YBCO films grown by reactive co-evaporation on ...

Home

Search

Collections

Journals

About

Contact us

My IOPscience

YBCO films grown by reactive co-evaporation on simplified IBAD-MgO coated conductor templates

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2010 Supercond. Sci. Technol. 23 014018 (http://iopscience.iop.org/0953-2048/23/1/014018) View the table of contents for this issue, or go to the journal homepage for more

Download details: IP Address: 103.8.221.253 This content was downloaded on 18/10/2013 at 12:57

Please note that terms and conditions apply.

IOP PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGY

Supercond. Sci. Technol. 23 (2010) 014018 (5pp)

doi:10.1088/0953-2048/23/1/014018

YBCO films grown by reactive co-evaporation on simplified IBAD-MgO coated conductor templates Vladimir Matias1 , E John Rowley1 , Yates Coulter1, B Maiorov1 , Terry Holesinger1 , Chris Yung2 , Viktor Glyantsev2 and Brian Moeckly2 1

Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 2 Superconductor Technologies Inc., Santa Barbara, CA 93111, USA E-mail: [email protected]

Received 6 September 2009 Published 9 December 2009 Online at stacks.iop.org/SUST/23/014018 Abstract We demonstrate coated conductors fabricated by reactive co-evaporation of YBa2 Cu3 O y (YBCO) by cyclic deposition and reaction (RCE-CDR) on ion-beam-assisted-deposition- (IBAD-) textured templates simplified by the elimination of the epitaxial buffer layer. Hastelloy substrates, both polished and unpolished, were used as a starting material for the IBAD templates. Y2 O3 bed layers were then deposited followed by IBAD-textured MgO and a thin homoepitaxial MgO layer. The MgO-terminated templates were used for direct deposition of YBCO by RCE-CDR. Critical current densities obtained for the undoped YBCO material are comparable to the best values measured previously with the use of LaMnO3 or SrTiO3 epitaxial buffer layers and state-of-the-art coated conductor results. The structural characterization data indicate a well oriented YBCO film with a robust template. Electrical measurements also indicate no weak links and a typical magnetic field behavior of undoped YBCO, characterized by a low density of naturally occurring strong pinning centers and correlations along the ab direction. (Some figures in this article are in colour only in the electronic version)

Theva [2, 3]. The method can be used for growth of other thin film materials [4]. The process has also been demonstrated for coated conductors by Prusseit and co-workers at Theva [5] and Youm and co-workers at KAIST in Korea [6]. More recently Los Alamos National Laboratory (LANL) demonstrated the RCE-CDR process on ion-beam-assisted-deposition(IBAD-) textured templates [7] and attained up to 950 A cm−1 at 75 K in single-layer YBCO thick films without any additions [8, 9]. Here we present new results on films made by this process resulting from a collaboration between LANL and STI. STI has focused on deposition directly on the MgO layer without the need for the additional epitaxial perovskite buffer layer such as SrTiO3 or LaMnO3 , that is typically embodied in

1. Introduction The method for reactive co-evaporation (RCE) of YBa2 Cu3 O y (YBCO) films was first elegantly implemented in an oscillatory manner by Kinder and co-workers [1]. They showed that by rapidly cycling deposition and oxygen reaction they could solve the critical issues of temperature uniformity, via a black-body-type heater, and oxygen pressure in a high vacuum deposition environment, via a differentially high oxygen pressure enclosed in the heater. We call this process cyclic deposition and reaction or CDR; see schematic in figure 1. Since then the CDR process has been used extensively in commercial environments for manufacturing of superconducting films on wafers, most notably at Superconductor Technologies Inc. (STI) and 0953-2048/10/014018+05$30.00

1

© 2010 IOP Publishing Ltd Printed in the UK

Supercond. Sci. Technol. 23 (2010) 014018

V Matias et al

has many years of experience in manufacturing of wafers using these processes. Control at the 1 at.% level is attainable, in particular by use of atomic absorption spectroscopy (AAS) for rate control [11]. The AAS technique is also inherently nonintrusive, can be used in a variety of deposition configurations and is amenable to km-length manufacturing. In our opinion the RCE-CDR process can be scaled up for manufacturing of long lengths of tapes.

2. Experimental details 2.1. Reactive co-evaporation Samples were fabricated by RCE-CDR at LANL and STI in a similar manner. In both cases samples rotate within a heater that has a high-pressure oxygen pocket and an opening for deposition. In the case of the LANL system the deposition is done during 10% of the cycle, whereas at STI it is 30%. We use elemental sources of yttrium, copper, and barium. At LANL yttrium and copper are evaporated by use of a differentiallypumped Pierce-type electron gun which is skipped between the two sources for heating. Barium is heated using 400 kHz RF induction. Atomic absorption is used to measure the vapor densities just beneath the tapes. These data are used in real time to adjust the RF power, as well as the electron beam scan patterns and residence times. At STI resistively heated sources are used for evaporation and quartz crystal monitors and atomic absorption are used for rate control. As discussed in the previous section, our samples cycle between a low-pressure region, where they receive a deposit from the sources, and a high-pressure region, where they are oxidized and YBCO is thermodynamically stable. The pressure and temperature conditions, as well as the cycling time were discussed in more detail in [9]. At LANL we use an instantaneous YBCO deposition rate of 6 nm s−1 and approximately 0.12 nm is deposited per cycle. In this way a 1 μm thick film can be produced in 30 min. In our current experiments, tape sections are mounted in the heater and rotate with the rotor to cycle the process, but there is no continuous tape motion for long-length deposition. The deposition temperature is between 750 and 800 ◦ C.

Figure 1. Schematic of the RCE-CDR process.

the standard IBAD architecture. The elimination of this layer simplifies the process significantly since the complexoxide layer deposition introduces an additional variable to an already complicated process. The templates in the work presented here consist of simple-oxide materials and are therefore simpler to fabricate, with broader processing windows. The complexity still exists for the deposition of the high temperature superconducting (HTS) layer which consists of 4–5 elements and tight processing windows. Nevertheless, we feel that the HTS deposition complexities are largely mitigated by the RCE-CDR process that provides inherent temperature and pressure stability. Temperature stability and uniformity are some of the key parameters for a reproducible HTS deposition process. The remaining critical parameters are compositional control and compositional uniformity. Compositional control depends on the deposition process used and in our co-evaporation process it has been advanced over the years by use of a variety of in situ sensors for precise rate monitoring and control [10]. STI

Figure 2. Schematic of the two templates used in this work. Template A utilizes electropolished Hastelloy substrate with a thin evaporated Y2 O3 bed layer followed by IBAD-MgO and a thin epi-MgO. Template B has unpolished Hastelloy substrate with a thick planarizing Y2 O3 layer also followed by IBAD-MgO and epi-MgO.

2


V Matias et al

Table 1. Table of samples and their critical currents.

Sample 1 (STI) Sample 2 (STI) Sample 3 (STI) Sample 4 (LANL)

YBCO thickness (nm)

IBAD template

Measurement temperature (K)

Jc (MA cm−2 )

Ic (A cm−1 )

N-value

700 3000 5300 980

A A B B

77 75 77 75

1.9 1.46 1.2 4.1

134 438 640 401

17 22 23 33

MgO by IBAD. A textured IBAD-MgO film of about 7 nm thickness was deposited on the Y2 O3 bed layer with an Ar ion-beam-assist at 1000 V while the tape was held at room temperature [13]. An epitaxial MgO layer of 25 nm was grown on top at approximately 600 ◦ C. Template B used an unpolished Hastelloy substrate that was planarized by a solution deposition planarization (SDP) process [14]. This process further simplifies the IBAD template by eliminating the need for the electropolishing step. We used 15 coatings of Y2 O3 or Y2 O3 –Al2 O3 (10%) mixtures to planarize the unpolished substrate. Using this process the substrate RMS roughness was reduced from 25 nm on a 5 μm × 5 μm scale to about 1.5 nm. The IBAD-MgO and epi-MgO layers were identical in template B as in A. In both templates typical FWHM of the mosaic spread for the MgO was 4◦ –5◦ in the plane of the film, and 1.5◦ out-of-plane as determined by x-ray diffraction analysis.

Figure 3. Superconducting transition temperature as measured inductively for sample 1 on template A.

3. Results Both templates were employed at STI for deposition of YBCO films. Some of the results are shown in table 1. The best results were obtained on template B. However, the thin buffer layer template A also produced very good coated conductors. We note that there is a total buffer layer thickness of only about 35 nm in the IBAD template A (Y2 O3 + MgO). We were originally concerned that there would be interdiffusion of the metal elements from the substrate, such

2.2. IBAD templates The IBAD-textured templates were based on the IBAD-MgO process [7, 12]. Two different architectures were used, as shown schematically in figure 2. Template A consisted of an electropolished Hastelloy substrate, 100 μm thick, on which a 6 nm thick Y2 O3 layer was deposited by evaporation. An appropriate ‘bed layer’ is necessary for proper texturing of

Figure 4. SIMS depth profile data on YBCO deposited on the thin IBAD template, architecture A.

3


V Matias et al

Figure 5. XRD phi scan of the {103} peaks of the YBCO for sample 4.

Figure 7. Critical current density magnetic field dependence for sample 4 (Jc (sf) = 4 MA cm−2 ) for H applied along the main crystallographic orientations.

Figure 6. TEM cross section micrograph of the 4 MA cm−2 sample. The YBCO layer is 1.0 μm thick and SDP layer is 1.2 μm. Figure 8. Critical current density as a function of the angle of the magnetic field with the c-axis for sample 4 at 1 T and 75 K.

as Cr and Mn, into the superconductor that may degrade the properties of the superconductor. However, this does not appear to be a significant problem, as evidenced by the high Jc values, sharp superconducting transition temperatures and secondary-ion mass spectroscopy (SIMS) depth profiling through the film. Figure 3 shows a typical inductively measured superconducting transition for the YBCO film deposited on template A. The Tc value of 88 K is typical for the co-evaporated YBCO films and the transition width is narrow, Tc ∼ 0.8 K. Figure 4 shows the SIMS data indicating no perceptible interdiffusion of the relevant elements. Although results on template A are surprisingly good in terms of the YBCO critical currents, we found template B to be better on average. In particular the highest- Jc sample made by RCE-CDR on template B, sample 4, with a thickness of 1.0 μm, had a Jc of 4 MA cm−2 , at 75 K and in self-field. This compares extremely favorably with the state-of-the-art films on coated conductors or single crystal substrates, as we discuss later. X-ray diffraction analysis of this sample revealed an in-

plane φ of 2.4◦ and an out-of-plane ω of 0.9◦ ; see figure 5. The same sample was analyzed by TEM to confirm thicknesses and quality of interfaces. Figure 6 shows a TEM cross sectional micrograph indicating sharp interfaces and showing the thin and dense, about 33 nm thick, MgO layer (IBAD + homoepi). One can also see in the picture that the thick SDP layer, in this case an amorphous Y2 O3 –Al2 O3 (10%) mixture, planarizes the rough metal surface. The highest- Ic film, sample 3, supported over 600 A cm−1 at 77 K. The film possessed an (005) θ –2θ value of 0.065◦ and a ω value of 0.36◦ . This superb crystallinity results even for a film over 5 μm thick, reflecting the excellent control afforded by the RCE-CDR growth process. Critical currents of sample 4 were also measured as a function of magnetic field at 75 K. Figure 7 shows the plot for a magnetic field applied perpendicular and parallel to the film normal. The magnetic field dependence is that of a 4


V Matias et al

5. Conclusion We have demonstrated high-quality YBCO films on simplified IBAD templates suitable for coated conductors by the reactive co-evaporation cyclic deposition and reaction process (RCECDR). The simplified IBAD-MgO templates consist of just an amorphous Y2 O3 and crystalline MgO layers with the YBCO grown directly on MgO. Transport properties in these films compare very favorably with the best PLD samples made on SrTiO3 single crystals.

Acknowledgments We would like to thank Paul Dowden at LANL for his help in this work. The work at LANL is funded by the Department of Energy Office of Electricity Delivery and Energy Reliability.

Figure 9. Comparison of the highest- Jc RCE-CDR sample on MgO with PLD-deposited YBCO on SrTiO3 versus film thickness from the literature, [16]. All the measurements were made at 75 K.

References [1] Berberich P, Assmann W, Prusseit W, Utz B and Kinder H 1993 J. Alloys Compounds 195 271–4 [2] Matijasevic V, Lu Z, Kaplan T and Huang C 1997 Eucas 1996 Conf. Proc. pp 189–91 [3] Prusseit W, Furtner S and Nemetschek R 2000 Supercond. Sci. Technol. 13 519–21 [4] Moeckly B H and Zhang Y M 2001 IEEE Trans. Appl. Supercond. 11 450–4 Peng L S-J, Heinig N F and Moeckly B H 2002 Mater. Res. Soc. Symp. Proc. 688 211 [5] Nemetschek R, Prusseit W, Holzapfel B, Eickemeyer J, DeBoer B, Miller U and Maher E 2002 Physica C 372–376 880–2 [6] Lee B S, Chung K C, Kim S M, Kim H J, Youm D and Park C 2004 Supercond. Sci. Technol. 17 580–4 [7] Matias V, Gibbons B J and Feldmann D M 2007 Physica C 460–462 312–5 [8] Storer J, Hänisch J, Sheehan C and Matias V 2007 MRS Proc. Spring 2007 Mtg 1001-M14-02 [9] Matias V, Hänisch J, Reagor D, Rowley E J and Sheehan C 2009 IEEE Trans. Appl. Supercond. 19 3172–4 [10] Matijasevic V and Slycke P 1998 Proc. SPIE 3481 190–5 [11] Wang W et al 1997 Appl. Phys. Lett. 71 31–3 [12] Wang C P, Do K B, Beasley M R, Geballe T H and Hammond R H 1997 Appl. Phys. Lett. 71 2955–7 [13] Matias V, Hänisch J, Rowley E J and Guth K 2009 J. Mater. Res. 24 125 [14] US Department of Energy 2009 Superconductivity for Electric Systems Peer Review http://www.htspeerreview.com/2009 V Matias presentation [15] Civale L et al 2004 J. Low Temp. Phys. 135 87–98 [16] Foltyn S, Civale L, MacManus-Driscoll J L, Jia Q, Maiorov B, Wang H and Maley M 2007 Nat. Mater. 6 631–42 [17] Maiorov B et al unpublished

state-of-the-art undoped YBCO sample. The critical field (power-law) exponent, α , for this sample is 0.56, similar to that for other undoped YBCO thin films [15]. Angular dependence of the magnetic field at 1 T and 75 K is shown in figure 8. Jc () shows a prominent peak ab ( = 90◦ ), indicative of correlated defects along ab-planes. Along the c-axis direction ( = 0◦ ) a smaller characteristic peak can be observed centered in this orientation similar to that of other undoped YBCO films [15].

4. Discussion These results demonstrate by proof of principle that RCECDR films can have high Jc values on simplified IBAD-MgO templates, especially with the SDP-planarized substrates. Our champion result compares very well with the best YBCO films without additions on SrTiO3 substrates. Figure 9 shows a direct comparison with the best PLD YBCO samples [16]. Additionally, the high degree of crystallinity and high N-values from Jc measurements that we have obtained in these films also indicate an absence of weak links in these materials [17]. It appears to us that this IBAD template is a very good platform on which coated conductors can be developed. We believe that the IBAD architecture consisting of a thick solution-deposited planarizing amorphous layer and a thin MgO textured crystalline layer, is ideal as the coated conductor template, since it can be fabricated quickly and at low cost and provides ample protection to the YBCO layer from detrimental species.

5