Dependencies of microstructure and stress on the thickness of GdBa2Cu3O7 − δ thin films fabricated by RF sputtering
© Wang et al.; licensee Springer. 2013
Received: 28 April 2013
Accepted: 30 May 2013
Published: 1 July 2013
GdBa2Cu3O7 − δ (GdBCO) films with different thicknesses from 200 to 2,100 nm are deposited on CeO2/yttria-stabilized zirconia (YSZ)/CeO2-buffered Ni-W substrates by radio-frequency magnetron sputtering. Both the X-ray diffraction and scanning electron microscopy analyses reveal that the a-axis grains appear at the upper layers of the films when the thickness reaches to 1,030 nm. The X-ray photoelectron spectroscopy measurement implies that the oxygen content is insufficient in upper layers beyond 1,030 nm for a thicker film. The Williamson-Hall method is used to observe the variation of film stress with increasing thickness of our films. It is found that the highest residual stresses exist in the thinnest film, while the lowest residual stresses exist in the 1,030-nm-thick film. With further increasing film thickness, the film residual stresses increase again. However, the critical current (Ic) of the GdBCO film first shows a nearly linear increase and then shows a more slowly enhancing to a final stagnation as film thickness increases from 200 to 1,030 nm and then to 2,100 nm. It is concluded that the roughness and stress are not the main reasons which cause the slow or no increase in Ic. Also, the thickness dependency of GdBa2Cu3O7 − δ films on the Ic is attributed to three main factors: a-axis grains, gaps between a-axis grains, and oxygen deficiency for the upper layers of a thick film.
KeywordsStress Thickness dependencies Superconducting performances
Second-generation high-temperature superconducting (HTS) coated conductors based on ReBa2Cu3O7 − δ (REBCO, RE = Y, Gd, Sm, etc., rare earths) films are coming into practical applications for motors, fault current limiters, generators, and transformers [1, 2]. High critical current (Ic) is needed for many HTS applications. Apparently, enhancing the thickness of (RE) BCO films is the simplest method. However, there is an obstacle for this way as there is a current density (Jc) decreasing phenomenon as films become thicker . Such a falloff of Jc is found in ReBa2Cu3O7 − δ films fabricated by different methods, such as pulsed laser deposition , hybrid liquid-phase epitaxy , Ba-F-based methods , and chemical solution deposition by ink-jet printing .
Several possible reasons for the so-called ‘thickness effect’ of Jc have been advanced. These include a-axis growth, the increase in surface roughness, and porosity. Another reasonable interpretation of the thickness effect of Jc has been proposed by Foltyn et al. . They attributed this to misfit dislocations near the interface between the superconductor and the substrate. The same research group reported that by inserting several thin CeO2 layers, the thickness effect can be overcome in some extent . The suppressed thickness effect may be due to much more interfaces between the superconductor and the substrate in the multilayer compared with the single layer. Variations of stress also may be an important factor for Jc to decrease with thickness. Xiong et al.  reported that variations of stress in yttrium barium copper oxide (YBCO) film resulted in first the increase and then the decrease of Jc with increasing film thickness. Similar results are found by Zeng et al. . Many groups have made their efforts to find methods to eliminate the thickness effect of Jc with enhancing film thickness. However, a much deeper understanding of the development of residual stress and microstructure in ReBa2Cu3O7 − δ films with different thicknesses is desired for the optimization of superconducting performance.
In the present work, GdBa2Cu3O7 − δ (GdBCO) films with different thicknesses are fabricated by radio-frequency magnetron sputtering (RF sputtering) in order to understand the problems mentioned above, particularly with respect to microstructure and residual stress. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) are performed to observe the texture, surface morphology, and oxygen content of GdBCO films. Meanwhile, the Williamson-Hall method is applied to calculate the residual stress in the studied films.
Biaxially textured Ni-5 at.% W alloy tapes from EVICO GmbH (Dresden, Germany) are used in these studies. The out-of-plane and in-plane texture are 6° and 7°, respectively. The thickness of the alloy tape is 70 μm, and the width is 10 mm. The root mean square roughness (RMS) is no more than 7 nm over a 50 μm × 50 μm area. CeO2, yttria-stabilized zirconia (YSZ), and CeO2 films are in sequence fabricated on Ni-W tapes by RF sputtering. Firstly, CeO2 is fabricated. The formed gas Ar (97%) + H2 (3%) served as the sputtering gas to prevent the oxidation of alloy tapes. The total pressure is 0.02 Pa.
After the fabrication of the CeO2 seed layer, a total pressure of O/Ar mixture gas of 30 Pa is introduced to the chamber. Then the YSZ layer is fabricated. The YSZ (8% ZO2) target is used in the experiment. The sputtering power is 40 and 50 W for the CeO2 seed layer and the YSZ layer, respectively. The growth temperature is 760°C for both the CeO2 seed layer and the YSZ layer. The substrate-target distance is about 50 mm for both the CeO2 seed layer and the YSZ layer. The fabrication time is 30 min for the CeO2 seed layer and 60 min for the YSZ layer. Secondly, the CeO2 cap layer is fabricated. The parameters for the CeO2 cap layer are identical to those for the CeO2 seed layer. The O/Ar ratio is 1:5 for both the YSZ layer and the CeO2 cap layer. The thicknesses of the CeO2 seed layer, the YSZ layer, and the CeO2 cap layer are about 30, 70, and 30 nm, respectively.
The microstructure features of CeO2/YSZ/CeO2-buffered Ni-W substrates are measured. The out-of-plane and in-plane are 4.3° and 7.0°, respectively. The AFM image shows a smooth and no-crack surface morphology of the CeO2 cap layer. At last, the GdBCO films are fabricated on CeO2/YSZ/CeO2-buffered Ni-W substrates by RF sputtering. During the GdBCO film fabrication, the substrate temperature, O/Ar mixed gas pressure, and sputtering power are 780°C, 25 Pa, and 80 W, respectively. The O/Ar is 1:1.
Seven samples with various thicknesses are fabricated. Film thickness is controlled by different sputtering times, while other parameters are fixed. The thickness for the studied samples is measured using a step profiler. The seven samples are 5 cm long and 1 cm wide. In order to get an average thickness of our samples, especially for the thicker films with a-axis outgrowths, ten points along the sample width direction are chosen for thickness measurement using the step profiler for every sample. The distance between the chosen points is 0.1 cm. The average thicknesses of our samples are 200, 390, 602, 810, 1,030, 1,450, and 2,100 nm, respectively. The thickness homogeneity along the length direction (not the width direction) is very good for the studied samples. Four films are used to analyze the development of the microstructure and stress of GdBCO films. Their thicknesses are 200, 1,030 1,450, and 2,100 nm, and they are named F200, F1030, F1450, and F2100, respectively. The microstructure and stress of the films are studied by XRD, SEM, AFM, and XPS analysis. The Ic is measured using the standard four-probe method. A voltage criterion of 1 μV/cm is used to determine Ic in the I-V curves.
Results and discussion
Film texture and surface morphology
As the thickness increases to 1,030 nm, rectangular-shaped outgrowths appear on the film surface. This implies a-axis grains of the GdBCO film. At the same time, both the size and number of pinholes become smaller (Figure 3b). The pinholes disappear for samples F1450 and F2100 (Figure 3c,d). The disappearance of pinholes for thicker GdBCO films can be attributed to a temperature decrease effect of top layers for thicker GdBCO films. Because the GdBCO film is a bad thermal conductor, the top layer will not be heated sufficiently. Hence, it is indicated that the disappearance of pinholes for thicker films probably results from a decrease of deposition temperature for the top layer. This explanation accords very well with our above discussion for the appearance of the pinholes in thinner films. The mechanism of the pinholes is still not clear. They will also damage the superconducting performance of the (RE) BCO films because they will decrease the effective supercurrent-carrying cross-sectional area.
From Figure 3c, it can be seen that the size and number of the rectangular-shaped outgrowths become bigger and more when the thickness increases to 1,450 nm. With the thickness increasing to 2,100 nm, the rectangular-shaped outgrowths are overlapped together. Some gaps are left between the grains. This will certainly lower the GdBCO films’ density and decrease the Jc value with increasing film thickness.
Stress analysis by means of the Williamson-Hall method
For the thinner GdBCO film, the film grows with lattice distortion, which results in compressive stresses. As the film thickness increases to a critical thickness, such as 1,030 nm, the GdBCO film grows with a standard lattice. Therefore, the compressive stresses are released. With the further increase of the thickness of GdBCO films, a-axis grains appear. At the same time, the bigger roughness value for thicker films will lead to tilted GdBCO grains. The two factors result in tensile stress emergence.
Oxygen content analysis by XPS
As mentioned above, the XPS measurement of GdBCO films with different thicknesses is equivalent to the XPS depth profiling measurement of sample F2100. The oxygen content is different for different depth layers for one thick film. For the bottom layer from 0 to about 1,030 nm, the oxygen content almost does not change. For the upper layers from 1,030 to 2,100 nm, the oxygen content reduces. The oxygen deficiency for the upper layers beyond 1,030 nm for thick films may result in bad superconductivity, which will be discussed in the next part.
The outgrowths on the thick films will obviously affect the results of the XPS measurement. The analysis area is 700 × 300 μm2, so the area will contain many outgrowths (see Figure 4c,d). The outgrowths will contribute to the signals of XPS measurements. The outgrowths are mainly consisting of a-axis GdBCO grains. The oxygen content reduction is accompanied with the emergence of a-axis grains for the upper layers of the thick film. It implies that the oxygen deficiency for the upper layers beyond 1,030 nm of thick films mainly results from a-axis grain emergence.
Superconducting performances of GdBCO films
Tao et al.  reported the Jc of YBCO film to be 1.6 × 106 A/cm2 at 77 K and self-field with a thickness of 1.2 μm by sputtering method on buffered Ni-5 at.% W substrates. Tran et al.  found that the 0.2-μm-thick GdBCO film had the highest Jc of 3.8 × 106 A/cm2 and the Jc value decreased to 4.2 × 105 A/cm2 as the film thickness increased to 0.55 μm. From our results, the Jc of the 1,450-nm-thick film can achieve as high as 2.0 × 106 A/cm2. At the same time, a nearly linear relationship between film thicknesses and Ic has been found when the film thickness is below 1,030 nm. A linear relationship between thickness and Ic is very important for achieving high current carrying ability for thick films. Recently, several ways have been developed to solve the thickness effect in (RE) BCO films. Using multilayer technology, Foltyn et al. have achieved Jc values of up to 4.0 × 106 A/cm2 in the film with a thickness of 3.5 μm, at_75 K, self-field on metal substrates . Tran et al. have overcome the rapid decrease of Jc value by BaSnO3 addition in (Gd) BCO films . Feldmann et al. achieved a Jc (75.6 K, self-field) of 5.2 × 106 A/cm2 in a single-layer 2.0-μm-thick YBCO film with BaZrO3 (BZO) and Y2O3 additions . Dürrschnabel et al. obtained the Jc of (Dy) BCO film to be 1.7 × 106 A/cm2 at 77 K and self-field with a thickness of 5.9 μm on inclined substrate-deposited MgO-buffered Hastelloy substrates . These research results are exciting. Our next research work will focus on finding methods to overcome the thickness effect in (RE) BCO films.
GdBCO films with different thicknesses are prepared on CeO2/YSZ/CeO2-buffered Ni-W substrates by means of RF sputtering. The stress and microstructure of the GdBCO films with various thicknesses are investigated by XRD, SEM, AFM, and XPS techniques. For the 200-nm-thick film, the highest Jc value of 4.0 MA/cm2 has been obtained. The highest Jc value is attributed to high-level compressive stresses for the 200-nm-thick film. A nearly linear relationship between Ic and film thickness is observed as the film thickness increases from 200 to 1,030 nm. It is realized that differences of stress and roughness do not affect the supercurrent carrying ability with increasing film thickness. We find that when the film thickness approaches to a certain value about 1,030 nm, the a-axis grains appear at the upper surface. As a result, more and more a-axis grains lead to lots of grain gaps, which will certainly reduce the effective supercurrent carrying cross section. In addition, oxygen deficiency is found for upper layers beyond 1,030 nm for F1450 and F2100. It can be understood that the slower increase of Ic for the 1,450-nm-thick film and no increase of Ic for the 2,100-nm-thick film are due to a-axis grains, gaps between a-axis grains, and oxygen deficiency for the upper layers of the thick film.
Atomic force microscopy
Critical current density
- RF sputtering:
Radio-frequency magnetron sputtering
Root mean square
This work is supported by the ITER Plan Project (grant no. 2011GB113004), Shanghai Science and Technology Committee (grant no. 11DZ1100402), Graduate Student Innovation Ability Training Special Fund projects (grant no. Z-072-004), National Science and Technology (grant no. 11204174), and Shanghai Youth Science and Technology The Phosphor Plan (tracking) (grant no. 11QH140100). The authors gratefully thank the Instrumental Analysis Center of Shanghai Jiao Tong University and MA-tek analytical lab for the competent technical assistance.
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