Defect structure of ZnGeP₂ crystals grown in the furnaces with tube diameters of 70 and 130 mm

The crystal structures of two ZnGeP₂ crystals grown by the Vertical Bridgman method in different furnaces were compared. The furnace tube diameter was 70 mm for the first crystal and 130 mm – for the second one. The diameter of both crystals was 30 mm. The radial temperature profiles of the furnaces show different curvatures in the growth zone. For the first crystal the profile was slightly concave, whereas, for the second one, it was flat. The structure was analyzed by X-ray diffractometry and topography methods. In the first crystal, the dislocation density changed from 10² cm⁻² in the initial growth region of the sample to 10⁴ cm⁻² at the last-to-freeze end. Low-angle grain boundaries were observed. The second crystal contained only single dislocations. Both crystals showed growth striae. After thermal annealing (both crystals were treated under identical conditions), the second crystal showed a much better optical transparency than the first one.