Page 236 - Chalcogenide Glasses for Infrared Optics

212 Cha pte r Ni ne

Report and in an Airforce Compendium by Charles Sahagian and
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Carl Pitha. These are only a few of the many such works reported
during the 1970s. The understanding of the critical factors related to
infrared optical materials use with lasers was advanced markedly.
The TI glass 1173 was under consideration because it could be
cast in large window form, something not possible using the Kodak
hot pressed ceramic technique. The technology had not developed at
this point to produce large plates of melt-formed crystalline materials.
Kodak Irtran 6, CdTe, transmitted with no absorption out to 30 µm,
farther than most all other materials being considered. As a candidate
material early in the Airforce Program, the size of CdTe produced
was emphasized. The Kodak limit was a 6-in diameter. The Airforce
Materials Laboratory was interested in any scheme to produce large
CdTe plates.

9.2 Previous Work at TI
TI at that time was growing single-crystal small-diameter CdTe disks
by conventional means to be used as substrates for growth of the new
infrared detector material HgCdTe in thin layers by liquid epitaxy.
The effort at that time, championed by Dick Reynolds of TI, was to
replace mercury-doped germanium as the detector material of choice
for infrared systems. The author in 1971 at TI submitted a proposal
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and won a program funded by the Airforce Materials Laboratory to
develop a method to prepare large plates of single-crystal CdTe suit-
able for application with high-energy CO lasers emitting at 10.6 µm.
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The method chosen was based on solution epitaxy growth on a
mosaic of single-crystal CdTe substrates. An excellent analysis of the
theoretical considerations governing solution growth is found in an
article by Tiller. In general, a solution of Te saturated with a specific
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concentration of Cd at a high temperature is cooled and placed in
contact with a substrate held at a lower temperature. The layer grows
on the substrate but slows as the solution is depleted, depending
upon diffusion for continued slow growth. Application to CdTe
growth is depicted in the TI process shown in Fig. 9.1. In the diagram,
the graphite boat used to hold the CdTe substrate is fitted with a
graphite slide that contains a solution chamber. The entire apparatus
is contained in a quartz chamber. Because the volatility of cadmium
is much greater than that of tellurium, a solution rich in tellurium
with cadmium concentration less than 20 percent was chosen. In
position 1, the entire apparatus is heated to a liquidus temperature
above that for the CdTe solution, above 800°C. Cooling starts and
proceeds until the desired solution temperature is reached and the
slide is moved to position 2, the substrate is covered by the solution,
and the layer begins to grow. Growth is slow and allowed to continue
for a period of time. As the solution saturated with Cd next to the
substrate becomes depleted, growth depends upon Cd diffusion