J.R. Liu

Damage caused by magnetic pressure at high trapped field in quasi-permanent magnets composed of melt-textured YBaCuO superconductor

Quasi-permanent magnets made of melt-textured YBa2Cu3O7 - delta (MT-YBCO) superconductor can now trap multi-tesla fields, B sub t. The interaction of the trapped field and the critical current causes an outward pressure, proportional to B sub t (2), which can crack the magnet. The authors have done an experiment to observe such cracking in a mini-magnet fabricated from four MT-YBCO discs activated at 49 K using an applied field of 14 T. They have compared the results to existing theories which describe magnetic pressure in a trapped-field magnet (TFM) previously activated. They find that a modification is needed to describe magnetic pressure during the process of activation. They present the experimental results and the expanded theory, based on the simple Bean model. Theory and experiment show good agreement. The authors find that cracking is more likely during activation, and conclude that 10 T is achievable in TFM`s composed of present materials. Cracking is most probable at the center of a TFM, with the cracks running radially outward.

Comparative study of CdS thin films deposited by single, continuous, and multiple dip chemical processes

We have used Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), Raman, and photoconductivity to characterize CdS thin films grown by single, continuous, and multiple dip chemical processes. XRD has further shown, without ambiguity, that grown CdS films, independent of the process, in an almost homogeneous reaction free basic aqueous bath have a zincblende crystal structure where reflections from (111), (200), (220), and (311) planes are clearly identified. RBS, Raman, and photoconductivity confirm the high stoichiometry and excellent structural properties with low optically active trap state density of single and continuous dip CdS films. However, they collectively suggest that multiple dip CdS films suffer from defects that act as carrier traps and lead to prolong photoconductivity decay in these films.

Smoothing of YBa2Cu3O7−δ films by ion cluster beam bombardment

Smoothing high-temperature superconductor (HTS) surfaces, especially HTS thin-film surfaces, is crucial for HTS thin-film device processing. In this letter, we describe a method to planarize the surface of a YBa2Cu3O7−δ HTS film down to a smoothness with a standard deviation of 1 nm or better. The method includes first smoothing the HTS surface by ion cluster beam bombardment, followed by annealing in oxygen ambient to regrow the damaged surface layer. Additional YBCO layers can be grown epitaxially on the treated surface, even without removing the top surface layer, which contained some residual damage after annealing. This method can be integrated into HTS circuit fabrication as a key step of planarization.

Large Jc enhancement by ion irradiation for thick YBa2Cu3O7−δ films prepared by photoassisted metalorganic chemical vapor deposition

Thick superconducting films of YBa2Cu3O7−δ grown by photoassisted metalorganic chemical vapor deposition that have shown a decrease of Jc with increasing thickness (>1 μm) have been exposed to ion irradiation in an attempt to enhance Jc. A 4.5 μm thick film irradiated with 1 MeV protons at a dose of 3×1014/cm2 has shown an increase of Jc by a factor of nearly 20–1.7×106u2002A/cm2 corroborating the proposal that a lack of pinning centers in the grown thick films, due to their high crystalline quality, contributed to the observed decrease of Jc with increasing film thickness.

High critical current density in epitaxial HgBa2CaCu2OX thin films

High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been reproducibly fabricated using cation-exchange method. The thin films have pure Hg-1212 phase and have smooth surface morphology. The superconducting transition temperatures of these films are in the range of 120–124 K. The critical current density Jc is up to 3.2×106u2009A/cm2 at 77 K and drops only by a factor of 2 at 100 K and self field. At 110 K, a Jc of 7.8×105u2009A/cm2 has been obtained. X-ray diffraction pole figures show that these films are epitaxially grown on LaAlO3(001) substrates, which is consistent with a χmin of 19% obtained using Rutherford backscattering/channeling analysis.

(α, α) Cross section on 11B for α-energies between 1.0 and 5.3 MeV

Abstract Cross sections for θlab = 165° of He-ion backscattering on 11B in the 1.0–5.3 MeV energy range were measured. Self-supported films of 500 A from enriched 11B were used as targets. A new resonance at 3.87 MeV was observed and the resonances at 4.19 and 4.25 MeV were resolved. Angular distributions were measured at useful resonance energies of 2.62, 3.69 and 3.87 MeV for ion beam analysis. The cross sections are presented in graphic and tabular forms with ±2% uncertainty in the relative cross section.

Cross sections for light element analysis by non-Rutherford scattering

Abstract The non-Rutherford cross sections of light elements, from Be to F, for θlab = 165° are reviewed. New results for cross section measurements of Be, B and other light elements at θlab = 165° in the energy range of 1.0–5.3 MeV are presented. Non-Rutherford backscattering for light elements and Rutherford backscattering for heavy elements were used to measure stoichiometries, depth profiles and light element contaminants in thin films of different compounds. The suitable energy range of the α-particles used for major light elements analysis is discussed. In the 3–10 MeV energy range, significant light element sensitivity enhancements were observed, while the heavy elements showed classical Rutherford behavior.

Point Defect Engineering and Its Application in Shallow Junction Formation

Point defect engineering using high-energy ion bombardment can be used as a method to inject vacancies near the surface region with excessive interstitials created near the end of the projected range deep inside the substrate. We demonstrate that implantation of MeV Si ions into a Si substrate can suppress boride-enhanced diffusion normally associated with a high B concentration layer. B atoms from a surface-deposited boron layer experienced a suppressed diffusion during annealing if the Si substrate was preimplanted with MeV Si ions, and the retardation of B diffusion became more effective with higher dosages. The concept of boron diffusion control was used as an approach to form ultrashallow junctions.

Rutherford backscattering spectrometry: reminiscences and progresses

This paper documents a few historical events important to the development of Rutherford backscattering spectrometry (RBS). The classical experiment of Geiger and the atomic model and theoretical understanding of Rutherford on alpha-scattering are fundamental to the backscattering technique. The rapid growth of atomic and nuclear physics from the 1920s to the 1960s with the development of tools and methods such as accelerators and solid state detectors all contributed to improvements in and proliferation of RBS applications. Research in semiconductors and in thin-film areas especially benefited from the development of RBS. Recent trends to extend RBS into a broader region of energies and a broader selection of projectiles will also be discussed.

Thermal neutron irradiation of 6Li doped Y1Ba2Cu3O7−δ and flux pinning enhancement

Abstract Up to 25 times enhancement of magnetic critical current density has been observed at 70 K and 2500 Oe for 3% 6 Li-doped sintered YBCO sample irradiated with thermal neutrons at a dose of 1 × 10 18 n/cm 2 . The energetic fusion fragment of 6 Li after capturing a thermal neutron can create a uniformly distributed radiation damage. The short life time and small quantity of the radioactive byproducts of the thermal neutron irradiation make this technique quite feasible for practical applications.