TlBa2Ca2Cu3Oy superconducting films on MgO with different morphologies

https://doi.org/10.1016/S0921-4534(02)01836-1Get rights and content

Abstract

TlBa2Ca2Cu3Oy (Tl-1223) superconducting films were prepared under identical conditions, on MgO substrates with different morphologies resulting from heat treatments at temperatures between 600 and 1350 °C. The superconducting films have almost the same morphology, in-plane alignment and composition, but different critical current densities Jc. Critical current density Jc determined at 77.3 K differs for films with various annealed MgO substrates, by a factor of 5 in zero field and 10 in 1 T. The behavior of Jc is discussed in relation with the flatness of the MgO surface, and with pinning effects induced by the Ca-segregates. For the present work, the best films were obtained for the un-annealed (as-received) substrates and for substrates treated at 1350 °C. In these samples, the substrate’s flatness and morphology with regular steps are essential for high quality of the superconducting films. In the films grown on MgO annealed between 800 and 1200 °C and showing rough surface and Ca-segregates, Jc was lower. However, Jc was increasing with heat treatment temperature of the substrate, possibly due to the Ca-segregates inducing or acting as pinning centers. The effects of Ca-segregates are of interest for maximization of the quality of the high-temperature superconducting films. In the literature, up to now, these effects have not been considered and therefore Ca-segregates removal has been recommended.

Introduction

MgO has a low dielectric loss coefficient tanδ (10−6) in the 100 GHz band [1] and a low dielectric constant of 9.6. These parameters recommend MgO as a good substrate or buffer for high-temperature superconducting (HTS) thin films applied to high frequency passive devices. High stability, low interdiffusion between MgO and HTS films comparing with other low loss substrates such as sapphire and Si, the lack of twinning (specific for some substrate materials), as well as the availability of large area MgO wafers at low price, are also important. The attractive features of MgO are shadowed by its large lattice mismatch with superconducting phases (around 9%), usually resulting in several in-plane misorientation angles. It is generally recognized that critical current density Jc is increasing, and surface resistance of the HTS films is decreasing when the degree of the in-plane alignment is increasing [2]. Second major problem is that MgO has several surface morphologies depending on annealing or annealing/etching/Ar+-ion sputtering conditions [3], [4], [5]. These treatments can be gathered according to their purpose:

  • (i)

    Atomically flat and clean surface of the substrate is required for higher quality of the films. For MgO, efforts are concentrated on removal of Ca-segregates, (observed above 850 °C) [3], [5], and other defects as pits and scratches from polished material [4].

  • (ii)

    Step-like regular surface of the substrate is necessary for preferential nucleation and rapid growth sites that lead to a controlled growth and a smooth surface of the HTS film [6], [7], [8], [9], [10], [11], [12], [13]. Step-like morphologies of the MgO substrate, can also improve epitaxiality and morphology of the HTS films [7].

  • (iii)

    MgO is sensitive to H2O-vapors and CO2 [14]. To remove water or absorbed gases it is important to treat the substrate (e.g. by heating).


The above-mentioned criteria can explain the different treatments of MgO described (and recommended) in literature till now. Temperatures, cycles of heating-etching, annealing time, atmosphere and polishing may significantly vary. For example, the temperatures recommended in the literature are from 500 to 1400 °C. Although many papers describe different methods and conditions for substrate treatment, it is sometimes difficult to find out if these are the optimum ones for the superconducting film considering a certain situation such as HTS material, method or approach of synthesis, etc.

The rather small quantity of available information on this subject led us to study the influence of MgO surface morphology on the properties of TlBa2Ca2Cu3Oy (Tl-1223) HTS films. In this work, we investigated Tl-1223 films grown on MgO substrates treated at different temperatures. For our study we have chosen Tl-1223 superconductor due to its promising superconducting parameters. Recently, it has been shown that Tl-1223 synthesized by high-pressure technique can attain critical temperatures of 133.5 K [15], close to the record Tc=135 K of HgBa2Ca2Cu3Oy phase [16]. Moreover, Senoussi et al. [17] reported a critical current density Jc of 105 A/cm2 at T=100 K and H=0 Oe, and 2×103 A/cm2 at T=100 K and H=5 kOe. At T<80 K and H⩽20 kOe they also found that some Tl-1223 with certain chemical substitutions exhibit very much the same Jc behavior as YBa2Cu3O7−δ (Y-123). In Refs. [18], [19], [20], [21], [22] different HTS are compared in terms of irreversibility field Birr (Jc=103 A/cm2 criterion) versus normalized temperature (1−T/Tc). It resulted that the so-called “single layer” materials (Y-123, Cu-1223P, Cu-1212P, (Cu,C)-1223, Tl-1223, Hg-1223, Pb-1223) are usually superior, from this point of view, to “double layer” phases (Tl-2223, Bi-2212, Bi-2223, (Hg,Tl)2Ba2Can−1CunOy, n=2–5). Tl-1223, having Ba substituted with Sr (Tl-1223Sr), has the second level of performance after Y-123, but a higher Tc. Phases Cu-1223P, Cu-1212P and (Cu,C)-1223 with high Tc and Birr were not considered in the above discussion since, till now, synthesis of these materials is very difficult. Therefore, we believe that Tl-1223/MgO films are of great interest for fabrication of microwave components. A short overview on the importance and potential of HTS for commercial microwave components is given for example in Ref. [23]. Our experimental data and observations might be useful for other HTS/MgO systems.

Section snippets

Experimental

Commercial (Fuuruchi, Japan) (200) MgO single crystal substrates were cut in pieces of 5×5 mm2 and annealed at 600, 800, 1000, 1200 or 1350 °C, in flowing O2 or N2, for 90 min. After annealing, all 10 substrates plus two un-annealed ones (as-received) have been used as a single batch for deposition of amorphous films at room temperature by rf sputtering, using 70 W rf power, in an atmosphere of 7 mTorr Ar+7 mTorr O2. The target for rf sputtering was prepared by solid-state reaction. A precursor

MgO substrates

AFM images on 2×2 μm2 area, in 2D and 3D representation, of the MgO substrates annealed in oxygen at various temperatures, are presented in Fig. 1. Also, roughness along the indicated lines is given. The as-received substrate is very flat, although in some cases pits up to 60–70 nm can be observed (Fig. 1a). At 600 °C (Fig. 1b), the roughness slightly increases from less than 1 nm to around 1–1.3 nm. The substrate treated at 800 °C (Fig. 1c) shows the same behavior, but Ca-segregates appeared,

Discussion

The best Jc and Tc were obtained in films grown on as-received and 1350 °C treated substrates (Fig. 5, Fig. 6b). As we have already seen, the flattest surfaces were observed by AFM on the same substrates (Fig. 1). Also, the normalized XRD peak intensities of the two samples were the highest, suggesting that flat MgO surface favors the growth of grains with better c-axis alignment and/or crystal perfection, resulting in higher Tc and Jc. Although the difference in Jc between the two films is

Conclusions

Tl-1223 superconducting films grown on MgO substrates annealed at different temperatures were investigated. Ca-segregates can act as pinning centers, but the effect of MgO surface reconstruction can increase roughness with negative impact on the quality of the film. MgO heat treatments recommended in literature for the HTS film preparation seem to be not optimized for obtaining higher Jc values, at least in the case of our Tl-1223/MgO system. Further study has to be carried out in this

Acknowledgements

P.B. and A.C. gratefully acknowledge STA fellowships. Authors thank to Dr. A. Negishi (AIST) for allowing the use of SEM-EDS equipment. This paper is dedicated to the memory of Prof. Ihara.

References (34)

  • S.L. King et al.

    J. Mat. Sci. Eng. B

    (1996)
  • K. Fukui et al.

    Surf. Sci.

    (1999)
  • M. Ye et al.

    Solid State Commun.

    (1997)
  • M. Kamei et al.

    Physica C

    (1992)
  • S. Kim et al.

    Thin Solid Films

    (1995)
  • S.L. King et al.

    Appl. Surf. Sci.

    (1995)
  • M.G. Norton et al.

    J. Cryst. Growth

    (1991)
  • S. Senoussi et al.

    Physica C

    (1997)
  • S. Adachi et al.

    Physica C

    (1999)
  • T. Shibata et al.

    Physica C

    (2001)
  • T. Tatsuki et al.

    Physica C

    (1999)
  • K. Fujinami et al.

    Physica C

    (1998)
  • B. Komiyama et al.

    IEE Trans. Microwave Theory Tech.

    (1991)
  • S. Koike et al.

    Trans. Mat. Res. Soc. Jpn.

    (1994)
  • F. Ahmed et al.

    J. Low Temp. Phys.

    (1996)
  • B.H. Moeckley et al.

    Appl. Phys. Lett.

    (1990)
  • Q. Meng et al.

    Appl. Phys. A

    (1999)
  • Cited by (0)

    View full text