The production of high quality crystals in space is a promising near-term application of microgravity processing. Gallium arsenide is the selected material for initial commercial production because of its inherent superior electronic properties, wide range of market applications, and broad base of on-going device development effort. Plausible product prices can absorb the high cost of space transportation for the initial flights provided by the Space Transportation System. The next step for bulk crystal growth, beyond the STS, is planned to come later with the use of free flyers or a space station, where real benefits are foreseen. The use of these vehicles, together with refinement and increasing automation of space-based crystal growth factories, will bring down costs and will support growing demands for high quality GaAs and other specialty electronic and electro-optical crystals grown in space

Randolph, R. L.

English

NASA Technical Reports Server

PRODUCING GALLIUM ARSENIDE CRYSTALS IN SPACE 
Richard L. Randolph 
Microgravity Research Associates, Inc. 
Coral Gables, Florida 33 134 
ABSTRACT 
The production of high quality crystals in space is a promising near- 
term application of microgravity processing. Gallium Arsenide is the selected 
material for initial commercial production because of its inherent superior 
electronic properties, wide range of market applications and broad base of 
on-going device development effort. Plausible product prices can absorb the 
high cost of space transportation for the initial flights provided by the 
Space Transportation System. The next step for bulk crystal growth, beyond 
the STS, will come later with the use of free flyers or a space station, where 
real benefits are foreseen. The use of these vehicles, together with refinement 
and increasing automation of space-based crystal growth "factories," will bring 
down costs and will support growing demands for high quality GaAs and other 
specialty electronic and electro-optical crystals grown in space. 
INTRODUCTION 
As the demand grows for faster and larger integrated circuits, interest 
is intensifying in electronic materials which offer promise of surpassing the 
limits of silicon based semiconductor technology. Gallium arsenide (GaAs), 
is generating particular interest because of its inherent advantages in terms 
of high switching speeds, low power dissipation, temperature tolerance and 
radiation resistance. Also, GaAs has the advantage that it emits coherent light. 
These advantages will continue to stimulate GaAs device development in 
the years ahead. Increasing application of GaAs semiconductor technology is to 
be expected in support of increasing performance demands for high speed signal 
and data processing, radiation hardened military systems, phased array radars, 
satellite communication, mainframe computers, fiber optic communications, 
VHSIC and VLSIC. 
GROUND-BASED METHODS OF CRYSTAL GROWTH 
Commercial crystal growth is accomplished by two processes, bulk and 
epitaxial. 
Bulk crystal growth methods can produce very large crystals, 200 pounds 
or more, but the process utilizes large, heavy furnaces, high temperatures 
(125O"C), high power (up to 300 KW) and high pressures (up to 150 atmospheres). 
Further, the crystals produced are plagued by imperfections and impurities. 
The crystal properties vary by over an order of magnitude per inch radially, 
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axially and azimuthally, and even so-called high-grade crystals contain 10 
15 
impurities and 105 vacancy sites per cubic centimeter. These imperfections 
degrade the electrical performance, reliability and useful life of electronic 
and electro-optical devices. 
Epitaxial crystal growth avoids many of the problems of bulk processing. 
Here the crsytal material is deposited layer upon layer on a crystal seed. The 
process achieves more uniform crystal structure but is very slow and produces 
only thin layers of small diameter (3 inch) crystals. Typically, epitaxial 
crystals have been layered upon slices of bulk crystal to fabricate devices, 
and is not compatible with growth of bulk quantities. 
Improvements in crystal growth technique for both bulk and epitaxial 
processes have proven very difficult to achieve. Even with silicon, a relatively 
simple material, improvements are still being sought after 30 years of develop- 
ment. The major barriers in bulk growth are in improving crystal uniformity 
and purity and in reducing defect density. The barriers for epitaxial growth 
are in scaling to larger sizes and increasing growth rate. 
The primary difficulty in surmounting these barriers lies in the fact 
that all growth techniques involve phase transformations from liquid to solid 
state and therefore involve density and temperature gradients. These gradients 
are the source of several different complex flow processes which cause spatial 
and temporal fluctuations in the growth of the crystal. Particularly significant 
are gravity driven convection currents which create disturbances that are 
particularly troublesome at the crystal growth interface. 
Many efforts have been made to suppress gravity driven convection 
currents, with only limited success. One method, developed at MIT, applies a 
strong magnetic field across the molten material to suppress the currents. 
This results in some slowing of convective flow by simulating an increase in 
the viscosity of the melt, but produces only p-type material. N-type or semi- 
insulating material has not been successfully produced by this method. Another 
technique employs very steep thermal gradients at the crystal growth interface. 
This approach has been effective in reducing stoichiometric oscillations and 
constitutional supercooling, but it cannot be scaled up in a gravitational 
environment without causing convection. Other methods are being tried and 
improvements are expected. However, because of the fundamental role played 
by gravity in fluid dynamics, improvements of Earth-based crystal growth 
processes are expected to be limited in scope, and concentrated mainly in 
structural improvements as opposed to compositional. In space, on the other 
hand, where the effects of gravity are all but eliminated there is promise that 
bulk quantities of epitaxial quality crystals can be successfully grown. 
LIQUID PHASE ELECTROEPITAXIAL CRYSTAL GROWTH 
The electroepitaxial method of crystal growth makes use of an electric 
current passed through the molten solution and the seed to force migration of 
the solute to the liquid-solid interface where growth occurs in epitaxial 
layers. Supersaturation takes place in the immediate vicinity of the growth 
interface and growth is controlled by the rate of transfer of solute, under 
the electric field, to the interface. This method appears particularly 
attractive for the processing in space of III-V crystals, such as GaAs and InP. 
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A major advantage is that crystals can be grown by this process far 
below the melting point of the compound. In the case of GaAs, crystals have 
been grown electroepitaxially at temperatures ranging from 800°C to 950°C. 
The lower temperature allows operation below the compositional instability 
region and reduces dissolved impurities. Also, the problem of arsenic vapor 
pressure is essentially eliminated. 
Further, the electric current allows control of growth rate and of 
doping concentration. Attainable growth rates under this process are adequate 
to support bulk production for commercial purposes. The electroepitaxial 
method, in the microgravity environment, permits the controlled growth of 
ternary and quarternary crystals. 
THE MARKET FOR GaAs 
Interest in GaAs as a desirable semiconductor material has existed 
over many years. As compared to Si, GaAs offers switching speeds up to 10 
times faster, power consumption of only 3 to 4 percent, higher and lower 
temperature tolerance, higher radiation resistance - and it emits coherent 
light. The poor quality of available material, however, discouraged GaAs 
device development effort until recently. Only in the past several years 
have better GaAs crystals become available from improved processing techniques. 
As a result, development activities aimed at marketable GaAs devices have 
accelerated rapidly. Some market analysts are now forecasting that world- 
wide sales of GaAs integrated circuits will exceed 3 billion dollars by 1995 
and approach 140 billion dollars by the year 2000. Important applications 
of GaAs devices are foreseen in support of advanced communications systems, 
high speed data processing, artificial intelligence, smart weapons and micro- 
wave signal processing, among others, where the special speed, power, temper- 
ature tolerance, radiation resistance and light emiting characteristics of the 
material will prove significant. 
Space produced GaAs will cost a great deal more than that produced on 
Earth, primarily because of the very high cost of space transportation. 
Applications for this special material are expected to be limited to devices 
requiring the utmost in electronic performance, reliability and lifetime, and 
where the cost of the material will represent only a minor portion of that of 
the finished system. These conditions will be present in such rapidly evolving 
areas as satellite communications, defense communications, mainline computers, 
artificial intelligence, high powered lasers, wafer scale integrated circuits 
and various smart weapons and defense systems. 
Although market demands for new materials are always difficult to 
predict, it is estimated that by 1990 requirements for space grown GaAs will 
be in the order of 30 to 50 kg. With expanding applications thereafter, and 
with other types of crystals being introduced to space processing. MRA 
expects the market to grow substantially thereafter. 
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ENHANCEMENT OF CRYSTAL GROWTH BY SPACE PROCESSING 
Semiconductor single crystals constitute the basic framework of solid 
state electronics, and their quality (chemical and structural) has been, and 
will continue to be, the rate determining factor in semiconductor device and 
system advances. Improvements in device characteristics over the years (in 
terms of speed, power, sensitivity, etc.) have stemmed directly from improve- 
ments of single crystal characteristics. Often, new device and system con- 
cepts have had to await needed advancements in single crystal quality and/or 
dimensions. The significance of crystal quality improvements made possible 
by space processing should be viewed in this light. 
Fortunately, crystal growth experiments in space have already demon- 
strated the marked improvement which can be achieved by growing the crystals 
in a microgravity environment. Also, improved capabilities for characterizing 
crystals and a better understanding of the art and science of crystal growth 
stemming from serious laboratory research in recent years, make possible a 
better evaluation of the quality enhancement achievable in space as well as 
the expected limits of Earth processing methods. 
Since no crystals have yet been grown in space using the electro- 
epitaxial method, actual measurements of the quality attainable must await 
the early experiments of the MRA joint endeavor. Ground based laboratory 
experiments, along with analysis of results of prior experiments in space 
using other growth techniques, do suggest that the quality of crystals grown 
electroepitaxially in space will be substantially enhanced over those grown 
on Earth. 
Particularly, it is expected that the experiments of the joint 
endeavor will demonstrate that bulk crystals produced electroepitaxially in 
space, as opposed to Earth grown crystals, will be compositionally and 
structurally homogeneous, striations eliminated, and the concentration of 
impurities and defects (of all types) greatly reduced. 
Since inhomogeneities, impurities and other defects degrade electronic 
performance of the crystal and lead to earlier breakdown, the availability of 
high quality crystals from space production can lead to important advances in 
device performance, reliability and useful life. 
THE NASA/MRA JOINT ENDEAVOR 
NASA and Microgravity Research Associates (MRA) entered their joint 
endeavor agreement in April 1983. The endeavor is to develop the electro- 
epitaxial method for growing semiconductor crystals in space. GaAs is 
identified as the crystal material of choice for this endeavor. 
The terms of the agreement specify that MRA will be responsible for 
developing the experiment to be flown and for providing the necessary growth 
furnaces and support equipment for conducting the experiments. Also, MRA is 
committed to commercialize the product after the completion of joint endeavor. 
NASA is responsible for providing seven flight opportunities without charge and 
for furnishing integration services. There is provision for an eighth flight 
if it is mutually agreed that it is needed. No exchange of funds is involved, 
85 
but NASA has access to the science resulting from the endeavor,and will 
receive some quantities of the crystal materials produced in space. Over 
the series of seven flight missions, the electroepitaxial process will be 
verified and refined and the furnace equipment will be tested, scaled up, 
improved and readied for post-endeavor commercial crystal growth operations 
in space. 
The last several flight missions of the endeavor (in the 1987-1989 
time period) will produce quantities of space grown crystals sufficient for 
wide-scale distribution to electronic materials laboratories for their 
evaluation, as well as to support initial sales to users of semiconductor 
crystal materials. 
SPACE TRANSPORATION SYSTEMS 
The Space Shuttle is well suited to support the research and develop- 
ment phases of the MRA program for producing bulk quantities of high quality 
GaAs crystals in space. All essential services, including electrical power, 
heat dissipation, and provision for monitoring and control systems, are 
present. These services will support the growth of 15, and possibly up to 
20 kilograms of GaAs crystal on flights which do not need to accomodate other 
significant users of electrical power. These production quantities will be 
sufficient for initial commercial production. As market requirements expand, 
however, the limited power available on the Shuttle and the relatively short 
duration of Shuttle missions will not be adequate to support needed pro- 
duction increases. By the early 1990s MRA must be looking to other space 
vehicles such as free flyers or a space station to accomodate its requirements 
for growing bulk quantities of crystals for the market. These vehicles will 
have greater amounts of electrical power available,and the growth process can 
continue over longer periods of time. 
The cost of space transportation is a1s.o a major consideration. 
Missions aboard the Space Shuttle, where furnaces and support equipment must 
be carried up and down, are very expensive and will require that the finished 
crystal product be marketed at a price much above that of crystals produced 
on Earth . The cost of production aboard Shuttle-serviced free flyers or a 
space station, where the furnaces will remain in orbit, are expected to be 
considerably less. This saving can reflect in lower market price which, in 
turn, will stimulate market demand. However, it should be noted that regardless 
of which mode is used, Shuttle sortie, or Shuttle-serviced free flyer or space 
station, transportation costs using the Shuttle Transportation System are the 
dominant part of production costs. These costs, unless significantly reduced, 
will keep the price of space products restrictively high. 
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Producing gallium arsenide crystals in space

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