiO2 and Y2O3 buffer layers on Si wafers

The preparation of YBCO thin films by a four ion beam co-deposition system

We present preliminary results for thin films grown with a recently commissioned ion beam codeposition system designed for in situ growth and etching of YBCO films. The apparatus consists of a high-vacuum chamber (10-7 Torr) equipped with four Kaufmann ion beam sources. Three of the sources are directed at targets for ion beam sputter deposition of materials; the fourth ion source is directed at the substrate for substrate precleaning, oxygen ion beam assisted deposition and ion beam etching. We have determined individual sputter deposition rates from Y2O3, BaF2 and copper targets as a function of ion beam currents on unheated substrates in argon background, normal background, oxygen background, and oxygen ion assist conditions. Films were studied by X-ray diffraction (XRD), Auger electron spectroscopy (AES), resistivity and Rutherford back-scattering analysis (RBS).

A simple high-temperature superconducting thin-film optical bolometer

The authors have demonstrated the operation of a simple optical bolometer by irradiating a constriction in a Y1Ba2Cu3O7-δ thin film with chopped laser light at λ=633 nm. The change in film resistivity was measured by lock-in detection of the AC component in the current-biased mode. Operating optimally near 77 K, the bolometer shows detectivity of 5 mV W-1 and NEP of 10-6 W Hz-1/2. The authors discuss the possibility of methods to improve these figures by at least three orders of magnitude

Superconducting YBa2Cu307 films for novel optoelectronic device structures

This short overview briefly summarizes the most important parameters for successful preparation and associated properties of thin films of YBa2Cu3O(7-delta) (YBCO) superconductors. The principles are illustrated by using the example of monotarget ion beam sputtering technique: YBCO films grown in situ on SrTiO3 show Tc(onset)

Preparation, processing and tunneling in YBa2Cu3O7-δ-Pb native-barrier structures

The authors have developed a procedure for the preparation of small (40×40 μm2) window-type YBa2Cu3O7-δ-Pb junctions with YSZ insulator and native tunnel barrier. They present the patterning technique of the two electrodes based on photolithography and wet etching. The nature of the barrier is found to be semiconducting. The tunneling measurements show gap-like feature of YBa2Cu3O7-δ at 8.5 meV and some additional features related to Pb and YBa2Cu3O7-δ phonon spectra

Window-type tunnel devices on YBa2Cu3O7-x thin films

The authors present the first YBa2Cu3O7-x-Pb tunnelling measurements using window-type tunnel devices. YBa2Cu3O7-x (YBCO) thin films are grown in-situ by dual Ar+ ion beam sputtering. Tunnel barriers are formed on a chemically etched YBCO surface. Tunnelling dV/dI(V) characteristics show reproducible weak gap-like structure 4.4±0.4 meV and 22±2 meV, the latter giving a 2Δ/kBTc value of about 6.5

Superconducting YBa2Cu3O7 films on Si and GaAs with conducting indium tin oxide buffer layers

Superconducting YBa2Cu3O7-delta (YBCO) thin films have been grown in situ by ion beam sputtering on Si and GaAs substrates with intermediate, conducting Indium Tin Oxide (ITO) buffer layers. Uniform, textured YBCO films on ITO exhibit Tc onset at 92K and Tc0 at 68K and 60K on Si and GaAs substrates respectively, the latter value is the highest Tc reported on GaAs. YBCO/ITO films exhibit metallic resistivity behavior. In situ YBCO films on SrTiO3 show Tc onset = 92K and Tc0 = 90.5K, transition widths are less than 1K. A simple optical bolometer has been constructed from YBCO films on SrTiO3. Tunnelling measurements have also been carried out using the first YBCO-Pb window-type tunnel junctions.