Page 238 - Chalcogenide Glasses for Infrared Optics

214 Cha pte r Ni ne

was designed and built. The chambers are all made entirely of quartz,
assembled in the TI glass shop before placement in the three furnaces.
The two source furnaces are controlled at temperatures above that of
the furnace around the center reaction zone. The two chambers serving
as sources are constructed so the saturated solutions come from the
bottom and contain no solid CdTe. The entire apparatus, furnaces
and reaction chamber, is placed on a rocking platform and rocked so
that solution liquid is moved back and forth across the substrates. As
the hot liquid leaves the source chamber, it cools and is depleted of
CdTe, adding to the layer on the substrates. At first the rocking rate
was 2 times per minute over a period of hours. One reaction run was
18 h with the source temperatures at 900°C and reaction chamber at
700°C. The results were very disappointing. The substrates did not
grow into a plate and, in fact, dissolved to a great extent. The conclu-
sion reached was that the temperature differences between substrate
and solutions were much too great and the rocking speed was too
fast. Small conventional runs were resumed to establish the proper
growth conditions and to demonstrate that single-crystal growth
across the substrates would occur. Methods used in forming 2 in ×
2 in mosaic CdTe substrates are illustrated in Fig. 9.3. Stockbarger
formed CdTe crystals squared up by sawing and then cut into
squares or long strips. If the beginning crystals are all single, a square
substrate is formed in one case or a substrate is formed from long
sawed strips. Even if the material is not all single, good results may
still be expected if the poly is large-grain. Ideally, if the crystal is
completely single, an orientation may be found that that is favorable
for CdTe growth. The graphite holder for the substrates shown in
Fig. 9.3 has graphite strips at both ends to hold the substrate in place
because its density is less than that of the solution and it would float
away as the solution moved across. In all the runs, it was found the
substrate pieces did not grow together. One possibility to be pur-
sued was to alloy the pieces together before starting. Otherwise, the
Te-rich solution flows into the cracks between the pieces, preventing
growth.
9.2.1 Conclusions Concerning This Effort

1. High-resistivity layers of CdTe will grow from Te solutions if
Cd concentration is 20 percent or greater in the temperature
range of 700 to 750°C.
2. The volatility of Te is such that a closed system must be used
to avoid Te loss and solution concentration changes.
3. Results indicate it is necessary to alloy mosaic pieces together
prior to layer deposition attempts.
4. The temperature difference between solution and substrate
must be kept low to maintain equilibrium conditions.