Page 259 - Chalcogenide Glasses for Infrared Optics

234 Cha pte r Ni ne

600°C 1250°C
Seed
Arsenic Melt

Heater coils
View through
alumina tube
Quartz wool

Quartz Quartz boat Seed
chamber Gallium Arsenic
vapor

Quartz support for boat Mullite support
Alumina tube tube
Hot Cool
Heater coils

Direction of heater
movement during growth
FIGURE 9.16 Diagrams depicting the horizontal Bridgman machine.

of quartz in shape and lined with quartz cloth. The boat may be
made from boron nitride (BN) to avoid Si as an impurity in the melt.
AMI purchased an expensive BN boat 4 in × 10 in × 1 in for plate pro-
duction. The process simply described is thus: as arsenic is heated, it
sublimes into the chamber containing the gallium and is absorbed,
reacting with the gallium. The gallium temperature is slowly raised
to the melting point of GaAs, 1238°C, and the arsenic continues to be
absorbed until the compounding is complete. A small amount of
arsenic in excess maintains the required vapor pressure that also pre-
vents the collapse of the quartz chamber on the melt side. When com-
pounding is complete, the heater begins to move, slowly lowering
the melt temperature beginning at the seed end. Thus, the crystal
growth is controlled at the melt-solid interface. This fact is very
important. In the plate casting process, control at the liquid-solid
interface is not possible, so the end plate may have very large grains