Large Plate CdTe Synthesis by Sealed Vessel Transport.

A crucial problem in the volume production of U. S. Government FLIR systems is the availability of mercury-cadmium-telluride HgCdTe detector arrays. The conventional method for preparing CdTe crystals is too slow. The purpose of this program will be to develop an alternative approach. A compounding process originally developed for the preparation of large plates of IR transmitting chalcogenide glass has been applied to cadmium telluride. A three zone furnace system is used to vaporize, condense and compound near stoichiometric quantities of 5-nines grade cadmium and tellurium. A controlled directional freeze results in circular plates up to 25 cm in diameter which are free of voids or gross inclusions and contain large areas of single crystal material. A heat treating process under cadmium and tellurium vapor has been developed which results in a 100 fold improvement in infrared transmission over non heat treated material. Absorption coefficients less than 0.002cm have been observed from 2.5 micrometers wavelength on samples ranging up to 2 cm in thickness. The attainment of near theoretical transmission, as expected, is dependent upon the sample thickness, anneal temperature, pressure of cadmium or tellurium used in the anneal, and length of time of treatment. Details of the compounding process as well as the annealing process are included in the text.