Ultralow Gradient HGF-Grown ZnGeP 2 and CdGeAs 2 and Their Optical Properties
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MRS BULLETIN/JULY 1998
even in low gradients) and growth along preferred directions to facilitate fabrication of device crystals. Finally growth was performed in a two-zone, transparent furnace in order to monitor and control the seeding-and-growth process. These key aspects of our approach20 have led to the reproducible growth of large crack-free single crystals of ZnGeP2 and CdGeAs2 measuring 19 mm in diameter by 140 mm in length. Purification of starting materials, careful control of composition during synthesis and growth, as well as optimized growth parameters and postgrowth annealing, have contributed to substantial reductions in absorption loss: two to five times lower than the best results reported previously. As a result, mid-infrared (ir) lasers based on these improved crystals have generated record-level multiwatt output powers at conversion efficiencies as high 21 22 as 50%. '
Compound Synthesis
ZnGeP2 The compound-synthesis approach used throughout the majority of our ZnGeP 2 development effort has been a simple direct synthesis from the constituent elements. High-purity (99.9999%) zinc (Johnson Matthey), germanium (Eagle Picher), and red phosphorus (RASA) were used as starting materials: Zn and Ge were supplied in powder (-100-200 mesh) and flake (-10 mesh) form, respectively, and the particle size of the phosphorus was reduced to less
than 2 mm by grinding in an inert atmosphere. The reason for minimizing particle size was to maximize surface area to enhance the reaction kinetics. The powders were weighed into pyrolyticboron-nitride (PBN)-coated graphite boats and loaded into heavy-walled quartz ampoules that were evacuated (
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