IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY,VOL. 11, NO. 1, MARCH 2001
2826
Superconducting Tapes Using ISD Buffer Layers Produced by Evaporation of MgO or Reactive Evaporation of Magnesium Ralf Metzger, Markus Bauer, Kai Numssen, Robert Semerad, Paul Berberich and Helmut Kinder
Abstract-To avoid large angle grain boundaries in YBazCu307(YBCO) thin films, biaxially textured buffer layers were grown by inclined substrate deposition. We use MgO as buffer layer material, which can be grown at rates up to 300 W m i n using either e-beam evaporation of MgO or reactive thermal evaporation of magnesium Employed substrates were Hastelloy C276 or stainless steel. Epitaxial YBCO was grown on these buffer layers by thermal co-evaporation. We reached YBCO in plane texture of 7" FWHM and critical currents along tape direction of 84 A on tapes with 8 mm width and 1.8 pm YBCO thickness which means a current density of 0.58 MA/cm'.
Index Terms-biaxial texture, coated conductors, inclined substrate deposition,MgO, YBCO
I. INTRODUCTION
L
ARGE scale production of YBCO coated metal tapes is
considered to be one of the most promising applications of high temperature superconductors. In order to obtain considerably high critical currents, superconducting films have to be biaxially textured [l]. This can be obtained by epitaxial growth on a biaxially textured surface. As common metal tapes do not fulfill this requirement, one either has to use particularly treated RABiTSTMtapes [2] or a biaxially textured buffer layer which can be grown on polycrystalline substrates by ion beam assisted deposition (IBAD, [3]) or inclined substrate deposition (ISD, [4],[5]). We use MgO buffer layers grown by inclined substrate deposition. Buffer layer quality is tested by x-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and the superconducting properties of YBCO films grown on it by thermal co-evaporation [6]. In this paper we want to give a short summary of our results under various buffer layer coating conditions which were made as preparatory work for the development of a new continuous tape-coater. The mechanism of texture evolution in MgO buffer layers due to substrate inclination is quite well understood [7]. Manuscript received September 17, 2000. This work was supported by Bayerische Forschungsstiftung. Ralf Metzger, Markus Bauer, Kai Numssen, Paul Berberich and Helmut Kinder are with the Physik Department E10, Technische Universitdt Miinchen, 85747 Garching, Germany (telephone: +49-89-289- 12660; fax: +49-89-289- 12414; e-mail:
[email protected]) Robert Semerad is with Theva Diinnschichttechnik GmbH, Hauptstr. Ib, 85386 Dietersheim, Germany
Starting from numerous single crystalline nuclei with random orientation, each nucleus grows by epitaxial alignment of MgO from the vapor phase. The inclined direction of incidence of the arriving vapor causes some orientations to grow faster in height than others. Monte Carlo simulations [7],[8] have shown, that optimum growth velocity is achieved for orientations with MgO [OOl] tilted by a texture angle p towards the direction of vapor incidence. The value of p rises with increasing substrate inclination angle a. This states a reason for an uniaxial texture, but MgO [ l l O ] can still align in any direction perpendicular to MgO [OOl]. Therefore, the afore mentioned simulations examined the growth velocity depending on the orientation of the MgO [1101 axis in that plane and found a slight advantage for an alignment parallel to the plane of vapor incidence. The advantage becomes more pronounced with increasing a. The taller columns can shadow some of their smaller neighbors, as pictured in Fig. 1, resulting in the extinction of the latter. Orientations with substantially lower growth rates die out earlier than those close to the optimum. In particular the out of plane alignment develops sharper and in an earlier state compared to the in plane orientation. Experimental results which are in good agreement with this growth model will be shown.
Fig. I . Texture evolution by epitaxial growth of randomly oriented grains with different growth velocity for different orientations.
1051-8223/01$10.00 0 2001 IEEE
2827
I
substrate quartz crystal monitor
0
I
w
vacuum pump
1
evaporation source
I
I
180
Fig. 2. Coating setup for the ISD process.
11. COATING SETUPAND RESULTS Inclined substrate deposition, as all physical vapor deposition methods, is done in high-vacuum environment. Within the vacuum chamber primarily an evaporation source, a quartz crystal monitor, a tilted substrate holder and a controllable oxygen inlet close to the substrate is needed, as shown in Fig. 2. Besides this we have a pivoted electrode in our chamber, used for originating a glow discharge in order to remove adsorbed water from the substrate surface. In principle one can obtain MgO buffer layers either by reactive evaporation of magnesium in an oxygen atmosphere or by evaporation of MgO material itself. Generally we use an e-beam gun for evaporation of MgO. In order to economize the process for the production of long tapes, we tried to use simple thermal evaporation of metallic magnesium instead. In the following we present results of both approaches. A. E-beam evaporation of MgO
Using MgO as buffer material the ISD layer is grown at room temperature with growing rates between 120 and 150 n d m i n for 600 s to 800 s. Our best results with respect to the superconducting properties were achieved with a tilt angle of 25" and an oxygen flow of 1 .sccm. Resulting films were analyzed by XRD, yielding a pole figure shown in Fig. 3. The cross near the [002] peak marks the deposition direction. Looking at the [200] and [020] peaks you can notice that the in plane peak width A$ of this peaks is considerably larger than their out of plane width Ax. By a more detailed inspection of the pole figures we obtained that the misorientated MgO grains were preferentially rotated around the [002] axis and not around the substrate normal = 0"). To determine in plane and out of plane alignment with respect to the rotation around the [002] axis, we use the MgO [220] peak ($ = 0" and = 90" - p) to measure A$ and
Fig. 3. Pole figure of MgO [200) with in plane width A@ and out of plane width A x of the MgO [200] or MgO [020]peak.
Varying the substrate inclination angle one can observe, as expected from Monte Carlo simulations, a rising texture angle with increasing substrate inclination (Fig. 4, boxes). The quality of the in plane orientation A$ seems to decline with decreasing substrate inclination (Fig. 5, boxes). The out of plane width (Fig. 5, circles), however, remains rather constant. In Fig. 4 and Fig. 5 also XRD data of YBCO, grown on the respective buffer layers are plotted. The texture angle and out of plane width of YBCO is nearly the same as for MgO which one should expect. Remarkable is the enhanced in plane texture of YBCO compared to the buffer layer. This is in agreement with previous results [7] showing a decrease of Mg0[220] in plane width with increasing film thickness. Therefore, we can assume that the texture quality of the buffer layer is best at its surface. The texture of the epitaxially grown YBCO film is determined only by this surface. However, misorientated MgO grains, which have been overgrown by neighboring grains, are still taken into account in the XRD measurement of the MgO peaks and worsen its width compared to the YBCO values.
401 35
m
0
B 0
(x
x
Ax.
20
25
30
35
40
45
50
55
a [degree] Fig. 4. Texture angle p depending on substrate inclination angle a.
2828 TABLE I RESULTSWITH MGO E-BEAM EVAPORATION
MgO
0 0
44 Ax
A
44 YBCO
MgO Bridge length 0
0
0
25
30
8
A
f
f
35
45
40
50
Bridge width
YBCO thickness
45 mm 40 mm 5"
E
jc
(77 K)
(77 K)
8mm 18OOnm 84A 8" 600nm 32A 0.6 mm 600nm 2.8 A Jc is the critical current along the tape direction.
0.58MA/cm2 0.69MNcm2 0.79 MA/cm*
The critical current of 84 A is the maximum value, we have achieved up to now. The corresponding film is also the thickest one we have fabricated
55
a [degree] Fig. 5. Texture quality of MgO and YBCO films depending on substrate inclination a during ISD process.
The most reliable information on the quality of the buffer layer is the quality of the superconductor grown on it. Usually we utilize 600 nm YBCO films produced by thermal coevaporation for characterization of our buffer layers, but up to 1.8 pm were deposited, too. On the YBCO film a protection layer of gold or silver with a thickness of about 50 nm is grown in situ. This layer also assures good electric contact for the resistive measurement. Resistive measurements were done by applying an increasing DC current to the tape. If the critical current density is trespassed, a steep increasing voltage drop can be observed along the tape. Fig. 6 shows a typical I-V plot for our YBCO films. Table I shows measurement data of three different samples. The first two samples were simply contacted at their ends by pressing indium pads on the protection layer. The voltage contacts were mounted in between. As criterion for determination of the critical current a voltage drop of lpV/cm was used. Due to its small dimensions of 10 x 10 mm, the third sample had to be pattemed by standard photolithography in order to measure j,.
B. Thermal evaporation of magnesium metal Due to its high vapor pressure, magnesium can be sublimated at a temperature of about 600°C, which allows the use of simple thermal evaporation. Initially we used resistively heated boats for that purpose. Later we changed to an evaporation cell (see Fig. 7) which consists basically of an one side opened stainless steel tube. Thermocoax heating wire is wound around this tube. An additional stainless steel cover pressed on the heater windings ensures good thermal contact. In order to save heating power, thermal radiation shields are added. The general coating setup does not change compared to Fig. 2.
0 0
lZ01 100
U
0
T = 77K 600 nm YBCO 10 x 8 mm bridge
Fig. 7. Evaporation source for magnesium metal.
0 0 0 U
0
B
ff
-5
0
5
10
15
20
25
30
35
Current [A]
Fig. 6. Current vs. voltage characteristic. Voltage is measured over 10".
Best superconducting properties were achieved with growth rates of 300 n d m i n and 3 p m MgO thickness. Using constant oxygen flow, the chamber pressure decreases more than one order of magnitude from 50 mPa to 2 mPa when evaporation of magnesium starts, due to its reaction with the oxygen. Table I1 shows our results for mechanically polished stainless steel or Hastelloy C276 substrates and for electropolished Hastelloy. Critical current densities do not depend significantly on the substrate.
2829 TABLE I1 RESULTSWITH MAGNESllJM METAL EVAPORATION Substrate Hastelloy el. pol. Hastelloy mech. pol. Stainless steel mech. pol.
A$ A’$ Mgo 11.5” 13.3~ 10.4”
YBCO
Tc [K]
jc(77K) [MA/cm2]
8.8” 9.3” 7.9”
84.9 85.0 84.8
0.31 0.24 0.24
111. CONCLUSION
MgO buffer layers grown by inclined substrate deposition have been proved to be well suited substrates for YBCO coated tapes. Thermal evaporation is fully compatible with the ISD process. YBCO films of 1.8 p m in thickness have been produced without any major decrease in critical current density, yielding a critical current of 84 A. It is planned to increase the film thickness further to increase this value. E-beam evaporation of MgO has been substituted with thermal evaporation of magnesium. Feasibility of the new approach has been shown successfully, even though critical currents still remain below the standard value. Also Hastelloy substrates can be replaced by mechanically polished stainless steel in principle. In this case an electropolishing process sufficient for ISD requirements is still needed.
REFERENCES Dimos D, Chaudhari P, Mannhardt J, “Superconducting transport properties of grain boundaries in YBazCu3O-r bicrystals”, in Phys. Rev. B, 41, no. 7, pp. 4038-4049, 1990. Norton D P, Goyal A, Budai J D, Christen D K, Kroeger D M, Specht E D, He Q, Saffian B, Paranthaman M, Klabunde C E, Lee D F, Sales B C, List F A, “YBazCusO7 on biaxially textured’nickel (001): an approach to superconducting tapes with high critical current density”, .in Science, vol. 274, no. 5288, pp. 755-757, 1996. Iijima Y, Tanabe N, Ikeno Y, K o b o 0, “Biaxially aligned YBazCu307., thin film tapes”, in Physica C , vol. 185-189, pt. 3, pp. 1959-60, 1991. Hasegawa K, Yoshida N, Fujino K, Mukai H, Hayashi K, Sato K, Honjo S, Sato Y, Ohkuma T, Ishii H, Iwata Y, Hara T, “In-plane aligned YBCO thin film tape fabricated by pulsed laser deposition”, in Proceedings of the 9th International Symposium on Superconductivity (ISS96), Springer-Verlag, Tokyo, Japan, vol. 2 , pp. 745-748, 1997. Bauer M, Semerad R, Kinder H, “YBCO films on metal substrates with biaxially aligned MgO buffer layers”, in IEEE-Transactions-onApplied-Superconductivity,vol. 9, no. 2, pt. 2 , pp.1502-1505, 1999. Kinder H, Berberich P, h s s e i t W, Rieder-Zecha S, Semerad R, Utz B, “YBCO film deposition on very large areas up to 20x20 cmz”, in Physica C, vol. 282-28, pp. 107-110, 1997 Bauer M, Metzger R, Semerad R, Berberich P, Kinder H, “Inclined substrate deposition by evaporation of magnesium oxide for coated conductors”, in Materials Research Society Symposium Proceedings, edited by Norton D, Schlom D, Newman N, Matthiesen D, Materials Research Society, Warrendale, Pennsylvania, 2000, vol. 587, p. 02.2.1 Bauer M, “Herstellung und Charakterisierung yon YBCO-Schichten und biaxial texturierten Pufferschichten auf technischen Substraten”, Ph.D. thesis, Technische Universitat Munchen, Germany, 1998
