CVD OF SUPERCONDUCTIVE YBa2 Cu3 O7-δ

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CVD OF SUPERCONDUCTIVE YBa2 Cu3 O7-δ

F. Schmaderer, G. Wahl

To cite this version:

F. Schmaderer, G. Wahl. CVD OF SUPERCONDUCTIVE YBa2 Cu3 O7-δ. Journal de Physique

Colloques, 1989, 50 (C5), pp.C5-119-C5-129. �10.1051/jphyscol:1989518�. �jpa-00229541�

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Colloque C 5 , suppl&ment au n05, Tome 50, mai 1989

CVD OF SUPERCONDUCTIVE YBa, Cu, 0, -

F. SCHMADERER and G. WAHL

Asea Brown Boveri AG, Corporate Research, Heidelberg, F.R.G.

Le dgp8t chimique en phase vapeur est une m&thode prometteuse pour produire des supraconducteurs ,81haute temp6rature. En effet cette technique permet le depot sous haut potentiel d90xyg&ne avec des vitesses Qlevees par rapport & celles obtenues pour les autres methodes de dQp8t. Sa potentialitb est grande et on peut revatir des substrats de g60m6trie complexe (ex: fibres). Les B-didtone chelates de Y, Ba, Cu ont &t6 utilises commme precurseurs pour le depat,, 2e YBa2Cu307-6. Pour definir le procede de depot.

llhydrodynamique du reacteur a

6t6

Abstract

Chemical Vapour Deposition is a very promising method to produce High-Tc superconductors because of the following reasons: the deposition is possible at a very high oxygen potential, the deposition rates can be high compared to those achieved by other deposition techniques and it has an excellent throwing power which is necessary if complicated substrates (e.g. fibers) have to be coated. As source materials for the YBa2Cu307-&-deposition experiments B-diketone chelates have been used for Y, Ba and Cu, respectively. In order to define the deposition process, the gas flow phenomena in the reactor were studied by Ti02 fume experiments and by computer simulations. The deposition experiments were carried out on singlecrystalline (100) SrTi03- and Y-stabilized polycrystalline ZrO2- substrates

(YSZ) showing in each case superconductivity above 94 Kelvin and current densities of 105 ~ c m - ~ at the maximum available magnetic field of B = 5.5 Tesla and 77 Kelvin when using SrTi03 substrates.

Introduction

The discovery of the new class of oxide-superconductors by J.G. Bednorz and A. Miiller /l/ has generated strong efforts in the developement of techniques to obtain thin films with appropriate superconducting properties such as sputtering, laser-, electron beam- and molecular beam evaporation and several wet chemical procedures / 2 / . Most of these processes have the disadvantage of a low oxygen potential during the deposition process which results in an oxygen deficiency of the coating. To obtain superconductivity

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989518

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the coatings have therefore to be posttreated at high temperatures (up to 950°C) with defined cooling rates. Chemical vapour deposition has the advantage that the deposition can be carried out at a high oxygen content in the gas phase even in pure oxygen if the precursor molecules are stable and no side reactions take place. In addition CVD is a molecular process which promotes oriented or even epitaxial grain growth which seems to be essential for this kind of superconductor.

YBa2Cu307-6 coatings produced by CVD are described by different authors /3-9/. But only very few measurements of the critical current densities exist /g/. The aim of this work was 1) to investigate the possibilities of this process for industrial applications. That means high deposition rates must be reached

-

Chemical Engineering Considerations

The stagnation flow reactor which was used for the deposition experiments is schematically shown in fig.1. The axisymetrical reactor consists of a quartz tube having a diameter of 5 cm, a gas inlet tube and a substrate holder which can be heated inductively. The reasons for using such a reactor geometry, where the gas flows perpendicularly onto the deposition surface from the bottom to the top, is the fact that chemical reactions of the reactive gas with the walls can be minimized and no recirculation effects takes place. This leads to an homogeneous mass transport toward the substrate surface, which can be seen in fig.2, where the gas flow is visualized with Ti02-fume. The stagnation flow reactors are well understood, many calculations about the gas flow in such reactors are carried out /10/11/12/. In the stagnation area a constant mass transport to the surface can be expected. The radius ro of this zone depends on the geometry of the reactor and was estimated by Houtman et al. /12/. In order to estimate the maximum deposition rate in the stagnation point (not limited by surface reactions) the transport through the gas phase from the gas inlet (molar fraction of the reactive components coi) to the deposition surface (molar fraction of the reactive components near the surface cSi) can be calculated by /13/

whereby Shi is the Sherwood number defined by

sample (015mm, 2mm)

gas inlet

Fig.1: Stagnation flow reactor

gas f low : 70 lh-l

Fig.2: TiO2-fume experiment

where ji is the molar deposition rate of the component i and n the total molar concentration in the gas inlet. The Reynolds number Re is defined by

( = viscosity in the gas inlet, P = density in the gas inlet, v = gas velocity in the gas inlet, d = diameter) and the Schmidt number Sc as

(Di = diffusion constant of the component i in the gas inlet). The correction factor g(Sci) is a factor in the range of 1 (710%) for the Sc range of interest 10 > Sc > 1 /13/. The factor 8 depends on the ratio H/d where H is the distance between deposition surface and the gas inlet. Fig.

3 shows our calculated values of B in comparison with measurements in liquids (Sc >> 1 ) and gases /13/. Our calculations were carried out in isothermal reactors with finite difference methods /10/14/.

As shown in /15/ for boundary layer approximations eq.1 should be approximately correct for non isothermal conditions too (difference smaller than a factor of two), if the gas inlet parameters are taken for the definition of Re, Sc and Sh. From the eq. (1) and (2) the deposition rate is given by:

~i B E ~ ~ 1 1 3 n(coi

-

ji =

In all these equations thermodiffusion effects are neglected.

The source materials, which have been used as precursors /16/, are:

Cu-acetylacetonate, [ ~ u ( a c a c ) ~ / , or

Cu-hexaf luoroacetylacetonate, CU( h f a ~ ) ~ ]

2,2,6,6-tetramethylheptandionate-(3,5)Y, [Y( thd 13], 2,2,6,6-tetramethylheptandionate-(3,5)Ba, [ ~ a ( t h d ) ~ ] ,

10

-

2.45 34000

P

0.05 0.1 1.0 10 40

nozzle height, Hld -

Fig.3: The factor B in the stagnation point versus the ratio H/d for different measurements /13/ and our calculations.

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For the estimation of the maximum deposition rate at a given gas inlet molar fraction coi and a given molar fraction csi at the surface the gas properties II and Di must be known. For II the value of oxygen /17/ was taken because mainly oxygen was used as carrier gas and the molar fractions of the precursors ci were ci < < 1. In order to determine the possible influence of thermodiffusion the thermodiffusion factors ai must be known.

Di and ai were estimated from the Lennard Jones distances oij and the Lennard Jones interaction energies cij between the molecules i and j according to the classical formulas given in /18/19/20/. The oij and cij were calculated from the values oi and ci between equal molecules /17/.

Except oi and ci for oxygen /18/ all other ci and oi were estimated by empirical methods /18/. o i were calculated from the Le-Bas volume Vgi

and ci from the boiling point Tgi:

k = Boltzmann constant.

The boiling points Tgi given in tab.1 were for Cu(acac)2 extrapolated from the measurements of Spee et al. l , for Y(thd)3 from /22/ and for Ba(thd)2 from fig.4 as indicated in this fig. From these parameters the diffusion coefficient Di in 0 2 and the factors of thermodiffusion ai were determined. The estimation of the influence of the thermodiffusion coefficients according to eq.27 in /13/ shows that the ratio

(j,: transport with and ja=o without thermodiffusion) is

for our molecules. Therefore we neglect this influence and eq.5 can be used.

Table 1:

1) according t o /21/ 4 ) according t o /18/

2 ) according t o /22/ 5 ) c a r r i e r gas is 02

3) extrapolated from Fig.4

Experiments

For the evaporation of the compounds three separate units were constructed having their own heating elements including temperature and gas flow control. This is necessary due to the different evaporation behaviour of the molecules and in order to control the stoichiometry in the layer by changing the temperature of the source and/or the carrier gas flow. Fig. 4 shows the molar evaporation rates nev of the B-diketone chelates used as source materials. The evaporation experiments were carried out in identical systems having the same flow rates of the carrier gas argon/oxygen (25 l/h) and same amount of precursor powder (1 g).

In the case of copper, two compounds were used initially. As can be seen Cu(hfac)2 shows a higher volatibility with a lower activation energy EA compared to Cu(acac)~. This can be explained by a lower association tendency in the case of Cu(hfac)2, where hydrogen atoms in the ligands are substituted by fluorine atoms, which causes smaller interactions between the molecules in the evaporator. A much stronger temperature dependence has been obtained in the case of Y(thd)3 and Ba(thd)2. While the activation energies are less different, the temperature for comparable evaporation rates differs very strongly. Fig. 4 shows that at the TB of Y(thd)g and Cu(acac)2 a evaporation rate of nev = 7*10-4 mol min-l is reached. With the help of this value the boiling temperature Tg of Ba(thd)2 can be extrapolated from this figure. For the deposition experiments of YBa2Cu307-6 we choose Cu(acac)2 as copper source instead of Cu(hfac)2 because of the more similar evaporation behaviour compared to Y(thd)g and Ba(thd)2. To avoid condensation, all parts between the evaporation system and the deposition region are held at temperatures some degrees higher than the highest evaporation temperature.

The results concerning the kinetics of the single oxide deposition of Y, Ba and Cu has been published elsewhere /16/. The evaporation temperatures T

d

(j=1,2,3) for the generation of a molar ratio of 1:2:3 of Y(thd)g. Ba(th 12 and Cu(acac)2, respectively, in the gas phase are indicated in fig.4. The following deposition conditions were used:

Td ⁼ 1173 K

~ ~ [ ~ ( t h d ) ~ ] = 423 K

~ ~ [ ~ a ( t h d ) ~ ] = 528 K

~~[cu(acac)~] = 435 K Ptot = 500 Pa

i ~ r = 25 l/h (STP) i ~ x = l0 l/h (STP

with Td = deposition temperature, ptot = total pressure, iox = iAr =

oxygen/argon gas flows over Cu(acac)2, Y(thd)g and Ba(thd)2, respectively.

For the deposition experiments Y-stabilized polycrystalline Zr02 (YSZ)- and (100)-oriented single crystalline SrTi03- substrates were used. After the deposition experiment the samples cooled down in pure oxygen (t = 20 min).

The deposition rate of the mixed oxides were in the range of &

-

( = j = 8*10-9 m01 min-l cm-2).

Results

Fig.5 shows scanning electron micrographs (SEM) of a coating deposited on a YSZ- substrate. The plate-like structure corresponds to a mainly (100)- and (001)- orientation of YBa2Cu307-6 compared to a randomly distributed material obtained with conventional powder technology as can be seen in Fig 6a and 6b, where the (110,103)-reflex at 2 8 ~ 3 2 . 8 ~ is dominating. At 28=30.7O (Fig.6b) small amounts of the tetrag~nal non-superconducting Y2BaCuOq are visible.

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Fig.4: Molar evaporation rates nev of Cu(acac)2, Cu(hfac)2, Ba(thd)2 and Y(thd)3 versus the reciprocal, absolute evaporation temperature TV.

i = 25 l/h (STP), Ptot = 500 Pa, T1,T2 and Tg indicates the evaporation temperatures for Y(thd)g, Ba(thd)2 and Cu(acac)~ used for the deposition experiments).

A much stronger (001)-orientation can be obtained if single crystalline SrTi03- subtrates are used (fig.6~). In this case the c-axis of the orthorhombic structure is mostly perpendicular to the substrate surface.

Increasing the deposition temperature up to 950°C leads to a nearly perfect (001)- orientation (fig.6d). Fig.7 shows a SEM-picture of a (001)- textured coating (Td = 9000C) with an average grain size of several um.

The electrical resistivity and the critical current density of the coatings were measured by using the conventional DC four-probe method. The contacts were made by silver paste. To minimize the stress of the electrical contacts due to the high currents, the coatings have been structured (100 urn current bridge) with an excimer laser ( X = 193 m ) . Fig.8 shows that

6a) randomly distributed bulk material

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9*104 counts

j *

Fig.6a,b,c,d: X-ray diffraction of YBa2Cu307-g bulk material

(6a), on YSZ- (Td=1173 K)(6b), and on SrTi03- substrates with Td = 1173 K(6c) and Td = 1223 K(6d).

both samples on Zr02- as well as on SrTi03- substrates are superconducting at a transition temperature of Tc > 94K directly after the deposition experiment and cooling down in oxygen. The critical current density jc at 77 Kelvin of YBa2Cu307-6 deposited on SrTi03- substrates versus the applied magnetic field B parallel to the surface is plotted in Fig.9 (5.5 Tesla was the maximum magnetic field available). These values of the critical current densities j c ( B ) are in the range of the results reported by K. Watanabe et a1./9/. The critical current density on YSZ- substrates was in the range of jc 1 0 ~ ~ c m - ~ at 77 Kelvin. The layer thickness of all samples was in the range of 1.5rm. The smaller current densities on YSZ are caused by the different grain structure of the superconducting layer because:

1) YBa2Cu307-g is very anisotropical (coherence length E=10-20nm in the a-b plane, Ex0.5-0.7nm in the c-direction /23/).

Fig.7: SEM- picture of YBa2Cu307-6 on SrTi03- substrates.

(Td = 1173 K)

TEMPERATURE 10 KELVIN

Fig.8: Transition curve of YBazCu307-6 on different substrates.

105

6 Tesla

Fig.9: Critical current density jc versus the magnetic field B.

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2 ) No orientation of the a-b-axis and not so strong preferential orientation in the c-direction perpendicular to the surface.

3 ) The grains are, as fig.5 shows, very often not good connected. There are gaps visible between the grains which increase the transition resistance /24/.

Discussion

These experiments have shown that it is possible to prepare this new kind of oxide superconductors by chemical vapour deposition with very high current densities. One of the problems are the small deposition rates. In order to estimate the causes for these small values the maximum possible deposition rate was calculated according to eq.(5). For the application of eq. 5 the following deposition parameters derived from the data in the last sections were used: coi = 1.4*10'~, Re = 134, Sc = 5 (typical value in n(5*102 Pa, 300 K) = 2.23*10-7 m01 cm-3, d = 1.55 cm, D = 7.00

z:q;'18

to

l/ptot-.

= 16.6*10 m01 rnin-l~m-~ is calculated. The comparison of this values with the measured value of j = 8*10-g m01 min-l cm-2 shows that we are probably in the mass controlled range and the very small deposition rate in comparison to the evaporation rate is caused by the gas flow and not so much by the kinetics on the surface. Therefore in order to increase the deposition rate the mass transport toward the substrate surface have to be increased.

Acknowledgement

The authors would like to thank W. Sick for the deposition experiments Dr.

A. Abeln, M. Hauck and B. Martin for the Tc- and jc- measurements and Dr.

J. Demny, Mrs. U. Feller and Mrs. D. Stenzel for X-ray and SEM- investigations.

Reference

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