Pulsed laser deposition (PLD) has become a potential method in fabricating highly quality superconducting thin films suitable for electronic applications such as in Josephson junction-based electronics and in second generation coated conductors [11, 12, 14, 20, 23]. PLD of high
In UV PLD of Bi-Sr-Ca-Cu-O films, the substrates are usually heated to 800°
Recently, we reported the fabrication of micron thick
The primary motivation of these previous works is to use the existing Nd:YAG laser in PLD experiments to avoid the complicated optics and gas systems of an excimer laser based PLD. Also, when fundamental wavelength of the Nd-YAG laser is used for deposition, films with the same chemical composition as the starting material can be fabricated, and therefore it is versatile in the deposition of multicomponent films. The film properties can be adjusted through heat treatment steps after deposition.
In this chapter, we examine effect of post heat treatment on the the morphology, composition, crystallinty of the
2. Infrared Nd:YAG laser ablation and post heat treatment
The laser system used in this study is a Q-switched Nd:YAG (Spectra Physics GCR 230) operating at 1064 nm at 10 Hz repetition rate with 8 ns pulsed duration. The deposition was performed in a stainless steel vacuum chamber continuously evacuated to maintain a pressure of 10-2 mbar. The solid state sintered target is placed 30 mm from the substrate and was rotated for uniform laser ablation. The Bi-Sr-Ca-Cu-O films were grown on (100) MgO substrate with laser fluence of 5.5 J/cm2, while Y-123 films on (100)
For Bi-Sr-Ca-Cu-O films, two heat profiles in ambient air was performed. Some were partial melted at 880 °C and some were heated at 940 °C for 15 minutes and rapidly quenched to room temperature. Both of the heat profiles are seconded with annealing at 850 °C for 2 hour. In the case of Y-123, three heat treatment steps were performed, first the films were re-sintered at 900 °C for 12 hrs followed by heating at 1000 °C for 15 minutes and rapid thermal quenched to room temperature. The last heat treatment involves, heating on a tube furnace with oxygen at 930 °C for 12 hours, and annealed at 450 °C for 2 hours in ambient air.
Scanning electron microscopy (SEM) were used to examine the surface morphological features and composition of the films. Film thickness was determined by SEM cross- sectional imaging. X-ray Diffraction (XRD) were used to investigate the composition and crystal properties of the film. To verify superconducting property of the films, linear four point probe resistance measurement were performed.
3. Post deposition heat treatment effects
3.1. Bi-Sr-Ca-Cu-O films
SEM surface micrographs of partial melted and rapidly quenched Bi-2212 films grown with 5.2
XRD measurements on the partial melted and rapidly quenched Bi-2212 films is shown in figure 2. The peaks are indexed using the card file no. 41-0317. Both films are highly c-axis oriented with minimal Bi-2201 impurity. However, sharper XRD peaks are observed for films subjected to partial melting indicating higher crystalline quality.
The rough morphology of films subjected to rapid quenching at very high temperature is due to very fast cooling of the Bi-2212 material. It has been observed that IR PLD Bi-2212 films require partial melting and annealing to allow uniform diffusion and migration of Bi-2212 materials on MgO substrate forming smoother film . This attributed to the micron- size spheroidal grains trasferred on the substrate by the IR laser during deposition . Hence, heat treatment is required to facilitate growth, flatten and densify the material producing much thinner films.
Figure 3 shows the resistance vs temperature measurement on the partial melted Bi-2212 films with transition temperature,
Figure 4 shows the SEM micrographs of (a) a representative as- deposited films grown using 2.0
At a higher magnification (fig. 4d), the surface of the oxygen annealed Y-123 film show that the grains interconnect laterally forming larger particulates with extended grain boundaries (indicated by squares). This is a common feature of Y-123 films especially when solid state sintered targets are used as the material source for the deposition [15, 21]. While the heat treatment helps in densifying the grains, it also aids in coalescence of grains and forming an alignment of the material relative to the substrate orientation . The appearance of rectangular grains could be a result of incongruent melting of Y-123 at rapid thermal heating at 1000 °C and the thickness of the films. The surface of the film also indicates that the film is not fully melted at that temperature. This is partly due to micron-sized grains transferred by IR laser pulses that needs higher temperature to completely melt. This also attributed to the high melting temperature of Y-123 of about 1200 °C. Hence, we can infer from the SEM image that film is at the initial stage of growth.
The observed morphology of the Y-123 film is due to the micron sized particulates generated by IR PLD. It has been reported that large YBCO particulate are difficult to merged into YBCO films completely especially in the surface of a thick film where the mobility and heat exchange rate are lower than that in the substrate surface, hence they will hinder the c-axis growth of surrounding film . Larger YBCO grains in the as-deposited films could provide sites for the nucleation of a-axis grains [25, 34]. Since large chunks of the target are able to arrive on the substrate surface forming inherently thick films, preferential a-axis formation is observed when you subject to heat treatment. In contrast, UV PLD generates ultrafine particles that lands on heated substrate forming thinner film with c-axis orientation. We are also unable to see micro-cracks initially observed in the as-deposited films. This is due to annealing that result to removal of pores and enlargement of grains [9, 10]. The growth of a-axis grains also contribute to dispersion of micro-cracks .
Figure 5 shows the XRD spectra of Y-123 films without and with oxygen annealing. The XRD pattern for both films are composed of c-axis and a-axis oriented Y-123 grains. It contains no other diffraction peaks from the precursors of the Y-123 material. This indicates high phase purity of the film. The reflection corresponding to (006) and STO (200) is hard to distinguish due to the small lattice mismatch of Y-123 with STO (about 2%). The value of c-lattice parameters for film not subjected to oxygen annealing is about c= 11.77 while for oxygen annealed film, c= 11.68. The value of the c-lattice for oxygen annealed film is similar to the oxygen rich Y-123 (c= 11.68) . The variation in c-lattice can be attributed to the decrease in the fraction of oxygen gas in YBCO material . Hence, oxygen annealing is necessary for the as-deposited films.
The mixture of a-axis and c-axis is due to the thickness of the film and partly due to incongruent melting of Y-123 at 1000 °C. It has been reported that Y-123 films possess mixture of a-axis and c-axis growth as a result of increasing film thickness . In contrast to previous reports that the critical thicknesses to obtain crack free Y-123 is about 2.2
Figure 6 shows the resistance vs. temperature measurement on Y-123 films. Without oxygen annealing the film shows semiconducting behavior (fig. 6a) . This attributed to oxygen deficiency in the film. In contrast, the oxygen annealed film (fig. 6b) shows superconducting transition temperature
The a-axis outgrowths are typically observed in the c-axis oriented thick films. The a-axis outgrowth inhibits transport of the superconducting currents in the films resulting into a low value of
In summary, the heat treament greatly influence the growth and surface morphology of IR PLD films. Since the films are deposited on un-heated substrates, the habit of the material upon heat treatment becomes an important parameter in choosing heat treatment profiles. Layering and grain movement during high temperature melting can be reduced by exposing the film at temperatures closer to the melting temperature and introduction of annealing steps in an environment conducive for uniform coalesnce and intake of oxygen. The temperature used for Y-Ba-Cu-O is low enough to see the initial stage of growth and will allow us to implement a heat profile that will melt and provide sufficient oxygen on the film.
The post heat treatment studies on IR Nd:YAG PLD films is an important stage in developing the technique to be a competitive and alternative technique in producing high
J. C. De Vero acknowledge the Philippine Commission on Higher Education, the Office of the Chancellor in collaboration with the Office of the Vice Chancellor for Research and Development, of the University of the Philippines Diliman, and the National Research Council of the Philippines for funding support.
Arnold J. Pfuch A. Borck J. Zach K. Seidel P. 1993Preparation and characterization of Bi2Sr2
Awaji S. Watanabe K. Badica P. Togano K. 2006Growth of superconducting
Blanca G. R. S. Vero J. C. D. Garcia W. O. Sarmago R. V. 2012 Enhanced flux pinning in ir pld grown bi-2212 films, doi:j.physc.2012.03.045.
Branescu M. Vailionis A. Huh J. Moldovan A. Socol G. 2007 Afm and complementary XRD measurements of in situ grown YBCO films obtained by pulsed laser deposition
Branescu M. Ward I. Huh J. Matsushita Y. Zeltzer G. 2008 Scanning electron microscopy and resistive transition of in-situ grown YBCO films by pulsed laser deposition
Chrisey D. B. Hubler G. K. 1994 Pulsed Laser Deposition of Thin FilmsJohnWiley and Sons Inc.
Christen H. M. 2005Pulsed Laser Deposition of YBa2Cu3O 7for Coated Conductor Applications: Current Status and Cost Issues in Second Generatoin HTS Conductors, A. Goyal (Ed.), Kluwer Academic Publisher.
Dam B. Rector J. Chang M. F. Kars S. de Groot D. G. Griessen R. 1997The laser ablation threshold of
De Vero J. Blanca G. Vitug J. Garcia W. Sarmago R. V. 2011 Stoichiometric transfer of material in the infrared pulsed laser deposition of yttrium doped Bi-2212 filmsPhysica C: Superconductivity 471 378 383
De Vero J. Gabayno J. Garcia W. Sarmago R. V. 2010Growth evolution of Bi2Sr2
Eason R. 2007 Pulsed Laser deposition of Thin Films: Applications- Led Growth of Functional MaterialsJohn Wiley and Sons, Inc.
Ichino Y. Yoshida Y. Yoshimura T. Takai Y. Yoshizumi M. Izumi T. Shiohara Y. 2010Potential of Nd: YAG pulsed laser deposition method for coated conductor production, Physica C: Superconductivity 470 1234 1237
Jannah A. N. Halim S. A. Adbullah H. 2009 Superconducting properties of bscco thin films by pulsed laser deposition
Kaneko S. Shimizu Y. Yuasa H. Ohya S. 2002Change in stoichiometry by laser-induced plasma deposition of high-tc superconducting thin films,
Kastner G. Schafer C. Senz S. Kaiser T. Hein M. A. Lorenz M. Hochmuth H. Hesse D. 1999Microstructure and microwave surface resistance of typical YBaCuO thin films on sapphire and LaAlO3, Superconductor Science and Technology 12 366 375
Kim C. H. Hong K. S. Kim I. Hahn T. Choi S. 2000Effects of target microstructure on pulsed laser deposited
Kim C. H. Hong K. S. Kim I. T. Hahn T. S. Choi S. S. 1999Comparison of microstructures of pulsed laser deposited
Kim S. Lee S. 1999 Characterization of YBCO superconducting films fabricated by pulsed laser deposition
Kume E. Fukino H. Zhao X. Sakai S. 2004Temperature dependence of composition ratio of bi2sr2
Kusumori T. Muto H. 1999Fabrication of high quality YBCO epitaxial films by ablation using the fourth harmonic of the Nd: YAG laser,
Kusumori T. Muto H. 2001Change in stoichiometry by laser-induced plasma deposition of high-Tc superconducting thin films,
Li S. Ritzer A. Arenholz E. Bauerle D. Huber W. Lengfellmer H. Prettl W. 1996Step-like growth ofBi2Sr2
Marechal C. Lazaze E. Seiler W. Perriere J. 1998 Growth mechanisms of laser deposited BiSrCaCuO films on MgO substrates
Mohan R. Sighn K. 2007Calcium and oxygen doping in
Morimoto K. Takezawa K. Minamikawa T. Yonezawa Y. Shimizu T. 1998 Low-temperature growth of YBCO thin films by pulsed laser ablation in reducing environment
Nie J. C. Yamasaki H. Develos-Bagarinao K. Nakagawa Y. 2007Surface morphology and microstructure of thick
Park J. H. Jeong Y. S. Lee S. Y. 1998 Investigation on the relation between the thickness and the orientation of epitaxially grown YBCO thin films by laser ablation
Prouteau C. Verbist K. Hamet J. Mervey B. Hervieu M. Raveau B. Tendeloo G. B. 1997 Microstructure of a-axis oriented YBCO films on SrTiO3 substrates using a new template layer La4BaCu5O13
Rossler R. Pedarnig J. D. a. C. J. 2001Bi2Sr2
Vonk V. van Reeuwijk S. J. Dekkers J. M. Harkema S. Rijnders A. J. H. M. Graafsma H. 2004 Strain-induced structural changes in thin YBa2Cu3O7-x films on SrTiO3 substrates
Yamada Y. Kamashima J. Wen J. G. Niiori Y. Hirabayashi I. 2000 Critical thickness and effective thermal expansion coefficient of YBCO crystalline film
Yavuz M. Uprety K. K. Subramanian G. Paliwal P. 2003 Preparation and characterization of BSCCO 2212 thin films
Zheng R. Campbell M. Ledingham K. Jia W. Scott J. Singhal R. 1997Diagnostic study of laser ablated
Zhu D. Huang J. Li H. M. S. Su X. 2009A new method to preserve the c-axis growth of thick