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AMI Infrar ed Crystalline Materials 231

and Haisty were issued a U.S. Patent covering the use of chromium
doping. The author and colleague Charlie Jones, developed an
absorption method to measure chromium levels in GaAs using the
16
Perkin Elmer 350 instrument. Later a method for measuring the depth
of mechanical damage in polished GaAs surfaces was developed 17
using the far infrared Perkin Elmer 301 spectrophotometer. When still
at TI, the author and George Cronin started a program to begin cast-
ing GaAs plates. This effort led to a TI proposal with the help of Bob
Crossland, an optics manager in the TI EO division, submitted to the
Air Force Avionics Laboratory at Wright Field. The program was
based on a simple casting plate approach. Results for the first phase
18
were reported in 1975. Harold Hafner of TI worked with George
Cronin on the program. The author was not involved directly, only in
supplying the infrared refractive index data. The program continued
into 1977 when it ended after the Airforce Materials Laboratory chose
the Raytheon ZnSe/ZnS window as the Airforce Avionics Window.
The author took one last trip in 1977 for TI to Wright Field in an
unsuccessful attempt to save the TI program. Later, work on casting
GaAs windows continued at TI for several years. The TI process
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involved placing precompounded material in a large round chamber,
of up to 30-in diameter, containing a plate mold. The chamber was
filled with an inert gas (usually argon) under pressure. The GaAs was
heated to the melting point and allowed to form a homogeneous
melt. Sometimes the melt was covered with boron oxide to suppress
the loss of arsenic. Cooling was slow to encourage large-grain formation.

9.8 Production of GaAs at AMI
While in the process of building up the crystal silicon growth capabil-
ity, AMI also acquired equipment used for GaAs growth. The exper-
tise of George Cronin was used in getting production started. The
first unit was a surplus Czochralski TI silicon puller modified for
growing GaAs crystals. The method used was designated LEC (for
liquid-encapsulated crystals). The method is by far the most devel-
oped and widely used today. A diagram depicting the method is
shown in Fig. 9.14. First, GaAs has a melting point of 1238°C which
occurs under a pressure of 0.7 atm of As vapor. Arsenic sublimes at
613°C and gallium boils at 2237°C. The chamber is sealed except for
an opening where an inert gas is allowed to enter. The gas is either
helium or the less expensive argon. The boron nitride (BN) crucible is
loaded with the required gallium and /or mixed with GaAs scrap.
Pure boron oxide (B O ) is placed in the top of the crucible. The
2 3
required mass of arsenic is placed in the top of the chamber. The gallium
essentially is not volatile. When the chamber is heated, the gallium in
the boron nitride (BN) crucible is heated, and the heated arsenic pro-
duces vapor that combines with the gallium and continues to be absorbed