**1. Introduction**

196 Heat Treatment – Conventional and Novel Applications

Journal of Crystal Growth 310: 3800-3803.

[37] Yang X, Li H, Bi Q, Cheng Y, Tang Q, Su L, Xu J (2008) Growth of Highly Sensitive Thermoluminescent Crystal α-Al2O3: C by the Temperature Gradient Technique.

> Pulsed laser deposition (PLD) has become a potential method in fabricating highly quality superconducting thin films suitable for electronic applications such as in Josephson junctionbased electronics and in second generation coated conductors [11, 12, 14, 20, 23]. PLD of high *Tc* superconductors generally utilize excimer lasers in the ultraviolet (UV) range [6, 11] . However, excimer lasers use toxic gases such as Cl and F for excitation. In contrast, flash lamp pumped Nd:YAG laser can provide stable power and better beam profile [14, 20]. Nd: YAG lasers are also easy to operate and have low maintenance costs [2, 14, 20]. To date, the third harmonic (355 nm) and fourth harmonic (266 nm) of the Nd:YAG has been used to grow high quality high-Tc superconducting films [12, 14, 20].

> In UV PLD of Bi-Sr-Ca-Cu-O films, the substrates are usually heated to 8000*C* followed by in-situ and ex-situ post heat treatments in gas atmospheres [1, 13, 22, 29]. Reports have shown that Bi- content of the film is greatly influenced by the substrate temperature and can be highly deficient at higher temperatures [1, 29]. In some cases, heat treatment during deposition results in the contamination of the film especially on Si substrates [6]. PLD of *YBa*2*Cu*3*O*7−*<sup>δ</sup>* using UV lasers produce films with *Tc* of 90 K require substrate heating ranging from 700- 8000C in a background *O*<sup>2</sup> gas (pressure of ranging from 100 to 200 mTorr) [6, 7, 32]. This is usually done since Y-Ba-Cu-O is highly dependent on oxygen content. This process is either performed in-situ or ex-situ and part of the post heat treatment [4, 18]. These results implies that post heat treatment is necessary to homogenize the composition of the film and improve the critical temperarure *Tc* [1, 4, 19]

> Recently, we reported the fabrication of micron thick *Bi*2*Sr*2*CaCu*2*O*8+*<sup>δ</sup>* (Bi-2212) and Yttrium doped Bi-22Y2 through PLD with a 1064 nm Nd:YAG laser [9, 10]. The films underwent heat treatment outside the PLD growth chamber to produce flat and highly c-axis oriented films with stoichiometries identical to the targets. In the case of Bi-2212, the measured *Tc* is

©2012 De Vero et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ©2012 De Vero et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

only about 58 K and for Bi-22Y2, the highest *Tc* is 90.5 K at 25% Y concentration [9, 10]. Y substituted Bi-2212 films grown by IR PLD show drop in magnetoresistance and improved critical current density *Jc* [3].

Bi-Sr-Ca-Cu-O is observed. The average thicknesses of the films are 9 *μ*m. In figure 1b, films heated at 940 ◦C for 15 min and the rapidly quenched films show pronounced layering and terraces of Bi-Sr-Ca-Cu-O. This makes the film rougher compared to partially melted Bi-2212 films. The rapidly quenched films is thinner compared to partial melted films with an average thickness of about 2 *μ*m. Both films show small amount of spheroidal particulates after heat

on Ceramic Superconducting Films Produced by Infrared Nd:YAG Pulsed Laser Deposition

199

Post Deposition Heat Treatment Eff ects

**Figure 1.** SEM surface micrographs of Bi-2212 films subjected to (a, c) partial melting and (b,d) rapid

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

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 [10]. This attributed to the micron- size spheroidal grains trasferred on the substrate by the IR laser during deposition [10]. Hence, heat treatment is required to facilitate growth, flatten and densify the material producing

Figure 3 shows the resistance vs temperature measurement on the partial melted Bi-2212 films with transition temperature, *Tc* of about 79 K. In our previos report, partial melting with subsequent annealing for 10 hours results into *Tc* of only about 58 K. The shorter annealing

quenching. Both films were subquently annealed.

much thinner films.

subjected to partial melting indicating higher crystalline quality.

treatment [10].

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 *Bi*2*Sr*2*CaCu*2*O*8+*<sup>δ</sup>* and *YBa*2*Cu*3*O*7−*<sup>δ</sup>* prepared by IR Nd:YAG PLD.
