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CRYSTAL GROWTH FROM MELTS IN 0-G ENVIRONMENT 

L. D. Fullmer and R. M. Housley 

NORTH AMERICAN ROCKWELL CORPORATION 
THOUSAND OAKS, CALIFORNIA 



ABSTRACT 

The value of highly perfect single crystals to 
technology is discussed. Many crystals used sell 
in the range $10,000 - $100,000 per po\ind. Many 
potentially useful crystals which cannot be 
grown on earth because the melt cannot be con- 
tained could be grown from containerless melts 
in 0-g. Very desirable increases in perfection 
of other crystals currently being used could 
probably be achieved in 0-g. The factors limiting 
perfection and their relation to gravity are 
discussed. Different techniques of growth in 0-g 
are discussed and a very versatile crystal pulling 
apparatus which combines the advantages of float 
zone refining and Czochralski growth is schematically 
described. 



PROMISE 

Of the many possibilities which have been suggested for space manu- 
facturing it seems to us that crystal growing offers the most promise 
of rapid economic success. Many crystals now used in technology for 
such purposes as lasers, laser modulators, semiconducting devices, 
transducers, substrates for large scale integrated circuits, etc., sell 
for prices in the range of $10,000 - $100,000 per pound. The fact that 
these prices are paid gives a measure of the technological value of the 
crystals . 



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Many potentially useful crystals have not realized their promise hecause 
it has not proven possihle so far to produce them of suitable size and 
perfection. In a number of cases this seems to he solely due to the 
unavailability of a crucible material that will hold a melt from which 
the crystal can be grown. Noteworthy examples of difficult to contain 
melts are transition metal oxides including rare earth iron garnets and 
compounds with high alkali and alkali earth content. 

We believe crystals of this class can be grown relatively easily from 
containerless melts in space and that the economic rewards from such 
an accomplishment would clearly be great . 

In many other cases the crystals currently used in technology are 
limited in perfection by fluctuations in composition and/or the 
accompanying strains. Semiconductors, for example, are very sensitive 
to dopant levels (carrier concentrations) and even small fluctuations 
can render the device useless. Ferroelectric crystals used for laser 
modulation, second harmonic generation, and other nonlinear effects are 
extremely sensitive to fluctuations in stoichiometry which induce 
domain structure and/or result in birefringence variations. This 
effect causes detrimental optical losses in the device. 

There are at least three well recognized causes for these composition 
fluctuations in melt grown crystals l) Faceting results from the 
irregular nucleation of new growth along low index planes in a low 
temperature gradient. 2) Constitutional supercooling results from the 
instability of the growth front to a perturbation in the composition of 
the melt under conditions of high temperature gradient. In extreme 
cases this leads to the formation of trails of precipitated solute. 
3) Temperature fluctuations leading to compositional fluctuations 
result from turbulent or oscillatory^ convection in the melt. 

These convective thermal oscillations during growth have long been , 
known to lead to oscillations in the carrier density of semiconductors. 
Growth striations due to this cause have also recently been positively 
identified in-'-'^ rare earth doped CaF2, Nd-CaWOl)., Ba2NaNb50i2, 
Nd-Y3Al50i2, Cr-Al203, and Cr-MgAl20l^ . 

Convection and the associated oscillations would of course be absent 
in 0-g. Absence of convection and the constraints imposed by a melt 
container in 0-g also offer the possibility of arranging the heater and 
insulation geometries in such a way that the freezing isotherm is not 
near parallel to any low index crystal planes and at the same time the 
temperature gradients are small. This would minimize problems due to 
both faceting and constitutional supercooling. Therefore, there is 
real promise that better quality crystals of these materials can be 
grown in o-g. 



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Modest improvements in crystal quality resulting in devices with 
improved characteristics or higher yields of acceptable devices could 
again have a tremendous economic importance. 

TECHNIQUES 

Conceptually the simplest way of growing crystals in 0-g is from a 
seeded free floating melt. This possibility was mentioned by several 
speakers last year. Consideration of surface energies indicates that 
the seed and growing crystal would always stay inside the spherical melt 
until the maximum dimension of the crystal corresponded to the diameter 
of the melt. Perfection of crystals obtained by this technique would 
be limited by faceting and constitutional supercooling as are other 
melt grown crystals. 

The cooling time necessary to grow high quality crystals would certainly 
be measured in hours. Clearly some method of keeping the melt centered 
in the furnace for this length of time would be needed. Magnetic 
induction and gas jets have been suggested as means for positioning such 
a melt. Both would probably lead to temperature fluctuations in the melt 
and hence imperfections in the crystal. 

In addition the constant attention of an operator would be necessary to 
keep the melt centered. Alternatively, a servo system could possibly 
be devised but this would involve an additional development of sensors 
and control circuitry. It might be difficult to design sensors and 
positioning devices compatible with a furnace which will provide the 
necessary temperatures and temperature gradients. 

All of this is not to say that good crystals could not be grown from 
a free floating melt. It is only to convince you that the modified 
Czochralski method which we propose is actually simpler as well as far 
more versatile. 

A schematic drawing of o\ar design is shown on slide 1. It can also be 
looked at as a modification of the floating zone principle to incor- 
porate the advantages of the Czochralski method. These are the use of 
a well crystallized seed and the ability to neck down the crystal to 
prevent the propagation of twin boundaries, dislocations, etc. 

The basic equipment would be two opposing crystal pullers with reversible 
and varying pull rates and a suitable heating arrangement with controls 
and programmer. The insulation would be designed and fabricated from 
suitable materials for the specific crystal pulled. A versatile puller 
would have interchangeable containers for a variety of atmospheres, i.e., 
reducing or oxidizing. This design would not require positioning of 
the melt . 



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Slide 2 shows the operation of the puller. The end of a large, pre- 
pared ingot or crystal would be positioned and melted in the furnace. 
The ingot could be prepared by pressing and/or sintering a combination 
of materials yielding the right chemical composition. After the melt 
is obtained and thermally stabilized, the opposite puller would intro- 
duce a seed into the melt. The melt and seed could be simultaneously 
moved within the furnace to establish the proper gradients, etc. The 
seed piiller would then "pull" the crystal as in the Czochralski tech- 
nique. As the material is removed from the melt, the feed rod puller 
could replenish the supply at a steady rate. We believe a crystal 
puller of the type described here could be engineered, built, and tested 
within a year, and hence could easily be ready to go on a 1973 flight. 
Space Manufacturing Process Chamber #2 appears to have enough room for 
equipment designed along fairly conventional lines. 

One final subject must be discussed. That is the source of heat which 
will be used. This has two aspects, the type of heating element in the 
fiornace and the ultimate power soxirce. Each useful crystal produced by 
melt growth will require from several kilowatt-hours to several tens of 
kilowatt-hours of energy. The only ultimate soiirces of power which 
appear practical are solar power and nuclear power. If nuclear power 
is used, the heating elements could be of any conventional type, i.e., 
resistance, electron beam, glow discharge, induction, etc. This has 
obvious advantages . 

If solar power is to be used, it seems that high priority should be 
given to design of such a furnace and a Space Manufactin-ing Process 
Chamber compatible with it. A polar orbit would probably be required 
for the successful use of solar power. Solar power might have advan- 
tages for certain, refractory insulating materials, but glow discharge 
heating should also be considered. 

Since in the 1973 tests only battery pack power will be available, 
experimental rions should be made with a relatively low melting point 
material to limit power consumption. The material should also have 
been thoroughly studied on earth so that the results can be interpreted 
unambiguously. Many materials qualify, for example, KCl. 

Production of say 100 crystals with a value of several million dollars 
would require of the order of 1000 kilowatt-hours of energy. Assuming 
this power is available, quantity production could begin in the 1975 
space station. 

A real evaluation of the cost effectiveness of crystal growth from melts 
in space depends on a realistic assessment of the power cost. 



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References 

1. B, Cockayne, J. Crystal Growth 3_, ^, 60 (1968), and references 
therein. 

2. W. R. Wilcox and L. D. Fullmer, J. Appl, Phys. 36, 2201 (I965). 

3. D. T. J. Hurle, J. Gillman, and E. J. Harp, Phil. Mag. lk_, 205 
(1966). 

k. D. T. J. Hurle, Phil. Mag. 13, 305 (1966), and references therein. 



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Pulier head 



Puller 



Insulation^ 



Pull rod- 
Puller head" 




Fig. I Pull for 0-6ravity Environment 



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Fig. 2 Procedure for Crystal Growth in 0-Gravity Puller 



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