1PO4-10 Fabrication and characterization of underdoped YBCO thin films

G.A. Farnan, M.P. McCurry, and D.G. Walmsley, Department of Pure and Apllied Physics, Queen's University, Belfast, BT7 1NN, Northern Ireland

Presenting Author: G.A. Farnan

Thin Films of YBa₂Cu₃O_7-d have been deposited on single crystal (100) MgO by Pulsed Laser Deposition. AFM imaging shows a spiral growth mode. The influence of target hardness and oxygenation, substrate surface morphology and deposition rate are discussed, and the best deposition conditions for optimally doped films are given. The films have been controllably deoxygenated by sintering at 550C then annealing in a reduced pressure of O₂. By altering the oxygen pressure we can vary T_c between 88K and 0K; the process is reproducible, controllable and reversible. XRD shows that reduction in T_c is accompanied by systematic elongation in c-axis length. These data facilitate an inference of d , which we believe is of comparable accuracy to other methods. Resistance versus temperature measurements show a departure from linearity at a temperature T* which has been widely identified with pseudogap formation. T* has been determined as a function of d .

1PO4-11 Scanning Tunneling Microscopy and Spectroscopy of Particulates on YBCO Thin Films Prepared by Laser Ablation

Y. Fan and Alexender G. Fitzgerald, Department of Applied Physics and Electronics & Mechanical Engineering, University of Dundee, Dundee DD1 4HN, Scotland, United Kingdom

Presenting Author: Y. Fan

Details of the morphology, microstructure, and tunneling beahiour of growth spirals, particulates and precipates on the laser ablated YBa₂Cu₃O_7-x thin films have been studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). High resolution STM images show that the particulates caused b laser ablation still preserve their original crystal structure. Some well defined crystal surfaces formed by cracking or cleaving can be clearly observed. These observations indicate that the material removal by laser ablation undergoes an explosion process. The existence of particulates on film seriously hinder the spiral growth or layer by layer growth of the crystal grains in their vicinity resulting in low critical current.

STS studies show that the tunnelling behaviour of these particulates at room temperature is also different from that of the surrounding films. Unlike the epitaxial growth spiral or layer-cake like YBCO crystal grains, when the observed films were negatively biased there was no detectable electron tunneling through the gap from the particulate surface to the tunneling tip. Some precipitates have a similar tunneling behaviour to the particulates. These distinct tunneling characteristics of the different components in the films can be used to distinguish superconducting or non-superconducting phases at the nanoscale level.

1PO4-12 Scanning Probe Microscopy Studies of the Substrate Annealing Effect on YBa₂Cu₃O_7-x High T_c Superconducting Thin Film Growth

Yongchang Fan and A.G. Fitzgerald, Department of Applied Physics and Electronics & Mechanical Engineering, University of Dundee, Dundee DD1 4HN, United Kingdom

Presenting Author: Y. Fan

Substrate annealing effects on the growth of YBa₂Cu₃O_7-x high T_c Superconducting thin films have been studied by AFM and STM observations of the thin films prepared on the non-annealed and annealed substrates. Comparative AFM and STM image observations of the YBCO thin films show that the well-defined substrate surface conditions obtained after high temperature annealing play a dominant role in governing the film growth mechanism which determines the film's microstructure and surface morphology. The high density of terrace steps formed in high temperature annealing process provide favourable nuclei sites for deposited species and favour the YBCO grains to grow in a two dimensional step flow mode. In general, the YBCO thin films grown on the high temperature annealed substrates have exhibited an improved superconducting properties with a higher transition temperature T_c and higher critical J_c values. These studies show that by processing the substrate properly, it is possible to manipulate the growth mode of YBCO thin films in a controllable manner to achieve the required electrical and surface morphological properties and this will become very important in developing future high T_c superconducting device based on the High T_c superconducting films.

1PO4-13 Switching of YBCO thin films into the dissipative state at high current densities

Catherine Villard 1, Daniel Bourgault 1, Pascal Tixador 1, Laurent Carbone 2, and Marie F. Devishmes 2. 1 Centre National de la Recherche Scientifique/ Corsortium de Recherche pour l'Emergence des Technologies Avancees, 25 Avenue des Martyrs, 38042 Grenoble Cedex 9, France. 2 Schneider Electric S.A. Usine A, rue Volta, 38050 Grenoble cedex 9, France.

Presenting Author: C. Villard

We report on the switching properties at high current densities of YBCO thin films shunted by an Ag layer.The experiments are performed either in liquid nitrogen or liquid argon (87K). Different transition regimes associated to specific electric field thresholds are observed. The nature of the thermal exchange with the bath and the role of the silver shunt and substrate are discussed. While only two regimes in the current-voltage characteristics are observed in liquid nitrogen, successively reversible and hysteretic, an intermediate behavior is observed at 87K associated with a total current derivation by the silver layer. The transition properties with different current injection modes (continuous or pulsed) are compared.

*1PO4-14 High critical currents and the dislocation formation mechanism in pulsed laser deposited YBa₂Cu₃O_7-d films

B. Dam, J.M. Huijbregtse, F.C. Klaassen, J.H. Rector, and R. Griessen, Faculty of Science, Division of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, NL-1081 HV Amsterdam, The Netherlands

Presenting Author: B. Dam

The high critical current densities in YBa₂Cu₃O_7-d thin films at low magnetic fields are due to strong pinning by natural linear defects [1]. Identifying dislocations by means of chemical etching, we find that the matching field increases linearly with the number of dislocations, which are predominantly formed in the first stage of epi-growth. Although we can tune the dislocation density by the deposition parameters, it was sofar not clear how these dislocations form. In films <10 nm these dislocations have already formed. Hence, the structural relaxation process (occuring at a thickness >10nm) is not responsible for the formation of dislocations normal to the film surface. Indeed, on MgO which has a much higher lattice mismatch, we find similar dislocation densities as on SrTiO₃. As secondary phases can also act as a dislocation source we investigated the nucleation of YBa₂Cu₃O_7-d on SrTiO₃. We find a remarkably high density of secondary phases in the first monolayer, which in some cases is ordered along the substrate steps. This opens the way to tune both the order and the density of dislocations by creating a high density of secondary phases at the substrate, before actually growing the superconducting film.

[1] B. Dam et al., Nature 399 (1999) 439.

1PO4-15 Fabrication of Y-Ba-Cu-O Micro-Particles by Excimer Laser Processing

G.P. Luo, C.L. Chen, and C.W. Chu, Texas Center for Superconductivity and Department of Physics, University of Houston, Houston, TX, 77204

Presenting Author: G.P. Luo

Crystallization of Y-Ba-Cu-O was investigated with a pulsed excimer laser irradiation on amorphous Y-Ba-Cu-O films. The amorphous Y-Ba-Cu-O films were synthesized on (001) silicon substrates by pulsed laser ablation at room temperature. Periodical distribution of Y-Ba-Cu-O grains were obtained with 5-20 laser pulses in energy density of 0.1-0.2 J/cm². Scanning electron microscopy studies revealed that semi-spherical grains were formed on the films, and the grains became stable for the films with 20 laser pulses. The diameters of the Y-Ba-Cu-O grains were found to be dependent on the film thickness as well as the total irradiation energy. Sub-micro Y-Ba-Cu-O semi-spherical particles with diameters of 250 nm have been obtained in 200 nm-thick Y-Ba-Cu-O films.

1PO4-16 One-Step Double-Side YBaCuO Films Deposited by Shadowed Off-Axis PLD

Eugene M. Kaidashev 1, Valery G. Dneprovski 1, and Irina N. Zakharchenko 2. 1 Mechanics and Applied Mathematics Research Institute, University of Rostov, Rostov-on-Don, 344090, Russia. 2 Institute of Physics, University of Rostov, Rostov-on-Don, 344090, Russia.

Presenting Author: E.M. Kaidashev

Single crystal YBa₂Cu₃O_7-x (YBCO) films were prepared by shadowed off-axis pulsed laser deposition on one and double sides of (001) MgO substrate. The shadowing of the growing surface from bombardment by the high energetic ions of laser plasma and film growth only from the diffusion flow very improve film surface morphology in the case of shadowed off-axis pulsed laser deposition. Another important advantage of shadowed off-axis PLD is the possibility of one-step double-sides film deposition. In the work the structure, morphology of a surface and superconductive properties of YBCO films were investigated. The substrate surface was placed in a position parallel to the plasma plume. A rectangle shadow mask was located between target and substrate reduced the quantity of large particles deposited from diffusion flow. The laser’s beam was scanned on the rotating YBCO target surface. The scanning line was parallel to the longest side of the mask. Film structure morphology and superconductivity properties was investigated in dependence of mask width. For optimization of film growth conditions, the research methods of the time-of-flight optical spectroscopy, X-ray analysis, SEM and researches of film superconductive properties were used. Smooth and droplet-free superconductivity YBCO films were prepared on (001) MgO substrates. Such a geometry of PLD may be used for one-step double-side thin films deposition for microwave applications and for the manufacture of YBCO surface – coated tapes.

1PO4-17 The growth mode and microstructure of Ag-doped YBa₂Cu₃O_7-d thin films prepared by dual beam pulsed-laser deposition

C.K. Ong, S.Y. Xu, L.P. You, and J. Li, Centre for Superconducting and Magnetic Materials, and Department of Physics, National University of Singapore, Lower Kent Ridge Road, 19260, Singapore

Presenting Author: C.K. Ong

Ag-doped c-axis YBa₂Cu₃O_7-d (Ag/YBCO) thin films were fabricated by using dual-beam pulsed-laser deposition (DBPLD), and were systematically characterized by means of 4-probe method, SEM, AFM, AES, m -PIXE, RBS, HREM, etc.

When a small amount of silver was doped, the current density J_c of the film was obviously enhanced. With the increase of silver amount in the film, the values of J_c of the films first increased, then decreased. Under high substrate temperature, we observed bar-like structures in the films, with lengths up to 150 m m. Most of them oriented with their longitudinal edges along or at 45° to the in-plane a/b axes of the film. Silver was found aggregated at the positions of the bars oriented along a/b axes of the film, but not around those oriented at 45° to the a/b axes. The bars mainly consisted of well-crystallized a-axis YBCO grains, leaving aside a mixture of polycrystalline and amorphous structure. Sharp interface structures with various angles between the bar and the YBCO c-axis film were observed. The formation of the bars was found closely related to the atomic steps and defects on the substrate surface.

A dopant-assisted the growth mechanism was presented to interpret the results.

1PO4-18 Fabrication of YBCO/STO/YBCO multilayer by PLD

T.J. Hwang 1, D.H. Ha 2, D.H. Kim 1, K.W. Lee 2, and Y.K. Park 2. 1 Department of Physics, Yeungnam University, Kyungsan, Kyungpook 712-749, Korea. 2 Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon, 305-600, Korea.

Presenting Author: K.W. Lee

We have measured the electrical properties of YBa₂Cu₃O_y/SrTiO₃/YBa₂Cu₃O_y (YBCO/STO/ YBCO) multilayer prepared by the pulsed laser deposition method under the various deposition conditions of insulating STO layer. Superconducting properties of both YBCO layers are sensitively affected by the deposition temperature, T_STO, of intermediate STO layer. T_c of bottom YBCO layer is decreased due to the oxygen loss for T_STO = 780° C, but the superconductivity of upper YBCO layer is severely deteriorated due to the poor crystalline quality for low T_STO. However, we have successfully fabricated YBCO/STO/YBCO multilayer with T_c(zero) above 85 K for both YBCO micro-bridges of 50 m m width by depositing the intermediate STO layer progressively at T_STO = 550° C and 780° C. The resistivity of both YBCO layers at room temperature is below 0.3 mW cm.

*1PO4-19 NdBaCuO thin films for Josephson devices

Paul Seidel, Alexander Waldauf, Kai-Uwe Barholz, Jens Scherbel, Frank Schmidl, and Sven Linzen, Institut für Festkoerperphysik, Friedrich-Schiller-Universitaet Jena, Helmholtzweg 5, D-07743 Jena, Germany

Presenting Author: P. Seidel

We have prepared NdBaCuO (Nd-123, NBCO) thin films for Josephson junctions and dc-SQUIDs on strontium titanate bicrystal substrates with a misorientation angle of 24 degrees. The superconducting films were fabricated by pulsed laser deposition (PLD). To increase the superconducting properties of the NBCO films, we used PrBaCuO (Pr-123, PBCO) as a template layer. Quality was characterized by X-ray diffraction, Rutherford backscattering spectroscopy and energy dispersive X-ray analysis. The highest T_c of these films was about 94 K, about 8 K higher than without this template layer. The minimum backscattering yield was about 4 percent and rocking curves of both NBCO as well as PBCO (005)-peaks had FWHM values of 0.03 degrees. Josephson junctions and DC-SQUIDs were patterned by ion beam etching. The Josephson junctions showed a clear resistively shunted junction (RSJ) type behaviour up to 60 K and SQUID modulation was observed.