GROWTH OF CDZNTE SUBSTRATES FOR HIGH PERFORMANCE IR DETECTORS

CDZNTE SUBSTRATES ARE REQUIRED FOR THE PRODUCTION OF CDHGTE IR DETECTORS. FOR THE SECOND GENERATION SYSTEMS REQUESTED BY THE ARMY, THE CDHGTE EPITAXIAL LAYER NEEDS TO BE GROWN ON LATTICE-MATCHED SUBSTRATES. HOWEVER, THE DEFECT DENSITY IN THE CURRENTLY AVAILABLE SUBSTRATES IS HIGH, AND THE EXISTENCE OF LARGE SIZES IS LIMITED. THE PROBLEM IN GROWTH OF CDZNTE CRYSTALS IS THE INABILITY TO PRODUCE AND MAINTAIN A CONVEX SOLID-LIQUID INTERFACE BY THE VERTICAL BRIDGMAN PROCESS. THE PROPOSED PROGRAM IS TO ADAPT THE HEAT EXCHANGER METHOD (HEM[TM]) FOR THE GROWTH OF CDZNTE SUBSTRATES. IT HAS BEEN SHOWN N CDTE CRYSTALS THAT GROWTH BY THE HEM RESULTS IN A CONVEX SOLID-LIQUID INTERFACE THAT CAN BE MAINTAINED DURING THE CRYSTAL GROWTH CYCLE. IT HAS ALSO BEEN DEMONSTRATED THAT CENTIMETER-SIZE GRAINS ARE PRODUCED EVEN IN UNSEEDED GROWTH. THE DEFECT DENSITY IN CDTE CRYSTAL GROWN BY THE HEM IS \ 10(3)/CM(2), MUCH LOWER THAN THE 10(5)/CM(2) IN COMMERCIALLY AVAILABLE SUBSTRATES. IT IS INTENDED TO USE SIMILAR PROCEDURES FOR CDZNTE SUBSTRATE GROWTH. BECAUSE OF THE SOLIDUS-LIQUIDUS SEPARATION IN THE CDZNTE SYSTEM, THE VARIATION IN ZN CONCENTRATION IS EXPECTED TO BE MINIMIZED BY EMPHASIZING HIGHER TEMPERATURE GRADIENTS DURING GROWTH. THE DEFECTS DUE TO TEMPERATURE-GRADIENTINDUCED STRESSES CAN BE MINIMIZED BY IN SITU ANNEALING AFTER THE HEM GROWTH AND PRIOR TO COOLDOWN. AT THE END OF THE PROPOSED PROGRAM, IT IS INTENDED TO ESTABLISH FEASIBILITY OF PRODUCING CDZNTE SUBSTRATES FOR IR DETECTOR SYSTEMS REQUIRED BY THE ARMY.