Chemical engineering of CVD of HTc superconductors

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Physlca C 209 (1993) 195-198 North-Holland

Chemical engineering of CVD of HTc superconductors G. Wahla, W. Decker=, M. Pulver=, R. Stollea, O. Y. Gorbenkob, A. R. KauP, Y. Y. Erokhinb, I. E. Graboyb, M. Sommerc, U. Vogtc alnstitut f. Oberfl~tchentechnik und Plasmatechnische Werkstoffentwicklung, TU Braunschweig, D-3300 Braunschweig bChem. Dept. MSU Moscow, Russia CASEA Brown Boveri, Heidelberg For CVD of HTc superconductors different sources (single or multiple source systems, flash evaporation, aerosol formation) are possible. After a comparison of some of these systems the CVD of Y1Ba2CusOx superconductors from aerosol with an ultrasonic nebulizer is decribed. As precursors thd complexes of Y, Ba, Cu (thd = 2.2.6.6.-tetramethyl-3.5.-heptanedionate) are used and as solvent diglyme. Deposition experiments on flat samples and at the walls of a cylindrical hole show that the process is controlled by transport in the gas phase. The layers are contaminated with C. The maximum transition temperature is 79 K. 1. I N T R O D U C T I O N

CVD processes for the deposition of HTc superconductors have the advantage in comparison with PVD methods to have a large throwing power. Therefore it should be possible to coat complicated forms like fiber bundles for the production of superconducting cables. In order to be successful a long time stability of the evaporation behaviour must be guaranteed. Therefore the evaporation system is a crucial unit for the CVD process. In the following first the problems of evaporation sytems are discussed and then the CVD experiments. 2. E V A P O R A T I O N S Y S T E M S

A conventional evaporation system is shown in Fig. 1.

Fig. 1: Typical evaporator system The gas flows above the evaporator surface. 0921-4534/93/$06 00 © 1993 - Elsevier Science Pubhshers B V

By the similanty analysis of the corresponding gas flow problem the following formula is derived: I = x o n o D d f(Re, Sc),

(1)

with the abbreviations: d a typical length e.g. the diameter of the evaporator, xo (xo <<1) the equilibrium molar fraction of the evaporating material, nothe molar density of the carder gas, D the diffusion coefficient of the precursor molecule, Sc = ~/QD: the Schmidt number ('q = viscosity, Q = density, for thd-complexes, app. Sc = 511,2/) and Re = Q v~d / ~: the Reynolds number (v = characteristic velocity of the gas). For very slow gas flows it can be assumed that the total gas reaches the equilibrium molar fraction xo, then eq. 1 has the form: I=

ocxon o D d R e S c ,

(2)

where the constant o~depends on the geometry of the evaporation system. According to formula (1) the evaporation is linearly dependent on the reciprocal total pressure as is verified for the evaporation of compounds used for CVD of superconductors 131. The function f( ) depends on the geometry of the evaporator and is influenced e. g. by changing of the filling factor h/d during the evaporation process, where h is the distance between the rim of the crucible and the filling All rights reserved

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G Wahl et al / Chemtcal engmeertng of CVD of HTc superconductors

level (Fig. 1). Fig. 2 shows the change of the total evaporation rate versus h/d., calculated for two dimensions (infinite perpendicularly to the paper surface in Fig. 1) /4/. In addition to the pressure and the evaporation geometry which influence the evaporation rate the third parameter is the temperature.

evaporation from different sources can only be used for the laboratory investigation of the process /1, 2, 5, 6/. The nebulization process should produce droplets with defined upper limit of the droplet diameter in order to guarantee a complete evaporation. This is possible with ultrasonic nebulizers/9/. In the following experiments with such a nebulizer are described. 3. CVD PROCESS

Fig. 2: Ratio of the evaporation rate I at h to the value at h = 0 (L/d = 2.86, H / d = 0.5, Re = 1.1) The strong temperature /5, 6/ dependence demonstrates that the temperature must be stabilized in the range of _+1 K to reach a stable evaporation. In some evaporation systems the evaporation rate is not constant with regard to time. This is the case for the evaporation of Ba(thd)~/7/. According to/7/the deposition rate decreases at temperatures higher than 220 °C. Two causes are possible 1) the polymerization of the material and 2) a hydrolysis of the compound. This limitates the deposition rate of the superconductor because the vapor pressure can not be increased with the temperature. All the factors mentioned show that it is very difficult to control the evaporation process. A further difficulty is that because of the different vapor pressures of the precursors for each precursor an extra evaporator must be used which have to be controlled separately. To overcome these problems different possibilities are discussed in the literature: 1) flash evaporation 181or 2) nebulization of a solution with all precursor compounds. The

The CVD processes were carried out in a deposition geometry shown in Fig. 3. After the nebulization the aerosol was conducted through a heater in order to evaporate the aerosol (system 1). In an other apparatus the aerosol was directly con-ducted into the reactor (system 2). Two kinds of substrates were used: samples 10.10 mm made of Y stabilized ZrO2, MgO and samples made of stainless steel with a hole in the sample (20 mm length, 4 mm diameter). These measurements are of practical importance because the deposition on complicated forms like fiber bundles as mentioned above. In addition the kinetics of the process can be studied by investigation of the deposition profile at the walls of the hole.

4

Fig. 3: Stagnation flow reactor (units in mm) The calculations/3, 4/ of the gas flow show that the flow is very small inside the hole. Therefore the deposition profile can be calculated analytically by solution of the Laplace equation.

G Wahl et al / Chemical engmeenng ofCVD of HTc superconductors

For the calculation it was assumed that the chelate compounds completely decompose on the deposition surface and the oxide is formed, that means that the concentration cs of the chelates was assumed to be cs = 0 on the surface. The deposition profile is then described by:

("0(=-') jd = D e o~e r

'

-e

BoJi(Bo)

(3)

SoL

where r is the radius of the hole, L the depth, D the diffusion constant, c o the concentration of the deposited component, B o == 2.405 the first zero point of the 0 order Bessel function of the first type, Jl the first order Bessel function of the first type. The theoretical profile compares very well with the profile calculated numerically in spite of the fact that the formula is mathematically not quite correct on the wall near the entrance. The depositions were carded out with the following typical conditions: total pressure 2000 Pa, Ar gas flow 0.13 tool/h, O 2 gas flow 1.5 tool/h, diglyme flow (0.2 mol/h with chelates 0.03 tool/I, temperature 680 - 950 °C. The process depends very weakly (10 % increase at 150 K temperature increase) on temperature as is typical for diffusion controlled processes. The measured and calculated profiles in the hole as is shown in Fig. 4 for the Cu deposition agree approximately with the calculations according to formula (3). Similar results were found for the other components. The deviations found in the case of Cu at 750 °C are probably caused by nucleation effects in the gas phase. The pressure dependence at Re = const. (Fig. 5) and the velocity dependence at p = const, of the deposition rate Jd (Jd proportional to p-1 and v is) is not in agreement with the diffusion controlled deposition (Jd independent of p and proportional to v~). Possible explanations are: incomplete evaporation of precursors in the free flowing gas, precursor degradation in the vapor phase. Similar experiments were made in system 2. In this system the deposition

197

decreased with increasing temperature. The reason for this phenomenon might be the incomplete evaporation of droplets or nucleation in the gas phase. •

!i o

!

-

!

•

!

-

i

i

theory

'

Im~C

1

o

Z

~

I

!

4

5

dept.h,mm Fig. 4: Comparison of experimental deposition profiles on the vertical walls of the hole with theoretical profile (for Cu(thd)2) NE o %

2

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Io

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Fig. 5: Pressure dependence of YBCO deposition In both systems heating effects during the deposition caused by burning of the solvent are observed. The values of the temperature increase are different for various solvents. Diglyme produces minor temperature increase AT < 15 - 25 K at temperatures < 850 °C and total pressures < 20 hPa. But increase to 22 hPa results in a strong increase of the temperature until AT = • 130 K. The strong dependence of AT on the total pressure is typical for chain reactions

G Wahl et a/ /Chemtcal engmeenng of CVD of HTc superconductors

198

in the gas phase. Even for large AT only partial oxidation of solvents occurs. Analysis of the exhaust gas frozen in the liquid nitrogen trap showed that 80 - 90 % of the solvent passed the reactor without damage. Carbon contamination was found by ESCA and AES in the range of some wt%. X-ray investigations were made with a sliding X-ray beam in order to increase the sensitivity, but no BaCOs was found indicating that the carbonate is amorphous in the layer. The carbon content probably influences the superconducting properties as Fig. 6 shows, where smaller critical temperatures were measured, in spite of the fact that the X-ray investigations showed mainly the superconducting phase. These samples were made in the temperature evolution range. 0L

r

=

i

w

R.o4 1 2 -Oa 3

t00 ZO

Te.~>ratur~, K

.

Fig. 6: Magnetic susceptibility data for films obtained in conditions of temperature evolution, temperature reached: 860 °C (1), 910 °C (2), 920 °C (3) The main aim of further investigations is the minimization of the carbon content. One possibility is the use of solvents with smaller quantity of donor atoms like tetrahydrofurane. Then higher Tc and high current densities can be expected and the method may be used for industrial applications.

REFERENCES 1. F. Schmederer, G. Wahl, Joum. d. Physique Coil C5, suppl. 5 (1989) 117 2. F. Schmaderer, R. Huber, H. Oetzmann, G. Wahl, Proc. XI Int. CVD, Conf. ed. by K. E. Spear, G. W. Cullen, 1990, ECS, Pennington, 211 3. W. Decker, A. N0rnberg, M. Pulver, R. Stolle, G. Wahl, Yu. Yu. Erokhin, O. Yu Gorbenko, I. E. Graboy, A. R. Kaul, M. Sommer, U. Vogt, Proc. of the XII Int. CVD Conf., ed. by K. Jensen, G W. Cullen, ECS in print 4. Fluent V3.03, Cream Inc. Fluent Europe, Hutton's Building, Sheffield, $1 4ES 5. F. Schmaderer, R. Huber, H. Oetzmann, G. Wahl, Appl. Surf. Sci. 46 (1990) 53 6. F. Schmaderer, R. Huber, H. Oetzmann, G. Wahl, Journ. d. Phys. IV Coil C2 Supp. on Journ. d. Phys. II, 7 (1991) C2-539 7. E. Fitzer, H. Oetzmann, F. Schmaderer, G. Wahl, Journ. d. Phys. IV, Coll. C2 Suppl. 2 (1991) C2-713 8. R. Hiskes, S. A. DiCarolis, J. L. Young, S. S Laderman, R. D. Jocowitz, R. C. Taber, Appl. Phys. Lett. 59 (1991) 606 9. H. Kuttruff, Physik und Technik des Ultraschalls, Hirzel Verl. Stuttgart 1988 ACKNOWLEDGEMENT The financial support provided by the DAAD and BMFT in Germany is gratefully acknowledged.