Page 257 - Chalcogenide Glasses for Infrared Optics

232 Cha pte r Ni ne

Gas pressure

B 2 O 3

Heaters

GaAs melt
Sealed crystal puller (LEC)

FIGURE 9.14 The Czochralski sealed crystal puller LEC method.

by the melt during the compounding process. When the material in
the crucible reaches the melting point of pure GaAs, stoichiometry is
complete under an atmosphere of over 500 mm of arsenic pressure.
As shown in Fig. 9.14, the melt is covered with a layer of liquid boron
oxide the purpose of which is to suppress the vaporization of arsenic
from the melt. The compounding process is very slow, taking hours.
The next step is to grow the crystal. A rotating seed crystal is inserted
through the melted B O and makes contact with the GaAs melt, and
2 3
the growth begins. From start to finish, the process is very difficult
and may take more than 12 h. After cooling, the grown crystal is
covered with a layer of boron oxide that is easily removed. The AMI
operator of this process, Bob Harp, a former colleague of George
Cronin, retired from TI, was very experienced. Crystals with 3- to 4-in
diameter were grown in this machine, smaller than what was
currently produced and used for devices in the industry, but useful
for small infrared windows and lenses. The process was expensive
and less efficient if not run every day.
Another unit, referred to as a vertical Bridgeman, was built by and
purchased from a local company. A diagram of the unit is shown in
Fig. 9.15. The vertical Bridgman contains a melt in a quartz crucible or
one made from BN, in a vertical position. The top chamber holds the
required arsenic for compounding and pressure control. While the
chamber is spinning, the heater is moved up so that cooling and
growth proceed from the bottom up. A seed can be placed in a
receptacle in the bottom of the crucible to produce a single crystal of
a specified orientation. Instead of moving the heater, a method deve-
loped later by Kelly Burke and Russell Kremer, of Crystal Specialties, 20
controlled the energy in each turn of the heater individually. In this
way, the thermal gradient can be moved up the crystal without
any physical movement disturbing the melt, producing a crystal of