Growth of biaxially textured buffer layers on rolled-Ni substrates by electron beam evaporation

Growth of biaxially textured buffer layers on rolled-Ni substrates by electron beam evaporation

[email protected] ELSEVIER Physica C 275 (1997) 266-272 Growth of biaxially textured buffer layers on rolled-Ni substrates by electron beam evaporation M. Pa...

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[email protected] ELSEVIER

Physica C 275 (1997) 266-272

Growth of biaxially textured buffer layers on rolled-Ni substrates by electron beam evaporation M. Paranthaman a,*, A. Goyal b, F.A. List b, E.D. Specht b, D.F. Lee b, P.M. Martin b, Qing He c, D.K. Christen c, D.P. Norton c, J.D. Budai c, D.M. Kroeger b a Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6100, USA b Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6100, USA c Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6100, USA

Received 8 October 1996; revised manuscriptreceived 11 December 1996

Abstract This paper describes the development of two buffer layer architectures on rolled-Ni substrates using an electron beam evaporation technique. The first buffer layer architecture consists of an epitaxial laminate of CeO2/Pd/Ni. The second alternative buffer layer consists of an epitaxial laminate of YSZ/CeO2/Ni. The cube (100) texture in the Ni was produced by cold-rolling followed by recrystallization. The CeO 2 films were grown epitaxially on both Pd-buffered and textured-Ni substrates. The YSZ films were grown epitaxially on CeO2-buffered Ni substrates. The crystallographic orientation of the Pd, CeO 2, and YSZ films were all (100). We also studied the effect of the CeO 2 layer thickness and crack formation on textured-Ni substrates. The layer thickness was found to be critical. For some thicknesses, cracks formed in the CeO 2 layer. The presence of YSZ layers on the CeO 2 layers seem to alleviate the cracks that are formed underneath. Our SEM studies showed that both CeO 2 (3-10 nm thick underlayer) and YSZ layers were smooth and continuous. Keywords: Buffer layers; Biaxial texture; Rolled-Ni substrates; E-beam evaporation

1. Introduction For conductors developed for high temperature and high field applications, YBa2Cu307_ ~. (referred to as YBCO) deposits are very promising. Also, Dimos et al. [1,2] have demonstrated from their YBCO bicrystal studies that high critical-current

* Corresponding author. Fax: + I 423 574 4939.

density, Jc, can only be obtained on oriented YBCO films with a high degree of texture both normal to and within the basal plane. Two approaches have been used to deposit biaxially textured YBCO films. The first approach was to grow biaxially textured Yttria Stabilized Zirconia (YSZ) buffer layers on polycrystalline Ni-based alloys such as Haynes 230, and Hastelloy C276 through an ion-beam assisted deposition (IBAD) process [3-6]. A high Jc of over 1 × l06 A / c m 2 at 75 K and zero field was obtained on l ixm thick YBCO films on Ni based alloys with

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M. Paranthaman et al. / Physica C 275 (1997) 266-272

textured YSZ buffer layers grown by IBAD [7]. The second approach was developed at Oak Ridge National Laboratory using the concept of rolling-assisted biaxiaily textured substrates (RABiTS) [8]. Our approach was to use rolling-induced texture to obtain biaxially textured fcc metal strips and to deposit epitaxially on the strips both buffer layers and superconductors to form a conductor. The Ni was chosen as the substrate because it readily develops the cube texture and is more oxidation resistant than Cu. Recently, we demonstrated that a Jc of 0.73 × 10 6 A / c m 2 at 77 K and zero field can be obtained for films with a layer sequence of Y B C O / Y S Z / C e O 2 / N i [9]. The crystallographic orientations for all the layers were (100). In this architecture, all oxide layers were grown by pulsed laser deposition (PLD). There is a clear need to develop a capability to deposit buffer and superconductor layers by other techniques. These layers must be grown as continuous epitaxial films on Ni. The purpose of the chemical buffer layers is to retard oxidation of Ni, to reduce the lattice mismatch between Ni and YBCO, and also to prevent diffusion of Ni into YBCO. Previous studies proved that CeO 2 [10-13], Pd [13,14], and YSZ [15] films could be grown epitaxially by vapor deposition techniques on single crystal substrates. In this paper, we describe our successful development of the growth of buffer layers on rolled-Ni substrates using an electron beam evaporation technique. In related work, these buffer layer architectures also have been developed using rf and dc magnetron sputtering; those results are reported separately [16]. Here, the deposition conditions for growing epitaxial Pd, CeO 2 and YSZ films on textured Ni substrates by electron beam evaporation are reported for the first time.

2. Experimental results and discussion The cube (100) texture in Ni (99.99%) was produced by cold-rolling to over 90% deformation followed by recrystailization at temperatures ranging from 400 to 1000°C [8]. The thickness of the textured-Ni substrate used was 125 I~m. The deposits were produced without any substrate polishing in an AIRCO Temescal CV-14 system with three electron guns. It was operated by a Temescal FDC-8000 Film

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Deposition Controller. The films were analyzed by detailed X-ray diffraction studies. A Philips Model XRG3100 diffractometer with Cu K~ radiation was used to record powder diffraction patterns. Texture was analyzed using a Rigaku rotating anode X-ray generator. A graphite monochromator selected Cu K~ radiation, and slits defined a 2 x 2 mm 2 incident beam. A four circle diffractometer was used to collect pole figures (in reflection) and to measure rocking curves. The out-of-plane alignment was measured by diffracting from (002) planes (parallel to the surface) and rotating the sample about a [100] axis (labeled to). In-plane alignment was measured by diffracting from (202) planes (inclined 45 to the surface) and rotating about the [001] axis (labeled 4'). SEM micrographs were taken using a Hitachi S-4100 Field Emission Scanning Electron Microscope. The beam voltage used was 15 kV. The experimental details for the growth of two buffer layer architectures on rolled-Ni substrates are described below. 2.1. CeO2 / P d / N i architecture 2.1.1. Growth o f Pd on Rolled-Ni substrates by e-beam evaporation

The as-rolled Ni substrates were cleaned ultrasonically with both acetone and methanol, and mounted on a substrate holder with a heater assembly in the e-beam system. After the vacuum had reached 1 × 10 - 6 Torr at room temperature, the substrates were in situ annealed at 400°C for 4 hours. The temperature of the substrate was measured using a thermocouple. Then the Pd layer was grown on the textured Ni at temperatures ranging from 100 to 500°C. The typical deposition rate for Pd was 0.5-1.0 n m / s e c at a pressure of 10 -6 Torr, and the final thickness was varied from 200 nm to 1 micron. The thickness of the film was measured by a quartz crystal monitor during the deposition. The 0 - 2 0 scan for a 400 nm thick Pd film deposited on Ni at 500°C showed the presence of a (001) oriented film. From the to and 4' scans for 400 nm thick Pd films which were deposited on Ni at 500°C, the Full-Width-Half-Maximum (FWHM) for Ni (002) and Pd (002) obtained are 7.3 ° and 4.1 °, and that of Ni (202) and Pd (202) are 8.8 ° and 7.4 °, respectively. The XRD results show that Pd can be deposited epitaxially on Ni.

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2.1.2. Growth of CeO 2 on Pal-buffered Ni substrates by e-beam evaporation Initially, biaxially textured Ni substrates were cleaned and Pd films were grown on them as discussed in the previous section. CeO 2 films were then deposited on the Pd-buffered Ni substrates. After the vacuum in the chamber had reached 1 × 10 - 6 Torr at room temperature, a mixture of 4% H 2 and 96% Ar was introduced until the pressure inside the chamber reached ~ 10 -4 Torr. The gas flow was controlled by a dc powered piezo-electric valve. The Pd-buffered Ni substrates were then annealed at ~ 600°C for 30 minutes at ~ 10-4 Torr. After annealing, the chamber was maintained at a pressure of 2 × 10 -5 Torr with a mixture of 4% H 2 and 96% At. The textured CeO z layers were grown on the Pd-buffered Ni at temperatures ranging from 300 to 750°C. The deposition rate for CeO 2 was 0.1 n m / s e c . The final thickness was varied from 50 nm to 200 nm. Cerium metal was used as the source. The crucibles used were usually Graphite. A 0 - 2 0 scan for a 100 nm thick CeO 2 film deposited on Pd-

buffered Ni at 400°C is shown in Fig. 1. The strong CeO 2 (200) lines show the presence of a good out-of-plane texture. The oxygen impurity present in the chamber is apparently enough to oxidize the film to form stoichiometric CeO 2. Fig. 2 shows the ~o and ~b scans for the same film. The F W H M for Ni (002), Pd (002) and CeO 2 (002) are 6.6 °, 4.6 ° and 5.9 °. The rocking curves for Pd and CeO2 are smooth because these are fine-grained films. The Ni substrate, by contrast is coarse-grained, so its rocking curves consist of many sharp peaks corresponding to individual grains. The F W H M from ~b scans for Ni (202), Pd (202) and CeO 2 (202) were 8.3 °, 7.8 ° and 10.0 °. The XRD results demonstrate that CeO 2 and Pd can be deposited epitaxially on Ni.

2.2. YSZ / CeO 2 / N i architecture 2.2.1. Growth of CeO 2 on rolled-Ni substrates by e-beam evaporation The electron beam evaporation technique was also used to deposit CeO 2 films directly on Ni. Biaxially

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Fig. 1. Room-temperature powder X-ray diffraction for 100 nm thick CeO 2 deposited on Pd-buffered Ni at 400°C.

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textured Ni substrates were mounted on a substrate holder which contained a heater assembly. After the vacuum in the chamber had reached 1 × 10 -6 Torr at room temperature, a mixture of 4% H 2 and 96% Ar was introduced until the pressure inside the chamber reached ~ 1 Tort. The Ni substrates were annealed at ~ 700°C for 60 minutes at ~ 1 Torr. During CeO 2 deposition, the chamber was maintained at a pressure of 2 × 10 -5 Torr with a mixture of 4% H 2 and 96% Ar. The textured CeO 2 layers were deposited on the Ni substrates at temperatures ranging from 300 to 750°C. The deposition rate for CeO 2 was 0.1 n m / s e c , and the final thickness was varied from 5 to 150 nm. The XRD results from the 0 - 2 0 scan, and also from the to and ~b scans for 100 nm thick CeO 2 films deposited on Ni at 600°C revealed good epitaxial texturing. The CeO 2 (002) from the 0 - 2 0 scan showed the presence of a good out-of-plane texture. The use of a mixture of 4% H 2 and 96% Ar gas presumably prevents the formation of NiO during the CeO 2 growth. These characteriza-

tions show that CeO 2 can be deposited epitaxially on Ni.

2.2.2. Growth of YSZ on CeO2-buffered Ni substrates by e-beam evaporation The electron beam evaporation technique was used to deposit YSZ on CeO2-buffered Ni substrates. Biaxially textured CeO2-buffered Ni substrates were cleaned with methanol, and mounted on a heated substrate holder in the e-beam system. After the vacuum in the chamber had reached 1 × 10 -6 Tort at room temperature, a gas mixture of 4% H 2 and 96% Ar was introduced until the pressure inside the chamber reached ~ 1 Torr. The CeO2-buffered Ni substrates were annealed at ~ 700°C for 60 minutes at that pressure. The chamber was then maintained at a pressure of 2 × 10 -5 Tort with a mixture of 4% H 2 and 96% At. The textured YSZ layers were grown on the CeO2-buffered Ni substrates at temperatures ranging from 650 to 750°C. The YSZ deposition rate was 0.1 n m / s e c , and the final thickness

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M. Paranthaman et a l . / Physica C 275 (1997) 266-272

270

YSZ ~200)

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epitaxially on CeO2-buffered Ni substrates. To date, our efforts to grow YSZ directly on rolled-Ni substrates have produced either randomly oriented YSZ or (111) oriented films.

2.3. Thickness dependence and crack formation in CeO2 layers CeO2/Ni, CeO 2 (111) pole figure

YSZ/CeO2/Ni, YSZ (202) pole figure

Fig. 5. CeO2 (111) and YSZ (202) pole figures for 100 nm thick YSZ deposited on 10 nm thick CeO2-bufferedNi at 600°C.

was varied from 50 nm to 150 nm. Yttria (10%) stabilized zirconia was used as the source. The 0 - 2 0 scan is shown in Fig. 3 and the to and ~b scans are shown in Fig. 4. These results were obtained on 100 nm thick YSZ films which were grown at 600°C. The strong YSZ (200) and CeO 2 (200) peaks shown in Fig. 3 indicate the presence of a good out-of-plane texture. The F W H M for Ni (002), CeO 2 (002), and YSZ (002) were 7.4 °, 6.6 °, and 6.8 °, and that of Ni (202), CeO 2 (202), and YSZ (202) are 9.5 °, 8.8 °, and 8.5 °, respectively. As shown in Fig. 5, the CeO z (111) and YSZ (202) pole figures demonstrate that the buffer layers are epitaxial with a single orientation. The XRD results show that YSZ can be grown

(a)

In our studies of CeO 2 layers of various thicknesses which had been deposited on rolled-Ni substrates, we found that the as-grown 100 nm thick CeO 2 layers were cracked whereas 50 nm thick CeO 2 layers were crack-free. The SEM micrographs which demonstrates these features are shown in Fig. 6. After the growth of YSZ on a 50 nm thick CeO 2 layer, the CeO 2 layers were also cracked but the YSZ layers were crack-free. The presence of YSZ layers on top of CeO 2 layers seem to alleviate the cracks that are formed underneath. The cerium oxide layer thickness was found to be critical. Both CeO 2 and YSZ layers were crack-free for a CeO 2 underlayer thickness of < 10 nm. Our SEM studies showed that both CeO 2 and YSZ layers were smooth and continuous. This demonstrates that a very thin layer of CeO 2 is needed to grow textured YSZ layers. Efforts are being made to demonstrate the growth of high Jc YBCO films on these buffer layers.

(b) r

Fig. 6. SEM micrographs for (a) 50 nm and (b) 100 nm thick CeO 2 films deposited directly on rolled-Ni substrates. The micrographs were taken at 50 k × magnification.

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3. Conclusions We developed two buffer layer architectures on textured-Ni substrates using the electron beam evaporation techniques. The two buffer layer sequences are C e O 2 / P d / N i and YSZ/CeO2/Ni. The cube (100) texture in Ni was produced by cold-rolling followed by recrystallization at temperatures ranging from 400 to 1000°C. The CeO 2 layer was deposited epitaxially on both Pd-buffered and textured Ni substrates. The YSZ layer was deposited epitaxially on CeO2-buffered Ni substrates. For thicker CeO 2 films on Ni substrates, crack formation was observed. Growth of a 3-10 nm thick CeO 2 layer prevents crack formation and also assists the epitaxial growth of YSZ films.

Acknowledgements Thanks are due to Dewey Easton for technical assistance with the e-beam system. The research was sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences and Office of Energy Efficiency and Renewable Energy, Office of Utility Technology-Superconductivity Program. The research was performed at the Oak Ridge National Laboratory, ORNL, managed by Lockheed Martin Energy Research Corporation for the U.S. Department of Energy under contract # DE-AC0596OR22464. References [1] D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoues, Phys. Rev. Lett. 61 (1988) 219.

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