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1 Introduction▲
2 Experimental▲
3 Results and discussion▲
4 Conclusion▲
图表▲

Rare Metals2018年第3期

Thickness modulation effect of CeO₂ layer for YBCO films grown by pulsed laser deposition

Xiang Wu Lin-Fei Liu Yan-Jie Yao Meng-Lin Wang Bin-Bin Wang Yi-Jie Li

Department of Physics and Astronomy,Shanghai Jiao Tong University

收稿日期：22 April 2016

基金：financially supported by the International Thermonuclear Experimental Reactor (ITER) Project from Ministry of Science and Technology of China (No.2011GB113004);the National High Technology Research and Development Program of China(No.2014AA032402);the Shanghai Commission of Science and Technology (Nos.11DZ1100402 and 13DZ0500100);the Natural Science Foundation of China(Nos.11204174 and 51372150);

Thickness modulation effect of CeO₂ layer for YBCO films grown by pulsed laser deposition

Xiang Wu Lin-Fei Liu Yan-Jie Yao Meng-Lin Wang Bin-Bin Wang Yi-Jie Li

Department of Physics and Astronomy,Shanghai Jiao Tong University

Abstract：

CeO₂ film plays an essential role in nucleation and growth of YBa₂ Cu₃ O_7-x(YBCO) films. In this work,the dependence of superconducting properties of YBCO on CeO₂ films with different thicknesses was investigated,in order to achieve fabrication of high-performance YBCO coated conductors in industrial scale. The crystalline structure and morphology of CeO2 films with thickness ranging from 21 to 563 nm were systematically characterized by means of X-ray diffraction(XRD), atomic force microscope(AFM) and reflection high-energy electron diffraction(RHEED). Additional focus was addressed on evolution of the surface quality of CeO₂ films with thickness increasing. The results show that at the optimal thickness of 221 nm, CeO₂ film exhibits sharp in-plane and out-of-plane texture with full width of half maximum(FWHM) values of 5.9° and 1.8°, respectively, and smooth surface with a mean root-mean-square(RMS) roughness value as low as 0.6 nm. Combing RHEED and transmission electron microscope(TEM) cross-sectional analysis, it is found that nucleation and growth of CeO₂ films at early stage remain in island growth mode with rougher surface,while further increasing the thickness beyond the optimal thickness leads to weak surface quality, consequently resulting in degradation of superconductor layers deposited subsequently. Eventually, a critical current density(J_c) as high as 4.6×10⁶ A·cm^-2(77 K, self-field) is achieved on a YBCO film on a thickness-modulated CeO₂/MgO/Y₂ O₃/Al₂ O₃/C276 architecture, demonstrating the advantages of CeO₂ films as buffer layer in high-throughput manufacture of coated conductors.

Keyword：

CeO₂ films; Thickness modulation; Pulsed laser deposition; Surface quality;

Author： Yi-Jie Li e-mail:yjli@sjtu.edu.cn;

Received： 22 April 2016

1 Introduction

Using textured metallic tapes instead of single crystal as substrate for growth of YBCO films is the first breakthrough in development of 2G high-temperature superconductor wires,which makes long-length conductors’fabrication possible [ 1, 2, 3] .Several techniques for developing biaxially aligned templates have been investigated,mainly including rolling-assisted biaxially textured substrates [ 4, 5] and ion-beam-assisted deposition (IBAD) [ 6, 7, 8] .Owing to the high quality of the texture and high throughput,the IBAD-MgO approach has been proved to be a promising technology in mass production.

In IBAD-MgO routes,several oxide films serving as buffer layers are needed in order to block the interdiffusion and enhance the compatibility between YBCO and metallic substrate,e.g.,ceria,alumina,yttrium oxide,yttria-stabilized zirconia (YSZ) and LaMnO₃ (LMO).Among them,CeO₂-based materials have been extensively studied and show significant advantages of obtaining high-performance YBCO films [ 9, 10, 11] .CeO₂ with cubic fee fluorite structure possesses high melting temperature (2500℃),small lattice mismatch with YBCO (0.52%),self-assembling crystalline structure and tenability of surface chemical activities by doping foreign element or post-processing.Key features such as morphology,texture,strain and stoichiometry of CeO₂ films made by different deposition techniques were broadly investigated.It is suggested that all these features may have strong influence on the nucleation and growth of YBCO films.Up to date,enormous effort has been made on deposition of CeO₂-based films on YSZ or on NiWsubstrates [ 12, 13, 14] .The structure transition when increasing the thickness seems the common feature regardless of deposition techniques [ 15] .The possibility of deposition of CeO₂ films directly on IB AD-MgO template was proved [ 16, 17] .Highly textured CeO₂ films have also been routinely obtained in industrial scale by multi-plume pulsed laser deposition (PLD) route [ 18, 19, 20] .Due to large strain at the interface of the IB AD-MgO and the CeO₂ layer,thickness effect of CeO₂ directly on IBAD-MgO remains poorly understood.

Irn this work,a series of CeO₂ films with different thicknesses directly deposited on IBAD-MgO substrates was prepared.The global and surface crystallographic orientation and the morphology of CeO₂ films were investigated.A nucleation and growth model was used to illustrate microstructural evolution in the CeO₂ films with thickness increasing.The correlations of the features of CeO₂ films and superconducting properties of YBCO layers were also established.

2 Experimental

The Hastelloy C276 tapes used in this study were 10 mm in width and 50μm in thickness,and the typical mean rootmean-square (RMS) value was less than 2 nm(5μm×5μm) on the tape surface after an electro-polishing process.The Al₂O₃,Y₂O₃ and MgO films were in sequence deposited on the C276 tapes by magnetron sputtering [ 21] and IB AD techniques,while both CeO₂ and YBCO films were deposited by multi-plume PLD processes.During PLD processes,a KrF exciter laser with a wavelength of 248 nm was used and the energy of the laser was 250 mJ with repetition rate of 60 Hz.The target-substrate distance was 4 cm.

CeO₂ films were deposited on IBAD-MgO at 750℃under an oxygen pressure of 0.5 Pa.The thickness of CeO₂film was controlled by deposition time,varying from 21 to563 nm.The YBCO films were deposited following the procedure published previously [ 22] .The thickness of YBCO films was fixed at 350 nm by adjusting deposition time.All the buffer layers and superconducting layer were deposited in reel-to-reel manner with production rates of higher than 100 m·h^-1.

X-ray diffractometer (XRD,D8 Discover with GADDS,Bruker Advanced,Cu Kαradiation operated at 40 mA and40 kV) with general area detector diffraction system was used to collect the phase and texture information of CeO₂and YBCO films.The structure of CeO₂ films was characterized by high-resolution field emission scanning electronic microscopy (FE-SEM,FEI Sirion 200),transmission electron microscopy (TEM,JEOL-3000F),reflection high-energy electron diffractometer (RHEED,PHYSITEC EG-1000) and atomic force microscope (AFM,BioScope,Veeco Instruments,Inc.).The critical current was measured by the conventional four-probe method with a criterion of 1μV·cm^-1 at 77 K,in self-field.

3 Results and discussion

First,XRDθ-2θpatterns of CeO₂ films with different thicknesses are shown in Fig.1.All the films exhibit CeO₂(002) peaks with negligible CeO₂ (111) peaks,indicating excellent crystallinity and strong c-axis growth.When increasing the thickness,the intensities of CeO₂ (002)peaks become stronger;moreover,an obvious peak shifting to higher 2θvalue is also observed.The latter suggests that CeO₂ c-axis lattice constant (a_k) decreases with thickness increasing,probably due to different strain conditions.The lattice strain along growth direction (c-axis),ε_k,can be defined as:

where a_k is the c-axis lattice constant of CeO₂ in the film and a₀ (0.541 nm) is the CeO₂ bulk lattice constant.With thickness varying from 21 to 563 nm,a clear decrease inε_kfrom 0.16%to almost 0%is observed,indicating that the film surface is nearly at strain-free state at thicker films.

FWHM values of out-of-plane and in-plane orientations of CeO₂ films with different thicknesses and those of corresponding YBCO films are summarized in Table 1.All the CeO₂ and YBCO films show sharp texture quality,as evidenced by the small FWHM values ofω-scan andφ-scans.The sharp texture quality of CeO₂ films can also be confirmed by typical CeO₂ (111) pole figure as shown in Fig.2.Besides,the significant texture improvement can be seen with the thickness of CeO₂ films increasing from 21 to221 nm,and little change when further increasing the thickness up to 563 nm.Such textural evolution of CeO₂films directly affects the epitaxial growth of YBCO films,i.e.,the texture quality of YBCO films with slight improvement basically inherit that of CeO₂ film underneath.Additionally,it is noticed that texture qualities of YBCO films vary when depositing on thick CeO₂ films with comparable biaxial texture (thickness from 221 to536 nm).The best texture is found in YBCO film deposited on 221-nm-thick CeO₂ film.This implies that the different nuclear and growth behaviors of YBCO films grown on these CeO₂ films are probably due to the surface conditions which are hardly reflected from XRD scans.

Fig.1 XRDθ-2θpatterns of CeO₂ films with different thicknesses

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Table 1 Texture and AFM analysis of CeO₂ films with different thicknesses and YBCO films deposited on these CeO₂ films

In order to shed the light on surface conditions in terms of morphology and crystallinity,AFM and RHEED techniques were used.First,the morphology of both original MgO film and CeO₂ films with different thicknesses is shown in Fig.3.RMS roughness and lateral grain size(G.S.) for all the films are listed in Table 1.CeO₂ films in broad range of thicknesses on IB AD-MgO exhibit a homogeneous,dense and crack-free morphology with granular grains,which is a typical feature of normal grain growth.For the 21-nm-thick CeO₂ film,the thinnest one,the smallest lateral grain size of about 35 nm and large amount of grain boundaries are observed (Fig.3b),leading to a largest mean height variation (R(z)) of 2.4 nm.With thickness increasing to 221 nm,the lateral grain size is boosted up to 77 nm and the surface tends to be flat and dense.Consequently,RMS roughness decreases from 1.5to 0.7 nm.With the thickness of CeO₂ film further increasing,the grain size is larger than 100 nm and the RMS value increases to 1.1 nm,because of deeper grain boundaries caused by the grain coarsening or thermal grooving effect [ 23] .It is therefore can be seen that the morphology is the result of a balance between grain boundary energy and free surface energy [ 24] .

In order to give a deeper insight into the surface crystallinity of CeO₂ films and understand the correlation of CeO₂ film and YBCO nucleation and growth,reflection high-energy electron diffraction (RHEED) was also preformed on a typical MgO and the CeO₂ samples.Figure 4shows ex situ RHEED patterns of CeO₂ with different thicknesses.For comparison,RHEED pattern of IBAD-MgO was also collected,as shown in Fig.4a.It reveals that IBAD-MgO film forms a single crystal-like structure on the surface under island growth mode,which is in agreement with previous publications reported in Refs. [ 25, 26, 27] .Clear diffraction spots with relatively weak intensity appear on RHEED pattern of 21-nm-thick CeO₂ film,while these spots change to discontinue robs arrays when the film thickness increases to 221 nm.This transition suggests that the growth mode of CeO₂ film changes from island growth mode to Stranski-Krastanov growth mode [ 28] .Further increasing film thickness leads to weaker diffraction intensity,but the shape of diffraction robs remains unchanged (as indicated by the intensity profile line shown in Fig.5).This surface structure transition is in the line with AFM observation and the texture quality of the YBCO films aforementioned.Although little difference is found among CeO₂ films with thickness of 221-563 nm,surface quality varies,which has strong influence on the formation of YBCO films deposited on top.Namely,the film with low surface roughness and well-defined diffraction RHEED pattern serves as the best candidate for the growth of YBCO.

Fig.2 XRD texture results of a 221-nm-thick CeO₂ film:aω-scan,bφ-scan and c (111) pole figure of CeO₂ films grown on IBAD-MgO with texture ofΔω=1.8℃andΔφ=5.9°

Fig.3 AFM images of a IBAD-MgO film and CeO₂ films with thickness of b 21 nm,c 221 nm and d 422 nm

Fig.4 RHEED patterns of a IBAD-MgO film and CeO₂ films with thickness of b 21 nm,c 221 nm and d 422 nm

Fig.5 Intensity profile of RHEED patterns across streak

Figure 6a shows TEM image of cross section of the YBCO film grown on 563-nm-thick CeO₂ film.Generally,both CeO₂ and the YBCO films present obverse column micros true ture which is a typical feature of PLD films.The clear CeO₂-YBCO interface also confirms the negligible interaction under the optimized growth conditions.It is worth noticing that lots of misfit dislocations (pointed by arrows in Fig.6a) are present at the bottom of CeO₂ film to accommodate large lattice mismatch between CeO₂ and the IBAD-MgO layer.With thickness increasing,misfit dislocations decrease and column grains become larger in lateral,which is also in agreement with AFM observations.A schematic cross section of the three layers of CeO₂/IBAD-MgO architectures is shown in Fig.6b.At the beginning,CeO₂ film consists of both small-φgrains and large-φgrains which lead to a poor texture.As thickness increases,CeO₂ grains eventually grow to dominant grains with smallφthrough competition with other smaller grains with poor texture.Continuing to increase the thickness of CeO₂ film,the other type of growth mechanism is activated,which results in the grain coarsening.It is therefore can be seen that the morphology is the result of a balance between grain boundary energy and free surface energy [ 29] .

Texture improvement in CeO₂ films with film thickness increasing which is generally called“self-epitaxy"was firstly discovered by Muroga et al. [ 30] in 2002 when studying CeO₂ films grown on Gd₂Zr₂O₇ buffered substrates.This phenomenon is believed to be very beneficial to obtain high epitaxial superconducting films at low-cost metallic substrate.Considering that self-epitaxy only occurs in IB AD routes,several researchers assumed that small lateral grain size is one of the prerequisites.Based on the mechanism of IB AD deposition,it is known that the grains with good texture quality undergo less ion bombardment and consequently less strain in the crystallites.Therefore,a crystallite with good texture is energetically favorable for further nucleation of the layer on the top.

Figure 7 shows voltage-current curves of YBCO films deposited on CeO₂ films with different thicknesses measured by conventional four-probe method at 77 K,selffield.It indicates that critical current (I_c) of YBCO is strongly dependent on the thickness of CeO₂ films.With the thickness of CeO₂ film increasing to 221 nm,the critical current increases from 6 to 165 A with the highest critical current density (J_c) of 4.6×10⁶ A·cm^-2 (77 K,self-field).With the thickness of CeO₂ film further increasing to 563 nm,the critical current decreases to65 A.As expected,the highest J_c values of YBCO film with the best in-plane and out-of-plane texture are strongly linked with the surface conditions of CeO₂ films.

Fig.6 TEM image of cross section of YBCO film deposited on CeO₂ film with thickness of 563 nm a and schematic cross sections of IB AD-MgO and CeO₂ layer b

Fig.7 Voltage-current characteristics of YBCO films deposited on CeO₂ with different thicknesses tested at 77 K and self-field using conventional four-probe method

4 Conclusion

CeO₂ films with different thicknesses were deposited on IBAD-MgO/Y₂O₃/Al₂O₃/C276 substrate by a multi-plume PLD technique.The relationship between microstructure and thickness of CeO₂ films was systematically investigated.For 221-nm-thick CeO₂ film,FWHM values of out-of-plane and in-plane are 1.8℃and 5.9℃,respectively.And the surface is smooth with a RMS value of smaller than 1 nm.Coordinated RHEED and TEM cross-sectional analysis show that CeO₂ films remain in island growth mode with rougher surface conditions at early stage,while further increasing the thickness beyond the optimal thickness leads to weak surface quality which would also result in degradation of superconductor layers deposited subsequently.Finally,the excellent YBCO film can be achieved with high I_c of 160 A and J_c of 4.6×10⁶ A.cm^-2 (77 K,self-field),which is deposited on the 221-nm-thick CeO₂ film.