Effect of seed layers on the preparation of SrTiO3 buffer layers on Ni tapes via sol–gel method

Effect of seed layers on the preparation of SrTiO3 buffer layers on Ni tapes via sol–gel method

Physica C 415 (2004) 57–61 www.elsevier.com/locate/physc Effect of seed layers on the preparation of SrTiO3 buffer layers on Ni tapes via sol–gel metho...

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Physica C 415 (2004) 57–61 www.elsevier.com/locate/physc

Effect of seed layers on the preparation of SrTiO3 buffer layers on Ni tapes via sol–gel method X.B. Zhu a

a,*

, L. Chen a, S.M. Liu a, W.H. Song a, Y.P. Sun a, K. Shi b, Z.Y. Sun b, S. Chen b, Z. Han b

Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China b Applied Superconductivity Research Center of Tsinghua University, Beijing 10084, PR China Received 8 January 2004; received in revised form 20 May 2004; accepted 25 July 2004 Available online 3 September 2004

Abstract SrTiO3 (STO) buffer layers with different STO seed layers on Ni (2 0 0) substrates are fabricated using the spinning coating technique. It is found that the seed layers can remarkably affect the orientation of the subsequent STO precursor layer and the relationship between the orientation of the STO layer and the seed layer is discussed. Ó 2004 Elsevier B.V. All rights reserved. PACS: 68.55. a Keywords: Buffer layer; SrTiO3; Sol–gel; Texture; YBCO

1. Introduction YBa2Cu3O7 d (YBCO) coated conductors have a great potential for applications due to their high critical current density and strong flux pinning at liquid nitrogen temperature [1]. Currently, two competing methods are being explored for fabrication of YBCO coated conductors; namely, the ion beam assisted deposition (IBAD) and the rolling

*

Corresponding author. Tel.: +86 551 5591436; fax: +86 55 5591434. E-mail address: [email protected] (X.B. Zhu).

assisted textured substrates (RABiTS) processes [2]. The RABiTS process is a non-vacuum method, with lower requirement to the equipments, being easy to be scaled up. Additionally, it has been reported that using an all-chemical method, YBCO coated conductors have critical current densities exceeding 106 A/cm2 at 77 K and zero applied magnetic field [3]. To fabricate YBCO coated conductors, it needs a suitable buffer layer which plays an important role in the sandwich structure for its ability to prevent diffusion between the metal substrate and the YBCO film as well as to transfer texture of the substrate. Until now, it has been reported that

0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.07.018

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various buffer layers can be grown on textured Ni substrates via solution-based methods [4–11]. Among these various buffer layers, SrTiO3 (STO) has the character of perovskite structure and is stable. In addition, the lattice parameter of 0.3905 nm of STO provides a very close lattice match with YBCO (2.3% mismatch) and Ni substrate (11.1% mismatch). Therefore, STO is considered to be one of the most suitable buffer materials for the preparation of YBCO coated conductors. Many research works have been published focusing on the sol–gel processing approaches for STO buffer layers [12,13]. To obtain high quality STO buffer layers, Siegal et al. [3,14] pointed out that a very thin template serving as seed layer was first grown to provide orientation followed by the growth of subsequent precursor STO layers to provide thickness. However, what solution concentration for seed layers is the most suitable to fabricate STO buffer layers has not been investigated until now. In this work, we have grown STO buffer layers on textured Ni substrates using the spin-coating technique from a solution-based precursor. The motivation of this study was to investigate the effects of solution concentration of the seed layers on the orientations of the subsequent precursor STO buffer layers by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) methods.

2. Experimental The detailed solution preparation for STO could be found elsewhere [14]. Briefly, the STO solutions were produced by reacting titanium isopropoxide with acetylacetone before combining it with a solution of Sr acetate dissolved in trifluoroacetic acid (TFA) and diluted by acetylacetone to the desired concentration. In our work, the seed layer solution concentration was 0.04, 0.065, 0.08 M and the solution concentration for the subsequent STO precursor layer was 0.25 M. A spinning rate of 4000 rpm and time of 60 s were used in the process of deposition both for the seed layers and the subsequent precursor layers on Ni (2 0 0) tapes, followed by a hot-plate treat-

ment at 300 °C. These dried films were heated quickly to 900 °C and kept at this temperature for 2 h under 4% H2/N2 atmosphere. The treatment for each layer, seed layers as well as subsequent precursor STO layers, was repeated up to the numbers of layers. Typically, a seed layer and two subsequent precursor layers (about 100 nm for each STO layer determined by AFM) were deposited for our samples. For the sake of description, we define here the seed layer samples fabricated using solution concentration of 0.04, 0.065, 0.08 M and no seed layer as samples As, Bs, Cs and Ds, respectively. Correspondingly, the STO samples deposited on these four seed layers are defined as samples A, B, C, and D, respectively. The microstructure was identified by an Inlens detector in a thermal field emission gun scanning electron microscopy (FEG-SEM) LEO1530 equipped with a HKL electron backscattering diffraction pattern (EBSD) detector and a Park Scientific Instruments designed Autoprobe CP type atomic force micrograph (AFM); A Philips XÕpert PRO X-ray diffractometer (XRD) with CuKa radiation was used to carry out the h–2h scan, the out-of-plane x scan and the in-plane u scan investigations.

3. Results and discussion The results (not shown here) of the XRD h–2h scan, the out-of-plane x scan (full width in half maximum, FWHM 7.2°) and the in-plane u scan (FHWM 8.1°) of our Ni (2 0 0) tapes indicates that the Ni (2 0 0) tapes have a strong cubic texture. A series of STO precursor buffer layers are deposited on different seed layers buffered Ni (2 0 0) tapes. The h–2h scans of STO/seed layer/ Ni as a function of seed layer solution concentrations are shown in Fig. 1(a)–(d). The peaks appearing at 32.1° and 46.2° belong to the (1 1 0) and (2 0 0) planes of STO buffer layers and the peaks appearing at 51.7° and 44.3° belong to the cubic Ni substrates. It can be seen that the strongest absolute intensity of STO (2 0 0) peak is attributed to the sample B. However, the STO (2 0 0) peaks of other samples are relatively weak. The results indicate that the crystalline qualities of STO

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Fig. 1. The h–2h scans of the samples A (a), B (b), C (c), and D (d).

buffer layers can be changed when the seed layers are different. The above difference of crystallization qualities of STO buffer layers can be uniquely

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attributed to the effect of seed layer solution concentrations because there are no other differences than the seed layer solution concentrations when fabricating these STO buffer layers. The SEM results (not shown here) show that sample A has a network structure, but that in sample B, the STO buffer layer consists of filaments, and that for sample C and D, the STO buffer layers are relatively dense compared with samples A and B. The differences between pictures for these samples can thus be only attributed to the effects of the seed layer solution concentrations. In order to trace the differences of XRD and SEM patterns of the above samples, the observations of AFM and SEM are carried out to discern the morphology of different seed layers. The SEM patterns are shown in Fig. 2(a)–(d). It can be seen that the grains of sample As and Bs are isolated, with grain sizes about 30–50 nm, and the distance

Fig. 2. The SEM patterns of the samples As, Bs, Cs and Ds, respectively.

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between two grains in sample As is larger than that in sample Bs, i.e., at the same area there are more grains in sample Bs than in sample As. In sample Cs, the grains are also isolated but sparser than in sample As and Bs. Additionally, the grain sizes of sample Cs are not uniform, with grain sizes varying from 40 to 200 nm; for sample Ds finally, there exist no isolated grains but connected crystalline grains. The difference can also be observed in AFM (not shown here). It has been known that the subsequent STO precursor layers nucleate at the grains of seed layers. When the numbers of grains in the seed layer decrease, i.e., the nucleate centers decrease, the crystallization of the subsequent STO precursor layers will degrade. Additionally, as discussed by

Schwartz et al. [15], in the fabricating process of highly oriented films the seed layer should be thin enough to suppress the grain growth along the thickness orientation. Combined with our experimental results, we suggest that although the grain growth along the thickness orientation of the seed layer fabricated with low solution concentration, i.e., 0.04 M in our experiments, is strongly suppressed, the nucleation centers for the subsequent STO layer are too sparse to induce all grains in the subsequent STO layer to nucleate at these oriented nucleation centers resulting in weak crystallization as well as weak orientation. At an intermediate solution concentration, i.e., 0.065 M in our experiments, the orientation of the seed layer is high due to the suppression of grain

Fig. 3. The SEM (a, b), the AFM (c) and the out-of-plane x scan pattern (d) of the sample B. From the SEM, it can be seen that the STO buffer layer consists of filaments with relatively dense structure. The root-mean-square (RMS) roughness of 10.1 nm obtained from the AFM indicates that the STO buffer layer is relatively smooth. The FWHM of 5.3° of the out-of-plane x scan reveals that the STO buffer layer has a strong out-of-plane orientation.

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growth along the thickness orientation, additionally, the nucleation centers for the subsequent STO layer are sufficient resulting in the strongest crystallization quality and orientation of the STO buffer layer in our experiments. At the high solution concentration of seed layers, i.e., 0.08 M and no seed layer condition, the decreasing of the nucleation centers for the subsequent STO layers as well as the suppression of grain growth along the thickness orientation in seed layers led to the weak crystallization qualities and orientations for the subsequent STO layers. Based on the analysis as well as the experimental results, we suggest that there exists an optimal solution concentration for the seed layers to obtain high quality STO buffer layers, i.e., 0.065 M in our experiments. As a result of the most excellent oriented STO buffer layers (sample B), SEM at different magnifications, AFM and the out-of-plane x scan are carried out and the results are depicted in Fig. 3. It can be observed that oriented, crack-free, relatively dense and smooth STO buffer layers can be obtained in our experiments.

4. Conclusion STO buffer layers were fabricated on different seed layers on textured Ni tapes using the spincoating technique. It was shown that the seed layer solution concentration remarkably influenced the orientations of the subsequent STO precursor layers. Either at a low or high seed layer solution concentration, the crystallization of subsequent STO precursor layers was insufficient which could be attributed to the small number of nucleation centers. Only at an intermediate seed layer solution concentration (0.065 M in our experiments), the STO buffer layers were highly (2 0 0)-oriented. AFM and SEM revealed that the STO buffer layers were relatively dense and smooth. Further works are being carried out to fabricate epitaxial,

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smoother, denser and thicker STO buffer layers for the preparation YBCO of coated conductors.

Acknowledgment This work was supported by the National Superconductivity 863 project under nos. 2002AA306211, 2002AA306281 and the Fundamental Bureau, Chinese Academy of Sciences. The authors thank J. Meng at Tsinghua University for the measurement of SEM.

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