Page 267 - Chalcogenide Glasses for Infrared Optics

242 Cha pte r Ni ne

Seed

Rectangular/square boat Transition
region
FIGURE 9.21 Boat with seed used to form GaAs face-oriented plate.

the only chance for success. Placement of the seed relative to the
melt was critical. The first two attempts failed while the third was
partially successful. The left drawing in Fig. 9.21 shows the design
of the quartz boat used. The right photograph shows the plate after
it was removed, ground, and polished. Note the first half from the
seed is almost all single with a twin line extending from the neck,
causing the lower half to be poly. A sample cut from the single area
evaluated by X-ray failed to confirm the <100> orientation. The
conclusion reached was that the horizontal Bridgman in present
form would not perform the desired oriented single-crystal growth.
Fundamentally, the modification of the equipment and procedures
would lead to eventual success.
As the LMCO design advanced, the shape of the required window
changed to that of a dome, not flat, and the size increased. The require-
ment was right at the edge of our capability. Glen Whaley designed
and fabricated a quartz chamber we all felt would produce a GaAs
blank suitable for fabrication of the dome-shaped window. We recog-
nized that there was risk involved. Because of the importance of this
program, we looked for a better solution.
Because of the good relationship Greg Whaley had built up with
Bob Ochrym, head of sales at Litton/Airtron, the largest supplier of
GaAs in the United States, they agreed to supply AMI with an
extremely large crystal. It had twinned during production so the price
was modest. The crystal had an 8-in diameter and weighed 25 kg. The
arrangement included the head of Airtron, Dave Miller, and head of
production, Larry Urick. AMI ground and polished the crystal and
evaluated its quality. The crystal then went to Exotic Materials for
fabrication and coating.
AMI has continued its relationship with the big producers and
still serves as a supplier of GaAs to Lockheed Martin for its
Arrowhead helicopter program. All material is 100 percent checked
for quality by AMI. Required blanks are polished, ir transmission
is measured, and tested by laser transmission for the presence of
precipitants.